Need a little help figuring something out

[diesel25lrs]

Lance,

Curious how did you do it when you went back and measured again without the bottle jacks to get suspension at ride height?

I should probably update this to add...do not use this method of setting your car up for measuring.













Ron entered the numbers I gave him after the initial measuring using this method, then got back to me to go over some discrepancies...things weren't adding up. So I put the car back on the front tires on the rack and remeasured all of the height numbers...and they were all quite different.

I'm guessing the bottle jacks kept the suspension from settling completely like it will when sitting on the tires themselves. Was a nice try I thought, but in action, did not work as hoped. Part of the learning experience I guess.

Eventually I plan on fabbing up or purchasing some steel plates to bolt onto the hubs that will extend down to the ramp to set the hub at ride height. They'll have to be a bit adjustable height wise and also be able to slide around a bit once on the ground surface to allow the suspension to settle completely. What they will do though is not only allow more access for pivot point measuring, but also provide a nice flat base to measure from for setting alignment specs once changes have been made. One or two trips to the alignment shop saved should more than pay for the stands I believe.

[SSLance]

Lance,

Curious how did you do it when you went back and measured again without the bottle jacks to get suspension at ride height?

After putting the car back down on the tires, I just had to get more creative with where and how I placed the laser levels to get the measurements from. Using the framing square with welding magnets to hold it true and square helped a lot to get into the tight spots.

Plus I wasn't as overwhelmed as I was in the beginning because I had already made all of the measurements once and pretty much knew what I needed and where to go to get them.

When starting from scratch, I was pretty frustrated fighting around the tires trying to get the first couple of sets of measurements. Over time I developed ways to get around things that made it easier to get around obstacles.

[diesel25lrs]

Thanks Lance,
I'm thinking that without lift access it will probably be worth it to just fab up some hub mounted stands of some sort...up and down adjustability will be key though.

[SSLance]

Yeah, you need the car 10" off of the ground at least as you have to be all over the underside of the car.

I'm going to build some crib stands to use with my 2 post lift at home to do this in the future.

[Ron Sutton]

Hey Guys,

I'm going riding quads with my girls now ... so I'll catch up with everyone when I get back Monday.

Take care all !!!

[Rod]

Hey Guys,

I'm going riding quads with my girls now ... so I'll catch up with everyone when I get back Monday.

Take care all !!!

Have fun Ron!!!

[rustomatic]

As math goes, I've always labeled geometry as my favorite, but this thread is making me like statistics a bit more (while slightly hating geometry). At least I've now found what is perhaps the greatest benefit of my relatively new screw-in lower ball joints: I can (somewhat easily) take them out and buy new ones that give me more geometrical quandaries to ponder.

Thanks Ron, Lance, ancient Greeks. I'll now happily return to planning the angle grinder destruction of the rear end of my transmission tunnel, which was necessitated by geometrical changes in my driveshaft/rear axle...

[SSLance]

Little progress this weekend... I got the rear springs in the car and drove it around a bit to feel it out. Then things went a bit wonky. Pressing the stock ball joints out of the new A-Arms went pretty well, but pressing the new Howe Ball Joints back in them gave me a few fits. Seems the housing kept wanting to go in the arm a bit crooked. Finally just sent them with my friend John to his machine shop to use the good press to get them to go in straight. This delay kept me from getting to anything else on the front this weekend though.

We've got an autocross this coming Sunday so I'll run it with the new rear springs, new front 36 mm sway bar and street tires...and run the camera on the sides pointed at all 4 wheels to do a direct comparison to the first video in this thread. Then after that I'll have a month off to complete the front end changes and really make a difference.

[Ron Sutton]

Little progress this weekend... I got the rear springs in the car and drove it around a bit to feel it out. Then things went a bit wonky. Pressing the stock ball joints out of the new A-Arms went pretty well, but pressing the new Howe Ball Joints back in them gave me a few fits. Seems the housing kept wanting to go in the arm a bit crooked. Finally just sent them with my friend John to his machine shop to use the good press to get them to go in straight. This delay kept me from getting to anything else on the front this weekend though.

We've got an autocross this coming Sunday so I'll run it with the new rear springs, new front 36 mm sway bar and street tires...and run the camera on the sides pointed at all 4 wheels to do a direct comparison to the first video in this thread. Then after that I'll have a month off to complete the front end changes and really make a difference.

Sound good Lance.

Obviously we still have the front geometry problems & roll center too low & off to one side. The front tires will still want to push, just maybe not as bad with the stiffer rear springs. Once you have time to correct the front end geometry, move the roll center & add caster ... the front end will have more grip, and that is when we'll see a noticeable lap time improvement.

I'm back from my quad trip, so I can answer questions if you have any.

We went on an epic 7-1/2 hour quad ride up several mountains in the Sierra Mountains in Nevada & California. From 7000' to 11,700' through a wide variety of rough terrain, rocks, creeks, broken shale. My wife flipped over once, but was OK. She's a sweet, but tough gal. We saw breath taking scenery on several mountaintops looking out over the range, went partway into old gold mines that were caved in, rock & wood cabins where miners lived. We were all worn out from the ride.

[SSLance]

Sound good Lance.

Obviously we still have the front geometry problems & roll center too low & off to one side. The front tires will still want to push, just maybe not as bad with the stiffer rear springs. Once you have time to correct the front end geometry, move the roll center & add caster ... the front end will have more grip, and that is when we'll see a noticeable lap time improvement.

I'm back from my quad trip, so I can answer questions if you have any.

We went on an epic 7-1/2 hour quad ride up several mountains in the Sierra Mountains in Nevada & California. From 7000' to 11,700' through a wide variety of rough terrain, rocks, creeks, broken shale. My wife flipped over once, but was OK. She's a sweet, but tough gal. We saw breath taking scenery on several mountaintops looking out over the range, went partway into old gold mines that were caved in, rock & wood cabins where miners lived. We were all worn out from the ride.

Yeah I know, I thought about just putting the LCAs (with stock ball joints) and the spring adjusters in by themselves, then going back after next weekend to finish everything else up but I really dislike doing things twice, especially when coil springs under pressure is involved. If everything would have went perfectly smooth, I might have got it all in last weekend, but that wasn't meant to be.

Besides, the weather was perfect here for a boat ride and just like yourself on the ATVs, sometimes you just have to get away and play for a while instead.

We can always work on the car when it's raining...right?

[Ron Sutton]

Yeah I know, I thought about just putting the LCAs (with stock ball joints) and the spring adjusters in by themselves, then going back after next weekend to finish everything else up but I really dislike doing things twice, especially when coil springs under pressure is involved. If everything would have went perfectly smooth, I might have got it all in last weekend, but that wasn't meant to be.

Besides, the weather was perfect here for a boat ride and just like yourself on the ATVs, sometimes you just have to get away and play for a while instead.

We can always work on the car when it's raining...right?

We can't use that reasoning out here ... because it doesn't rain from May to November.

.

[rod cole]

Ron from what I understand from what you stated. Could a person put the vehicle on turn plates and add caster until the spindle/wheel stayed level to the max available turn radius. Thus achieving what you are doing with math? And does the correct amount of caster for a given spindle give the same camber whether turned in or out? Thanks Rod

[Ron Sutton]

Hi Rod,

Ron from what I understand from what you stated. Could a person put the vehicle on turn plates and add caster until the spindle/wheel stayed level to the max available turn radius.
I'm not 100% certain of your question, but If I understand it right, the answer is ...

If you place the wheel at zero camber ... and set the degree of caster to exactly match the degree of KPI in the spindle ... then as you rotate the wheel & spindle forward ... it will remain straight up & down ... plumb ... 0.0° ... meaning the caster counteracted the KPI. But when you rotate the wheel & spindle backwards ... the degree of caster & the degree of KPI in the spindle COMBINE to lean the tire towards the inside of the corner.

Did that make sense?

Thus achieving what you are doing with math?
Correct.


And does the correct amount of caster for a given spindle give the same camber whether turned in or out?
That's a tricky question. What is the correct amount of caster? That will vary somewhat with track, driver & philosophies of the set-up person.

For tight tracks, I like to run 1.0° to 1.75° more caster than spindle KPI. On bigger, faster tracks, with high speed sweeping corners, that may make it too free on entry or middle ... frankly anytime the driver uses excessive steering input.

But at no time does caster keep the dynamic camber the same if you turn the wheel forward versus backwards. As stated above ... If you place the wheel at zero camber ... and set the degree of caster to exactly match the degree of KPI in the spindle ... then as you rotate the wheel & spindle forward ... it will remain straight up & down ... plumb ... 0.0° ... meaning the caster counteracted the KPI. But when you rotate the wheel & spindle backwards ... the degree of caster & the degree of KPI in the spindle COMBINE to lean the tire towards the inside of the corner.

If that didn't make sense, just bring up the parts that don't & I'll be happy to explain it.

Bets wishes.




Thanks Rod

[rod cole]

So what I am shooting for is when the left wheel is turned into a right corner. I want the wheel to stay 0 or negative camber by adding caster to offset KPI yes? Thanks Rod

[Ron Sutton]

So what I am shooting for is when the left wheel is turned into a right corner. I want the wheel to stay 0 or negative camber by adding caster to offset KPI yes? Thanks Rod

There is more to it than that. You don't want to rely solely on caster. You want to build your steering geometry with a "happy combination" of KPI, caster, caster gain, camber, camber gain ... while taking into account the body/chassis roll angle.

I'll repost the info on the concept ... then the process I use to build a "happy combination" ... and after you read it, if anything isn't clear, post your questions & we'll discuss it deeper. Here goes ...

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Front End Geometry

Most everyone knows camber, caster & KPI/SAI work together, but most don’t really understand HOW they work together & how they affect each other. I’ll do my best to explain it, but we’ll need to peel the onion one layer at a time, so bear with me.

For those that don’t know what this is, KPI stands for King Pin Inclination & SAI stands for Steering Angle Inclination. They mean the same thing.

KPI was a term coined back in the day of solid front axles when spindles actually used king pins. Steering Angle Inclination is a more correct modern term & is calculated simply by running a theoretical line through the upper & lower ball joints & comparing that angle to the actual spindle pin the hub spins on (rolling axis in the photo). (I use both terms because many race car guys are used to the older term of KPI.) .

Think of caster as “dynamic camber” … since caster has no affect on angle of the tires & wheels … until you turn the steering. Then caster is tipping the top of BOTH tires towards the inside of the corner you’re turning into (Good).

KPI angle is important to keep the scrub radius lower. You can look at the illustration above & imagine how big the scrub radius would be if the KPI was straight up & down. If the front tire stays in the same location more KPI makes the scrub radius smaller. Less KPI reduces the KPI.

A bigger scrub radius means the tire is farther from the steering pivot … making the arc bigger that tire has to make in order to pivot. Not a big deal at low speeds, but in track conditions, at higher speeds, with the tire at its limit of grip … when you turn the wheel more, you are “torquing” the tire tread around a big axis to turn. This “rips” the tread across the pavement, causing the tire to lose a degree of grip. The bigger the scrub radius ... the higher degree of grip is being lost when you turn the steering. A zero scrub radius means the center of the steering pivot & the center of the tire tread are the same. This pivots the tire right in the center of the tread reducing lost traction to the minumum. KPI has pros & cons.

Think of KPI as a different kind of “dynamic camber” … since it also has no affect on the angle of the tires & wheels … until you turn the steering. But unlike caster, it is not tipping both tires towards the inside of the corner you’re turning into. KPI is tipping the top of the outside tire out towards the outside of the corner you’re turning into (BAD) and tipping the top of the inside tire in towards the inside of the corner (Good).

When the KPI/Caster Split favors the KPI … the tire & wheel, on the outside of corners, goes into a state of positive camber (BAD) … rolling over on the outside part of the tread and sidewall of the tire … with the inside part of the tread becoming unloaded. Basically, at this point, the actual tread making contact with the pavement (contact patch) gets narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had a full contact patch.

Now let’s talk about the tire on the inside of the corner. Some cars roll so much the inside suspension goes into a “droop” or state of extension … and if that car has negative camber gain built in … the droop actually helps the inside tire stand straighter. For cars don’t roll as much … and that compress the suspension on the inside tire & wheel when cornering, the negative camber gain on the tire on the inside of the corner is tilting that inside tire the wrong way. It is rolling over on the inside part of the tread and sidewall of the tire … with the outside part of the tread becoming unloaded. Also making the contact patch narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had more contact patch.

So your front tires that were already at their limit of grip … just lost a significant amount of contact patch & essentially got narrower … and lost even more front traction … creating a push or understeer condition.

The amount of dynamic camber loss is minimal with slight amounts of steering input on large sweeping corners, but grows exponentially worse with higher rates of steering input (front wheel steering angle) on tighter corners. More caster would help both situations … creating more dynamic camber the correct way for both tires … keeping the tire contact patches flatter on the track surface. But how much is enough? Read on.

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Now let’s discuss how Caster & KPI can work together for an optimum combination …

Racing spindles run the gamut of KPI angles. In oval track stock cars where the control arms are pretty average in length & the wheels are out wide … making the scrub radius bigger … racers typically run spindles with 8-12 degrees of KPI. In Trans Am & other road race cars where the control arms are longer & the wheels have a lot of back spacing … making the scrub radius smaller if not zero … racers typically run spindles with 3-5 degrees of KPI.

For the street & Pro Touring market, there are a lot of spindle options … usually with KPI somewhat in the middle of the extremes outlined in the racing paragraph above. Many Ford, Mustang II, Wilwood Pro Spindle & the ATS spindle from Speedtech all have 8.0 degrees of KPI. A lot of GM spindles have 8.75. I believe the C6 is 9.15. For our conversation … and math purposes … let’s use a spindle with 8.0 degrees of KPI/SAI as our sample.

If you were to set both the caster & camber at zero … and rotated the spindle 90 degrees each direction … the difference would be 2x the KPI/SAI angle of 8.0 degrees … so in this case 16.0 degrees.

We know the wheels don’t turn anywhere near 90 degrees, but this example makes everything more clear. Please humor me & follow along closely, because I’m about to share something that is one of the most overlooked keys to proper cornering set-up. We will account for the ACTUAL steering turning radius later.

If you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 8.0 degrees of camber loss (goes into positive camber). Bad … very bad for the outside tire of a corner. :(

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel also experience 8.0 degrees of camber loss (goes into positive camber). But this good for the inside tire of a corner.

Reminder, obviously we are not turning the wheel 90 degrees in the real world, so don’t lock in on the numbers “too much” … just the concept.

Caster is different. If we set caster at 8.0 degrees positive (top to the rear) & leave KPI/SAI out of the equation, as if we had a spindle with zero KPI/SAI … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 8.0 degrees of camber gain (goes into negative camber). The right direction for the outside tire in a corner.

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 8.0 degrees of camber loss (goes into positive camber). And this is the right direction for the inside tire of a corner.

So … caster helps both the inside & outside wheel & tire.

Here’s the most important piece of info to know at this point. It is the first & most important key to getting the front tires to use their full contract patch when cornering … increasing front end grip & turning speed. Drum roll please …

Caster offsets KPI/SAI on the wheel & tire on the outside corner … and compounds (adds to) KPI/SAI on the wheel & tire on the inside corner.
Read that again. It’s very important.


This is called KPI/Caster Split. When the Caster & KPI are equal … the caster offsets the negative effects of the spindle KPI on the outside wheel ... and compound the advantages of the KPI on the inside wheel. When the KPI is greater than the caster (unless the car has a TON of Camber) the outside wheel is going to lose camber as the steering is turned & roll over on the outside front tire. Ugly.

The greater the split is favoring the KPI, the worse the problem. On the other hand if the KPI/Caster split favors the caster … meaning the caster is slightly greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires stay flatter to the road, have more grip, better turning & higher corner speeds.

Sooo … if we set the car up using spindles with 8.0 degrees of KPI/SAI and 8.0 degrees of caster … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 0 degrees of camber gain or loss. Frankly it is zero, no matter what degree you rotate it to the front, because 8.0 degrees of caster counteracts … or neutralizes … the 8.0 degrees of KPI/SAI.

If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experiences 16.0 degrees of camber loss (goes into positive camber). This is the right direction for the inside tire of a corner … way too much ... but we’re not turning 90 degrees. We’re turning somewhere from 0 to 25 degrees. What if the wheels were turning 15 degrees? … that’s 1/6 of 90 degrees … times 16.0 … equals 2.67 degrees … the right direction.

At this early point in peeling the layers of the onion ... we have:
Outside Front Tire at 0.0 degrees (OK)
Inside Front Tire +2.67 degrees (Good)

You’re probably going “Hmmmm” … but we don’t have the whole picture yet.
We have a lot of other geometry to factor in. Remember, we’re peeling this onion a layer at a time, so we’ll get to camber gain, chassis/body roll angle & static camber in steps.

Camber gain & chassis roll angle are next. Chassis roll angle hurts the contact patch of both tires. Camber gain (towards negative) helps the contact patch for your outside tire & hurts the contact patch for the inside tire.

Chassis/Body Roll Angle
Stock production cars have a HIGH roll angle when pushed to their limits. Pro level race cars running high travel/low roll angle suspensions … obviously have pretty low roll angle. But’re we’re talking modified PT cars for track purposes. Way better than stock production cars, but not quite race cars.

Because higher roll angles are an enemy of proper geometry & optimum contact patch … for this conversation let’s say we’re running a milder version of the high travel/low roll suspension … and have 1.5 degrees chassis/body roll in the corners. Obviously any chassis/body roll is ot the outside of the corner … and therefore hurtful to the contact patch angle of both tires.

If we add chassis/body roll angle into our numbers above, now we have
Outside Front Tire +1.50 degrees (Bad)
Inside Front Tire +1.17 degrees (Good)

Camber Gain
On the outside tire, if you worked out your camber gain to be 1.5 degrees negative “in dive” … and assuming we have a modern low roll angle suspension with chassis/body roll angle of 1.5 degrees … those two just neutralized each other. The inside tire, of this car in the same corner, is compressed, but not as far, so it doesn’t have as much camber gain towards negative (reminder: camber gain towards negative is bad on the inside tire).

Let’s say we end up with 2/3 the compression travel on the inside tire & end up with 1.0 degrees negative camber gain (the bad direction for the inside tire) … so what does that do for us at this level of “onion peeling”?

Dynamically we have:
Outside Front Tire 0.0 degrees (OK)
Inside Front Tire +0.17 degrees (OK)
Not optimum yet, but we’re going the right direction & we’re not done yet.

The next layer of the onion is static camber.
You need SOME static camber … to help with initial steering turn-in responsiveness. Just don’t get greedy. In road racing or AutoX where you’re turning left & right, static camber is like camber gain. It helps the contact patch on the outside tire & hurts on the inside tire. For this example, let’s add 1.0 degrees of static camber.

Now with static camber added … with your car hard in the corner … suspension in dive, wheel turned 15 degrees for a tight corner …

Dynamically we have:
Outside Front Tire -1.0 degrees (Good)
Inside Front Tire -0.83 degrees (Bad)
Not optimum yet, but we’re we’re not done yet.

Now, here is another part I love. You simply add caster until the contact patches of both tires are flat & happy. And from this point the math is easy.

Add 1.0 degree of caster and …
Outside Front Tire -2.0 degrees (Good)
Inside Front Tire +0.17 degrees (Good)

Add 1.25 degree of caster and …
Outside Front Tire -2.25 degrees (Very Good)
Inside Front Tire -0.42 degrees (Very Good)

Add 1.5 degree of caster and …
Outside Front Tire -2.5 degrees (Very Good)
Inside Front Tire -0.67 degrees (Very Good)

There are many factors that will define your optimum set-up, but this creates a baseline that is darn close.

**P.S. I like to end up “around” 1.5-2 degrees more dynamic camber on the outside tire, since the outside tire is loaded so much more.

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There is a little more involved in this, when you involve exact steering angles for specific corners. The good news is … when you run tighter corners requiring more steering angle than 15 degrees … the caster increases the dynamic camber to help the tires maintain flat contact patches. I use a spread sheet I developed to plug in all the info & know exactly what dynamic camber I have at different steering angles & different camber gain & different suspension travels.

The best way I have found to work out a front end setting is to start with KPI/SAI & caster … then bring in camber gain … and finally static camber … to achieve the optimum dynamic camber for BOTH tires. Regardless of how you get there ... all of these geometry pieces need to work together in harmony to achieve full, optimum contact patches for both front tires in hard cornering situations … for optimum cornering grip & speed.

It probably is clearer now why getting advice on one setting that worked for a buddy’s car … without knowing the whole picture … can be misleading. As a tuner, I couldn’t imagine setting the caster without knowing the spindle KPI & the car’s camber gain … and then of course testing on track with tire crayon on the edges every run (plus taking tire temps).

It’s been said a zillion times. It’s the whole package, not one part or one setting.

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Remember the KPI/Caster Split concept ... if the caster is slightly greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires have more grip, better turning & higher corner speeds.

When I'm designing a front suspension for a specific class with rules on what spindle we can run, I pick the best spindle available under the rules and design everything else to either fix or compliment that spindle. Factory spindles usually have KPI/SAI ranging from 7-10 degrees. When I have to run a factory spindle ... I know I'm going to end up with 1-2 degrees of caster more than the KPI.

I designed & raced NASCAR Modifieds with Factory GM #2 spindles with 8.75 KPI. 10-10.25 degrees of caster produced awesome results. We had a crew chief go off the range with set-ups & try 6-7 degrees of caster, but the cars always pushed in mid corner ... and snapped loose on exit. He was used to running less caster, but didn't take into account the KPI of the spindles we had to run.

The whole combination of KPI/SAI, caster, caster gain, camber, camber gain, Ackerman, toe, steering ratio, etc. ... ALL have to be designed together for optimum cornering performance. All of them are important, but the KPI/Caster Split is critical & often not fully understood.

When I'm designing a front suspension with no rules on what spindle we can run, I design the spindles & have them built. Then I'm not trying to fix anything ... and everything else in the front suspension can be designed to compliment that spindle. For a road racing car, I designed the spindle with 3 degrees of KPI/SAI ... and designed the rest of the the front suspension around what is called a "zero scrub" set-up ... & we ended up with 4.0 degrees of caster for optimum handling. This car did NOT require a high caster number, because the spindle KPI was lower. What is optimum for tight cornering is having the KPI/Caster Split slightly favoring the Caster.

Another successful car I designed with 5 degrees of KPI/SAI ... ended up with optimum handling with 6.5-7.0 degrees of caster ... depending on the track. Again, the key was the KPI/Caster Split slightly favoring the Caster.

This higher amount of caster seems odd to most veteran mechanics & street car guys used to running 1-3 degrees of caster. But when you look at the newer Corvettes, Vipers, BMW’s, Mercedes, Etc. … you'll see they run a lot more caster than what most think of as "typical”. The Factory GM specs for the C6 ZR1 is 7.7-8.3 degrees of caster. But owners that compete & win in the C6's increase the caster to 9.5-10.5 degrees of caster ... with the KPI/Caster Split favoring the Caster.

The common denominator is we are almost always running more caster than KPI/SAI ... if we can. . We run spindles with lower KPI where we can too ... but it requires running wheels with DEEP back spacing to get the scrub radius low (or sometimes zero). So simply ordering a spindle with smaller KPI is NOT a bolt-on solution.

There are a lot of spindle options, including custom spindles, which cost less than most folks think. One BIG question in this equation though is Scrub Radius. For optimum AutoX performance. Scrub Radius is important. The smaller the better … down to zero … and KPI/SAI plays a big role in this.

The first goal with Scrub Radius … if possible … is reducing it, so the tire is not having to torque itself around a big radius when you turn the wheel. If that is not possible … the 2nd goal would be … at least not making it bigger. Caster, KPI/SAI, wheel back spacing & Scrub Radius … all need to be worked out together. You should not arbitrarily pick a spec for any one of these things without considering how they affect each other.

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How do you work out a front end geometry combination?

All of us know that each front suspension geometry setting affects the overall picture ... and to a degree ... each other. As a race car Designer & Crew Chief, I can't look at just one individual setting. I have to look at the spindle KPI/SAI, Caster, Caster Gain/Loss, Camber, Camber Gain/Loss as a team ... a team of geometry devices that I need to work together in harmony to improve how the tires contact the road dynamically.

Having done this for years, designing, building, tuning & racing a lot of cars, in just about every type of racing, has given me some firsthand insight into how all these things work. So now it's a little easier & quicker for me to "get a set-up there" to the sweet spot. I have a process that gets me there quickest ... with the least back-n-forth.

First … some guidelines:
Zero scrub radius with long control arms & deep backspaced wheels is optimum, but many race series rules prevent us from achieving this, with rules on the LCA. If the rules allow, we're running long control arms & a low KPI spindle on deep backspaced wheels & achieving low to zero scrub radius.

But when the rules restricting our LCA choices prevent us getting the ball joints "out there" … making us choose between track width & scrub radius … we are “usually” going with wheels with less backspacing to achieve the maximum track width allowed by rules … then running higher KPI spindles to reduce the scrub radius as much as we can.

I've had people ask me why don't we give up track width to achieve a lower or zero scrub radius ... but in the big picture ... track width trumps scrub radius, up to a point. Everything has its limits & there are exceptions for everything. Tight AutoX courses sometimes favor narrower cars with narrower track widths. But most anything faster than that … like road course … favors a wider track width. When you have no rules or limitations, you making everything optimum. When rules prevent that, you’re shooting for the best overall compromise.

I have a step-by-step process I follow ...
with the goal being optimizing the contact patch of both tires, while turning hard on tight corners of AutoX tracks or Road Courses .

A. I have to work out the Spindle KPI/SAI with the length of the A-arms, tire width & wheel backspacing ... to end up with a desirable scrub radius.

When I’m working out a car for class rules, those rules often limit what you can do for spindles & wheels. In more unlimited series there are less rules. On the street, there are no rules, except the ones you impose based on your desires, budget & priorities.

If I have a scenario where we are running front wheels with a lot of backspacing … that gets the KPI/SAI closer to the center of the tire … reducing Scrub Radius. “Zero Scrub” is optimum, but challenging to achieve for packaging reasons. It requires very wide wheels & deep backspacing. You can go too far ... with REALLY DEEP back spaced wheels & HIGH KPI/SAI spindles … you can get into a situation of negative Scrub Radius, which isn’t desired.

I recently designed a front end with 12” wide wheels & 10” of backspacing. A 8-10 degree KPI/SAI spindle would have put it into negative Scrub Radius. The spindle design & hub design also play a part on the packaging with the distance from the steering axis to the wheel hub face. With the hub we used & a 5 degree spindle, it created a zero scrub radius package.

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B. Once I know the spindle KPI/SAI, that guides me on how much “static” caster I need to build in to achieve a KPI/Caster Split favoring the caster.

Optimally, we need the 1.0-2.0 degrees more caster than KPI/SAI angle. A lot of Ford passenger cars run 8.0 KPI, so we’re shooting for 9.0+. A lot of GM spindles are 8.75 KPI, so we’re shooting for 10.0 +/-. If we can run a lower KPI/SAI spindle … like in the example above using a 5 degree KPI spindle … we’ll need 6.0+ degrees of caster.

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C. I need to work out if we want caster gain, or for the caster to stay the same, as the front suspension compresses fully under braking & turning (Term = "Dive") Think of this as “dynamic” caster, because it only happens when the suspension is compressed.

Quick Primer: When the front suspension compresses, if the angles of the lower control arm cause the lower Ball Joint to move forward and/or the angles of the upper control arm cause the upper Ball Joint to move back … that creates camber GAIN … in dive (compression). When the front suspension compresses, if the angles of the lower control arm cause the lower Ball Joint to move back and/or the angles of the upper control arm cause the upper Ball Joint to move forward … that creates camber LOSS … in dive (compression). We never want loss. When the front suspension compresses, if the angles of the lower control arm and/or upper control arm combine for no change in the ball joint locations … that is considered Caster Neutral. *Ask if you want the “how” explained more in-depth.

If we can get all the “static” caster we need, we will typically run only a small amount of caster gain. If we can’t get enough static caster, we need to build in more of caster gain to help us get to the desired total number.

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D. If I can get the caster I want ... statically and/or with gain ... so the KPI/Caster Split favors the caster 1.0+ ... I can run less Camber, which is the goal. We're always going to run SOME static camber (negative) … say –0.5 at a MINIMUM … and we always want SOME Camber gain … but if we don’t end up with a KPI/Caster Split favoring the Caster by 1.0+ degrees … then we need to make the difference with Static Camber & Camber Gain.

This is NOT ideal, because Camber by itself helps the angle of the outer tire achieve optimum contact patch … and hurts the angle of the inner tire, preventing optimum contact patch. The more static camber & dynamic camber we have to run … to make the outer tire work best … the more it hurts the contact patch of the inner tire.

If we can get to the optimum angle with a KPI/Caster split favoring the caster by 1.0+ degrees … either statically or in dive … we will run smaller amounts of static camber & camber gain. This is optimum. But if we don’t … we’ll run all the camber we need to … to make that outer tire WORK.

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E. Once I know how much camber I need for that combo, I work out how much of it is going to be static camber & how much is going to be camber gain.

We always want some static camber. -0.5 is like the minimum. I like -1.0 to -2.0 … IF everything above falls into place.

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F. How we get the camber gain, & how much, affects the car's static & dynamic roll center ... so they have to work as a team too.

Our desired roll center plays a role in this decision, because all the A-Arm angles creating Instant Centers determines where the static Roll Center is ... and where it goes dynamically in dive.

It should be clear now why I don't start with Camber.

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All of this ... is to optimize both front tire's contact patch with the asphalt ... In the car's dynamic states when it's driven HARD ... meaning turning, braking, rolling, unwinding & accelerating to the limits of the car, tires & driver ... sometimes beyond.

I realize this is a lot to digest. Don’t be afraid to ask questions where ever I wasn’t clear.

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*Caster gain is achieved when the front A-arm geometry is set for anti-dive. Zero caster loss or gain is achieved with zero anti-dive. Caster loss is achieved when the front A-arm geometry is set for the opposite of anti-dive … called “pro-dive.”

** The more static camber you have to run to optimize the outside tire, the more you’re hurting the inside tire. I like to get 50%-60% of the total camber desired … through camber gain. Here is why: The suspension on the inside of the corner is not compressed as far as the suspension on the outside corner. So effectively … the inside tire is not getting as much negative camber gain to fight & overcome.

*** Don’t get greedy with static camber. Yes it improves initial turn-in steering response, which is good. But two things:
1. Camber helps the outside tire & hurts the inside tire. If you run too much static camber, you can’t get the inside tire to work optimum.
2. There are a LOT of other things that help turn-in response … so use some of those … & don’t get greedy with static camber.

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You know how people say a little information can be dangerous? This is one of those times.

If your PT car has a Moderate to High scrub radius … you have to be careful with how much caster you can put in the car … because caster combined with a moderate to high scrub radius creates a “jacking effect” when you turn the wheels. Dynamically, this jacking effect “de-wedges” the car … loading the inside front & outside rear tires more … while also unloading the inside rear tire & increasing the degree the car diagonally rocks & loads the outside front tire.

All of this helps the car to turn better. But go too far … and the car will get loose on entry. This is where track tuning comes into play. If you have a moderate to high scrub radius … sneek up on the caster you put in the car … until you get the car “free” on entry … then reduce the caster a tick … or tune something else to allow you to keep that amount of caster, so the car turns well in the middle. But do NOT keep a set-up that makes the rear loose & stepping out on corner entry.

Make sense?


In my experience & by my personal scale ...
0-1/4" = Zero
1/4"-1" = Very Low
1-2" = Low
2-3" = Moderate
3-5" = High
5-7" = Very High
7" + = Extreme

* But this relative to the situation. When we designed a new Modified chassis that dropped the Scrub Radius from over 7" to under 5" ... we thought of that as "low scrub radius” ... for the NASCAR Modified class we raced in.

[rod cole]

Got it. Currently have stock G-body spindles 3/4 taller lower ball joint and 1/2 taller upper with 1/2 shorter upper arms. Looks like in youre example I am going to need 9 plus degrees of caster to offset the spindles most likely requiring moving the lower ball joint forward to reach these numbers. Thanks for you,re expertize. Rod

[Ron Sutton]

Got it. Currently have stock G-body spindles 3/4 taller lower ball joint and 1/2 taller upper with 1/2 shorter upper arms. Looks like in youre example I am going to need 9 plus degrees of caster to offset the spindles most likely requiring moving the lower ball joint forward to reach these numbers. Thanks for you,re expertize. Rod

You are on track with the amount & method for tight cornering.

[SSLance]

So just to update where I'm at with the car now... I've made a couple of the changes Ron suggested already as I had the parts on hand and available time to make them. I put the 36 mm sway bar up front before the last autocross 2 weeks ago and I put the new rear springs in before this week's autocross. Now that I've got a month off before the next event, I'm ready to tackle the major pieces of the recommended changes, new LCAs, taller lower ball joints, repositioned LCA mounting holes, coil spring adjusters to raise ride height and adjust weight, the bump steer kit and the associated alignment settings once the parts are in place.

I ran the bigger front bar last event with the R-comp tires I'd been running most of the season. I could tell the car was staying flatter, not rolling over near as much and this seemed to help keep the front planted better. I ran a camera pointed at the front suspension that event and discovered that not only is my frame flexing around quite a bit, but also I was running way too low of a tire pressure in the R-comps and was really rolling over the sidewalls and the tire slip angle was huge. Data collection is huge and the GoPro helps tremendously with this.

I then put the stiffer springs in the rear along with the adjusters. An unexpected consequence of this addition to me was about an inch of ride height increase in the back, even with the adjuster flange back out and the springs just set on the base of the adjuster. I wanted to drive it with the stiffer springs in anyway to see how it drove, knowing the increase in ride height would upset other parts of the car and that this would be temporary until I finished up the front end changes. Here is how it sat before yesterday's event.



Little too much rake which I'm certain had a negative effect on the roll center. I don't want to spend too much time on what it screwed up as it's all going to change again very soon. I also ran my street tires yesterday for the first time since April, brand new sticky Rivals up front and wore out Nitto 555s out back.

I kind of figured the mis-mash of changes would be entertaining, add in a Gymkhana style course setup for this event...and well...it was.

I was WAY loose entering into corners. The front was stuck pretty well, I can't remember ever losing or pushing the front tires more than just a little bit of squealing but I was struggling getting into corners which was messing me up pretty good. It seemed like the car wasn't rolling over near as much though, with the big bar up front and stiffer springs out back, it seemed to run more flat. And traction off of the corners was much better as well, it didn't want to seem to try to spin the tires off of a corner as long as I wasn't already sliding the rears because of loose in.

After my second run (with some road rage mixed in from a missed cone\DNF) we had an oil down from a blown power steering line and about a 45 minute delay. With nothing to do and still a bit fuming over my last run, I texted Ron with an update on my progress. He was by his phone and texted me right back. I explained how it was going, about being loose in...and he helped me with a few driving tips to help counteract the condition for the last two runs.

I can't tell everyone here how much I appreciated his help at that time. What an unbelievable resource, and the fact that he is texting me, on a Sunday afternoon, from halfway across the Country...blew my mind. Thanks Ron!

Ron told me to brake a bit earlier and a bit softer and to work on picking the throttle up sooner on exit. In my mind...that meant, slow down...to go faster....and it worked. I picked up almost 2 seconds on my 3rd run and it just felt better overall. Picked up another 2\10s on my 4th run but also a cone.

Here are my runs from the weekend http://youtu.be/xRvExmeX82U

And here are the final best raw times in order

http://www.kcrscca.org/results/solo/...event9_raw.htm

1985 Chevrolet Monte Carlo SS Maroon (1) 71.176 (2) 70.841+dnf (3) 69.134 (4) 68.929+1

So anyway, now the real fun can begin. I'll start disassembly of the front end this week and will document here the making of the major changes Ron prescribed and then if all goes well, get some testing video and then event video from our next autocross on Sept 15th.

My personal goal is to get this car to run as fast on street tires as similar cars I race against run on R-Comps and I think it can be done. Especially if the driver keeps his head in the game.

[Ron Sutton]

Great update Lance.

I was a little leery of you running just 2 of the 9 changes we planned. Sometimes that gets real ugly.

Here, the result was actually pretty good.

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On a somewhat different note, as you move to a "Tweener High Travel/Lower Roll Set-up" ... you'll need to adapt your driving style as you did this last weekend. This type of set-up doesn't allow the driver to charge the corner as hard ... meaning it doesn't like late, hard braking. You'll need to brake a "little" sooner. How much sooner ... 5', 15' etc ... needs to be worked out in on-track testing.

And you need to brake a lot softer & a little longer. Again, how soft & how long will come to you from on-track test driving. You will want to slow the car "less" ... as you will be able to carry more corner speed with this set-up ... and that is where half of the lap time advantage comes from.

The other half of the lap time advantage comes on the corner exit. You will be able to get the car turned sooner & better. And you will be able to get on the throttle earlier & roll it to full throttle quicker. This is a big deal to the overall lap time. Bigger than most people think.

The simple summary of the this set-up will be:
a. Brake a little earlier, a lot softer & a little longer.
b. Slow the car less & carry more corner speed.
c. Get the car turned better & sooner.
d. Pick up the throttle earlier & roll it on quicker.

[SSLance]

Are you being easier on the brakes to keep the car from diving and unloading the rear tires on corner entry? Basically letting the car settle back to ride height before initial turn into the corner?

In other words, with this type of setup, where in the corner do you want the weight to transfer to the front, then the outside, then the rear on exit?