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Old 01-05-2013, 10:35 AM
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gow589 gow589 is offline
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The amount of Ackerman is very different for a street car or a track car. The track itself can dictate what is the best Aackerman. Ackerman originated form horse and buggies. When buggy made a turn they made the inside wheel turn more which actually matched the center of the turn. If they didn't, the horse would pull the wheels sideways.

In my 1976 Cutlass, it is a boat. They were making these cars both sporty and so Grandma could drive. If you look at any large mid 70's boat like these the inside wheel turns more making it easy to get around in a grocery parking lot. It does however make it interesting if you jerk the wheel at speed (ask me how I know).

In a race car you want the toe to be correct for the straight away and correct for the turn which depends on speed and the arc of the turn. Often racers find themselves wanting more toe out in a turn which means they have too much toe out in the straight away making the straight always dicey. To counter this ackerman is changed by moving the rack forward or aft so you get the desired toe in for the straight away.

For most street driven cars, we deal with what ever we have, how ever it is for what ever reason. To change the Ackerman more often is done in the design of the car.

Bump Steer

I spent a lot of time getting bump steer out of a Pantera. The stock rack is wrong and is an inherent problem in the Pantera. There is a bump steer fix which changes the height of the rack but ultimatly does not fix bump steer...it does however change the impact location. It makes bump not too much of a problem but it makes droop 2x as bad. The arc is just shifted.

I first needed to measure everything. I cut tubing and bolted them in each suspension point. Then I built and welded a frame to tie them all together. I removed the frame and used it to locate all suspension points. Put this info in a program; suspensionprog.




From the program I determined the appropriate height and width of the rack. Reality is often different then the accumulated errors from this kind of excessive; which meant the need to fine tune.

I converted to power steering which an entirely different rack. My a-arms are built from scratch and I modified the distance between the pivot points on the rack to match the information form the software program. I cut one end to not only to adjust the length but I made a threaded shaft so I could adjust it. When the apropriat spot was found, I welded it in place.





Using a HF laser mounted to the rim (not the tire whcih is not always uniform) I was able to achieve ZERO bump steer:



Now this is really the short version of everything done; the very short version. In fine tuning the rack I made a chart and was able to decipher the problem for tuning. The problem is not always as simple as this chart but this chart generally can lead you in the right direction. This chart is for the right wheel. If your bump does what is shown, it shows you what needs to be changed in your ride height or your pivot distance to fine tune the problem:



NOW, if your only problem is rack width, you can measure how much you are off by turning your rack to change the pivot location and finding the zero spot, doing the same with the other side then measuring how far you had to turn your rack to get the result. I can re-explain that if it did not make sense. (words are hard to understand some times).
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