I went through each article Grumpy referenced above, and found these three references to bump steer and geometry:
(The underlining and italics are by me....)
http://www.hotrod.com/techarticles/hrdp ... ewall.html
"As the front suspension moves up (compresses) and down (extends), the lower control arm moves in an arc around its pivot point on the chassis. The tie rod, connected to the lower control arm through the steering arms, also moves in an arc. If these two arcs are different, the wheel will turn slightly either in or out as the suspension moves through its travel. The result is a change in toe-in, which makes the car dart one way or another with no driver input. In extreme cases, it can be very dangerous."
"The two main things that affect bumpsteer
are the angles and the lengths of the tie-rod ends and lower control arms. Ideally, they should be the same length and perfectly parallel to each other, but that's not always practical. Most factory suspensions are pretty good in regard to bumpsteer (since it's a safety and therefore a liability item for the OEs). Because an aftermarket rack-and-pinion swap obviously changes the location and length of the tie rods, there is a good chance that it will adversely affect bumpsteer if not adjusted properly, especially on lowered cars."
http://www.cachassisworks.com/Attachmen ... DS_WEB.pdf
"A common issue with many custom steering and suspension conversions is
achieving correct steering geometry using the limited selection of OEM and
aftermarket rack and pinions. To solve this problem the g-Machine front-steer,
manual rack and pinion can be sized
from 15.5” to 29.5” (measured from inner tierod
pivots), in one-inch increments, without affecting rack travel, steering effort, or
reliability. This enables correct steering geometry for vehicles ranging from compacts
to full-size pickups and allows the assembly to serve as a direct-replacement
upgrade for 24.5” Mustang II racks."
http://www.steeroids.com/html/technical_info.html
"TIE ROD ENDS
Bump Steer Adjustment
Bump Steer (or Bump Steering) is a term used to describe when a vehicle hits
uneven or rough surfaces and causes the car to wander or become darty. What
causes that? Basically,
if the tie rods and the lower control arms are not the same
length and parallel to each other when the suspension travels up and down they
will be traveling at a different radius from each other. This causes the alignment to
toe in and toe out as the suspension moves, causing the car to wander and dart
while on rough surfaces – which is annoying and dangerous!
Every Steeroids Rack & Pinion kit comes with adjustable tie rod ends that include
a series of three spacers which allow you to
fine tune the angle of your tie rods to
match the angle of the lower control arms – helping eliminate bump steer.
Correct Adjustment of Tie Rod Ends to Eliminate Bump Steer
Add and remove spacers provided to move the outer tie rod end up and down. The
goal is to get the steering tie rod assembly parallel with the lower control arm
pivot points.
NOTE – you are NOT trying to get the tie rod assembly parallel with
the ground. Be sure to make adjustments when the vehicle is resting on its own
weight."
There is a possible minor error in the statements quoted above. On many of the suspension systems I have looked at, the steering arms are located on their own individual arc path that is somewhere between the longer arc path of the lower control arms and the shorter arc path of the upper control arms. It seems to me that the arc path and angularity of the R&P end links needs to parallel and match the length of the actual arc path of the steering arm connection point, the point where the R&P end links join the steering arms. In all cases that I have actually looked closely at it, the arc and angularity of the steering arm connection point is actually slightly shorter then the arc of the lower control arm, and sometimes the upper control arm has a slightly different angularity than the lower control arm does, so the angularity of the arc for the steering arm control points is also somewhere between those of the two control arms, but closer to the lower control arm.
I'm putting 2" dropped spindles in my '55 project, with aftermarket tubular control arms and coil-overs. The control arms have additional castor built into them. However the steering arms are in their original stock location on the spindles so the stock geometry should apply. Am looking at R&P options but haven't decided on a specific direction as yet.
The stock steering center link and outer tie rod links are still there, so I can take measurements and try to find something that matches the dimensions of the original. Or if a supplier can convince me that a variation from the original will give better results, I'm willing to consider that also.
Aloha,
Willy