Quote:
Originally Posted by DennisMenace
I am about to install a KW V3 with 19 mm raised spindles. Just ordered the Tarret Bump Steer Kit. Any advice how I can identify the „ideal“ bump steer correction for my car? I assume I cannot just take the 42 mm and apply it to my car but rather need to adjust it according to my ride height. Thanks a lot for any advice!
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It depends on the ride height and tire spec
stock ride height spec is 108+/5m a - b, call it 110mm for this discussion
b is the actual height of the chassis
a is the axle height which is determined by the tire, lets say this is fixed @ 306mm+/- which is what a 205/55 x16 tire would provide.
so 306 -110 = 196mm for chassis height
c & d, I need to measure but they should be roughly equal stock
lower the car from stock by say 20mm
now 306 - 130 = 176mm for chassis height
here is where the 19mm raised spindle comes in
What the raised spindle does is restores 20mm of bump travel
and changes the angle of the A-arm which determines where the instant center is, which in turn determines the roll center height, which wants to be under but as close as possible to the CoM height, horizontal is the lowest angle that you want the A-arm to be, this is bad but it's worse when the outer end is above the inner end
this diagram compares 2 different ride heights, w/ 2 different A -arm angles. The top one is preferred as the roll center is closest to the CoM, A 1 and A2 are the critical datums. A1 < A2 which is a good thing.
c is now the same as before but d is down 20mm this makes the arcs vary more which is where roll steer comes from
so if you add a 12mm rack pacer d is now only down 8mm
to eliminate that last 8mm or more, you need an adjustable height steering knuckle
I good starting point to reduce roll steer would be roughly equal heights for c & d, though a full bump sweep is necessary to see exactly what you have .