Bump Steer

[DavidNJ]

Search and found virtually no references. On a short wheelbase IRS car, this should be very important.

With so many different rear suspensions, I am wondering how people do it. There seems to be very little adjustability. Ditto the cars with solid rear axles.

In the front, do people use rod ends on the spindle, shim the rack, or both?

Thanks,

David

[Woodz428]

Bump steer is most generally a front suspension reference, I imagine it could be created by some IRS geometry issues. It is usually created by the lower arm and the steering arm traveling in different arcs, and usually happens when hitting "bumps", as it were. Having the steering link run parallel and having the same length as the lower arm minimizes or eliminates the issue. After it is already constructed the most that can be done is offset busings for the rack, to raise or lower it to make it more parallel to the arm. Most of the bump steer issues should be worked out in the lay out, because if it is set up badly enough a complete redesign is necessary.Fine tuning is the most you can do after the fact, and if it is a drastic problem it can never be "tuned" out.

[dlampe]

Here is a thread that goes through a ton of detail about the process both front and rear. http://www.gasholes.com/smf/index.php?topic=3232.0 I think it is after page 30 some where. I used this process and discovered that my car was in really bad shape. I used a .097 aluminum spacer under my rack to fix the problem. I have always read the .020 was the acceptible limit per inch of travel on the front. The rear will allow a little more, maybe .030 but that is it! The car will not handle as well if these perameters are not checked. Good luck. This is a big project. I worked on mine for a month!

[DavidNJ]

Woodz428,

Bump steer affects both ends. In some ways it is more critical in the rear, since you can't steer the rear with the wheel. While excessive bump steer can make a car darty, it is also a primary contributor to roll steer. Semi-trailing arms suspensions inheritently had bad roll steer, causing trailing throttle oversteer. Modern multi-links don't, such as the Ford rears used as the basis for Superformance, JBL, Hurricane, FFR, TurnTen, etc.

However, in bump steer we are talking about changes less that .125", even less than .050", over rather large dimensions. Outside of F1, and some sports racers, measurement and adjustment is the only way to fine tune it.

Race cars have mechansims to adjust bump steer built in. Street cars don't. Cobras sort of straddle the middle. With the large number of Cobra used on track days, the low volume welding fabrication (compare say JBL to McLaren F1), and the involvement of the owners in assembly and details, I would have thought bump steer would be high on the list.

dlampe, Morris seems to be the only one who is talking about bump steer. And if reading correctly, it seems he cut and rewelded the suspension mounting points to be able to adjust it.

[sideoiler10]

FinishLine has a bump steer kit for the mustang II type tubular A-arm suspension. Don't have my catalog in front of me now, but I think it is around $110 to $120 for the set of 2.