Anti squat?

[Eric Howell]

So what's a good % to shoot for on a street car?

With the 3 link I'm planing out it keeps coming up between 55% and 68%. I was originaly shooting for 75% to 85% but it's not going to happen.

Will about 65% be sufficent?
:help!:

[parsonsj]

More anti-squat isn't necessarily better. More anti-squat means less brake bite from the rear tires. A good starter number is 50-60% to provide a decent compromise between forward acceleration ability and rear brake bite.

jp

[BillyShope]

Certainly, if you were interested only in dragracing, you'd want to avoid oscillatory loading of the rear tires during launch and, therefore, you'd seek an anti-squat value of 100%. Though I do not agree with the idea that rear braking performance is inversely proportional to percent anti-squat, I do concede that the 100% value is not all that important in other forms of motorsports.

If you are fabbing a 3link, I would encourage you to consider the asymmetric design used in the early C-Type Jaguar. While this might not be the last successful sports racer with RWD and a beam axle, it is certainly the best known. The asymmetry, if properly done, can perfectly cancel driveshaft torque effects. The workbook which accompanies "Race Car Vehicle Dynamics" contains a description of the design by Norman Smith, a former Jaguar engineer. He doesn't cover the percent anti-squat and, unfortunately, little can be determined from the drawings and conceptual sketch included.

With an asymmetric 3link, the "odd" link can be either above or below the other two links. (Packaging problems are usually minimized with the odd link below.) My site includes a spreadsheet for 3link setup which provides dynamic driveshaft torque cancelation and 100% anti-squat.

Finally, don't scrimp on tubing size! This is not the place to worry about a few extra ounces. Always favor the thicker wall tubing.

http://home.earthlink.net/~whshope

[Eric Howell]

Thanks Bill, I know your site well and yes it will have the upper link offset to the right. My SVSA length is about 72" with 66.85% anti squat so brake hop shouldn't be any concern. I will have plenty of adjustment on the UCA to compensate for any mis calculation of CG.

[parsonsj]

Here's another thread on anti-squat, including some thoughts on how more that 50% a/s negatively affects rear brake performance via brake hop.

https://www.pro-touring.com/forum/showthread.php?t=12696&page=2

jp

[BillyShope]

Yes, John, I contributed a couple of posts to that thread, but I didn't include comments on rear braking performance.

To clarify my position, I would state that rear braking performance, considered in steady state conditions, is not affected by percent anti-squat. On the other hand, as you increase anti-squat, the vertical force component at the instant center has a greater change with a given change in rear brake force, thus increasing the possibility of some nasty transients. If you had said something about increased tendency for rear wheel hop during braking with increased anti-squat, I wouldn't have commented.

Someone in that other thread said something about not wanting to "lift" the car during acceleration. There is no lifting of the rear of the car, of course, until the anti-squat exceeds 100%. During acceleration, the front of the car will always rise. But, when the anti-squat is less than 100%, the rear of the car squats and the front rises even more during acceleration. When more than 100%, the rear rises and the front still rises, but not as much as it would with 100% anti-squat. (That change at the front of the car is often neglected in discussions of anti-squat.)

(Pardon that last paragraph. I'm an old man and I tend to ramble.)

http://home.earthlink.net/~whshope

[parsonsj]

Billy,

Sounds good. I've been to your site a couple of times even. Excellent information there.

jp