bump steer- definition?

[gchappel]

Would someone please define bump steer?
What does it do or feel like- especially on the track.
My car was lowered by PO, and on fast sweeping turns- like turn 1 at sebring the steering wheel "chatters" or jerks a little from side to side as the front wheels hit bumps during a turn. Is that what bump steer feels like?
Thanks- still have alot to learn here

[efeinsmith]

Bump steer occurs because, when you lower the car, you change the steering geometry between the r&p box and the wheels. When you hit a bump and the wheels change vertically, the wheel to steering box angle changes (due to the fact that the actual distance from the centerline is fixed by the a-arm). So the wheel steering angle effectively changes with movement (i.e. toe). The preferred position is for the r&p box to be on the same plane as the tie rod end pick up on the wheel hub.

When spacers are used, it moves the r&p unit back in to the proper relationship with the angles.

[wckrause]

If when driving in a straight line, the steering wheel jerks back and forth as you go over a bump, then you have bump steer.

You don't have to be in a turn to feel this.

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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

[orbmedia]

Bump steer is the change in toe as the suspension moves through it's range. It can occur both front and rear suspension. It has very little to do with actual bumps in the road. When race engineers talk about bump steer the deflections they are talking about are those encountered during braking and cornering.

Bump steer is often tuned one way or another (not simply eliminated) to give specific driving characteristics to a race car.

Real race cars have suspension links that can be adjusted to play with the bump steer and roll center.

Part of the confusion that most Porsche people have with bump steer is because of whole highly popularized "bump-steer" kit spacers sold which cures a common bump-steer component on a 911. They think that's all there is to it. it's not.