bump steer

Time to check and adjust bump-steer.

For the uninitiated, bump-steer is a condition where a wheel self steers as the spring is compressed while passing over a bump or while cornering or braking. You can imagine that the car will be hard to control if the wheels toe outwards every time you brake hard for a turn. Having the wheels toeing IN under the same circumstances is not as bad as toeing OUT but it's still not desirable. Whether or not this condition exists can be measured and usually it can be adjusted away by raising or lowering the steering-rack or the tie-rod ends until the wheels stop self steering as they move up and down with suspension travel.

Since this car started with a bunch of components and a clean sheet of paper, it's important to check for,and measure for, bump steer and to adjust the condition away. The cross-member which the the steering rack attaches to was designed to adjust up and down in anticipation of having to deal with bump-steer.

I borrowed the bump-steer checking outfit shown in the photos. I don't like it as much as the one I used to own (I loaned it to someone and never got it back - note to self!) but it certainly does the job. Basically, a flat aluminum plate, with a piece of measuring tape attached to each end, is bolted to the hub, substituting for the wheel. Dial indicators touch the horizontal center-line of the plate with the suspension in a neutral (at ride height) position on a small bottle jack. The coil-over (or in this case the metal strut) is removed and the suspension is jacked up and down slowly while the dial indicators tell you if (and how much and in which direction) the plate steers. In my case the plate (wheel) was toeing out under suspension compression (bump) and the steering rack had to be shimmed upwards slightly. Adjusting away the self steering under bump is more important than under droop since under real world conditions, if the wheels are in droop, the car is coming of the ground and the tire doesn't have much traction to do any steering until you come back down anyway.

A good visual way of finding out if bump-steer in your car is at least minimal, is to check that the tie-rods are parallel with the lower control arms and of similar length. If they don't match like that, the tie rods and lower control arms will travel in different arcs and cause the wheels to change direction on their own.

The suspension geometry on the rear of this car will, by design, not have any serious bump-steer issues (or adjustments for it) but every car is different. Bump steer can happen on the rear suspension of some cars and the condition can be much worse, since you can't manually steer the rear end of the car. I've experienced body roll induced over-steer (roll over-steer) and it's not funny. When it first happens, and you spin the car repeatedly, you think you forgot how to drive fast.