Front Suspension Geometry and Tire Wear

[Pappy]

Okay Guys, I am going to stick my neck out with where I think we are with front suspension geometry. This is my opinion only, subject to any opinion or disagreement you think is appropriate, and certainly not presented as the authoritative answer. I have spent the last 4 years designing, fabricating, and testing a modern high travel, low roll suspension for another track car. My professed knowledge was guided by an individual I consider to be “the Pro” – which we can discuss if you want to send me a PM.
The ACR suspension is fairly “low roll” by design – lots of roll stiffness and aero that forces both sides of the car down. In a low roll suspension, both the outside tire and inside tire share some of the cornering load, so the “dynamic” (travel conditions at the compressed, loaded height and turn angle) geometry of both front wheels come into play. The dynamic conditions take into account wheel loading, slip angle, and both dynamic (not static) camber and dynamic (not static) toe. There are some rules of thumb (sometimes closely guarded, but available on Hilary’s server) that define dynamic camber and caster in relation to the King Pin Inclination (KPI) angle, and the optimum tire slip angle is dictated by the tire manufacturer. To keep things simple, and to quote the Pro, “Camber is good for the outside tire, KPI is good for the inside tire, and caster is good for both tires in a turn". As an aside, increased caster translates to increased negative camber on the outside wheel in a turn, and to decreased negative camber on the inside tire (a good thing, especially if that tire is loaded).
My next caveat: I am delinquent in providing factual track testing with the ACR. Lame excuse – it is hot out here and the track events are pretty much shut down, I am still track prepping the car, and I have the front suspension all apart to measure all of the pick-up points and various angles I need to enter into a suspension program.
Now my preliminary assessment: I am leaning toward two problems – too much static camber and a shock package (or rebound settings) that won’t hold the nose down through the apex to take advantage of the camber gain that is due to caster gain with travel. Too much static camber, without the nose being pinned down to take camber out of the inside tire in the turn, causes extreme scuffing of the INSIDE of the INSIDE tire during a turn. That tire, especially a stiff, short sidewall tire, is not at the proper slip angle for the amount of work it is being asked to perform. Adding camber to fix the problem in a turn results in scuffing of the inside edge of the outside tire and further hurts the situation on the inside tire. I will work to confirm this theory when I can get access to a semi-flat oval (like Phoenix International Raceway) where I can run a 15 minute session in one direction only and check the front tire temps and wear. My basic intuition tells me that since no one appears to be tearing up the outside edge of the outside tire, they are not close to “too little” camber.
Anyway, that’s my story and I am sticking to it. Feel free (and please do) take shots at me and my assessment. I promise to provide real empirical data when I can.
The following photos show: 1) 3.5 degrees of outside tire camber (starting with 2.5 degrees static) with the wheels turned 15 degrees. I will assume an additional 2 degrees of negative camber with a compressed suspension (maybe more - due to caster gain), for a total of 5.5 degrees of negative camber, less any roll angle. That’s a lot if there isn't much roll angle! 2) .25 degrees of negative camber on the inside tire with the tires turned 15 degrees (uncompressed). I would like to see more positive camber. 3) A destroyed tire at PCOTY that appears to show that the tire was worn from the inside out, indicating that the damage was primarily to the inside edge of the inside tire. When I can get on a flat oval I will be able to tell you if this hypothesis is correct.
In the meantime, I welcome your inputs and hope this stimulate conversation with the real experienced Viper racers.
Pappy

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[mjorgensen]

Pretty much how I have felt since 08' when testing my non aero version (although I did the same with all the other cars I had owned and tracked), never ran more than -1.9 in the front especially at an autocross, (more inside tire work available and a flatter foot print on the outside for braking and initial turn in). Sure the outside wears faster, but with the inside wear it always evened out the overall wear once you found the ideal pressures for that combination. Be prepared for some aggressive push back though for sure from others, but like I tell every customer most people have their own style of driving and one set up does not fit all.

[ACR Steve]

My set up is -3 camber ft and -.25 toe
I am thinking of going to -2.8 and 0 toe to avoid some inside scrub at turn in.

[Arizona Vipers]

Great post Pappy! Lots of great data here. I can't wait for others to start commenting on this. Subscribed!

[allans]

So Pappy, Based on your current feelings, what set-up would you start with ? Thank you. Allan

[Pappy]

So Pappy, Based on your current feelings, what set-up would you start with ? Thank you. Allan

Factory settings to establish a baseline, then less static camber (2 degrees or so to start) and more caster (6.5 - 7 degrees, or more if the KPI angle dictates) and check tire temps. When I start overheating or tearing up the outside edge, I'll back off. My biggest concern right now is how to get the shocks to pin the nose down better from braking through the apex, and I still have to get enough track time in this car to figure out how much and what speeds the front aero is contributing significantly to pinning the nose. Slower rebound may require revalving or even different shocks. Again, this will be my approach, but is not backed by any empirical data to date.

Pappy

[allans]

Thanks Pappy, I'm working on a new set-up with MCS shocks, valved and sprung for the ACRE. Also going to 18 " Forgelines and R7's in the front and reducing negative camber. Will let all know findings. Best, Allan

[Pappy]

Thanks Pappy, I'm working on a new set-up with MCS shocks, valved and sprung for the ACRE. Also going to 18 " Forgelines and R7's in the front and reducing negative camber. Will let all know findings. Best, Allan

Thanks Allan. I am anxious to hear your results. I think the shocks will be key to unleashing some real front end grip.

Pappy

[ACR Steve]

I have had 0 issues with front end grip. The car is insanely neutral on track. If anything it transitions to a tad over over steer which is what you want.

[XSnake]

Thanks Pappy, I'm working on a new set-up with MCS shocks, valved and sprung for the ACRE. Also going to 18 " Forgelines and R7's in the front and reducing negative camber. Will let all know findings. Best, Allan

Courious as to why hou would try and reinvent the wheel with new suspension. The bilsteins have been designed, engineered and tested for the ACR.

[mjorgensen]

Courious as to why hou would try and reinvent the wheel with new suspension. The bilsteins have been designed, engineered and tested for the ACR.

I'll let Allan elaborate, but he knows what the MCS have to offer, that's why he went with them in the first place. You know other people that are changing the suspensions also so nothing new. We use much different spring rates then others though and we have a half dozen new ACR's on MCS right out of the box just because the owners had them previously and know how incredible they are.

[Bob Woodhouse]

Pappy, I am curious what you know about that tire you photo'd from PCOTY. Is it a good reference? Some questions: Was it running under inflated? Or being drug sideways from gross misalignment or broken suspension? Way too much wear on the inside beyond the tread surface to suggest normal suspension dynamics was a key player. Also note the rubber peel left at the edge of the tread grooves is on both sides leading one to again suggest under inflation as the tire got drug sideways in both left and right corners.

Your analysis and depth in your article is awesome by the way. Love it, You are teaching us lessor peeps a lot. Thank you.

[TrackAire]

Pappy, I am curious what you know about that tire you photo'd from PCOTY. Is it a good reference? Some questions: Was it running under inflated? Or being drug sideways from gross misalignment or broken suspension? Way too much wear on the inside beyond the tread surface to suggest normal suspension dynamics was a key player. Also note the rubber peel left at the edge of the tread grooves is on both sides leading one to again suggest under inflation as the tire got drug sideways in both left and right corners.

Your analysis and depth in your article is awesome by the way. Love it, You are teaching us lessor peeps a lot. Thank you.

Can this be a case of a tire breaking the bead and losing the majority of air pressure yet the tire still stays on the rim and the minuscule sidewall height has no problem holding up the vehicles weight? Your point of the tires leading edge wearing out has me thinking under inflation or near no pressure to allow the sidewall to roll over that much.

The front sidewall ratio of the ACR has to be the smallest sidewall height of any production car I can think of......very little room for the sidewall to squish and wiggle.

Regardless, this is a very interesting situation where we will all hopefully learn some answers.

[mjorgensen]

I don't think the tire being under inflated when he came off track would have gotten by him so don't see that as option. I have seen a tire un-bead itself and nothing subtle about it. Would be awesome for someone to mount a Go Pro under the car shooting at the contact patch to see what the tire is doing under load and on the inside corner when unloading.

[Pappy]

Pappy, I am curious what you know about that tire you photo'd from PCOTY. Is it a good reference? Some questions: Was it running under inflated? Or being drug sideways from gross misalignment or broken suspension? Way too much wear on the inside beyond the tread surface to suggest normal suspension dynamics was a key player. Also note the rubber peel left at the edge of the tread grooves is on both sides leading one to again suggest under inflation as the tire got drug sideways in both left and right corners.

Your analysis and depth in your article is awesome by the way. Love it, You are teaching us lessor peeps a lot. Thank you.

Thanks for the compliment Bob. I think your questions about the PCOTY tire are important. I don't believe there was any misalignment or damage to the car, but it was being driven by multiple drivers - some not so good. As Mark suggested, different drivers have different styles. If you look at the pictures Venom V posted of his ACR on track at Thunderhill (http://driveviper.com/forums/threads/14226-Thunderhill-5-mile-fun-in-the-sun) you will see significantly different camber angles when the car is loaded with the nose down and when unloaded while accelerating. A driver who brakes early and lets off well before the corner will have a higher front travel height and will have different camber angles while he is turning in than will a late braker who still has the nose down at corner entry. This would not necessarily be true if the shock valving sucked the nose down to the track and held it at least through the apex. I am trying to achieve a consistent dynamic camber and toe with the nose in a predictable attitude (splitter on or near the track) and a given turn angle (generally based on the angle required for the most critical turns on a particular track). Caster gain is important to help reduce the inside tire's negative camber once the wheels are turned and can give the desired outside tire proper negative camber with less static camber dialed in. (As Mark said - brakes better and turns in better.) If the nose is up, camber gain (due to travel - compression) is minimized and the inside tire will suffer on the inside edge if it is doing any work at all. To TrackAire's point, a short, stiff sidewall exasperates the problem. Maybe an 18 inch tire with a taller sidewall is a good crutch.

These are just my thoughts based on experience with other platforms. Just hope they are good food for thought. I think a properly set-up ACR (alignment, shocks, tires) will start to tear up the track records Winkler set with a stock car.

Pappy

[MomentaryRacing]

OP observations are consistent with our experience.

Front grip going away too early in non-aero corners, and excessive inside/inside tire wear is what we saw repeatedly in the data and tire logs. We are experimenting with stiffer front spring rates to control the front, and less rear brake bias (it's excessive anyway) with RB kit being developed.

More front splitter helps, but not in lower speed corners.

Higher pressures and less static neg camber in the front, to keep the car from scrubbing/destroying the inside of the inside tire, have been go-to "band aids"

[Pappy]

OP observations are consistent with our experience.

Front grip going away too early in non-aero corners, and excessive inside/inside tire wear is what we saw repeatedly in the data and tire logs. We are experimenting with stiffer front spring rates to control the front, and less rear brake bias (it's excessive anyway) with RB kit being developed.

More front splitter helps, but not in lower speed corners.

Higher pressures and less static neg camber in the front, to keep the car from scrubbing/destroying the inside of the inside tire, have been go-to "band aids"

Have you tried pinning the nose down with higher rebound resistance and/or a different shock package? I assume the lower aero-influenced corners are not compressing the suspension or perhaps letting the nose come up too early and affecting the geometry??? I agree that more roll resistance might be required, but stiffer springs might hurt more than help. I am working on getting and keeping the nose down to take advantage of the dynamic caster to help with the inside tire's geometry in corners.

Looking forward to your thoughts. Thanks.

Pappy

[allans]

Installed my MCS Shocks and did a complete new set-up. Ride Height set to 3.75 f and 5.5 r. Camber -2.5 f, 1.7 r. Caster 6.0 f, .7 r. Toe .10 f, .15 r. Went to Track (95*). Car handled really well. camber was much better in front, with pretty even across the tread tire wear. Understeer was much better and easier to manage. Rear camber at -1.7 can be reduced. Maybe -1.5. Overall the car felt very well planted and in spite of the high temps I was easily able to match my previous "best" times. I set up the shocks with a best guess on damper settings and am sure there is more improvement to be found there. Overall i'm very happy with this set- up and will continue to refine. I have 18" front Forgelines coming and hope to gain even more with them, and Hoosiers. We will see. Best, Allan

[Arizona Vipers]

Installed my MCS Shocks and did a complete new set-up. Ride Height set to 3.75 f and 5.5 r. Camber -2.5 f, 1.7 r. Caster 6.0 f, .7 r. Toe .10 f, .15 r. Went to Track (95*). Car handled really well. camber was much better in front, with pretty even across the tread tire wear. Understeer was much better and easier to manage. Rear camber at -1.7 can be reduced. Maybe -1.5. Overall the car felt very well planted and in spite of the high temps I was easily able to match my previous "best" times. I set up the shocks with a best guess on damper settings and am sure there is more improvement to be found there. Overall i'm very happy with this set- up and will continue to refine. I have 18" front Forgelines coming and hope to gain even more with them, and Hoosiers. We will see. Best, Allan

Wow. Great info. What are you spring rates front and rear?

[allans]

600/1300 same as stock. 18" Forgelines due this week. Allan

[MomentaryRacing]

Pappy, more rebound damping did not seem to help. Actually, suspension travel/frequency data shows over-damping, if anything. We tracked down a number of corner entry and tire/brake wear issues to excessive rear brake bias (so not enough weight transfer forward on entry, is our guess), so that's the next experiment to run.

In other words, it's not that weight transfers away from the front too quickly, it's that there's not enough dynamic weight transfer forward to begin with (working theory, not authoritative statement, testing will clarify things).

Is there a geometry issue contributing to or causing this? Perhaps. That's one area where there is no simple test. We do plan to work with a chassis engineer to go over some of these things, but that's likely a winter project. While the season is on, there's more value in testing things that can be adjusted with relative ease, and letting data analysis determine the changes to carry forward.

We were fortunate enough to have access to pro race engineer to review our data from a couple of races, his recommendation was to work on the brake bias, which is what we are focusing on, in collaboration with RB.



This is on stock gen4 KWt2, which, going by the suspension/chassis travel data, do perfectly competent job with our spring package.

[Pappy]

Pappy, more rebound damping did not seem to help. Actually, suspension travel/frequency data shows over-damping, if anything. We tracked down a number of corner entry and tire/brake wear issues to excessive rear brake bias (so not enough weight transfer forward on entry, is our guess), so that's the next experiment to run.

In other words, it's not that weight transfers away from the front too quickly, it's that there's not enough dynamic weight transfer forward to begin with (working theory, not authoritative statement, testing will clarify things).

Is there a geometry issue contributing to or causing this? Perhaps. That's one area where there is no simple test. We do plan to work with a chassis engineer to go over some of these things, but that's likely a winter project. While the season is on, there's more value in testing things that can be adjusted with relative ease, and letting data analysis determine the changes to carry forward.

We were fortunate enough to have access to pro race engineer to review our data from a couple of races, his recommendation was to work on the brake bias, which is what we are focusing on, in collaboration with RB.



This is on stock gen4 KWt2, which, going by the suspension/chassis travel data, do perfectly competent job with our spring package.


Thanks for the feedback. Very interesting observations. I assume you are logging the suspension/shock travel data against track position? I would be interested in compression at or approaching mid-corner. Do you think the brake bias problem is keeping you from getting full compression at all? If you are not getting full compression (splitter close to the track) I could see where lots of rebound damping would show signs of over-damping. I still want to get the nose fully compressed with applied aero and/or braking and keep it there until beginning corner exit. This should hold better dynamic caster which should in turn help the inside tire, assuming it is loaded. Let us know how the brake bias testing goes. I should have some better shock data by the week of SEMA.

Pappy