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What is recommended Anti Squat percentage for track S197.

I have a 2011 Mustang GT 5.0 that I've used mostly for track days for the past year. I've lowered the car 1-1/2" front and rear using Ford Racing P springs. I also installed Whiteline upper and lower rear control arms among other mods. When I run car at track I just don't trust that rear of car won't jump out on me. I've been track driving since 1978 so MY rear end is pretty sensitive by this point. I find myself babying the car off the corners. Just recently I started thinking about why the car behaves this way. I went back to basics and realized Whiteline setup was a mistake. The rear lower control arm bracket is set up for too much anti squat. the lower arms angle upward quite a bit resulting in a large amount of anti squat. With this set up under acceleration the rear suspension is getting loads transferred immediately through the suspension links instead of more gradually through the springs and shocks. This may be good for drag racing but not for the track. I've ordered BMR rear lower control arm brackets with 3 location positions so I can level out the lower control arms and significantly reduce the anti squat. How much anti squat do you track guys run? The rule of thumb is 50%. I'm curious as to what the consensus is.
 
3,803
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I honestly don't know anyone who has actually measured that..
These cars really love to squat in the rear and lift the inside front tire on corner exit.. the trailing arms that are used in most of the IMSA, PWC are simply stock replacement units from Ford racing.
If you search the posts on the SCCA Runoffs you will see many examples of this, in not only Phoenix cars, but others as well.
qGKqzwc.jpg
 

racer47

Still winning after 30+ years
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I've spent a lot of time messing with anti squat. I have about 600 whp and need to hook it up to be able to use it. But its not just the lower arms. You need to also look at the 3rd link. With the rear lowered, the 3rd link points down quite a bit. Plus the instant center length is also important, longer is better, within reason.

I've tried many combinations and have settled on roughly 80-90% squat with a long instant center, about 65". Its hard to measure everything exactly but since all the mounts are holes, not slots, you just pick the best combo and not worry about the exact squat number.

I ended up with the BMR rear brackets, set at one hole below stock. Depending on how low you are, the lower links will be level to pointed up slightly. Plus I went with the BMR 3 hole upper bracket, set on the top hole. That extra inch or so of height on the front of the 3rd link pushes out (or lengthens) the instant center quiet a bit and cuts down on the anti-squat. The upper link still points down, just not as steeply.

This combo is hugely better than stock and hooks up significantly better than every other S197 that I've driven or raced against. If your upper is stock and your lowers point up a lot, you are probably well over 100% with a very short i.c. You can check your numbers here.


 
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I've spent a lot of time messing with anti squat. I have about 600 whp and need to hook it up to be able to use it. But its not just the lower arms. You need to also look at the 3rd link. With the rear lowered, the 3rd link points down quite a bit. Plus the instant center length is also important, longer is better, within reason.

I've tried many combinations and have settled on roughly 80-90% squat with a long instant center, about 65". Its hard to measure everything exactly but since all the mounts are holes, not slots, you just pick the best combo and not worry about the exact squat number.

I ended up with the BMR rear brackets, set at one hole below stock. Depending on how low you are, the lower links will be level to pointed up slightly. Plus I went with the BMR 3 hole upper bracket, set on the top hole. That extra inch or so of height on the front of the 3rd link pushes out (or lengthens) the instant center quiet a bit and cuts down on the anti-squat. The upper link still points down, just not as steeply.

This combo is hugely better than stock and hooks up significantly better than every other S197 that I've driven or raced against. If your upper is stock and your lowers point up a lot, you are probably well over 100% with a very short i.c. You can check your numbers here.




BMR published an article re Anti Squat using a 2011 Mustang as it's reference case (https://www.dragzine.com/tech-stori...spension-adjusting-instant-center-anti-squat/). It used [https://www.baselinesuspensions.com/instant-center.php] to calculate Anti Squat values for different cases on the Mustang. Using that model and correcting for 1-1/2' front and rear lowering, 26" tire and my measured LCA front and rear floor heights I came up with 67.16% Anti Squat and an instant length of 104.54". Currently I have a J&M upper control arm bracket with the upper link in the top most hole. I don't think any bracket could mount upper link higher because you would not be able to get socket on bolts. My front LCA bolt is 5.93" from floor and rear LCA bolt is 4.56" from floor with Whiteline bracket. By switching from Whiteline rear LCA bracket to BMR bracket I will be able to raise rear LCA height by approximately an inch to 5.56" making the LCA almost horizontal. This change re the model will produce an Anti Squat of 30.21% and an instant center length of 168.67". It is obvious that the LCA angle is critical in effecting Anti Squat.

With my current set up re the BMR model I have 67.1% Anti Squat. I am going to switch to the BMR rear LCA bracket and mount the LCA in the mid point hole position which should give me 30.21% Anti Squat. It's interesting how different models can give such different results for Anti Squat values. I'll run my modified BMR parameters on the model you used and see what results I get.
 
BMR published an article re Anti Squat using a 2011 Mustang as it's reference case (https://www.dragzine.com/tech-stori...spension-adjusting-instant-center-anti-squat/). It used [https://www.baselinesuspensions.com/instant-center.php] to calculate Anti Squat values for different cases on the Mustang. Using that model and correcting for 1-1/2' front and rear lowering, 26" tire and my measured LCA front and rear floor heights I came up with 67.16% Anti Squat and an instant length of 104.54". Currently I have a J&M upper control arm bracket with the upper link in the top most hole. I don't think any bracket could mount upper link higher because you would not be able to get socket on bolts. My front LCA bolt is 5.93" from floor and rear LCA bolt is 4.56" from floor with Whiteline bracket. By switching from Whiteline rear LCA bracket to BMR bracket I will be able to raise rear LCA height by approximately an inch to 5.56" making the LCA almost horizontal. This change re the model will produce an Anti Squat of 30.21% and an instant center length of 168.67". It is obvious that the LCA angle is critical in effecting Anti Squat.

With my current set up re the BMR model I have 67.1% Anti Squat. I am going to switch to the BMR rear LCA bracket and mount the LCA in the mid point hole position which should give me 30.21% Anti Squat. It's interesting how different models can give such different results for Anti Squat values. I'll run my modified BMR parameters on the model you used and see what results I get.
I just checked and to my surprise I found we used the same Anti Squat calculator. Maybe the difference is that the BMR 2011 Mustang numbers I used as a base line are quite different than what you used. What is evident is that small changes in dimensions can have a big difference in the results you gat.
 
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You changed a lot of stuff and jumped to quickly to the conclusion that anti-squat is your problem. 67.5% anti-squat is not a lot. I suspect your other changes, or combination of changes are part of the problem.

What type of bushings are you using in the rear upper and lower control arms? You change spring rates. Did you adjust your roll bar settings? I am going to guess you used poly bushings in the control arms - which will bind and cause issues. With the spring change, you may have too much rear roll stiffness compared to the front if you didn't change roll bar settings.

Did you torque the control arm bushings with the suspension fully loaded? If not, you may have introduced bind and artificially increase rear roll rates.

The 50% rule of thumb is tied closely to roll oversteer due to LCA angle changes. Roll oversteer is a corner entry and mid-corner issue, not corner exit and corner exit throttle application issue.
 
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I also forgot to mention your rear bump stops. When you lowered your car you removed 1.5" of rear suspension travel. Are you sure you are not hitting the bumpstops at corner exit? Did you trim or install shorter bump stops? If you hit the bump stops, your rear roll rates will go through the roof and the car will oversteer. It will also be unstable over hard curbing.
 
I also forgot to mention your rear bump stops. When you lowered your car you removed 1.5" of rear suspension travel. Are you sure you are not hitting the bumpstops at corner exit? Did you trim or install shorter bump stops? If you hit the bump stops, your rear roll rates will go through the roof and the car will oversteer. It will also be unstable over hard curbing.
In reply to your questions both the upper and lower rear control arms have poly bushings and were torqued under load at vehicle ride height, wheels off, jacks under axle, vehicle level. I've used poly bushings on many cars lighter than the Mustang and never had a bind problem but that does not mean I would dismiss the possibility. I also cut the Ford Racing bump stop in half to allow for adequate frame clearance. My car was a Brembo GT so I have the 35mm front bar and the 24mm rear bar. I've already ordered a 20mm rear bar. That brings me to the question of spring rates. I went back and checked and realized I had installed Ford Racing "K" springs along with Koni sport shocks. The problem I have is trying to find the real spring rates of these springs. I don't have a tester and the available documentation has rates all over the place. One source says the factory Brembo GT rates were 131F and 167R, Vorshlag says they tested them at 125F and 181R. The "K" springs were listed at 228F and 199.5R. Vorshlag tested them at 188F and 221R. If Vorshlag is right I think my rear springs may be problem as they would appear to too stiff. My car is premium package street car that I use for track days. I'm guessing it weighs close to 3700 pounds. Voeshlag sells a height adjustable rear spring kit with a 175#'in spring. Vorshlag also tested the Ford "P" rear spring at 165#/in which are a lot cheaper than the Vorshlag kit. Any suggestions?
 
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Did you use poly for both of the upper control arm bushings? Hopefully the poly is just on the body side mount and the axle side still has the factory rubber. Poly on both sides does bind, but shouldn't make the car oversteer.

I would go with the published Ford rates for the springs. If you were running a square tire setup I would suspect the car might have a tendency to oversteer with the swap to K-springs. When I originally installed Steeda Boss springs (225F and 195R) in my car I had to increase the front sway bar stiffness to correct oversteer with a square tire setup. I did have Ford LCA relocation brackets on the car and tried both the upper and lower positions without any notable oversteer issues. When I installed a BMR sperical upper, I raised the body side mounting position to increase the length of the instant center.

I would look at sway bar tuning. My preference in the past with lighter spring rates was to stiffen up the front bar via an adjustable bar. But, a smaller rear bar works too - and the Ford 20mm is not that expensive.

I also assume you installed camber plates with the new front struts and dialed in around -2.0 camber (about as aggressive as you can get with out prematurely wearing out a set of street tires).
 
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I forgot to mention that increasing the front roll rates from the factory settings initially yields increased front grip. The factory rates are too soft. Higher rates keep you in a better part of the camber curve and hence the increase in grip. This goes against what any chassis tuning book will tell you. But, those books and pricipals are assuming the vehicle is not under sprung to start with.
 

Mad Hatter

Gotta go Faster
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Here is a long colorful thread on the same subject.

 

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