Bump steer

[THR]

anyone had any experiecnes with bump steer?
my car does it from the rear, which also is torque steer.

anyone checked how much there car bump steers?

any comments?

[KC]

What we call bump steer in RC car racing describes something in the front of the car more than the rear. It usually results from too little caster in the front suspension and incorrect ackerman angle. If the rear of your car is stepping out during cornering over bumps then you are suffering from inadequate damping on the rear. The dampers are not absorbing enough of the bump to keep the rear tire in contact with the road. This can be changed by altering the spring rates and the rebound of the damper.

[enzo]

Bump steer is a change in where the tire points as it goes up & down. It has nothing to do with caster or ackerman - it is determined by the layout of the steering arm reletive to the a-arms. As the wheel rises or droops, the outer ends of the a-arms & steering arm swing through arcs, and move slightly inboard or outboard reletive to the starting position. If the steering arm is too long, too short, or set at the wrong starting angle (outboard joint height reletive to inboard joint), the arc it will describe won't be compatible with what's going on with the a-arms, and the wheel will change where it points.

Multi-link rear suspensions are subject to this also.

Most people try to eliminate all bump steer. We actually design a slight amount into the system - in bump, the wheel toes in slightly more (a few thousanths). This helps make the car a little more stable when encountering bumps while cornering.

[KC]

I stand corrected on it. I forgot about the relationship of the lengths of the upper control arm and steering control arm having a lot to do with steering change under supension copmpression.

[Tony Harman]

Enzo's got it right on describing bump steer. I run a single seater where everything is adjustable, we spent a lot of time dialling out the bump steer on the rear by adjusting the effective caster angle of the rear uprights. Of course Caster / Camber / Track are all linked so it's a slow old process to get it right. Depends what you're running THR whether you have the ability to adjust it out. (THR - does that stand for Tony Harman Racing then ???)
Enzo - interesting that you actually design in a little bump steer, want to expand more and give away some secrets ?, do you set that for both front and rear, how much, what sort of cars are you running.

[enzo]

I assume that you are running road coarses & not ovals - on ovals, bump steer is a big tuning factor. On road courses, it is a factor, but not so big as to warrant getting anal about it.

Some cars can adjust rear bump by changing rear caster - just be careful that you always get the desired setting with caster positive, not negative. On some older designs, you could get the desired bump with the caster either way, but having it set negative made a very diabolical car!

The reason behind setting the rear for slight amount of toe-in on bump ( about 1/64" in 1' of bump) and slightly less toe-in on droop is to make the rear more stable as it encounters bumpr while cornering - you are insuring that they steer INTO the corner as the encounter the bump. On the outer, highly loaded tire, this will increase the slip angle slightly, which will give that tire a bit more cornering capability as it hits the bump.

For the front - on road courses we set it the same ( tire toes in on bump) for the same reasons.

When I say that it toes in on bump, I'm talking relative to the initial static settings. Naturally the front initial settings are toe out, and the rear are toe in.

For tight courses where you have to get the car to rotate very quickly, setting the car for initial toe out in the rear can help - just be careful, it can sometimes make it hard to stop the rotation !

I work in CART part time as a consultant ( mostly Lights, but occasionally with the big boys), but spend most of my time with FF, F2000, FV, F. Atlantic, & F3.

[THR]

THR = Team Hobbit Racing
http://www.teamhobbit.com
u can see my car on this site (paul's clan)
the rear has macperson srut
and bump steers a bit, which translates to torque steer as the power is let on and off.

to fix it i need to raise the ties to the rear. which go from the hub to the subframe.
and alter the tow.

i am running, 2mm tow in F
4mm tow in rear.

dissagree that cars always run tow out at the front, enzo
depends on weight distribution. and driving style. course. etc.
bump steer is horrible enzo! more than 10thou over 2" travel u can feel. and have to make spring changes to suit it. or limit travel.