can someone tell me what bump steering is?

 

Bump steer. When the front suspension moves up or down, the toe angle can change if the tie-rod is the wrong length or angled improperly. This is usually felt as skittishness over bumps, especially bumps mid-corner. It's like the car is steering itself. Bump steer can also happen in the rear, but it's more complex.

Bump steer is often designed in so that the front tires toe out when the suspension is compressed, and the rear tires toe in when the suspension is compressed. This is called "roll understeer" because as the body rolls, you have to turn in more. Roll oversteer is just the opposite, and makes a car feel very jumpy. Imagine roll oversteer - the car leans as you corner, so the car turns in more, so it corners harder, so it leans more, so it turns in more, etc. Roll understeer is a lot safer feeling.

 

To "bump-steer" a car we remove the front springs, and disconnect the front bar. We then move one front wheel up and down through its normal full travel range, measuring the toe change in 1/2" travel increments. Then the toe changes are graphed.
Best interpretation I have found is the charts in "Advanced Race Car Suspension Development" by Steve Smith. You compare your results with the charts and it tells you if:
Outer tie rod ends are too high or low,
Inner tie rod ends are too high or low,
Tie rod/link is too long or short.

You then do whatever is needed to correct the problem.
We aim for less than 1/16" total (2 wheels) front toe change in at least 4" of travel.

 

oops