As the front suspension compresses under hard braking and/or bumps
what actually occurs that causes the bump steer ? Does the the front
wheel toe in actually change to toe out during the wheel travel or is
it camber related or what ?

Thanks >>>>> Mike

 

Tires play a big role. Post run-flats, I don't seem to have bump steer or the rear axle sideways ho that I used to have when I hit concrette tar strips on the freeway.

 

Bump steer occurs when one wheel is displaced vertically upwards when hitting a bump and this causes a change in toe. The "darting" that occurs is exacerbated by the fact that the displaced wheel is more highly loaded, and thus the tire on that wheel has more traction during the bump. Similarly "droop" steer is not felt so strongly since that tire has less traction when encountering a dip in the road.

 

Which way does the toe change ?

 

Usually goe to toe-in on bump. Thus, a bump on the right front makes the car dart left and vise versa.

 

Bump steer is prolly one of the most misunderstood conditions in our cars.

What causes bump steer is the fact that the tie rod connecting the spindle to the steering rack actually travels through a different (longer) arc than the spindle. The A-arms are actually shorter than the tire rod therefore they have that different arc. The Corvette actually has two front ends if you look closely the rear tie rods are actually anchors to hold the rear spindles in place.

The bump steer does cause a toe change. The car is normally aligned with the suspension setting at some normal/neutral position. The toe is set to some value to run straight at that neutral position. As the car goes down the road and the chassis rises and falls over the irregularities in the road the toe is constantly changing both negative and positive to the original setting. Usually when the suspension unloads it toes the tires out and when it loads it causes toe in. This happens on both the front and rear of the car.

This is where the Baer Bump steer kits come in as you can actually shim the tie rod end to compensate for the different arcs and actually have a neutral or zero bump steer condition through the suspension travel. The art of installing the shims and testing the alignment is a long and intensive exercise on an alignment rack. You have to align the car for normal then jack the car to unload the suspension and check the alignment record the toe, then use a come-along and jack the car to full suspension compression and record the toe. The put the car back to normal, install the shims and re-align and go through the process again until all the recordings are the same.

As our cars come from the factory they are set up pretty much ok. If you are going to do serious autocross and track events than you need to worry about it.

Bump steer is not the “kick out” of the car as it chatters over rough road with a stiff suspension and runcraps or the car pushing in turns due to rough roads. Here you need softer tires, shocks and maybe even lighter springs. But you will pay for it in cornering performance.

The darting condition that a lot of people experience especially under braking on a ruted road surface is caused by our wide stiff tires what happens is the tire actually is riding the side of a ridge in the road, but because it is so stiff and wide it will tend to try and run down to the bottom of the rut and you feel it in the steering wheel. bump steer has a little bit to do with this but not as much as the tire stiffness, width and road surface.

 

FM
Fcking Magic

 

Nice Blue Dragon! Great expalantion. Thanks