john hennessy wrote:
if you project a line thru the top and bottom a arms they will converge at the instant center,
the rack hight is somewhat irelavant as long as the track rod is also on a line running to the same point,
if the ball joint on the rack is on an imaginary line between the upper and lower a arm pivots.
alas if the track rod has to be longer due to the fixed position on the spindle the track will change during suspension movement
I'm still not following you, I think you are seriously overcomplicating the whole thing.
If you are using a front upright from a production vehicle, place your rack at the same height as the outer tie rod pivot with the suspension at its normal ride height, in your desired alignment. Anywhere else and you introduce more bump steer. It really is that simple.
The instant center you refer to is always moving relative to the chassis as the suspension cycles through its range. It doesn't sit static in the chassis as the suspension moves around. When discussing bump steer you should only be concerned where it is at normal ride height. That will minimize the toe changes as it goes into jounce and droop. Since most of us are using production spindles/uprights the steering arm location is already determined, thus we really should only be concerned about the height of the tie rod end pivot at the end of the steering arm is at ride height in proper alignment. (caster/camber/toe/etc.) That's where the rack should go.