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MG MGA - What is 'Bump Steer'
|I have heard this phrase many times and finally realized that I do not know what it means. Is there a simple explanation of what it is and what happens when it is present, in an MGA?|
|Turn steering to go around a corner. When you hit a bump the suspension moves up/down. If this motion changes the steering angle you get bump steer.|
I have driven MGA a quarter million miles in all kinds of road conditions, and have done a lot of autocross competition. I have never noticed any tendency for the MGA to bump steer with stock suspension set up.
Lowering the front suspension by shortening the coil springs could do that, because it changes angle of the unequal length suspension arms (also changes the roll center). Changing to MGB front suspension could do that, because it introduces negative camber and requires shorter tie rod links.
Minimizing bump steer requires keeping the suspension arms and tie rod links close to parallel at all times. When at rest the lower A-arm and tie rod link should be level, and the tire rod should be close to straight across left to right. Arc motion of the steering arm moves the outer end of the tie rode fore and aft a bit with steering motion. Pointed straight ahead the outer ball joint should be slightly forward. When turned to full lock the outer ball joint should be slightly rearward. That's a close as you can be to straight across average.
There is a very common issue with incorrect geometry in the Ackerman steering mechanism. When you make a tight turn the inside wheel should steer to a greater angle than the outside wheel, because the inside tire has to follow a shorter radius in the turn. This rule is often violated when the inside wheel does not turn in far enough, and the front tires are fighting each other trying to take a different track. You may notice this when backing up at slow speed and full lock when the tires might squeak a little. There is also some resistance to rolling in the same condition, so you may notice it more difficult to push the car by hand with steering at full lock. This is not much of a problem at small steering angles, and it does not affect bump steer, as long as the tie rod remains parallel to the lower suspension arm.
|As expected, Barney is absolutely correct.|
The term "Bump Steer" gives a pretty good clue as to the meaning.
A car with bump steer will change direction on its own in response to a wheel hitting a bump in the road even though the driver has not moved the steering wheel.
The bump steer on my MGA made it a quite scary on high speed bends, a bump would make the car jerk to one side quite severely. On occasion it would be suddenly a foot over the white line after hitting a bump! Not a good feeling.
According to Barney it appears that I had the worst combination of both shortened coil springs and negative camber MGB lower wishbone arms.
I have recently changed the wishbone arms back to standard MGA ones (although I still have the lowered springs fitted) and this has pretty much cured the bump steer completely.
The car is now so much nicer to drive than it was before that I wish I had changed the arms years ago.
This thread was discussed on 11/08/2012
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