[DeTomaso] Bump Steer

[GW]

I am getting to my front suspension. I am still waiting for my A-Arm 
Jigs to be laser cut. Business is first so my projects get done when we 
get time on the laser.

This has been a long ongoing project on the side burner...till now.

As you may know I am doing a power steering conversion. This is a rack 
which was used on small cars and sold as a conversion for an MGB. It 
uses an electric pump which can be turned on or off which means the rack 
can be manual or electric.

One thing I have really worked on is getting the geometry correct. As 
many of you know shimming the rack for bump steer is not a fix but a 
patch. All it does is shift the parabolic arch to where it is less 
noticeable in bump; far worse in droop.

The real problem in bump steer is the rack length it self; specifically 
the length between the tie rod pivot. I started by pulling in suspension 
dimensions. I bolted tubing into the suspension mounts and welded it all 
up in a frame. I pulled it off the car and used it to measure suspension 
geometry:

http://www.rc-tech.net/pantera1/frontsus/jig.jpg

I took the numbers and loaded the program Susp Calc the best I can. I 
played with rack length and height in the program. I tweaked one, then 
the other zeroing in on the best of both. Basically, rack height will 
determine the balance of bump steer, rack length will determine  how 
much bump steer. The two do work together. By working them into the 
funnel so to speak I came up with numbers of total variance of  .01 
degree or less in bump and .03 degree or less in droop. In my book 1/100 
of a degree is zero.

Here are the numbers:
http://www.rc-tech.net/pantera1/frontsus/bump.jpg

The key number is 11.862.....x2 = 23.724". That is the width of the 
pivot point in the steering rack. The stock rack is about 28".

Now garbage in is garbage out. How close is my measurements? ! could be 
within a 1/16", I could be within a 1/4". The rack may need to be 23", 
or it may need to be 24" or somewhere in between. The final tweak is on 
the car.

To do this had to cut the power rack. The power rack was too short to 
begin with. I needed a way to change the length of the steering rack to 
make in car measurements. One issue is welding on the original ball 
joint melts the inner liner and is not re-build able. My solution was to 
use a ball joint off an MGB rack I had. It has the same threads on the 
tie-rod and the shortened length will be exactly what I need.

What I did was cut down a threaded tractor implement pin and weld it on. 
The nut was welded to the ball joint end. To do so allows me to check 
travel at different lengths. I have about 3/4" travel. The idea is I am 
starting too long. I can check the travel at 2 different distances, then 
shorten the threaded shaft till I get in range. If I had 100% confidence 
in what  have transfered to the computer program I would cut it down to 
23.724"....In fact I know better. I need proof of concept on the car. 
Here are some photos

http://www.rc-tech.net/pantera1/frontsus/sus.htm

Gary