Plotting the Position of the Rack

I used Steve Smith's excellent book called "The Trans Am and Corvette Chassis Design/Theory/Construction" as a guide for measuring bumpsteer. I've graphed a few of the many tests (over a dozen) that I've taken and provided a quick explanation under each. Each test takes about an hour to do and I've spent at least 6 hours modifying the jig.

test 1

Test 1: This indicates that the rack jig (inner tie rods) is mounted too high. If it slanted to the right, the rack jig would be mounted too low.

test 2

When you see an "S" shaped graph, it indicates that there is play in the steering. In my case, one wheel bearing was loose and the right hand side tie rod needed tightening.

test 3

Here I've lowered the jig but not enough. Still have the left going slant. Notice the straight vertical section. The jig was interfering with the lower control arm and jamming the control arm in position. I had to knotch the jig to clear the control arms.

test 4

Here's the jig almost at the right vertical height, it could be a little lower. Now we have a nice curve appearing. This means that the tie rods are too long. Initially, I had the inner tie rods spaced 2" apart - the same as the rack spacing. The tie rods should be about 6" apart which will shorten them up about 2" each.

test 5

This test shows that the correct height has been found by the almost mirror image between bump and rebound. There still is the issue of tie rod length to deal with. If the curve opened to the right side, this would indicate that the tie rods are too short.

There's a couple of ways of "shortening" a tie rod without physically shortening it. You can move the rack farther forward. This may work, unfortunately, it didn't in my case. It looks like I'll have to widen the jig's inner tie rod mounts and shorten the tie rod lengths.

Before I do this, I'm going to do some measurements. The tie rods should run parallel to the lower control arm. This would be determined by a line drawn through the lower control arm's ball joint to the center of the lower control arm's frame mount.

The height of the tie rod should run parallel to the height of the lower control arm. I measured the difference in height from the ball joint and the outer tie rod end and it measured 1". The diffence between the inner tie rod end and the lower control arm inner arc axis should be the same. Now to rebuild the jig to these dimensions and test it in the next week.

Surprisingly, the correct steering geometry followed the upper control arm's arc and not the lower control arm's. The following picture shows the upper control arm and the rnp tie rod connecting to the steering arm.

tie rod

RnP tie rod follows upper control arm's arc.

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Copyright April, 2011 Eugene Blanchard