[Jac Mac]

Just a couple of points on Johns explanations.

Originally Posted by john chesnut

Paul, Welcome to the intricate world of suspension geometry!! When I began building cars I knew nothing about it. I had bought a kit that handled terribly. In my search to find out why I studied suspension geometry. I am no expert but I have learned alot and the cars I build are getting alot better.
I have a couple of thoughts about your numbers. Obviously your bump steer numbers are terrible. But, so many aspects of the geometry has an effect on each other that the height of the steering rack may not be the sole culprit. You may know the things that I will point out, forgive me if you do.
The inner tie rod of the steering rack needs to be in the exact same plane as the upper and lower aframes. If you were to draw a line from the upper aframe inner pivot point and the lower aframe inner pivot point, the inner tie rod should hit the center of that line. If its not you will not correct bump steer with the steering rack height.

This is basicly true but you have to allow for the ackerman if any, which is usually built into the steering arm. Any distance this is offset from a line drawn thru the top/bottom ball joints or pivots on the upright when viewed from front/rear must be added or subtracted from the rack length on the 'line' you mention drawn between the chassis mount points for the a-arms

A roll center below the pavement is good. The greater the length from the center of the frame this is the better the car will handle. Essentially the greater the length the smoother the camber transitions will be.

Not sure what you mean by this length from chassis center, unless you mean the instantaneous center of the upper/lower a-arms which is used to calculate the roll center, Paul, a pic of the car now with the angles of the top /bottom A-Arms etc would help us advise further
John

cheers