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Old 12-31-2012, 09:48 AM
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(I see just prior to my posting this, there is a pretty good explanation just put up...but I'll hit "post" anyway in case saying things over again helps.)

I'm not sure if Ackerman geometry is quite what you're dealing with here, that is about steered-wheel angles and generally pictured "from above"...for anything having to do with bump steer you're going to be picturing it from ahead or behind.

Whatever the actual suspension or steering member/pivot-to-pivot lengths may be, you can get close to a bump-steer-free deal by having the steering tie rod be the same length as the lower control arm, and at the same angle as the lower control arm (as seen from directly in front or from the rear and going by the pivot locations), if it's at the same level above the ground as said member....or...if the tie rod were at the level of the upper control arm, then you'd want it at that same length and angle of that upper arm then. If the tie rod height falls somewhere between the two like with a Mustang II spindle, split things up proportionately. This is a bit of an over-simplification and skips a couple considerations but like I said, will have you in the ball park.

Anti-dive geometry where the upper control arm pivot is angled back (as seen from the side) causes the spindle to "rotate" slightly as it goes through it's travel and makes zero bumpsteer theoretically not possible, but that's splitting hairs a bit.

So with your front suspension sitting level (hopefully the lower a-arms will be near level, with the uppers angled down toward the middle slightly), kneel down and have a look, imagining lines created through the pivot points and not being fooled by some shape of the a-arms. The angle and lenth of the tie rod needs to be working with that of the a-arms.
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