"bump steer" is not used in Belgium to explain somekind of mechanical problem.
Can anyone explain it to me?
As far as I read, it happens when swapping struts from one car to another. But what happens? How do you recognize it while driving etc?

if the length of the steering's tie rod assembly is a different length to the lower control arm, or if the two are not parrellel and travel in different arcs when the suspension compresses, it can steer the wheels more when it does compress.

It's hard to explain, but a shorter arm travelling the same vertical distance from a fixed inner pivot point will deflect more laterally than a longer arm traveling the same vertical distance. So for example if the steering's tie rod assembly is shorter than control arm you fitted in the strut conversion, you will end up having the wheel turn more as the suspension compresses - eg when you hit a bump.

Also, the outer end of an arm will have a greater amount of lateral movement for a given vertical movement the further away it is from horizontal. So if the arms are the same length but not parellel, the outer ends will move laterally by different amounts and steer the wheel over the course of suspension travel.

L18_B100 got it right.

This "rambling" is my explanation to help our mate understand bumpsteer.

When the strut goes up and down, it goes through an arc, as do the steering tie-rods.

If you don't move the crossmember mount points for the lower arms, then the "draglink" (centre steering rod that the tierods connect to) will remain suited to the crossmember (As much as the factory intended).

If you moved the mounting points in toward the centre, you should use a proportionally shorter draglink (Which means big money fabricating a new one).

Replacing lower arms (as with stanza lower arms) you need to run slightly longer tierods, to suit the longer lower arms. But the pivot point on the crossmember stays the same, so bump steer is not affected.

I know about bump steer from the days of offroad 10th scale R/C model car racing.

The Tamiya Hotshot used to have terrible bump steer on the front from factory. The front and rear double wishbone suspension had quite a few adjustments.

Bumpsteer isn't good, and should be eliminated by changing the steering pivot points (or toe-in/out pivot points for rear suspension). By removing the springs from the struts, and connecting up the steering to the balljoint, you will see what kind of bumpsteer to expect.

It's a combination of tradeoffs (or careful measurements) to get the desired camber/castor/bumpsteer/wheel-track/ride-height.

Having Bumpsteer, means that when you hit a dip in a road, both wheel will either turn in, or turn out at the same time, producing skidding, or unpredictable change of direction. It will either slow you down, or make you crash, depending on how hard you are holding onto the steering wheel and how fast you are travelling.

If you hit a bump on only one wheel, especially if cornering hard, it can send you into a temporary understeer or oversteer, depending on which wheel and whether or not you are getting excessive toe-in or toe-out on suspension compression.

Chris

this might be easier to digest!

bump steer is a change in toe caused by suspension travel.

Ideally, under all bump and roll conditions, the wheels should remain parallel to each other...

Good one..well said..

Chris

sorry to ask but what do i need to change to fit stanza struts into a 1200 sedan??
tierods what are they and what should i change em with??
cheers heaps
Dwayne.

Nothing, you just wind out the adjustment on the steering bits that connect to the "bottom of the struts". I think I used the 1200 one on the inner tierod ends, and the stanza ones on the outer tierod ends.

Chris.

To answer the question that was asked in the title.
Bump steer is the condition where the vehicle moves off the intended, or "steered" line during suspension movement.
You will know it if you have it. You hit a bump & the car wants to spear off to one side or the other. Sometimes a lot, other cars will do this only a little. The degree is dependant on the size of the bump, & the amount of steering geometry error in the individual car.

The others have offered up excelent explanations for WHY this phenomenon occurs, but the car wanting to steer itself when going over a bump is WHAT it is.

ahh ok cheers heaps doods.