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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, Miliken calls it "Total Lateral Load TransferDistribution". 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.