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Sleeper68
07-19-2020, 09:15 PM
I've been working on setting up my Camaro again after a front roll center height change. The car pushes quite badly now and does not roll much in the front. As a result the outer edge of the front tires is much hotter than the rest of the tire. So it appears that my roll rate split (front/rear) is too high. This brought up a thought about the relation of roll rate front to rear.

Some call this the magic number, some call it the balance, Milliken calls it "Total Lateral Load Transfer Distribution". The roll rate split is expressed as the percentage of the roll gradient taken up by the front suspension of the car.

What kind of numbers are y'all running? Do you set up your car for mid corner understeer, corner entry oversteer, etc?

dontlifttoshift
07-20-2020, 05:12 AM
Pushes where? Entry or exit or middle. Also, is this autox or trackday or both.

What were your roll rates before and after the roll center change?

Sleeper68
07-20-2020, 05:55 AM
Pushes where? Entry or exit or middle. Also, is this autox or trackday or both.

What were your roll rates before and after the roll center change?

The car pushes everywhere: entry, mid corner, exit. It is worst on entry and mid corner. This mainly pertains to autocross.

Previous front roll rate was about 3°/g. Not sure what it is now, I will have to calculate it. Probably somewhere near 2.75°/g now.

Front roll center was moved up about 1-1/8". This was mainly a consequence of improving the camber gain curve up front and was not the main objective, but it comes with the territory on a factory subframe car.

SSLance
07-20-2020, 07:32 AM
Two things stand out to me by what you's said so far.

First, the outer edges of the front tires hot is a clue you don't have enough caster in the car. When you turn the steering, the KPI overrides the caster and the outside front goes positive camber. Second, by raising the front roll center you've changed the diagonal roll and therefore the inside rear tire may not be disengaging enough and is still pushing hard enough to override the outside front tire.

With out good pictures of the car on entry and mid turn, it is really hard to tell...but the two above would surely make it push like a dump truck if extreme enough.

SSLance
07-20-2020, 07:39 AM
^ That is what the tuner in me would think and try to react to at the track.

Regarding actual TLLD numbers you are looking for when setting up at the shop, mine have changed so much since the last time it was put into simulation software I don't have an actual number any longer. I just know we work really hard at getting the front geometry perfect for the best contact patch at turn in and also getting the inside rear to release at just the right time and for the correct amount of time. These front heavy big HP cars we are messing with take a bit of a different approach that the small light 50/50 cars that are meant to do this kind of thing (and suspension gurus mainly talk about adjusting).

dontlifttoshift
07-20-2020, 07:49 AM
I'm not an engineer, I think it's important to note that. I am a firm believer that most push on entry is driver induced and have beat that drum for years here.........but you say it's tight everywhere and worse since you made the change in roll center.

That's not a huge change in roll center and I would have thought it would have been offset by better camber gain. Conventional theory would be with the higher roll center the car would want less spring or bar or both to get the roll rate back in line with the weight transfer at that end. This is math I have never done but comparing lateral weight transfer on the front before and after the RC change should tell you what you want your wheel rate to be in roll and then you can make an appropriate change.

Understeer through the middle is the worst......can't mash the gas if you are waiting for the front end to point the right direction. In general, loose is fast, you just need fast hands to keep up with it. I don't have those so neutral to loose on throttle is easiest for me to drive.

Sleeper68
07-20-2020, 09:08 AM
Two things stand out to me by what you's said so far.

First, the outer edges of the front tires hot is a clue you don't have enough caster in the car. When you turn the steering, the KPI overrides the caster and the outside front goes positive camber. Second, by raising the front roll center you've changed the diagonal roll and therefore the inside rear tire may not be disengaging enough and is still pushing hard enough to override the outside front tire.

With out good pictures of the car on entry and mid turn, it is really hard to tell...but the two above would surely make it push like a dump truck if extreme enough.

I don't think the issue is castor related since I have not changed that setting in years really. It is very likely the issue is related to the parallelism (or lack thereof) between the mass centroid axis and the roll center axis. Even so, I think the main issue is related to how the front and rear carry the lateral load.

I have attached pictures. The two of the car turning right are from the most recent event. You can see camber on the outside front appears to be positive. The front is rolling, but the front suspension is not really compressing. The picture of the car turning left is from March before the front RC change.

Static Alignment for all pictures:
-1.5° ±0.1° Camber
+6.1° ±0.1° Castor

0.000" ±0.015" Toe


^ That is what the tuner in me would think and try to react to at the track.

Regarding actual TLLD numbers you are looking for when setting up at the shop, mine have changed so much since the last time it was put into simulation software I don't have an actual number any longer. I just know we work really hard at getting the front geometry perfect for the best contact patch at turn in and also getting the inside rear to release at just the right time and for the correct amount of time. These front heavy big HP cars we are messing with take a bit of a different approach that the small light 50/50 cars that are meant to do this kind of thing (and suspension gurus mainly talk about adjusting).

Most suggest a TLLD related to the static front/rear weight distribution, and a roll axis parallel to the mass centroid axis. This tends to keep the car pretty well balanced regardless of static weight distribution front to rear. Only minor tweaks from there.

My fix for this issue is to raise the rear RC in proportion to the front RC movement to get the roll axis more in parallel with the mass centroid axis as it was before, and to add about -1/2° of camber. That should stiffen the rear up and hopefully migrate the balance away from understeer.

There is an autox next weekend where I'll test out the change fully. I am doing some street testing this week, but you can only go so far with that.

I appreciate the suggestions. Keep em coming. Very interested to see how other set up their rides to match their driving style. Learning as much as I can is always the goal.

Sleeper68
07-20-2020, 09:21 AM
I'm not an engineer, I think it's important to note that. I am a firm believer that most push on entry is driver induced and have beat that drum for years here.........but you say it's tight everywhere and worse since you made the change in roll center.

That's not a huge change in roll center and I would have thought it would have been offset by better camber gain. Conventional theory would be with the higher roll center the car would want less spring or bar or both to get the roll rate back in line with the weight transfer at that end. This is math I have never done but comparing lateral weight transfer on the front before and after the RC change should tell you what you want your wheel rate to be in roll and then you can make an appropriate change.

Understeer through the middle is the worst......can't mash the gas if you are waiting for the front end to point the right direction. In general, loose is fast, you just need fast hands to keep up with it. I don't have those so neutral to loose on throttle is easiest for me to drive.

I also thought the camber gain improvement (from -0.35°/in to -0.78°/in in bump, right off ride height) would offset the RC change as well. It appears I was wrong. Yes, I could reduce the front spring rate or front ARB rate to soften the front back up. My thought was to raise the rear RC up to match the front to get the balance back. The overall roll gradient would be less but not by much. The better camber gain curve should allow for less static camber with the stiffer total roll rate.

And yes, it was awful. The entry understeer made slaloms hard. Had to use alot of throttle and brake to get the car to rotate which made me sloppy.

The push in the pictures above was also annoying for the same reason you said. Made it hard to get in the throttle early. I tried to use throttle steering again which sometimes made me push out more than I wanted.

stab6902
07-20-2020, 10:31 AM
I am also surprised that your camber gain/roll center height change made the car push so much more. What did you do to increase camber gain? Did it have much impact on bump steer? Could your inside front be toe-ing in now under roll, giving you less front end grip than you had before?

CSG
07-20-2020, 10:33 AM
If you don't mind a less educated person joining in...
With the car pushing at entry it does not surprise me that you can't take advantage of a better camber curve. Once the tire slips you are not really going to transfer much weight to that corner. I like playing with the math and modeling suspension in CAD but I know I can get caught up in the numbers and they don't always transfer completely to the real world. As you already know you changed the balance of the car and I don't think you can really tell what you achieved unless you take some wheel rate away from that end (assuming you at least have a decent roll axis inclination). Like Donny said loose is fast. I have typically ran 1 hour wheel to wheel races and I can't drive a loose car that long at the level needed without messing up but man can I get a fast lap time.

Sleeper68
07-20-2020, 12:39 PM
I am also surprised that your camber gain/roll center height change made the car push so much more. What did you do to increase camber gain? Did it have much impact on bump steer? Could your inside front be toe-ing in now under roll, giving you less front end grip than you had before?

The UCA pivot was moved down 1/2" in rear - 5/8" in front, a variant of the Guldstrand mod if you will. This increases the angle of the UCA and makes camber gain more aggressive, by doing so the FVSA shortens and the RC moves up. Yes this change did affect the bumpsteer curve. I have adjustable height tie rod ends so I moved the outer tie rod down to a point where bumpsteer was minimized around ride height after moving the RC.

Camaros typically toe-out in jounce and toe-in in bump when the outer tie rod is not low enough. Conceivably "camaro style" bumpsteer could cause understeer if it was bad enough.

stab6902
07-20-2020, 01:00 PM
The UCA pivot was moved down 1/2" in rear - 5/8" in front, a variant of the Guldstrand mod if you will. This increases the angle of the UCA and makes camber gain more aggressive, by doing so the FVSA shortens and the RC moves up. Yes this change did affect the bumpsteer curve. I have adjustable height tie rod ends so I moved the outer tie rod down to a point where bumpsteer was minimized around ride height after moving the RC.

Camaros typically toe-out in jounce and toe-in in bump when the outer tie rod is not low enough. Conceivably "camaro style" bumpsteer could cause understeer if it was bad enough.

Okay good - sounds like you really thought it through. Some people switch to tall lower ball joints to increase camber gain, which exacerbates the stock bumpsteer issue if you don't make complementary changes to the tie rod ends.

dontlifttoshift
07-20-2020, 01:16 PM
My camber gain is more aggressive than yours and I run -2.7* static with about 7* caster. Yokohamas want more camber yet. Pic is on Goodyears and while it looks a tick positive, the tires wore well all weekend.
https://static1.pt-content.com/images/pt/2020/07/110317230_2742570539310029_6834837314811-1.jpg


I would soften the front before I raised the rear roll center. In general I want to work on the end of the car that is not working......removing grip from one end to make the other feel better is not the way to go fast. If the front sucks, that's what needs fixing, then work on the rear to find more speed. However...

You already made changes so lets see what happens.

Sleeper68
07-20-2020, 01:44 PM
As you already know you changed the balance of the car and I don't think you can really tell what you achieved unless you take some wheel rate away from that end

Or add wheel rate to the rear?


(assuming you at least have a decent roll axis inclination). Like Donny said loose is fast. I have typically ran 1 hour wheel to wheel races and I can't drive a loose car that long at the level needed without messing up but man can I get a fast lap time.

I think my roll axis inclination is incorrect now since the front RC moved up and the rear did not move.

Sleeper68
07-20-2020, 02:12 PM
My camber gain is more aggressive than yours and I run -2.7* static with about 7* caster. Yokohamas want more camber yet. Pic is on Goodyears and while it looks a tick positive, the tires wore well all weekend.
https://static1.pt-content.com/images/pt/2020/07/110317230_2742570539310029_6834837314811-1.jpg


Car looks good in that corner. Rear is not rolling much more than the front. Outside front seems very happy. Inside front has a lot of camber to it.

This is close to how my car cornered before the change. Looks like your setup is stiffer than mine. My ride freq's are pretty low compared to alot of others, about 1.5Hz front 1.5Hz rear. This correlates to 600lb/in front springs and 185lb/in rear springs with my corner weights and motion ratios. Wheel rates: ~210lb/in front , ~100lb/in rear.


I would soften the front before I raised the rear roll center. In general I want to work on the end of the car that is not working......removing grip from one end to make the other feel better is not the way to go fast. If the front sucks, that's what needs fixing, then work on the rear to find more speed. However...

You already made changes so lets see what happens.

I essentially made the front a little stiffer, in addition to the camber gain improvement the car needed so I could run less static camber. I ran about -3° camber when I was on 245s. I went to less camber with the 275 Rivals to improve braking because the car would lock up the inside front badly before with so much static camber.

Why do you think the front sucks? I see it as the front and rear are not jiving well together.

Let's say my roll rate before was 3°/g front 3.5°/g rear and now it is 2.5°/g front 3.5°/g rear. The much stiffer front now could possibly be the cause of the understeer.

Now let's say your setup is 2°/g front 2.5°/g rear. Your setup is still stiffer than mine, but since the roll rates are more in line with your car's static weight distribution the balance is good.

So one could argue that by making my rear stiffer, I improve the balance of the car. This doesn't necessarily mean I am removing grip from the rear because my setup is still softer than yours.

Obviously the numbers may be off but I think this is what is going on. What do you think?

SSLance
07-20-2020, 02:14 PM
I think my roll axis inclination is incorrect now since the front RC moved up and the rear did not move.


I would agree with that, the pictures really show it.

See how the inside rear is up and front is planted here?

https://www.pro-touring.com/attachment.php?attachmentid=177850&stc=1&d=1595263504

And see how the inside rear is planted and the front is rolling more than the rear here?

https://www.pro-touring.com/attachment.php?attachmentid=177848&stc=1&d=1595263479


This is what my car looks like on corner entry, this is a 1.2G turn at about 50 mph entry.

177859

177860

Notice how the outside front tire stays in negative camber even with roll and steering input and how the rear is rolling more than the front releasing the inside rear tire.

My front tires wear completely even and the car doesn't push anywhere. It's set about about -1.5* static camber and 9.75* positive caster. The front roll center is about ground high and migrates under the inside front tire with roll, the rear roll center is about 18" above the ground.

SSLance
07-20-2020, 02:27 PM
This picture shows clearly why I like running a bunch of caster. See the contact patch of the inside front tire. This along with a super stiff front sway bar really hook the front of this car up.

177861

It also corrects the contact patch of the outside front without having to run a ton of static camber which hurts braking as well as inside front contact patch.

dontlifttoshift
07-20-2020, 02:59 PM
Lance, I really don't see how you can make those assumptions (about Sleepers car) without the context of what is happening with the throttle or brake pedals. I'll let you guys talk about RC migration and ride frequencies and how much more the front is rolling than the back or vice versa. I have nothing to offer there.

Sleeper, you said the front [pushes all the time] sucks, not me. Before you had this issue, if you raised the rear roll center did the car loosen up?.......that wasn't because it had more grip. It may understeer less, but I doubt it will be faster. It's all theory till proven otherwise.

SSLance
07-20-2020, 03:26 PM
I agree Donny, lotta wild arse assumptions made in my diagnosis. I'll try to be better about that. :D

That said, the front tire contact patch when turned is not optimal and I'd work on fixing that before changing the rear roll center.

Sleeper68
07-20-2020, 05:35 PM
Sleeper, you said the front [pushes all the time] sucks, not me. Before you had this issue, if you raised the rear roll center did the car loosen up?.......that wasn't because it had more grip. It may understeer less, but I doubt it will be faster. It's all theory till proven otherwise.

The car was at neutral throttle in all three pictures.

I haven't moved the rear RC then tested it in competition. Regardless, we know moving it up would loosen the car up.

A tight car made looser will almost certainly be faster assuming we don't overdo it. A loose car made looser probably won't be faster. Don't know if we're in the theory realm with that one, I don't think so. Seems to me that has been proven time and time again over the years.

I understand your point that softer=more mechanical grip, and I agree. I'm not advocating for a super stiff ride here. I think my setup is softer than alot of people's on here.

What would be your suggested next step?


I agree Donny, lotta wild arse assumptions made in my diagnosis. I'll try to be better about that. :D

That said, the front tire contact patch when turned is not optimal and I'd work on fixing that before changing the rear roll center.

In an effort to keep the numbers out of it:

I agree the front outside tire is not happy. I do not agree that it is the fault of the front suspension entirely.

The front and rear suspension interact and need to work well together. It seems reasonable that a change to the front of a car necissitates a change to the rear in the same direction if the car was fairly neutral before.

I don't see an additional couple degrees of caster solving the problem. That amount of caster will have no appreciable effect at the steering angles we are dealing with (0-30° or so), in my opinion. It would take a full 90° of steering to get the added 1 or so degrees of camber from the caster/SAI split.

Your front RC is very low compared to mine. I think that is why your stiff front ARB works. Our rear RC's are comparable.

What would be your suggested next step?

I appreciate everyone's input. Discussions like this are good and strengthen our understanding of how these complex metal boxes on wheels work.

dontlifttoshift
07-21-2020, 05:00 AM
I mean, you're not wrong about "the front changed, so lets make the rear match" I would just prefer to get back the grip that was lost in the front before I took it away from the rear to get the balance back.

My next step would be to get the car as low as possible, add enough spring to keep from crashing the bumpstops and slam all camber and caster I could into the alignment with maybe a tick of toe out........Then run the biggest front bar I could so the car is slight understeer steady state and then add a little tiny rear sway bar so that I could adjust to oversteer as needed.........but I am a knuckle dragger and would rather drink than do math.

Like I said earlier, you made a change, lets see what happens. At the same time, like Lance said, your contact patch is broken. You are giving up all of your camber with body roll and so you are stuck riding on the outside edge......it is 100% a front suspension problem and your roll center is only a very small part of that.

Are you on crazy stiff shocks?

CSG
07-21-2020, 05:10 AM
I kind of do it the same as Donny. I set my front ride height where I want it making sure to keep the front roll center above ground at a minimum. I then set rear height. I will then set rear roll center to have whatever roll axis inclination I have guessed will work. I will then adjust corners somewhat for corner balance. I add in all the castor I can get which for me is normally 6 degrees. I drive the car and log tire temps. I start adding negative camber to whatever point the temps tell me (different per side). That makes my baseline runs. Then I start playing with wheel rate. I do like having pics of the car in action for tuning but in my opinion the car needs to be close before I start using that.

SSLance
07-21-2020, 06:50 AM
You are giving up all of your camber with body roll and so you are stuck riding on the outside edge......it is 100% a front suspension problem and your roll center is only a very small part of that.




Agree completely. Your car can only go around a corner as fast as the front tires will let it. My personal theory is not much different than Donny's, get the front as good as you can get it and tune the back to balance it to the front.







I don't see an additional couple degrees of caster solving the problem. That amount of caster will have no appreciable effect at the steering angles we are dealing with (0-30° or so), in my opinion. It would take a full 90° of steering to get the added 1 or so degrees of camber from the caster/SAI split.

Your front RC is very low compared to mine. I think that is why your stiff front ARB works. Our rear RC's are comparable.

What would be your suggested next step?



What caster do you have in the car now? And what spindles?

My car static camber is around -1.5*, with 9.75* of caster it gains to about -4* camber at 15* steering input and goes to about -5* when bump and body roll is added in. Caster makes a HUGE difference if you put enough in.

A good rule of thumb is to have enough caster to mask the KPI of your spindle. Say if you have a G body metric spindle that is around 9* of KPI, you need around 9* of caster to keep the camber from going positive with steering input. The amount of dive in braking and resulting camber gain also plays into this, it all has to work together.

My redneck way of setting this up is to pull the springs, set the car on turn plates and start cycling the suspension and putting steering input into it to see how it reacts with the nekkid eye. Bump the suspension 2" at the wheel and put 10-15* steering into it and put a camber gauge on and set it so the outside front is leaning into the turn. Beautiful thing about caster is, it will also lean the inside front tire into the turn whereas camber will do the opposite. (again, body roll and camber gain come into play as well but you get the point).

Sleeper68
07-21-2020, 09:01 AM
Like I said earlier, you made a change, lets see what happens. At the same time, like Lance said, your contact patch is broken. You are giving up all of your camber with body roll and so you are stuck riding on the outside edge......it is 100% a front suspension problem and your roll center is only a very small part of that.

Are you on crazy stiff shocks?

Yes we will see. I think getting the roll axis inclination like it used to be will make the car happy again, and I think it will handle better because of the improved camber gain, and thus more camber on the outside front while loaded up.

Yes the outside front contact patch is not good. That is why the car is understeering, we can all agree on that. Why the contact patch is not good is the topic of debate.

Two major car attributes changed between the two events pictured: 1. the front RC height and, 2. the camber gain curve. The camber gain curve improved so the only thing left that could cause issues is the front RC height and how it interacts with the rest of the car. To say that the roll center is a small part doesn't make sense to me because it is the only possible negative change from before.

My opinion on what we are looking at in the two later pictures is excessive jacking forces, due to a number of different things. The first major thing is the RC height. Jacking forces are determined by track width, acceleration, mass, CG height, and RC height. By moving the RC up, you increase geometric weight transfer "jacking" and decrease elastic weight transfer "sprung reaction". Because the springs are compressing less, the camber is not changing much. You can see this in the pictures. The front suspension is barely articulating. Instead the car is pivoting about the outside front tire. The second major thing is roll axis inclination. The rear is rolling about its roll axis much more than in the autox in March. The jacking forces are literally "picking the car up" and unloading the inside front. Thus, the rear is taking much more of the weight from the front. If we give that weight back to the front, from the rear, the jacking is counteracted and the front suspension articulates. From the articulation we gain camber, and the outside front is happy again. There are two ways to "give" weight back to the front, rear sway bar stiffening or rear RC height addition.

The shocks are Varishocks, the SS model. They are not very stiff IMO, probably somewhere near 0.6-0.7 damping ratio in the lower piston speeds. They are non-adjustable.


Agree completely. Your car can only go around a corner as fast as the front tires will let it. My personal theory is not much different than Donny's, get the front as good as you can get it and tune the back to balance it to the front.

What caster do you have in the car now? And what spindles?

You can only go around a corner as fast as whatever tire(s) lets go first. That could be any of the 4. In this case, it is my outside front. In other cases it could be inside front, outside rear, or inside rear if you have a FWD car (Ford Focus, Civic, etc.)

The car has about +6.1° of caster in it. Spindle is a GM short spindle. SAI is about 8.7°.

iadr
07-25-2020, 11:31 AM
Sleeper- you are filling your posts with words to fight the very good advice you got: to work on this through caster. Frankly I won't write more, because I am in awe of the politeness shown by others to your passive aggressiveness in this thread.

Sleeper68
07-25-2020, 03:45 PM
I appreciate everyone's input. Discussions like this are good and strengthen our understanding of how these complex metal boxes on wheels work.


I appreciate the suggestions. Keep em coming. Very interested to see how other set up their rides to match their driving style. Learning as much as I can is always the goal.


Sleeper- you are filling your posts with words to fight the very good advice you got: to work on this through caster. Frankly I won't write more, because I am in awe of the politeness shown by others to your passive aggressiveness in this thread.

I am sorry you feel that way. I tried to make it clear that I am open to everyone's ideas. I am always willing to learn from others and enjoys doing so. I also try to show my appreciation when others share their ideas. Even so, that does not mean I agree with those ideas.

I am not a passive aggressive person. Certainly it has been a goal to make my thoughts known on the subject so that we can can have a discussion.

It is not my intention for any of my posts to be taken as adversarial or patronizing. I love forums because they are a place where people of common interests can discuss and learn from each other.

Hopefully this helps to bring some context to my posts.

dontlifttoshift
07-28-2020, 07:05 AM
My fix for this issue is to raise the rear RC in proportion to the front RC movement to get the roll axis more in parallel with the mass centroid axis as it was before, and to add about -1/2° of camber. That should stiffen the rear up and hopefully migrate the balance away from understeer.

There is an autox next weekend where I'll test out the change fully. I am doing some street testing this week, but you can only go so far with that.



So what happened?

Sleeper68
07-28-2020, 07:55 AM
So what happened?

The rear RC was raised 1" from 12.5" above ground to 13.5" above ground. Kept the rear ARB full stiff. The front got some more camber and I changed the toe. Alignment for the event:

Camber: -2°
Caster: +6.1°
Toe: 1/8" out , measured at the rim edges, approx 0.4°

The car did really well overall. There were only two entries for CAMT so the event guys put me in CAMC. Ended up 3rd, about 0.45s off of 1st place. I have attached the results for CAMC.

The car would still push on some corner entries but I think it was driver induced to some degree. The car seems very well balanced now. I could get some more rotation when letting off neutral throttle slightly in some corners. Probably not loose enough for autocross, but I think it would do fantastic on a road course. This course favored right hand turns heavily but the car still worked the right hand side tires to a good degree based on tire temps. Unfortunately I was unable to get an infrared temp gun in time for the event. I have one on the way and will have it for the next event to log tire temps.

I will try to attach a link to a video of my last run (fastest) in the next post. I drove alot better overall too I think this time. Much smoother than last time and tried to shorten distance wherever possible. There is still alot of dead time in there where I need to be accelerating (either positive or negative) usually on corner entry. Definitely could have won the class if I used that dead time better. There is still a bunch of time out there.

Thanks to all for the suggestions. Certainly open to driving critiques.

dontlifttoshift
07-28-2020, 08:37 AM
Good! Keep chasing front grip.

Are you going to DriveAutoX at NCM? I won't be there but a good friend will be with a LeMans Blue 69 Corvette. Get a ride along or 10.

I can't offer driving critique, I've never seen you drive. Here are some tips.

Don't Coast! Your feet (in my case foot, I don't left foot brake) should always be pushing on a pedal to get the most acceleration in either direction.

Slow in = fast out. First one to the throttle wins.

Brake. Turn in. Pick up the throttle. Really concentrate on those being three separate things, with seat time, they will blend together seamlessly. This is especially important on a pushy car, lots of people think they are trail braking but they are really just asking too much of the front tires. So they end up grinding tires through the whole corner while the tenths click off and are very late to the throttle then.

Hit some cones, it's okay. But not with the front, try to run them over with the back tire.

Sleeper68
07-28-2020, 09:37 AM
I can't offer driving critique, I've never seen you drive. Here are some tips.

Video I mentioned:

https://youtu.be/Yu5EgJxH_7k

This may provide some insight

Thank you

dontlifttoshift
07-28-2020, 10:11 AM
Amateur analysis forthcoming.

Settle your hands down. The only time you needed to shuffle was for the rotunda.

Right off the bat, you were late with the steering input for the chicago box so you threw a bunch of wheel in it and did it harshly. Setting up further left would have given you more room to make that smoother.

That big right hand sweeper......why U no throttle? Generally late to the throttle every where.

If you added just 1 mile an hour everywhere, you would have won the class.

Lance will have more, he watches more video than I do.

SSLance
07-28-2020, 12:06 PM
Car looked good, pretty composed. I agree with Donny it needs to be driven harder. When I say "harder" I mean with more conviction on carrying speed, not necessarily harder on your inputs. Think attacking the course more but doing it in the smoothest way possible.

I can't tell if it's from the setup (left heavy car) or your steering inputs (harsher to the right) but the front tires don't like to turn right vs turning left. Try to be smoother with your initial steering inputs especially when turning right.

Entering the rotunda at about 17 seconds, the front was stuck pretty good until you added a bit more steering angle and the fronts protested immediately. This is where more caster would override the spindle KPI and help that outside front contact patch. In fact, as I watch the video back, every time your left hand gets up over the top of the steering wheel, the front tires lose grip.

Do you have coil overs on the rear? Ever corner balance the car with you in driver's seat?

Put a camber gauge on that left front in your driveway or garage and turn the steering wheel to the right until your tape on the wheel goes past 3 o'clock (it'll be even more accurate with driver weight in seat) and watch it go to positive camber at that point. Then put more caster in it and try that again. Trust me...you'll like it. And be much faster.

Sleeper68
07-29-2020, 05:45 AM
Settle your hands down. The only time you needed to shuffle was for the rotunda.

Right off the bat, you were late with the steering input for the chicago box so you threw a bunch of wheel in it and did it harshly. Setting up further left would have given you more room to make that smoother.

That big right hand sweeper......why U no throttle? Generally late to the throttle every where.

If you added just 1 mile an hour everywhere, you would have won the class.

Seems like a good assessment to me.

I agree. Late on turn in in some places, late on the throttle alot. I think the car can handle alot more throttle and using the brakes/accelerator to get the car to turn in better and rotate seemed to work when I managed to do it.

As far as the Chicago box, not sure how much more left I could get. Definitely late. I think I was being too careful to hit the inside cone with the right rear. Was probably a good 2-3 feet of the cone on the right.

I didn't use the throttle much on the sweeper because the front tires were talking. I will try adding throttle on turns like that next time and see what happens.

Sleeper68
07-29-2020, 06:14 AM
Car looked good, pretty composed. I agree with Donny it needs to be driven harder. When I say "harder" I mean with more conviction on carrying speed, not necessarily harder on your inputs. Think attacking the course more but doing it in the smoothest way possible.

I can't tell if it's from the setup (left heavy car) or your steering inputs (harsher to the right) but the front tires don't like to turn right vs turning left. Try to be smoother with your initial steering inputs especially when turning right.

Entering the rotunda at about 17 seconds, the front was stuck pretty good until you added a bit more steering angle and the fronts protested immediately. This is where more caster would override the spindle KPI and help that outside front contact patch. In fact, as I watch the video back, every time your left hand gets up over the top of the steering wheel, the front tires lose grip.

Do you have coil overs on the rear? Ever corner balance the car with you in driver's seat?

Put a camber gauge on that left front in your driveway or garage and turn the steering wheel to the right until your tape on the wheel goes past 3 o'clock (it'll be even more accurate with driver weight in seat) and watch it go to positive camber at that point. Then put more caster in it and try that again. Trust me...you'll like it. And be much faster.

Absolutely. More speed everywhere, more deliberate and accurate inputs while staying smooth.

The car does not have any coilovers. The rear is leaf sprung and the front uses traditional coil springs in the stock location.

I have checked corner weights, albeit before the Watts link and larger wheels. Should be close. Car was heavy on RF and LR by about 60lb compared to the complementary side. Static front weight dist. with me in it was about 56%. Car should be closer to 55% now and should be about 54% soon after I move the battery to the back.

I really need a carbon hood but man are they expensive. I'm hoping a blemished one comes up for sale soon.

For some reason, which I have not figured out yet why, the car doesn't really lose camber when you turn the wheel. It actually gains camber on the outside and loses it on the inside, pics attached. I don't think the push from more steering angle is related to the SAI/caster relationship, but it could be. I'll check and see what it does.

SSLance
07-29-2020, 08:11 AM
That actually doesn't look too bad. I wonder if it stays like that in dive and roll?

If the fronts are staying like that mid turn (in dive\roll), the next thing I'd be trying is a way to get the car to release the inside rear between turn in and apex. With a watts maybe lower the rear roll center so the body has a bigger lever to push with or if adjustable sway bar maybe stiffen it up some? My weapon of choice is generally more rear rebound on the shock to accomplish this.

Actually, scratch that... I'd need to see video of what the rear and front tires are doing while turning each direction before deciding on what changes to make. Seeing that it seems to turn left much better than turn right, a wedge or cross weight adjustment may help more.

How often do you take a passenger on runs with you? I'm betting on a right turn favoring course like the above, a passenger would net you at least a second if not more off your lap times. I once ran a very similar course on that pad at an Optima event and having a passenger bumped me up enough to get my name listed on TV in the autocross results.


https://youtu.be/TBvhqvUDphY?t=32

Geez, my car pushed SO bad back then!!

hquackenboss
07-29-2020, 09:14 PM
@sleepeer68

I read your thread, and I want to suggest a sequence of changes. To start, I have some observations about your car based on the photos, and your depiction it understeers everywhere.

You don’t need to calculate your roll angle. You have enough information to actually measure it.

I pasted a copy of the picture that you said is a 1.2G 50MPH corner and measured the roll angle. I did this as follows:
1. I pasted the picture into PowerPoint. If you don’t have Microsoft Office, you can use Google Slides, construct a triangle, and calculate the angle with high school trigonometry. With PowerPoint, you can do it without any calculations, by:
1 . draw a horizontal line. You can make the line width as wide as you want and pick a bright color.
2. I rotated the photo and moved the horizontal line until the contact patches of the inner and outer front tires were on the horizontal line. With a little trial and error, typing in decimal fraction angles, and move the horizontal line until the it is aligned with the contact patches of the inside and outside front tire, you get a picture like I have uploaded. It is a little hard to discern exactly where the outside contact patch is, so I might be a little off, and I recommend you reproduce my analysis on your own computer.

1. Based on my analysis, while I don’t know the actually cornering force, the measured roll angle (you can see the measurement from PowerPoint to the right of the picture) is (180 degrees - 177.6 degrees) = 2.4 degrees. My conclusion is you are in the right ballpark.
2. It may be too much of a stretch to judge conclusively that your car understands just from the picture, but since the roll angle is high enough to indicate the lateral force is pretty high, your front steering angle is pretty high, so yes, based on the photo, it looks like it understeers.

Recommendations:

1. I would not raise the rear roll center height under any circumstances. Accepting the trade-off of an increase in front roll center height to get a camber gain improvement is one thing, but it is the wrong thing to do to balance the steady-state handling.

2. I wouldn’t change the spring rates, or geometry at least, yet. Optimizing spring rates in these kinds of cars is harder than it looks, and if you haven’t done anything crazy, there are more important things. Same answer for making further geometry changes. Your geometry isn't way out of whack from what good practice is, I think you are still at the point where you can make dramatic improvements without messing with geometry.

3. Your alignment specs with this geometry (-1.5 degrees negative camber, ~6 degrees of caster) are not way off. I would leave them until you at least do the following:

4. I would increase the rear anti-roll bar (ARB) stiffness a bunch, to the point where it clearly oversteers. I would do this with no other changes to start. If I understood your posts, you said slightly increased the spring rate. Whether or not you did, I would not back that off. I would, to repeat what I just wrote, increase the rear ARB stiffness.

5. If you cannot do (1) right away, try reducing the front tire pressures. Especially for autocrossing, you can get to the low 20 PSI range. This will reduce the ride rate (the tire is also a spring, and reducing the pressure lowers the spring rate of the tire). But you are going to need to to (1).

6. Regarding locking your inside front tire, you should realize that if the car understeers, it is going to be more prone to inside front lock up because of the lateral and longitudinal load transfer. If you still have problems after increasing the rear ARB stiffness, then there are a few things you can do. First, is to reduce pitch under braking (this might not be enough, but it is easy to do if you have adjustable shocks compared to alternatives.
a. Increase front damper (shock) compression stiffness. If you happen to have 4-way adjustable shocks, adjust the low-speed compression stiffer.
b. Increase rear damper rebound (extension) stiffness. Again, if you have 4-way adjustable shocks, adjust the low-speed rebound stiffer.

7. The front end of the car looks a little high. I can see your headers are kind of low, and I doubt you want to scrape going over speed bumps, but I would lower it. This does three things: First, since with this geometry, the rate of negative camber increase under compression increases the more it is compressed, lowering it will further increase the negative camber on the outside tire for a given roll angle. If you have modeled your geometry, you can confirm this. if you don't think this is correct, please post a curve. Even better, if you do happen to take the front springs out, cycle one of the front assembly with the springs out (so you can move it from full droop to full compression, and plot it. See number 8 (below) regarding ball joints.

If you lower the car, which will increase the rate of camber change as the outside front suspension compresses, this will reduce the positive camber on the outside front tire. The other two things this does, is lower the CG of the car, and lowers the roll center.

8. Given you have a way of measuring and adjusting bump steer (I mean measuring it on the car, not just from the model), then I would use a taller upper ball joint, which will increase the negative camber gain. In fact, you can also use a taller lower ball joint, which will have the effect of lowering the car, and further increasing the rate of negative camber gain under compression. As a general rule, if the toe steer (bump steer) goes very slightly positive toe out under compression, then you get a sort of automatic counter-steer, which can make it a little easier to stay at max lateral without so many small steering corrections.

9.. I am guessing you have aftermarket lower control arms, which have some form of non-rubber bushing. If in the off chance you are using stock lower control arms and bushings, your cornering forces are high enough to compress them to the point where your actual camber on your outside front wheel is more positive than your calculation. The stock arms are probably fine, but you need to replace the bushings, and my strong preference is not urethane. I am focusing on the lower control arm bushings because they see higher load than the upper control arm bushings do, but both are important. Also check for any slop in the ball joints. A number of aftermarket units have enough slop to cause problems. Also, if you do have solid bushings, , it is crucially important if you have solid bushings that they move freely. Even urethane ones can be lubricated with a high-pressure lube, and some have grease fittings.

10. Beyond the above, there are a set of relatively easy things you can do with spring rate tuning using jounce bumpers, and some other things, but rather than provide more opinions, the first step is to get to a near-neutral balanced car by increasing rear roll stiffness, and then the other things listed.

I have an observation in general about comments on the thread, and what I see from people that look at Milliken Moment Diagrams, and models with load transfer equations, spring rate calculations and so on. Bill (the now deceased father) and Doug Milliken's book, Race Car Vehicle Dynamics, is a fabulous book, but it has so much information, that it is pretty difficult to appreciate the big picture, and to figure out which changes have the biggest impact, and what to do first. Also, it doesn’t emphasize some crucially important to keep three things in mind, which can screw up the handling: deflection, slop, and stiction. Ball joints and steering linkages have slop, welds in aftermarket parts can crack, solid bushings can bind, and rubber ones can deflect.

Finally, I am a stranger on this message board, so you might ask, who is this yahoo with all these opinions? Here is an article I wrote that describes how I got my start: https://www.pontiacv8.com/blog/2019/4/25/memories-of-the-72-scca-trans-am-series-firebird-by-harry-quackenboss I also built a C4 street prepared Corvette that was national championship competitive, and these days I advise a collegiate Formula SAE team that in the past few years finished first among 80 North American colleges, and finished 6th of 120 global colleges.

Sleeper68
07-30-2020, 03:41 AM
If the fronts are staying like that mid turn (in dive\roll), the next thing I'd be trying is a way to get the car to release the inside rear between turn in and apex. With a watts maybe lower the rear roll center so the body has a bigger lever to push with or if adjustable sway bar maybe stiffen it up some? My weapon of choice is generally more rear rebound on the shock to accomplish this.

How often do you take a passenger on runs with you? I'm betting on a right turn favoring course like the above, a passenger would net you at least a second if not more off your lap times. I once ran a very similar course on that pad at an Optima event and having a passenger bumped me up enough to get my name listed on TV in the autocross results.

Raising the rear RC and stiffening the rear ARB both make the rear stiffer. Unfortunately, my rear bar is at full stiff right now. I never like to be at the edge of the adjustment range but it'll have to be there until I can figure out something else. The car would definitely benefit from more low speed damping force, especially in the rear. Double adjustable varishocks are on the list for upgrades, hopefully soon. I am very happy with my current varishocks, they just don't have quite enough damping force.

I very often have friends as passengers. I used to be like you and go faster with the better right/left weight distribution, but have found in the last year that I go 3 to 6 tenths faster on average without the extra weight. Very interesting. Some things I guess we'll never understand

Pretty good driving in the video. Smooth and deliberate. I need to be more like that, although I think it will come. So far I haven't even gone to two events in a row with the same setup in the car. Once I learn the car again I think I'll get to a point where I can trust it more.

Sleeper68
07-30-2020, 05:03 AM
I pasted a copy of the picture that you said is a 1.2G 50MPH corner and measured the roll angle.

Thanks for the reply Harry. The "1.2g 50mph" actually came from Lance in regard to the photos he posted. I am unsure of the cornering load on my car in that picture but I would guess somewhere near 0.9-1.0g based on previous data. The car would do 1.1-1.2g back in March under steady state conditions.



1. Based on my analysis, while I don’t know the actually cornering force, the measured roll angle (you can see the measurement from PowerPoint to the right of the picture) is (180 degrees - 177.6 degrees) = 2.4 degrees. My conclusion is you are in the right ballpark.

I agree. The total roll stiffness is pretty close, especially for a streetcar. The balance was the issue in this photo.



2. It may be too much of a stretch to judge conclusively that your car understands just from the picture, but since the roll angle is high enough to indicate the lateral force is pretty high, your front steering angle is pretty high, so yes, based on the photo, it looks like it understeers.


The car is absolutely understeering in the photo. That was the main problem at the time.



Recommendations:

1. I would not raise the rear roll center height under any circumstances. Accepting the trade-off of an increase in front roll center height to get a camber gain improvement is one thing, but it is the wrong thing to do to balance the steady-state handling.


For what reason exactly? Moving roll centers are absolutely necessary to make old cars handle because many of them have roll centers under ground.
My current RC locations are approximately above ground by: 2.5" Front, 13.5" Rear (static).
The positions when the photo was taken: 2.5" Front, 12.5" Rear (static).
The positions when the photo in march was taken: 1.3" Front, 12.5" Rear (static).



2. I wouldn’t change the spring rates, or geometry at least, yet. Optimizing spring rates in these kinds of cars is harder than it looks, and if you haven’t done anything crazy, there are more important things. Same answer for making further geometry changes. Your geometry isn't way out of whack from what good practice is, I think you are still at the point where you can make dramatic improvements without messing with geometry.

3. Your alignment specs with this geometry (-1.5 degrees negative camber, ~6 degrees of caster) are not way off. I would leave them until you at least do the following:

4. I would increase the rear anti-roll bar (ARB) stiffness a bunch, to the point where it clearly oversteers. I would do this with no other changes to start. If I understood your posts, you said slightly increased the spring rate. Whether or not you did, I would not back that off. I would, to repeat what I just wrote, increase the rear ARB stiffness.


How does stiffening the rear bar differ greatly from raising the rear RC?
I have not changed spring rates in 5 years or so. I do think the car would benefit from a little more rear spring rate however, but it is difficult/expensive to achieve with leaf springs.



5. If you cannot do (1) right away, try reducing the front tire pressures. Especially for autocrossing, you can get to the low 20 PSI range. This will reduce the ride rate (the tire is also a spring, and reducing the pressure lowers the spring rate of the tire). But you are going to need to to (1).


At the time this picture was taken, the front left is badly overloaded and the camber wrt to ground is poor. Thus the sidewall is tucking under badly. Lower tire pressure might be a little faster, but it would destroy the tires.

Tire pressure for the event was: 33psig front , 31psig rear

It was set this way to preserve the tire outer edge based on how much the tires were rolling over.



6. Regarding locking your inside front tire, you should realize that if the car understeers, it is going to be more prone to inside front lock up because of the lateral and longitudinal load transfer. If you still have problems after increasing the rear ARB stiffness, then there are a few things you can do. First, is to reduce pitch under braking (this might not be enough, but it is easy to do if you have adjustable shocks compared to alternatives.
a. Increase front damper (shock) compression stiffness. If you happen to have 4-way adjustable shocks, adjust the low-speed compression stiffer.
b. Increase rear damper rebound (extension) stiffness. Again, if you have 4-way adjustable shocks, adjust the low-speed rebound stiffer.


I was able to tune this out mainly through better driving and increased rear braking bias. The shocks I have are non adjustable.



7. The front end of the car looks a little high. I can see your headers are kind of low, and I doubt you want to scrape going over speed bumps, but I would lower it. This does three things: First, since with this geometry, the rate of negative camber increase under compression increases the more it is compressed, lowering it will further increase the negative camber on the outside tire for a given roll angle. If you have modeled your geometry, you can confirm this. if you don't think this is correct, please post a curve. Even better, if you do happen to take the front springs out, cycle one of the front assembly with the springs out (so you can move it from full droop to full compression, and plot it. See number 8 (below) regarding ball joints.

If you lower the car, which will increase the rate of camber change as the outside front suspension compresses, this will reduce the positive camber on the outside front tire. The other two things this does, is lower the CG of the car, and lowers the roll center.


The car is pretty low in the front. I have attached a picture of the static ride height when prepped for competition. If anything, I think the rear needs to be lowered more.



8. Given you have a way of measuring and adjusting bump steer (I mean measuring it on the car, not just from the model), then I would use a taller upper ball joint, which will increase the negative camber gain. In fact, you can also use a taller lower ball joint, which will have the effect of lowering the car, and further increasing the rate of negative camber gain under compression. As a general rule, if the toe steer (bump steer) goes very slightly positive toe out under compression, then you get a sort of automatic counter-steer, which can make it a little easier to stay at max lateral without so many small steering corrections.


The car already has a tall upper ball joint (0.9") which, in general, increases camber gain and raises the front roll center. Tall lower ball joint on 1st gen camaros typically aren't great because they very negatively effect the bump steer curve. If the BJ is too tall, you wont be able to get the outer tie rod end low enough to correct the bumpsteer due to wheel clearance issues.



9. I am guessing you have aftermarket lower control arms, which have some form of non-rubber bushing. If in the off chance you are using stock lower control arms and bushings, your cornering forces are high enough to compress them to the point where your actual camber on your outside front wheel is more positive than your calculation. The stock arms are probably fine, but you need to replace the bushings, and my strong preference is not urethane. I am focusing on the lower control arm bushings because they see higher load than the upper control arm bushings do, but both are important. Also check for any slop in the ball joints. A number of aftermarket units have enough slop to cause problems. Also, if you do have solid bushings, , it is crucially important if you have solid bushings that they move freely. Even urethane ones can be lubricated with a high-pressure lube, and some have grease fittings.


All front control arms are aftermarket w/ delrin bushings. The inner sleeve that interfaces with the LCA pivot bolt is a steel tube with a wall thickness of about 1/4". The lower control arms actually pivot about this sleeve. The bushing are greasable and are greased every few months. I too agree that urethane is not a great bushing material in many cases. The BJs do not have any humanly perceivable slop and are mode by Howe. The are very good balljoints, probably some of the best you can buy.



I have an observation in general about comments on the thread, and what I see from people that look at Milliken Moment Diagrams, and models with load transfer equations, spring rate calculations and so on. Bill (the now deceased father) and Doug Milliken's book, Race Car Vehicle Dynamics, is a fabulous book, but it has so much information, that it is pretty difficult to appreciate the big picture, and to figure out which changes have the biggest impact, and what to do first. Also, it doesn’t emphasize some crucially important to keep three things in mind, which can screw up the handling: deflection, slop, and stiction. Ball joints and steering linkages have slop, welds in aftermarket parts can crack, solid bushings can bind, and rubber ones can deflect.


I agree it can be somewhat "textbookish". I have found Tune to Win by Carroll Smith is better in this regard and gives specific cases for adjustment. In addition the order in which information is presented correlates directly to its importance in the grand scheme of vehicle dynamics.



Finally, I am a stranger on this message board, so you might ask, who is this yahoo with all these opinions? Here is an article I wrote that describes how I got my start: https://www.pontiacv8.com/blog/2019/4/25/memories-of-the-72-scca-trans-am-series-firebird-by-harry-quackenboss I also built a C4 street prepared Corvette that was national championship competitive, and these days I advise a collegiate Formula SAE team that in the past few years finished first among 80 North American colleges, and finished 6th of 120 global colleges.

I love FSAE. Wish I could have done it in college. Unfortunately, my university did not have a team. Which team do you work with?

stab6902
07-30-2020, 06:19 AM
Finally, I am a stranger on this message board, so you might ask, who is this yahoo with all these opinions? Here is an article I wrote that describes how I got my start: https://www.pontiacv8.com/blog/2019/4/25/memories-of-the-72-scca-trans-am-series-firebird-by-harry-quackenboss I also built a C4 street prepared Corvette that was national championship competitive, and these days I advise a collegiate Formula SAE team that in the past few years finished first among 80 North American colleges, and finished 6th of 120 global colleges.

I just wanted to take a minute to welcome you to the board, Harry. You have an impressive resume and your technical knowledge is helpful to everyone who is following this thread. Please stick around and post more often!

SSLance
07-30-2020, 07:10 AM
Pretty good driving in the video. Smooth and deliberate. I need to be more like that, although I think it will come. So far I haven't even gone to two events in a row with the same setup in the car. Once I learn the car again I think I'll get to a point where I can trust it more.

Thanks... In 2014 I changed the complete suspension setup in my car 3 times during the racing season, and still won a Regional Championship, so I get the different car every time at the track deal.

Here is a video from the last autocross event I ran back in February just to show the difference between back in 2015 and today in both driver and car ability.


https://youtu.be/uHxmjm9Q0vQ

Many different iterations between then and today but just to show with a bit of time and knowledge, you can adjust the car to work well with your style of racing.

dontlifttoshift
07-30-2020, 08:41 AM
How does stiffening the rear bar differ greatly from raising the rear RC?

When you add roll rate from either more spring or the ARB, you load the outside tire......and I think the inside front to some degree.

That doesn't happen when you raise the roll center

Sleeper68
07-30-2020, 09:31 AM
When you add roll rate from either more spring or the ARB, you load the outside tire......and I think the inside front to some degree.

That doesn't happen when you raise the roll center

Raising a RC will load the outside tire of that end of the car, as will an increase in roll rate (ARB or ride springs). The difference is mainly that an increase in roll rate of an end increases the total roll rate of the system (i.e. the car). A RC change will merely "move around" the weight transfer you already have. A rear RC moved up will load the outside rear more, the inside rear less, and will reduce front weight transfer which unloads the outside front and loads the inside front. Just as you said.

From Dennis Grant:

"Weight transfer has two components:

Unsprung Weight Transfer: This is the contribution to weight transfer from the unsprung mass of the car.
Sprung Weight Transfer: This is the contribution to weight transfer from the sprung mass of the car, which itself is broken into two sub-components:

Geometric Weight Transfer: This is the contribution to weight transfer from the lever that comes of the difference between where the suspension naturally wants the sprung mass to roll (the roll centre) and where the sprung mass naturally wants to roll on its own (the projection of thecentre of gravity onto the ground plane)
Elastic Weight Transfer: This is the contribution to weight transfer from the reaction force of the springs and bars as they attempt to resist the spring mass roll.
These three components are additive, meaning that total weight transfer is the sum of all three components.

As a first guess, one might assume that the sprung mass works effectively the same as the unsprung mass, so weight transfer would be the sprung mass (Ms) times the lateral acceleration (LatA) times the height of the CG of the sprung mass (CGhs) divided by the track - and you'd almost be right.

The problem is determining the length of the lever through which the cornering force acts. For the unsprung mass, it's the projection of the CG on the ground (the ground on the tires being the centre of rotation). But for an articulated suspension, the point around which the suspension wants to roll is controlled by the interaction of the suspension links and pivots with each other.

Think of it this way - if you push a door on the handle, it will pivot around the hinges. If you take the door off the hinges and stand it up on an ice rink and then push the handle, it will (assuming it doesn't fall over) pivot in place, somewhere near the centre (where the projection of the CG onto the ice surface is). The suspension works the same way as the door hinges.

The trick then becomes figuring out where that pivot point - the roll centre lies. And that's where things start to get REALLY complicated.

Firstly, because locating the roll centre is really not a straightforward process. Secondly, because there is an intrinsic amount of weight transfer that is a function of the difference in height between the roll centre and the ground (the pivot point for the unsprung mass) and a second component based on the difference in height between the height of the roll centre (RCh) and the height of the CG of the sprung mass.

So to help calculate these sub-components, we call the first the Geometric Weight Transfer and the second the Elastic Weight Transfer.

Some textbooks refer to the Geometric Weight Transfer as "jacking force" and it can be helpful to keep that term in the back of your mind - because based on where the roll centre lies, that jacking force can be positive, negative, or neutral. And to further muddy the waters, the roll centre can (probably will!) move as the suspension articulates!

Breaking these down into equations, we get:

WTg = Ms * LatA * RCh / T

WTe = Ms * LatA * (CGhs - RCh) / T

Now if you pay attention to these equations, you can see that the location of the roll centre is really very important:

If the roll centre height is on the ground (0) then the geometric weight transfer is zero (no jacking force) and the elastic weight transfer is maximum;
If the roll centre height is on the CG, then jacking force is maximum and the sprung mass won't roll (the car will try and flip over the outside tire);
In between these two extremes, you get an intermediate result; and
If the roll centre is below the ground, you get "anti-jacking";"

Rod
07-30-2020, 12:26 PM
fun thread .....

Sleeper68
07-31-2020, 06:15 AM
fun thread .....

What are your thoughts Rodney?

Also, didn't see you at the TRSCCA Borderwars event, was hoping to.

Sleeper68
08-23-2020, 07:33 PM
Slight improvement in driving. CAMT turnout was 2 cars so I ran an "open" class called STM (street tire modified). Took 1st by 0.418s of a field of 8 .

Fastest run, 44.281 s , 5th run of 6, ETRSCCA, Bristol Motor Speedway:

https://youtu.be/OQcu2xM2zko

http://live.etrscca.org/STM.htm

Open to any thoughts.

I will hopefully have pictures of what the car was doing in corners in a couple days. I think think the roll couple is off...working on a way to stiffen the rear easily. A rear spring change is a major pain (and expensive), softening the front is not the solution IMO.

SSLance
08-24-2020, 07:08 AM
Holey long soft FAST course design!! Kind of hard to diagnose how the car is doing in turns when there aren't any!! :D

Sleeper68
09-09-2020, 07:16 AM
Holey long soft FAST course design!! Kind of hard to diagnose how the car is doing in turns when there aren't any!! :D

The course was definitely friendly to putting down power, but there were certainly places where alot of cornering force was being generated.

Here's a picture showing the front suspension compressing as it should.

Sleeper68
05-18-2021, 07:27 AM
Been awhile since my last post. I figured I would document my tuning progression to help others with the same type of issues I have been having.

The event after the one above was in Georgia at a CRSCCA event. Tight but fun course. Prior to the event I made a change to the car to reduce understeer (increase oversteer) by stiffening the rear via the use of rod end turnbuckles anti roll bar endlinks as opposed to the polyurethane bushed endlinks I had been using. In addition to this I increased the camber angle to -2.2° from -1.5°. Toe was set at 1/8" out (total toe). Car did great. helped alot with the understeer but still had slight turn in understeer. Ended up 5th PAX with a big heavy car. Link to video below:

https://youtu.be/g3PCvbclDA0

After the CRSCCA event I went to an event pretty far away from home (5hrs). That was the first time the car was on a trailer in 6 years. Car made it two runs before blowing the muncie trans. Good thing it was trailered. Next event not until 2021....

Event #1 in 2021 was a local ETRSCCA event at Bristol. Very cold, had alot of trouble getting the rivals up to temperature. Highs in the mid 40s when I was running. Car did decent but had to baby it due to the cold tires. Placed 2nd in CAMT. Link to video below.

https://youtu.be/3KSN1t5oynk

Event #2 in 2021 was an autocross guys event. Tighter course. Car did well but still had some understeering issues. Mid corner and corner entry. No change from previous setup.

https://youtu.be/ZZLDgcWE1Io

Event #3 in 2021 was at Bristol again, ETRSCCA. Just before the event I made a change. Moved the battery to the passenger side of the trunk, removed the front bumper, and lowered the front RC about 0.5". RC movement was achieved by changing the upper ball joint pin to a 0.5" tall version from a 0.9" tall version. Bumpsteer was corrected after the change. Alignment set at -2.0° camber, +6.5° castor, 1/8" toe out. Won CAMT by 0.57sec. 14th PAX.

https://youtu.be/NAzwijUgC04

Event #4 was a very tight course at Smokies Stadium with ETRSCCA this past weekend. Prior to the event I installed some double adjustable rear dampers from Varishock I had. The dampers I had been using previously were also varishocks but they were non-adjustable. The doubles are capable of much more damping force than the non adjustable versions. I set the bump damping as close to the non adjustable shocks as I could. For rebound I went approx. 2 clicks stiffer as far as I could tell. Did some street slalom testing and set the rear rebound where the turn-in understeer was reduced to a minimum without making the car overly loose. Placed 1st in CAMT by 0.082s and 20th PAX. My driving was pretty sloppy and on my fastest run I messed up the hairpin badly. I think the shocks improved the braking initial "bite" of the car alot but I was timid on the brakes as to not lockup. Need to drive the car harder with more conviction, trust it more, and be more smooth. This should help me keep a better and more consistent line and stay tighter to cones that need staying tight to. I also need to brake harder. Video below.

https://youtu.be/haloDI0p5Bs

Please feel free to critique driving and car behavior. Additionally I am open to any and all suggestions.

Thank you

Sleeper68
08-17-2021, 04:49 AM
More updates. After Event #4 I found that the bushings in the rear dampers were virtually destroyed. This seemed to happen because the the shock was initially mounted "cantilever style" which put the bushing in a big bind. I did not run the car like this but only mocked it up. Ended up making clevis mounts to keep the bushing from binding but the bushings were old and the damage was done. Got new bushings after the event. One was beaten out of the shock and the other was almost gone.

Event #5 was also an SCCA event @ Bristol Motor Speedway. Very fast course, placed 2nd in CAMT, 18th PAX, 52.872s Car felt alot better here but still pushes. I found that I can get the car to rotate is some situations with throttle, but in others it wants to push.

https://www.youtube.com/watch?v=W4fypAEUd48

Event #6: SCCA National Champ Tour @ Bristol. Day 1 was rough. Car pushed alot, could not get heat in the tires in 3 runs, and my driving sucked. I was way too timid. 7th out of 11 in CAMT, 208th PAX, 59.601s.

https://www.youtube.com/watch?v=3MTUs7z9aJA

Day 2 I did alot better but stayed in 7th of 11 due to the addition of time from both days. I bumped up the rear tire pressure 2psi and stiffened rear rebound 2 clicks. This reduced the grip in the rear (not what I wanted to do but was forced to at the time) but made the car alot easier to drive. It turned in alot better. Additionally I drove the car a lot harder and used the throttle more.

https://www.youtube.com/watch?v=p9eYX6oQzeA

At the latest event I worked on my driving. No setup changes. 1st in CAMT, 12th PAX, 17th RAW.

https://www.youtube.com/watch?v=6rFlDfnxd40

The next event is this coming weekned. I did some thinking over the past few weeks and came up with an idea, that alot of others have talked about, to use jounce bumpers (aka bump stops) to tune the front end stiffness easily since I am on leaf springs and regular, factory style front springs. I purchased an array of bumpers of varying stiffnesses to test out.

The first thing I am going to try it to remove the front jounce bumpers completely. Looking at pictures of the car mid corner it became clear that the car was in some way pivoting on the outside front tire to some degree. My theory is that this is due in part to the jounce bumper being loaded, not allowing the front end to articulate as much as I would like and thus not taking advantage of the "improved" camber curve as described in previous posts. The distance between the bumper and the bumper "landing" pad is about 1/2" - 3/4" at ride height. Not alot of movement allowed. You can see this somewhat in the attached picture. Outside front tire appears heavily loaded and camber relative to ground plane is poor - possibly even positive. Alignment is -2.0° Camber +6.5° Castor 1/8" total toe out. Tire pressures 33.5psig front 33.0psig rear on 275/35R18 BFG Rival S 1.5s.

iadr
08-17-2021, 05:15 AM
I'm reading, watching, but off the top of my head pre morning coffee, I don't have much input.

Thought experiment- what happens if you go to a stiffer front spring, and hold the car-as-a whole, flatter with that. Theory says should increase push, right? What if it doesn't actually do so in practice? I think your odds are better than 50-50.
Can you explain your taking out the .9" tall balljoint in favour of a .5" tall one? I trust you give your set up much more thought than I can just walking in here, but it is also true that you felt (and I agree) that that meant a need for more static neg camber, and also that you now note the outer tire is in need of more neg from somewhere ( maybe caster is a last grasp option?). Basically, in following the saying, "you don't understand something fully until you have to explain it", can you throw out a few sentences on what change in feel you aimed for, and consider if you got it?
I'd have lowered the front end at that point- cut say a quarter of a coil off- you'd change the turn in, get your rear end looser, and get deeper into the camber curve. Not by large amounts of any, but all factors are directionally the same so add up. That is definitely "hack"..., but this is a car that despite all the work and thinking we do in this "niche" of performance, has only a fair to middling and largely unchangeable geometry set, by definition. At an OE or custom suspension level, I think playing with rear roll center at that point might have helped, so what do you have you can do with stock rear parts? I'm not a Camaro guy- you do have a panhard bar in these, right?

Sleeper68
08-17-2021, 06:38 AM
Thank you for the reply iadr; I appreciate it. To answer your questions:


Thought experiment- what happens if you go to a stiffer front spring, and hold the car-as-a whole, flatter with that. Theory says should increase push, right? What if it doesn't actually do so in practice? I think your odds are better than 50-50.

Keeping with the theory I have, the car would push more, likely everywhere. Not particularly easy to do on my car; however, with easily changeable jounce bumpers (instead of ride springs) I could try a stiffer front end. Not sure what you mean by the odds are better than 50-50 - towards more understeer or towards more oversteer with more front spring?


Can you explain your taking out the .9" tall balljoint in favour of a .5" tall one? Basically, in following the saying, "you don't understand something fully until you have to explain it", can you throw out a few sentences on what change in feel you aimed for, and consider if you got it?

SETUP 1: The first significant front RC change I did was achieved by going to a 0.9" tall upper ball joint from a factory one on otherwise stock geometry (raised roll center ~ 1.25" and made camber gain negative instead of positive - 0.3°/in). This was in 2015. -3.0° camber , +6.0° castor , 0.0" toe

SETUP 2: The second front RC change was done in 2020 by lowering the inner pivot of the upper control arm about 1/2" rear hole 5/8" front hole (raised front roll center ~1.1" and made camber gain more aggressive 0.8°/in). This also had the effect of reducing front anti-dive since it is fairly excessive in first gen camaros. The reason for the second change was because I found I had to run alot of front camber (2.7°-3.0° neg) to keep the front tires happy but this came at the cost of poor front end grip in braking. I attributed this poor braking performance to the excessive static camber after playing with brake pressure proportion (ended up with full bias to rear tires in braking, still locked up front tires with even small amounts of trail brake). The idea was to increase camber gain and thus reduce the need for static camber. The inadvertent effect was a stiffer front end due to the higher front roll center. -1.5° camber , +6.5° castor , 0.0" toe.

SETUP 2.5: Rear RC moved up to 13.5" from 12.5". Rear ARB endlinks changed to rod end style from polyurethane-bushed style. -2.0° camber , +6.5° castor , 1/8" total toe out

SETUP 3: The third front RC change was performed in 2021 by going to a 0.5" tall upper ball joint from 0.9" tall (lowered front roll center ~0.5" and made camber gain less aggressive - 0.6°/in). This change was made in an attempt to "split the difference" between change one and two. This height jived with the rear RC height of 13.5" quite well for the roll stiffness's I have been able to achieve with my current springs and bars. The rear is as stiff as I can get it with the bars, leaf springs, and rear RC I have. -2.1° camber , +6.7° castor , 1/8" total toe out.

As far as I can tell, each time I raised the front RC, the car pushed more, anf the front was stiffer. Each time I raised the rear RC, the car pushed less and the rear was stiffer. The time I lowered the front RC, the car pushed less and the front was softer - up to a point. The point appears to be where the outside front jounce bumper starts to really "come on" and become really stiff. The jounce bumpers can be thought of as progressive rate springs.


I trust you give your set up much more thought than I can just walking in here, but it is also true that you felt (and I agree) that that meant a need for more static neg camber, and also that you now note the outer tire is in need of more neg from somewhere ( maybe caster is a last grasp option?).

The car wanted a certain amount of dynamic camber relative to the ground. The RC adjustments were preformed in an attempt to achieve this dynamic camber setting with LESS static camber. So far the adjustments have been semi-successful, but I appear to have been limited by the lack of rear roll stiffness - mainly due to a lack of rear ride stiffness (approx 1.4Hz / 175lb/in rear, front is 1.5Hz / 600lb/in).


I'd have lowered the front end at that point- cut say a quarter of a coil off- you'd change the turn in, get your rear end looser, and get deeper into the camber curve. Not by large amounts of any, but all factors are directionally the same so add up. That is definitely "hack"..., but this is a car that despite all the work and thinking we do in this "niche" of performance, has only a fair to middling and largely unchangeable geometry set, by definition.

What is your rationale that lowering the front end will get the rear looser other than increasing front dynamic camber? I have heard other people mention lowering the front to help with push. Unfortunately, the car would be very low and would scrape everywhere if I lowered the front much more. Could the same thing be achieved by raising the rear?


At an OE or custom suspension level, I think playing with rear roll center at that point might have helped, so what do you have you can do with stock rear parts? I'm not a Camaro guy- you do have a panhard bar in these, right?

The car has a rear Watt's link. They (1st gen camaros) do not come with a lateral axle locating device from the factory actually. This is a aftermarket unit from Jim Fay (Fays2). I did actually adjust the rear RC in proportion to the movement of the front RC from setup one to setup two and it helped but was not enough. The rear RC is as high as it will go without some fairly significant fabrication.

Thanks again for the reply. Love to have discussions like these. Other people's perspectives can really open your eyes.

dontlifttoshift
08-17-2021, 07:49 AM
You are giving away all of your camber gain in roll. A tire will make the most grip in negative camber, not flat.

Your wheel rate is too low so you get the above. Spring rubbers are cheap and easy.

Once the suspension bottoms on the stop and the inside starts to lift, your new roll center is the contact patch of the outside tire. Removing the bumpstop is going to make that significantly worse. Stay off the stop and enjoy better turn in.

Roll rates, roll axis distribution, and the rest of the science and math really only matters in steady state cornering. How much of that is there in autocross? You are trying to math your way into a perfect setup for a situation that exists for less than 5% of a run.

Back at the top of this page, read my post and reconcile it with what you just said.


As far as I can tell, each time I raised the front RC, the car pushed more, and the front was stiffer. Each time I raised the rear RC, the car pushed less and the rear was stiffer.

The front pushed after raising the roll center because you weren't loading the tire. The front wasn't stiffer, it just felt that way because you shortened the lever arm between CoG and the RC, but you loaded the tire less. Same in the back. You will have more total grip if you drop the rear roll center. Yes, the car will feel tight because you made your rear traction circle bigger so now we have excess traction in the rear. That brings me to me final random thought.

If we have excess traction, or grip, available in the rear, what can we do? Lets look at the circle.

191506

A tire gives us 100% available traction in any given direction but if we ask the tire to multi task it has to split up that 100%. So there we sit in our rear wheel drive car as the front tires grind away wondering what can we do to loosen the car up. The answer is underneath your right foot. By using the rear tires to accelerate the car, we remove available lateral grip from the rear tires and then we achieve balance. You can pick up the throttle earlier and harder and will carry more speed to the next element.

TL;DR More front spring, lower rear roll center, step on the gas. :)

Sleeper68
08-24-2021, 02:09 PM
Had an event this past weekend. Tried softening the front jounce bumpers by running a 3/4" hole saw through the center of them. It reduced the effective rate by about a factor or 2. Numbers posted below, compression is in inches and reaction force is in pounds.

Compression...OLD...NEW.....DRILLED

0".................0.........0.........0
0.5"..............30.......75........20
1.0"..............75.......150......40
1.25"............120.....225.......60
1.5"..............200.....350......110
1.75"............325.....650.......215

The car felt better but the problem is not fully solved. The next thing I am going to try is a bit more drastic....Adding a leaf to the rear springs with the help of a spring shop that makes custom springs and rebuilds leaf springs. Additionally, I am going to change out the front leaf spring bushings from rubber to aluminum or Delrin to increase the torsion on the spring in roll and thus increase the rear roll stiffness. Hopefully this extra rear ride and roll rate will allow me to get the rear sway bar off of full stiff and somewhere in the middle.

Here's a vid of my fastest. The accelerometer data is offset right, I forgot to recalibrate it that day. Car still pushes sometimes, especially in sweepers but the turn in is way better and I am able to get the car to rotate with the throttle now, whereas before it would tend to push with heavy throttle application. All indications point to the need for more rear stiffness or less front stiffness. Since the front is decently soft (1.5Hz undamped) I have opted to stiffen the rear (currently 1.2-1.4Hz undamped).

https://youtu.be/H_XrWLbo4r8

Thanks to all. Still open to driving critiques. I was pretty sloppy here.

Sleeper68
09-07-2021, 12:53 PM
Another update, and good news for that matter:

I changed out the leaf springs' front eyelet bushing to a Delrin-aluminum bushing/bearing from rubber this weekend. Made a fairly significant difference in terms of how the car drives. Now turn in is more precise, at least as far as I can perceive. The car also feels more neutral mid corner. All of this testing was performed at 80% max effort and with the new, stiffer front jounce bumpers. It is my hypothesis that the reason the car is turning better is becuase the balance is better (the rear is now stiffer since the bushing no longer flexes like it did and the leaf springs are forced to flex more torsionally).

I was able to induce yaw better than I had felt before the front roll center change without heavy throttle application. Light (maybe 20-30%) throttle while cornering hard would get the car to rotate around better while also accelerating. Additionally I was able to feel lift-off oversteer mid corner if I tried to induce it. Before lift-off would usually cause transient understeer until the front tires "caught up" after the car slowed down enough to stop understeering, causing me to have to wait for what seemed like an eternity. Also, like before, the car has tons of rear grip while accelerating. If I find the car is over rotating for what I want at the time I can apply throttle or counter-steer to get the car going straighter.


You are giving away all of your camber gain in roll. A tire will make the most grip in negative camber, not flat.

Your wheel rate is too low so you get the above. Spring rubbers are cheap and easy.

Once the suspension bottoms on the stop and the inside starts to lift, your new roll center is the contact patch of the outside tire. Removing the bumpstop is going to make that significantly worse. Stay off the stop and enjoy better turn in. I agree, this was (is still possibly?) part of the issue. The front end was pivoting on the outside front tire (not compressing the front suspension adequately thus gaining little camber relative to the chassis but losing chamber relative to the ground because of the chassis roll about the outside front tire). I believe car was understeering for two reasons: 1. Jounce bumpers engaging causing pivot and roll center change 2. weight transfer distribution was off. The front was carrying more weight transfer than it should have and the rear was carrying less than it should have - causing the rear to have more grip (less weight transfer) and the front to have less grip (more weight transfer)



Roll rates, roll axis distribution, and the rest of the science and math really only matters in steady state cornering. How much of that is there in autocross? You are trying to math your way into a perfect setup for a situation that exists for less than 5% of a run.

I think the prime issue is the car's balance being poor in steady state - and even though steady state is rarely "purely" achieved in autocross, it's contribution is present any time the car is cornering, which is virtually always in autocross. I think it really comes down to making the car corner better. More front grip is better even if I reduce rear grip since the front was the limiting factor. Now the front and rear tires break loose at more or less the same time (neutral balance).


The front pushed after raising the roll center because you weren't loading the tire. The front wasn't stiffer, it just felt that way because you shortened the lever arm between CoG and the RC, but you loaded the tire less. Same in the back. You will have more total grip if you drop the rear roll center. Yes, the car will feel tight because you made your rear traction circle bigger so now we have excess traction in the rear. That brings me to me final random thought.

I disagree. The front pushed after raising the roll center because doing so made the front stiffer, which made it carry more weight transfer, which loaded the outside front more and inside front less, reducing mechanical grip. It also increased front jacking force, which reduced chassis roll for the same lateral acceleration. However, since the front was carrying more weight transfer because it was so much stiffer than the rear, it had less grip and rolled less, reducing suspension compression and thus dynamic camber relative to the ground. The jounce bumpers definitely didn't help.


If we have excess traction, or grip, available in the rear, what can we do? Lets look at the circle.

A tire gives us 100% available traction in any given direction but if we ask the tire to multi task it has to split up that 100%. So there we sit in our rear wheel drive car as the front tires grind away wondering what can we do to loosen the car up. The answer is underneath your right foot. By using the rear tires to accelerate the car, we remove available lateral grip from the rear tires and then we achieve balance. You can pick up the throttle earlier and harder and will carry more speed to the next element.

TL;DR More front spring, lower rear roll center, step on the gas. :)

Yes, the traction circle tells us there is meat left on the bones in the rear; however, I am mainly having understeer issues on corner entry and mid corner, no corner exit. It is difficult to use the throttle to induce less understeer (more oversteer) in the middle and entrance of a corner. I do understand your point regarding conserving rear grip for throttle application since the car is RWD, but as of now I have plenty of rear bite.

As you and many others have said before, loose (oversteer biased, more front grip than rear) is fast because the car rotates. The car was very tight. Making it tighter will make it - and me - slower, in my opinion.

More front spring does not appear to be the solution, in and of itself at least, from what testing has shown. The rear is too soft - or the front is too stiff depending on how you look at it. Either way the two axles weren't working together before, and now they are working better with one another.

I have an event on the 19th and 26th where I will be able to test out the leaf spring bushing change better. Will report back with my findings.

Thanks again to all who have contributed to this discussion thus far. Please feel free to continue to discuss. I really appreciate it and I think we have all learned something - I know I have.

stab6902
09-08-2021, 07:17 AM
All indications point to the need for more rear stiffness or less front stiffness. Since the front is decently soft (1.5Hz undamped) I have opted to stiffen the rear (currently 1.2-1.4Hz undamped).



Are those measured ride frequencies, or calculated? When I actually measured my ride frequencies, I was surprised to find they were quite a bit higher than calculated, especially in the rear. My car (1968 Firebird, 3410 lb, 58% front) has a rear ride frequency of 1.75 Hz with stock 5 leaf rear springs, which have a published rate of 110 lb/in.

To measure the ride frequencies, I took the shocks off one end of the car, put the other end on full stiff (to minimize coupling effects), and bounced it. It'll be kind of like pushing a swing - it's fairly easy to feel the natural frequency. The quick and dirty way is to count bounces in like a 30 sec timespan, but I got fancy and used the accelerometer on my phone and then analyzed the data in Matlab. I think it's worth measuring (if you haven't already) to give you a baseline.

Sleeper68
09-08-2021, 02:07 PM
Are those measured ride frequencies, or calculated?

Calculated, using measured values - namely corner weight, motion ratio, and spring rate (normalized) using the std. equation for natural frequency ω=√(k*MR/m) , where ω is the natural frequency of the sprung mass, k is the spring rate, MR is the motion ratio, and m is the sprung mass.


When I actually measured my ride frequencies, I was surprised to find they were quite a bit higher than calculated, especially in the rear. My car (1968 Firebird, 3410 lb, 58% front) has a rear ride frequency of 1.75 Hz with stock 5 leaf rear springs, which have a published rate of 110 lb/in.

To measure the ride frequencies, I took the shocks off one end of the car, put the other end on full stiff (to minimize coupling effects), and bounced it. It'll be kind of like pushing a swing - it's fairly easy to feel the natural frequency. The quick and dirty way is to count bounces in like a 30 sec timespan, but I got fancy and used the accelerometer on my phone and then analyzed the data in Matlab. I think it's worth measuring (if you haven't already) to give you a baseline.

That is very interesting. Your measured value and theoretical value for natural frequency vary quite drastically in the rear. My calcs show you should have been in the 1.56Hz range for rear ride frequency (assuming a motion ratio of 1.0), however with a motion ratio of ~0.7 (the actual leaf spring simplified motion ratio in roll) you get 1.1Hz. This brings up an interesting point - the wheel:spring motion ratio for a solid axle changes depending if the sprung mass is moving in heave or roll, or some combination of the two. This may be why you got a higher measured value than "expected", you were moving the sprung mass in heave. Additionally, as you probably already know, leaf springs have a progressive rate when installed with shackles like G1 and G2 F bodies have. This may have contributed to the higher measured ride rate/frequency as well.

To be clear, my estimate of my rear ride frequency (1.3Hz) actually uses the measured "pure" roll motion ratio of ~0.7:1. Using a heave motion ratio of 1.0:1 I get 1.85Hz rear ride freq for my car.

I will check the rear ride frequency using your method on my car when I have a chance and report back.

stab6902
09-08-2021, 05:27 PM
Calculated, using measured values - namely corner weight, motion ratio, and spring rate (normalized) using the std. equation for natural frequency ω=√(k*MR/m) , where ω is the natural frequency of the sprung mass, k is the spring rate, MR is the motion ratio, and m is the sprung mass.



That is very interesting. Your measured value and theoretical value for natural frequency vary quite drastically in the rear. My calcs show you should have been in the 1.56Hz range for rear ride frequency (assuming a motion ratio of 1.0), however with a motion ratio of ~0.7 (the actual leaf spring simplified motion ratio in roll) you get 1.1Hz. This brings up an interesting point - the wheel:spring motion ratio for a solid axle changes depending if the sprung mass is moving in heave or roll, or some combination of the two. This may be why you got a higher measured value than "expected", you were moving the sprung mass in heave. Additionally, as you probably already know, leaf springs have a progressive rate when installed with shackles like G1 and G2 F bodies have. This may have contributed to the higher measured ride rate/frequency as well.

To be clear, my estimate of my rear ride frequency (1.3Hz) actually uses the measured "pure" roll motion ratio of ~0.7:1. Using a heave motion ratio of 1.0:1 I get 1.85Hz rear ride freq for my car.

I will check the rear ride frequency using your method on my car when I have a chance and report back.

I was assuming you were talking about heave ride frequencies all along, so things make more sense now. I'm not used to people talking about roll ride frequencies, but I can see why you were thinking along those lines given the problem you're trying to solve.

For what it's worth, my measured front ride frequency was also higher than calculated, as were the ride frequencies on both ends of my 1972 Skylark. The biggest delta was with the leaf springs though. They are progressive of course, but I couldn't measure a difference between light bouncing (+/- 0.5") and heavy bouncing (+/- 1"). I'm using the same standard equation as you, but always for heave as I'm trying to dial in ride comfort. More caveats - I'm running rubber bushings everywhere , which could add a very slight amount of wheel rate, and there's some estimation involved in my unsprung weights (I'm not taking into account half the weight of the control arms etc), but I think they're close.

Anyway, I'm curious what you measure. Sorry I'm not more help on your specific issue - I'm kind of on the opposite end of the spectrum, trying to improve handling performance on street cruisers while still maintaining a comfortable ride.

Sleeper68
09-22-2021, 01:57 PM
Thank you for the contribution Ryan, I appreciate it.


I was assuming you were talking about heave ride frequencies all along, so things make more sense now. I'm not used to people talking about roll ride frequencies, but I can see why you were thinking along those lines given the problem you're trying to solve.

For what it's worth, my measured front ride frequency was also higher than calculated, as were the ride frequencies on both ends of my 1972 Skylark. The biggest delta was with the leaf springs though. They are progressive of course, but I couldn't measure a difference between light bouncing (+/- 0.5") and heavy bouncing (+/- 1"). I'm using the same standard equation as you, but always for heave as I'm trying to dial in ride comfort. More caveats - I'm running rubber bushings everywhere , which could add a very slight amount of wheel rate, and there's some estimation involved in my unsprung weights (I'm not taking into account half the weight of the control arms etc), but I think they're close.

Anyway, I'm curious what you measure. Sorry I'm not more help on your specific issue - I'm kind of on the opposite end of the spectrum, trying to improve handling performance on street cruisers while still maintaining a comfortable ride.

I have not been able to measure the front and rear heave or roll rates yet. I will do so as soon as I can. I am still unsure how to easily measure the front and rear roll rates, those are a little more tricky.

Good news: the aforementioned leaf spring front eyelet bushing change did seem to net a benefit on course. I tried it out Sunday at a new (to me) venue with the Highlands Sports Car Club HSCC. This was a tight course. I found that front grip while braking, turn in feel, roll response rate (total), yaw rate (velocity), and front tire outer edge wear were improved. These improvements were really only qualitative in nature. This was the first time the car had been on this surface and temps were lower than usual. Next weekend I have an event on a familiar surface so we should be able get some more quantitative data. I can say this, the car definitely feels better than it was, and maybe better than it ever has been. Part of that may be driving, part of it may be real , and part of it may be placebo. It is also important to note that I used the new-stiff front jounce bumpers at this event.

Here is the video from this last Sunday, 5th PAX, 6th RAW:

https://youtu.be/a_rXDVMWofE

My theory is that the TLLTD (Total Lateral Load Transfer Distribution) and the roll couple were not playing well together after I moved the front roll center up (a consequence of making the front camber gain curve more aggressive). What I mean by this is that the front roll rate was too high compared to the roll couple to allow proper front suspension compression while accelerating laterally and by extension reducing dynamic camber to the ground plane (car pivoting on outside front tire). By moving the front roll center up, the car experienced increased front jacking forces, increased front load transfer (also reduced rear load transfer), and flattened the roll axis inclination. Other dynamic effects such as roll understeer induced by leaf spring rake may have affected this as well since rear roll increased due to the stiffer front axle. Stiffening the rear and reintroducing inclination to the roll axis increased rear weight transfer and reduced front weight transfer. What I have yet to be able to check is front/rear roll suspension compression ratio. Setup 2, 2.5, and 3 exhibited little front suspension compression. I would like to see what the front end is doing now. It feels like it is compressing more and the car is starting to yaw more quickly after steering input but I need better data.

For completeness, the suspension tuning progression:

SETUP 1: The first significant front RC change I did was achieved by going to a 0.9" tall upper ball joint from a factory one on otherwise stock geometry (raised roll center ~ 1.25" and made camber gain negative instead of positive - 0.3°/in). This was in 2015. -3.0° camber , +6.0° castor , 0.0" toe

SETUP 2: The second front RC change was done in 2020 by lowering the inner pivot of the upper control arm about 1/2" rear hole 5/8" front hole (raised front roll center ~1.1" and made camber gain more aggressive 0.8°/in). This also had the effect of reducing front anti-dive since it is fairly excessive in first gen camaros. The reason for the second change was because I found I had to run alot of front camber (2.7°-3.0° neg) to keep the front tires happy but this came at the cost of poor front end grip in braking. I attributed this poor braking performance to the excessive static camber after playing with brake pressure proportion (ended up with full bias to rear tires in braking, still locked up front tires with even small amounts of trail brake). The idea was to increase camber gain and thus reduce the need for static camber. The inadvertent effect was a stiffer front end due to the higher front roll center. -1.5° camber , +6.5° castor , 0.0" toe.

SETUP 2.5: Rear RC moved up to 13.5" from 12.5". Rear ARB endlinks changed to rod end style from polyurethane-bushed style. -2.0° camber , +6.5° castor , 1/8" total toe out

SETUP 3: The third front RC change was performed in 2021 by going to a 0.5" tall upper ball joint from 0.9" tall (lowered front roll center ~0.5" and made camber gain less aggressive - 0.6°/in). This change was made in an attempt to "split the difference" between change one and two. -2.1° camber , +6.7° castor , 1/8" total toe out.

SETUP 4: Change to softer front jounce bumpers. Increased front damper bump travel (reduced rebound travel) by moving the lower damper mount down in relation to the lower control arm. -2.1° camber , +6.7° castor , 1/8" total toe out

SETUP 5: Front eyelet bushing of leaf springs changed to aluminum/delrin/steel bushing with thrust washers from steel/rubber/steel bushing. This eliminated any appreciable compliance in the bushing and forced the rear leaf springs to comply with roll, effectively increasing the rear roll rate. -2.1° camber , +6.7° castor , 1/8" total toe out.

jetmech442
02-21-2022, 11:14 AM
A tire will make the most grip in negative camber, not flat.


I've been quietly following this thread for a while and appreciate the excellent mix of technical and hands-on discussion. Your first statement has me scratching my head, not because I have a different opinion, just want to understand it. On the dunning-Kruger map, I'm at the peak of Mt. Stupid in terms of the contact patch, and how even and uneven loading can affect traction, as well as how friction is non-linear with load(I know enough to get most of it wrong lol). Any thoughts you guys could share might help me on my descent from that glorious mountain lol.

iadr
02-21-2022, 12:09 PM
First off, Dunning-Kruger is a "gate keeping"/Credentialist bit of gaslighting. It is best un-wound by observing the ferocity with which the elite try to undercut your perceptions: ie "What are you going to believe, me or your lying eyes?" Think it was just co-incidence that it came out at the time of internet adoption? It was in fact a direct response, as in: "Can't have the plebe's thinking their newfound, newly shared perceptions of the way the world is run have validity, can we?" It's a particularly/typically ugly application of academe against the human intellect, and should be left in the corrupt sewers it came from.


And I think you're confusion is that when "we" say set up negative camber we're doing so without allowing for the distortion of the tire carcass.

So in action, whatever keeps the tread surface making generally the most effective contact with the road, is good. Simple physics says having much of the weight borne by the outer half of the tread shouldn't be all that bad. Empirical discovery shows there is actual bonding taking place, some say molecular or materials bonding. That means every millimeter squared of contact patch is better.

Sleeper68
02-28-2022, 07:48 AM
I've been quietly following this thread for a while and appreciate the excellent mix of technical and hands-on discussion. Your first statement has me scratching my head, not because I have a different opinion, just want to understand it. On the dunning-Kruger map, I'm at the peak of Mt. Stupid in terms of the contact patch, and how even and uneven loading can affect traction, as well as how friction is non-linear with load(I know enough to get most of it wrong lol). Any thoughts you guys could share might help me on my descent from that glorious mountain lol.

I'm no psychologist, but based on the fact you have the understanding that your knowledge of a particular subject could be better, I imagine you are not overestimating your ability. That being said, I think we can all learn something from each other. The best explanations I have gotten regarding tire dynamics at a basic level came from the book Tune to Win by Carroll Smith. You should read it; the entire thing. KYLE.ENGINEERS on YouTube also has a few videos on general tire frictional coefficient effects as a function of normal force and "slip" angle. Obviously this neglects the added variable of dynamic camber angle. He also worked for Mercedes F1 for a couple of years.

Since we are but lowly consumers, and do not own race teams, we are generally relegated to our own tire testing to obtain good tire data. Different tires like different dynamic camber angles for different loading situations (slip angle and normal force). I am switching to a new tire this year, RT660 from the BFG Rival S 1.5, so I will be performing my own tire testing to see what the Falkens like. Like iadr said, the tire likes what the tire likes. The best set of loading conditions for your car are what net the greatest average cornering forces on the same day, with the same conditions, on the same surface. Now, this ALSO neglects other considerations such as longitudinal tire forces (acceleration and braking) and any combo of the former on transverse or "cornering" forces. It has been my experience that more negative dynamic camber angle improves transverse but hurts longitudinal tire characteristics. Since most of us have solid rear axles, we can mainly concern ourselves with the front end in terms of alignments settings. I will be going to a test and tune next week before my first event to get a feel for the 660s on the surface I normally run on throughout the year. I will report back with my findings. SO far my experience with the 660 is it is less forgiving than the rivals, but is far better at communicating its status. Additionally, the 660s respond much better at lower temperatures and they heat up faster, good for a single driver like me. I imagine this will translate to a shorter tire life, but my rivals lasted about 2 seasons so I am ok with that.


It's a particularly/typically ugly application of academe against the human intellect, and should be left in the corrupt sewers it came from.
Yes.


So in action, whatever keeps the tread surface making generally the most effective contact with the road, is good.
I agree.


Empirical discovery shows there is actual bonding taking place, some say molecular or materials bonding. That means every millimeter squared of contact patch is better.
I would expand this to say that the every bit of additional contact patch with a significant pressure acting over its area matters. Distortion of the contact patch and its associated pressure gradient against the road or "pneumatic trail" is very important and is the ultimate output of all tire loading inputs. The resultant tire force is dependent entirely on the contact area, its pressure gradient with the resultant force acting at the contact patch center of pressure, the dynamic frictional coefficient at that time, and the force of the car acting on the tire. All of the input factors that lead to the resultant tire force and pneumatic trail are complexly tied together, and again, are generally characterized empirically as iadr said, especially in our case. One can educate himself on general trends for radial tires, but getting hard numbers from models is virtually impossible for laymen such as myself. It should also be noted that a tire will give you back whatever force to command of it up until it starts to "slip". I do not mean slip angle, what I mean by that is the tire will create a frictional force equal that of its portion of the force the car exerts on it to keep the car on a prescribed arc until its available frictional coefficient starts to fall off. Most tires follow curves similar to the one I have attached. Tires described as "peaky" usually have a friction peak that is taller but narrower and thus are harder to drive at the limit but are theoretically faster. This is represented by the blue line. Tires described as "forgiving" usually have a friction peak that is lower but wider and thus are easier to drive at the limit. This would be represented by the green or red line. The picture is not the best example, but you get the idea.

The center of pressure of the contact patch is generally behind the unloaded tire center if positive castor is present. This trailing effect is what generates self-alignment torque, which is what you feel in the wheel with your hand and is the torque that straightens the tires when you take your hands off the steering wheel. It is also the torque that falls away as the tire starts to become unloaded and starts slipping. This phenomenon is the basis of "feel" in the wheel and is one way experienced drivers stay on the limit of adhesion of the tires.

Suffice to say, the best way to educate oneself on tire dynamics, in my opinion, is to read books devoted to the subject. Specifically those books written by experienced race team members, drivers, or engineers such a Carroll Smith.

stab6902
02-28-2022, 10:28 AM
Good post Sleeper68.

@jetmech442 - Here's a good short explanation of tire dynamics with some nice, simple figures that are worth a thousand words: http://racingcardynamics.com/racing-tires-lateral-force/. It's no substitute for a whole book/chapter on the subject, but it's a good high level summary to get started.

One key point that really helped me when I started learning about tires is that the rate of grip increase trails off as vertical load increases. It's not linear like they taught us in basic physics. This validates the "common sense" rule of thumb that, all other things equal, a car with a lower center of gravity, wider track width, or lower weight will be faster.
197477.

Sleeper68
02-28-2022, 12:13 PM
Good post Sleeper68.

Thank you Ryan.


@jetmech442 - Here's a good short explanation of tire dynamics with some nice, simple figures that are worth a thousand words: http://racingcardynamics.com/racing-tires-lateral-force/. It's no substitute for a whole book/chapter on the subject, but it's a good high level summary to get started.

Just read through this. Exceptional primer to tires. I am going to explore this publication in its entirety now. Thank you for sharing.


One key point that really helped me when I started learning about tires is that the rate of grip increase trails off as vertical load increases. It's not linear like they taught us in basic physics. This validates the "common sense" rule of thumb that, all other things equal, a car with a lower center of gravity, wider track width, or lower weight will be faster.
Yes, and this is the exact reason why more weight transfer over a pair of tires hurts their tractive capacity. This is also why the generalized, and in my experience, underexplained statement "softer suspensions have more mechanical grip" is true; although is misleading to some degree. A softer end of the car will transfer less weight and thus have more mechanical grip, at the expense of the other end of the car. The end of the statement is the part that is often not explained.

To expand: A pair of tires have the most tractive capacity (aka potential for grip) when they are equally loaded. As weight is transferred from one tire to the other, the total capacity of the pair is reduced. Since wider track, lower center of gravity (CG), and less weight reduce WEIGHT TRANSFER, the pair (or set of 4) tires have more tractive capacity and thus the car has the ability to be faster through any given corner. This is why aerodynamic downforce is known as free weight since it increases tire load, does not really affect weight transfer, and does not have to be accelerated.

So to relate all of this to the original discussion: The Magic Number, which Milliken covers and calls Total Lateral Weight Transfer Distribution, is the ratio of roll stiffness (aka roll gradient in °/g) of the front to the rear. The roll couple is the moment exerted on the sprung mass while cornering and is what loads the outside suspension, unloads the inside suspension, and makes the car roll. The car as a whole has a roll couple but one can break it down into front and rear. The Magic Number and split between the front and rear roll couple is the greatest influencer of a car's propensity to understeer or oversteer (aka the car's balance).

I was having a lot of issues with balance, namely excessive understeer. After reading a lot to understand the mechanism by which anti-roll bars (aka sway bars) worked and how they changed a car's balance I came upon the magic number. What I learned was that, all else being equal, a car will transfer more weight on the front and less on the rear if the front roll stiffness is increased. More front weight transfer = more likely to understeer. One can increase weight transfer of one end of the car by increasing roll rate (sway bars or ride springs), lowering the CG (lowering the car), or by raising the roll center.

jetmech442
03-08-2022, 08:59 AM
First off, Dunning-Kruger is a "gate keeping"/Credentialist bit of gaslighting. It is best un-wound by observing the ferocity with which the elite try to undercut your perceptions: ie "What are you going to believe, me or your lying eyes?" Think it was just co-incidence that it came out at the time of internet adoption? It was in fact a direct response, as in: "Can't have the plebe's thinking their newfound, newly shared perceptions of the way the world is run have validity, can we?" It's a particularly/typically ugly application of academe against the human intellect, and should be left in the corrupt sewers it came from.

I took a while to respond to this. My first reaction to this aggressive take was to dismiss it as another woke/partisan/conservative/liberal(choose your insult) freakout. But I figured it was a strong reaction to have on a forum amongst car guys talking about tires..so I gave it time tried to read up. You have to go a ways down on google to find an article by Johnathan Jerry who shows that the effect can be seen in large scale random generated scenarios and thus the effect might not be part of the human psychi afterall, and I agree with that now. I don't think any literature should be used to dismiss someones opinion outright, but I do still find the graph a useful reflection of where I might evaluate my knowledge on a subject. Heat transfer and thermal radiation...I've got a solid, successful career and feel confident I can speak and teach it...tire camber curves, I still feel like I am sliding down Mt stoopid.

So thanks iadr for alerting me that the D-K effect is not fully accepted by the science community( in fact it seems more studies are being done to try and eliminate it's regression to the mean issue). I think we, as a country, need to get back to presenting differing views in a way that doesn't alienate the other side immediately and force them to dig in.

Speaking of camber curves and traction...



The best explanations I have gotten regarding tire dynamics at a basic level came from the book Tune to Win by Carroll Smith. You should read it; the entire thing. KYLE.ENGINEERS on YouTube also has a few videos on general tire frictional coefficient effects as a function of normal force and "slip" angle.

Big Fan of Kyle Engineers! He got me started on tire basics, as well as the book "How to build a high performance miata". I see "tune to win" on amazon, I scoop that up at your suggestion.

As with all your posts Ryan, I need to read them some more when the house (and my mind) is quite to fully absorb what your saying. Your posts are always clear and you choose words carefully, and sometimes I miss the nuance. I appreciate the effort you guys put into the posts.





Good post Sleeper68.

@jetmech442 - Here's a good short explanation of tire dynamics with some nice, simple figures that are worth a thousand words: http://racingcardynamics.com/racing-tires-lateral-force/. It's no substitute for a whole book/chapter on the subject, but it's a good high level summary to get started.

One key point that really helped me when I started learning about tires is that the rate of grip increase trails off as vertical load increases. It's not linear like they taught us in basic physics. This validates the "common sense" rule of thumb that, all other things equal, a car with a lower center of gravity, wider track width, or lower weight will be faster.

I'm on board with this concept. I read an article that explained the traction curve becomes more non-linear the closer you get to the tires load index.

So for a Hankook RS4, a 225/40R19 has a load index of 1279 lbs. a tire with more width, but a 235/35R19(slightly more width, but slightly less sidewall) has a lower index by ~70 lbs. In general, the slightly narrower tire would "fall off" less quickly than the 235 as more lateral load is transferred to it. But perhaps the 235 has slightly more traction due to its width so the fall off is a wash. Open to thoughts/criticism of this train of thought....

Thanks again for everyones response to me just jumping in and asking a question. Discussions like these are what continues to make this forum so great.

Sleeper68
02-08-2023, 04:59 AM
This post is provide an update as to how the car and myself have been performing since the last post.

List of events since last post:

HSSC 03 OCT 2021, BFG Rival S 1.5, Setup 5: https://youtu.be/L6FEjmYwQUY Very cold. Tires were about out of life here.

TRSCCA OctoberFAST Day 1 31 OCT 2021, Wet, RT660, Setup 6: https://youtu.be/Iv36rB08GYQ Falkens worked great. Alot of turn-in understeer but it was wet.

TRSCCA OctoberFAST Day 2 01 NOV 2021, Dry, RT660, Setup 6: https://youtu.be/H83PK4gJ9JI Love how fast the RT660s heat up. Great for a one driver car.

ETRSCCA Points #1 06 MAR 2022 BRISTOL, Setup 7: https://youtu.be/P1UkFrvwsqs First event post-spindle failure. First time and bristol (high speed courses) with the new tires.

ETRSCCA POINTS #2 03 APR 2022 SMOKIES STADIUM: https://youtu.be/ahyv41IntUs Starting to learn how the car drives with the new Falkens. They are more precise and provide far more feedback than the BFGs.

ETRSCCA POINTS #3 15 MAY 2022 BRISTOL: https://youtu.be/h5ZU228ThRs

ETRSCCA POINTS #4 29 MAY 2022 SMOKIES STADIUM: https://youtu.be/G0QG2SxoYRw

ETRSCCA POINTS #5 12 JUN 2022 BRISTOL: https://youtu.be/zXOY9T7Akzc The falkens can certainly get greasy when it is super hot outside, especially in the rear when applying power at the end of the run. Setup 7 is definitely working the outside rear tire more than before based on temps. Could also be driving improvement.

ETRSCCA POINTS #6 10 JUL 2022 SMOKIES STADIUM: https://youtu.be/gWhB4XF6apg

ETRSCCA POINTS #8B 24 JUL 2022 BRISTOL: https://youtu.be/ERG2jg3YfOk

ETRSCCA POINTS #9 21 AUG 2022 BRISTOL: https://youtu.be/gnW6uy7s5L8

ETRSCCA POINTS #10 09 OCT BRISTOL: https://youtu.be/sAUohwkN96A

As you can see from these videos, the car understeers far less and generally rotates better; however, it still understeers in steady state cornering. I think the balance (TLLTD) is still off, but I think there is another contributing factor. More on that later.

Thanks to contributions from members here at pro-touring.com and some driver improvements, I have been able to win the 2021 and 2022 ETRSCCA points championship for CAMT. I want to take this opportunity to thank everyone who has contributed to this thread. Public discourse, in my opinion, is very good for improving all of our understanding of vehicle dynamics.

My next area of focus for the car is fixing the steering geometry and the excessive steering arm flex the car has. The steering arms are made by Chassisworks of 7075-T73 Aluminum https://www.cachassisworks.com/p-2950-camaro-67-69-f-body-nova-68-72-x-body-z28-quick-ratio-steering-arm-billet-aluminum.aspx. The fact the steering arm is aluminum and because of the very low position of the outer tie rod end (approx 0.2" from the wheel inner lip), the steering arm flexes. This excessive flex (approx 0.2" of movement of the outer tie rod before the wheel moves when car is static on the ground) makes the car very hard to place while racing on the limit.

So, my plan currently is to use a higher inner tie rod pivot to allow the outer tie rod to move up. Currently, I want to use the Ridetech TruTurn centerlink/draglink instead of modifying a factory one or making my own. This will reduce the cantilever distance to the outer tie rod from the spindle. Additionally, I will need new steering arms to accommodate this so I will be making my own steering arms from steel with very similar Ackermann geometry to the Chassisworks units. The combination of the fact that steel is stiffer than the aluminum and the reduced cantilever should make for a much stiffer steering system.

Now, back to why I think the car is understeering in steady state: It could be the balance is still biased to understeer due to weight transfer, and shock settings are improving turn-in and transient balance; hiding the steady state balance issue. There could be another issue though, one related to the known steering arm flex. It COULD be that once the car is cornering hard and virtually steady, the steering arms (especially the outside steering arm) are flexing so much that the dynamic toe has changed to be very toe-ed in. Maybe this is part of the steady state understeer issue? Maybe I should return the inner upper control arm pivot point to the OEM holes and maintain the 0.5" tall or 0.9" tall UBJ?

What do our fine members at pro-touring.com think?

I think we all will agree that the steering arm flex should be reduced as much as possible.



The suspension tuning progression:

SETUP 1: The first significant front RC change I did was achieved by going to a 0.9" tall upper ball joint from a factory one on otherwise stock geometry (raised roll center ~ 1.25" and made camber gain negative instead of positive - 0.3°/in). This was in 2015. -3.0° camber , +6.0° castor , 0.0" toe

SETUP 2: The second front RC change was done in 2020 by lowering the inner pivot of the upper control arm about 1/2" rear hole 5/8" front hole (raised front roll center ~1.1" and made camber gain more aggressive 0.8°/in). This also had the effect of reducing front anti-dive since it is fairly excessive in first gen camaros. The reason for the second change was because I found I had to run alot of front camber (2.7°-3.0° neg) to keep the front tires happy but this came at the cost of poor front end grip in braking. I attributed this poor braking performance to the excessive static camber after playing with brake pressure proportion (ended up with full bias to rear tires in braking, still locked up front tires with even small amounts of trail brake). The idea was to increase camber gain and thus reduce the need for static camber. The inadvertent effect was a stiffer front end due to the higher front roll center. -1.5° camber , +6.5° castor , 0.0" toe.

SETUP 2.5: Rear RC moved up to 13.5" from 12.5". Rear ARB endlinks changed to rod end style from polyurethane-bushed style. -2.0° camber , +6.5° castor , 1/8" total toe out

SETUP 3: The third front RC change was performed in 2021 by going to a 0.5" tall upper ball joint from 0.9" tall (lowered front roll center ~0.5" and made camber gain less aggressive - 0.6°/in). This change was made in an attempt to "split the difference" between change one and two. -2.1° camber , +6.7° castor , 1/8" total toe out.

SETUP 4: Change to softer front jounce bumpers. Increased front damper bump travel (reduced rebound travel) by moving the lower damper mount down in relation to the lower control arm. -2.1° camber , +6.7° castor , 1/8" total toe out

SETUP 5: Front eyelet bushing of leaf springs changed to aluminum/delrin/steel bushing with thrust washers from steel/rubber/steel bushing. This eliminated any appreciable compliance in the bushing and forced the rear leaf springs to comply with roll, effectively increasing the rear roll rate. -2.1° camber , +6.7° castor , 1/8" total toe out.

SETUP 6: Switched from BFG Rival S 1.5 to Falken RT660 tires (same size 275/35R18). Reduced toe out to zero. -2.1° camber , +6.7° castor , 0" total toe out. This change occurred just before an event at NCM Racetrack in Bowling Green, KY in OCT of 2021.

SETUP 7: Post-spindle failure. -2.25° Camber, +6.75° Castor, 0" total toe.

iadr
02-08-2023, 12:56 PM
A concern is that arm:
https://www.cachassisworks.com/p-2950-camaro-67-69-f-body-nova-68-72-x-body-z28-quick-ratio-steering-arm-billet-aluminum.aspx
being made of aluminum, that type of .2in flex 100% guarantees eventual breakage. It's a matter of time- that said, could be a long time, but at .2inch I don't think it would take long at all. Even 0.040" under much greater (ie active) load would worry me if I were the manufacturer.

Now rather than condemn a product, I'm going to go the other way. Are you absolutely sure the arm itself is flexing? It's not within the wheel bearing or even tire carcass?
Where is it flexing? You believe it's bending off the solidly mounted rear spindle bolt? (this is rear steer, right?) That's the only way I could see possible- I see it as solidly mounted, then a cantilever off the rear bolt. You're saying you document that at standstill? Makes no sense. If it were that much of a wet noodle, the first hard corner or hard corner with a bump in it, and it would be gone.

It just doesn't make sense to me- the size of that, & that the equivalent parts made by Stielow and DSS and others with access to modelling are if anything less beefy- that it would flex problematically. If it were a drag race part meant to guide pizza cutters over a few dozen miles of runs, adapted over to protouring, OK. But that is specifically made for Protouring.

I'm not naive- faulty products make it out there, occasionally even at OEM level (pad knock back and wheel bearing issues the last few years), but no offence, just my gut here, I'm suspicious this is as much of one as you currently think it to be.

*sorry if anyone was trying to write a reply. As I thought about it, I edited my posted several times.

Sleeper68
02-09-2023, 12:39 PM
A concern is that arm:
https://www.cachassisworks.com/p-2950-camaro-67-69-f-body-nova-68-72-x-body-z28-quick-ratio-steering-arm-billet-aluminum.aspx
being made of aluminum, that type of .2in flex 100% guarantees eventual breakage. It's a matter of time- that said, could be a long time, but at .2inch I don't think it would take long at all. Even 0.040" under much greater (ie active) load would worry me if I were the manufacturer.

I agree, this is another reason I am changing to steel steering arms for this geometry. Aluminum has a finite fatigue life, regardless of loading condition. This being a fully-reversed load, it is virtually worst-case. Steel CAN have an infinite fatigue life if the stress is kept low enough and stress risers are properly addressed.


Now rather than condemn a product, I'm going to go the other way. Are you absolutely sure the arm itself is flexing? It's not within the wheel bearing or even tire carcass?

I am not absolutely sure. Other sources of compliance may exist, certainly. The fact I could watch the distance between the tie rod and the wheel change while moving the steering wheel with the car static, told me alot of this compliance in the steering system was coming from the steering arms. Wheel bearing play could cause this, but I precisely set wheel bearing preload with a bearing spacer and torque wrench. Also, wheel bearings are replaced every season, so I doubt much of the compliance comes from the bearing. This, is in no way, a dig at Chassisworks. I love their products and I do believe they put alot of thought and time into their products. It is, however, my belief that the majority of the compliance is coming from the steering arms.


Where is it flexing? You believe it's bending off the solidly mounted rear spindle bolt? (this is rear steer, right?) That's the only way I could see possible- I see it as solidly mounted, then a cantilever off the rear bolt. You're saying you document that at standstill? Makes no sense. If it were that much of a wet noodle, the first hard corner or hard corner with a bump in it, and it would be gone.

Yes, it appears the steering arm is bending, in two different axes. One axis is the steering axis, but the other, more significant, axis is a horizontal (more or less) line at the rear spindle mounting bolt. One important note: most users of this arm will see less flex than I do because they use it as intended: with a factory tie rod end. I have lowered my outer tie rod end about 0.75" from the factory position to correct bumpsteer, further increasing the cantilever on the arm. 1st gen F and X bodies are rear-steer, yes.

I need to apologize, as I was not clear about where and how the ~0.2" flex was measured.

1. Displacement of the zerk fitting of the outer tie rod end was measured with a dial indicator. No indicator was placed on the wheel.
2. The car was sitting at ride height on concrete floors.
3. ~0.2" (I don't remember the exact number) is the TOTAL displacement displacement of the zerk fitting on the outer tie rod end of one side of the car when turning the wheel both left and right up until visual indication of wheel movement was detected.

This was not a robust test method, nor are these displacement values representative of what may others would experience. The main reason for this is because my outer tie rod end is quite a bit lower than the factory steering geometry, which is what these arms were designed for. So low, in fact, that the wheel has contacted the tie rod when I had my bumpsteer virtually corrected. I had to move the tie rod end up 0.05" to prevent rubbing under hard road loads.

I plan to re-measure this displacement when the car is static with the current setup. I will amend my test method by placing an indicator on both the wheel rim and the tie rod end. Rod end displacement will be recorded in both directions when wheel displacement is 0.010" from straight ahead. The 0.2" number will likely shrink following this procedure. I will also repeat the test 3 or more times to ensure repeatability. Then, the new setup (steel arms with les vertical cantilever and trutrack draglink) will be tested in the same manner and flex difference can be directly compared. If further sources of steering compliance exist after this, they will be identified and reduced as much as possible.

Regardless of the fact this test was not super scientific, I can tell you the steering system flex is significant. How I know this is, when driving down the highway, one can oscillate the steering wheel violently about 30° both left and right from center with hardly no effect to straight ahead motion. This tells me something, or many things, is/are flexing. You can watch the whole steering system move with the steering wheel with the car static on the ground up until the steering arm. The tie rod moves, but the wheel won't (up to a point obviously).

Maybe I can take a video to show you guys what I am seeing. It is very possible I'm missing something.


It just doesn't make sense to me- the size of that, & that the equivalent parts made by Stielow and DSS and others with access to modelling are if anything less beefy- that it would flex problematically. If it were a drag race part meant to guide pizza cutters over a few dozen miles of runs, adapted over to protouring, OK. But that is specifically made for Protouring.

I have been unable to find any comparable products from DSE or Stielow. If you have found some I would love to take a look at them and possibly procure them. The only steering arms I see on DSE's site for 67-69 camaro comes on their hydroformed subframe assembly. They appear to be made from steel forgings. Pics attached.

iadr
02-09-2023, 01:30 PM
I like how you examine things. Kudo's- it's often said and certainly true of me, that one doesn't understand something until one explains it to someone. I can see you working through this.

I will keep it short through because I'm procrastinating putting a family members car back together after an overheat (she didn't know or listen to my rushed explanation that no heat in a Canada winter means your antifreeze is low, and that "failure to react" set off a series of cascading failures). Got to get in the garage.

The Stielow reference was to his using a fabricated arm on a Corvette spindle on one, & maybe two of his builds- I have a soft cover book by him and a ghost writer somewhere with multiple pics of that... that I made distinct note of as I was figuring out a front end plan for a largely still born project. He's smart and I've always regarded him as being able to "run it by" the right people and maybe more significantly, their computers when he needed to. That said some of his cars were almost pure SEMA type builds and not run in anger more than a few hours.

I also distinctly remember seeing what appeared to be steel *tube* (!) steering arms on a highly regarded Pro-touring car- some time ago, or maybe at a/the "other" site. I don't have a link, though I'm pretty sure have photos saved on my hard drive.

I've also seen off road buggies built with fabricated spindles and steering arms, and those guys are, obviously hard on their stuff when it comes to instantaneous G-forces, even if the tires seem cushiony and the ground less grippy, they still have huge leverage back from road surface to tie rods, with the 35-40" tires, not to mention the impacts. They were using welded steel plate, maybe 3/8, but more likely 5/16 or even 1/4 making a hollow, lengthy box-fabrication containing/joining 3 steel blocks- 2 mount-point blocks & a tapered "block" for the tie rod to go in. (Not recommending this, just saying...).
Circle track has examples, too. IDK, but I could believe those are only equal to street strength, but you don't hear of breakages much at all outside of contact.

I've followed self built cars of multiple genres- "locost" lotus7 replicas being one, and gt40's/supercar replicas being another. Lots of fabricated spindles/steering arms. That said you never know if what you see done is actually effective (safe). The few accidents I can recall were from over driving and straight up loss of control, and a locost where he tacked it together to do a motion test then forget to pull it apart and do full welds. The original Factory5 '34 Ford car had a public development failure its rocker front suspension. More I think because the rocker itself by definition has exactly double the suspension forces acting on its pivot.

That said, the Chassisworks piece looks beefy. I mean, yes, you can see cut paths were a factor in design but not in a bad way. But in thinking about it, the relatively few OEM bolt ons I can think of seeing/touching have all been always steel and always forged.

I'm staying subscribed to this thread and I am curious about seeing resolution and understanding.

Olds.PhD
02-10-2023, 04:08 PM
Maybe I can take a video to show you guys what I am seeing. It is very possible I'm missing something.

I mean, as a mechanical engineer, you could just break out your old beam calcs and find out that there is absolutely no way that arm is flexing 0.2" without breaking. It'd have to be significantly thinner to withstand that kind of deflection. And that's a single turn - no chance it'd survive even low cycle fatigue.

By measuring to the tie rod, you're including that rod end joint in your slop. Seems far more likely to have a bad tie rod end than a steering arm flexing that much. Either that or the steering arm isn't actually tight against the spindle, but that would likely be catastrophic and very obvious.

Video would be good.

Sleeper68
02-13-2023, 07:07 AM
I mean, as a mechanical engineer, you could just break out your old beam calcs and find out that there is absolutely no way that arm is flexing 0.2" without breaking. It'd have to be significantly thinner to withstand that kind of deflection. And that's a single turn - no chance it'd survive even low cycle fatigue.

Olds,

I understand your point. As stated previously, 0.2" refers to the total displacement (both loading directions, 0.1" from center) of the zerk fitting on the bottom of the outer tie rod end. This means that the zerk fitting moved about 0.1" from the unloaded position in both directions. This also means that the steering arm itself did not move the entirety of this distance. I did not measure steering arm deflection at any point. Additionally, as mentioned previously, this test procedure was not very robust and the likelihood for error is high. A test will be performed soon with a more robust procedure. I agree 0.2" steering arm movement does not pass the smell test. The reality is the steering arm does not move that much, the zerk fitting at the bottom (end) of the cantilever moves half that much (maybe). The steering arm furthest point probably moves more like 0.040-0.050". Regardless, this will be fleshed out with a proper test soon.


By measuring to the tie rod, you're including that rod end joint in your slop. Seems far more likely to have a bad tie rod end than a steering arm flexing that much. Either that or the steering arm isn't actually tight against the spindle, but that would likely be catastrophic and very obvious.

Video would be good.

The likelihood that there is any slop in the outer tie rod end is very low, in my opinion. The reason I believe this is because the outer tie rod end is made by Howe and has a user set pre-load or lash. I set the rod end up with some preload (as recommended by Howe). There certainly will be some flex of the rod end stud, however. It is approximately 5/8" in diameter and is made from alloy steel. The bolts attaching the steering arm to the spindle are 1/2-20 grade 8 and all are tight.

I have attached a few pictures of the modified steering arm including the outer tie rod end and the wheel for reference. You will be able to see that the rod end is very low near the wheel. Deflection at the bottom zerk fitting will be about double of the deflection of the steering arm lowest point.

Sleeper68
02-14-2023, 01:19 PM
There is a new development in the understeer saga. While underneath the car checking fasteners, I found that my rear sway bar was cracked. Pictures attached below. I am uncertain when this happened. The car has not really been driven since the last SCCA event on 09 October 2022 (video: https://youtu.be/sAUohwkN96A). I think crack started to significantly open up during the run in this video. You can see that car yaw fairly well early in the run, but at 0:43 I start a higher speed right hand sweeper. In the previous 4 runs, I was able to stay off the cone wall to the left by about 2 feet, but on my fifth run (shown in the video) I push out and hit a cone in the wall. SoloStorm data shows cornering G's to be comparable, as well as corner entry speed. Max corner accelerations for run 4 and 5 were 1.05 and 1.00 g respectively. Corner entry speeds for run 4 and 5 were 54.1 and 53.3 mph respectively.

The car was able to make counter centrifugal force, but it wasn't able to maintain the correct arc at the same speed. My current hypothesis is that the rear bar failed just before, or going into this turn on run 5, causing me to push out and tag a cone in the wall. The ARB (anti-roll bar) is made by Hellwig. They are sending me a replacement bar for free under warranty. I have had both the front and rear bar for about 7-1/2 years.

If this hypothesis is correct, I believe it would support the argument that the car needs the rear stiffness it has (or maybe more) since losing the rear bar would impart more weight transfer on the front and thus reduce the front end's tractive capacity.

What do our fine pro-touring.com members think?

Thanks again to all who have contributed to this discussion.

Sleeper68
04-26-2023, 05:03 AM
The broken sway bar has been fixed and some other issues have come up. Two events have been completed so far this year with not the best results. Setups below:

SETUP 7: Post-spindle failure. -2.25° Camber, +6.75° Castor, 0" total toe.

SETUP 7.5: Broken/cracked rear sway bar. -2.25° Camber?, +6.75° Castor?, 0" total toe?.

SETUP 8: Replace broken rear sway bar. -1.5° Camber, +6.50° Castor, 0" total toe. This setup snuck up on me. The the change in camber happened gradually overtime and seemed to result from loose upper ball joints. This prompted an evaluation of all Howe adjustable rod ends and ball joint on the car. I found that all four (both upper ball joints and both outer tie rod ends) were slightly loose to the point of lash. This was addressed and corrected in SETUP 9. It is unclear how long they ball joints had been loose. The car pushed at 2023 Event #1 with this setup and was difficult to place on the racing line. Tires were fairly cold at that even due to semi-wet and dusty course, low ambient temps, and low solar angle.

SETUP 9: -2.5° Camber, +6.8° Castor, 0" total toe. A different toe measurement method was used (toe bars on camber caster gauge, straight edge on opposite wheel, tape measure). This method is far easier to prepare but I fear it is far less accurate. Previous toe measurement and adjustment had been facilitated with a set of parallel strings attached to jack stands that had been aligned to the center longitudinal axis of the car. Toe was measured by taking the difference between front and rear distances to the wheel lip from the string on each side using a machinist scale. I am switching back to the old method but I am going to streamline it by making two tubes with notches at equal distances to locate the string eyes.

Event #1: Car pushed, my driving was sloppy, tires were cold, track was wet in places and very dirty/graveled. I had it in me to win but I hit a cone in a wet section on my fastest raw run.

https://youtu.be/370CngHJhpY

Event #2: Loose ball joints fixed, camber changed to -2.5°, car was far looser than before. The surface was cold, I was sloppy and not used to this setup, and mine lines were wide in places which killed me on a slow course like this one. I think there is room to improve the car's balance with with a rear shock or rear sway bar adjustment. Toe could be compromised and not actually 0". Need to re-check with the original, validated toe measurement method.

https://youtu.be/pOWlzmbBzkc