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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 control arm angles will have a profound affect, as will the inner tie rod location of your steering rack. The upper aframe angle will effect the camber loss or gain through the range of travel. If it is wrong it will make it almost impossible to eliminate bump steer. The correct angle depends on the components used, ride height, ect. The angle, forward or back, of the spindles tie rod location in relation to the steering rack will have a huge effect on Ackerman and bump steer as well. 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.
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.
Another crucial aspect of steering is the scrub radius. Essentially if you were to draw a line through the upper and lower ball joint the point where it hits the floor should be the center of your tire contact. This would be a zero scrub radius. Usually on Cobras the center of the tire contact point is several inches out from where the ball joint line hits the pavement. This difference in length acts as a lever on your suspension components and steering components. Scrub radius is aggrevated by the use of deep dish wheels. The effect is high steering effort and alot of kickback through the steering wheel. With a large scrub radius the car will feel twitchy. A car with a zero scrub radius require very little effort to steer, even with out power steering.
I have cut off and redone the front clip on several cars to correct these types of problems. Sometimes I wondered if it was worth it. But, when you drive a car that has both the front and rear suspension nailed it is exhilirating and worth it. Have fun! John
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