"Bumpsteer is a really big thing," says Butcher. "I think it's indicative of a lot of things. Whenever you're measuring anything on a race car, if you can measure things to the next increment as compared to your competitor, it's going to give you that much more of an advantage, and it's going to help your car stay on the long run.
"For bumpsteer at different tracks, you might want to run different amounts. You might run next to none at some tracks, but for entry stability you might want to run 0.015-0.020 inch bump-out. Whatever numbers you choose to run, like all adjustments on a car, it's a cut-and-try deal, something that's specific to your driver and your location. But you want to make sure that you know what that number is so you can repeat the process the next time around. Everything's relative in racing: If you know what you did last time, you'll have a better shot at winning the next one."
Excessive bump-out, over 0.050 inch, for example, can slow your car down. You should strive to get the best bump numbers possible, Butcher adds, even if it means replacing parts.
"Quite simply, all bumpsteer is, is lining up arcs and pivot points and lengths so that the wheel stays straight unless the driver wants to turn," says Butcher. "With any kind of car, you just have to make sure that those pivot points line up with each other and that the arcs are somewhat similar to each other so that you can fine-tune with shims at the tie rod. Or maybe you have to space up the steering rack if it's a car with a rack, or move the steering box around a little bit if it's that kind of deal.
"It's one of those adjustments, again, that if you have an understanding of what's going on-it's not magic or something like what's happening inside an automatic transmission-it's really quite simple. Once you understand how to line up those arcs, then doing the adjustments becomes much easier."
Symptom: Toes-out in compression and in on rebound, all in one direction.
Cure: Increase shim on outer tie rod or lower the inner tie rod.
Symptom: Toes-in on compression and out in rebound, all in one direction.
Cure: More shim at outer tie rod or raise the inner tie rod.
Symptom: Always toes-in during both compression and rebound.
Cure: Lengthen the tie rod.
Symptom: Always toes-out on compression and rebound.
Cure: Shorten the tie rod.
Symptom: Toes-out on compression, then in on rebound, and then starts back out with more rebound travel.
Cure: Less shim at outer tie rod and shorten tie rod.
Symptom: Toes-in on compression, then moves out on rebound, and then starts back in with more rebound travel.
Cure: More shim at outer tie rod and lengthen tie rod.
* Source: Longacre Racing Products