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I have 99 FRC with a continual front wheel toe-ing out problem. I can get a good alignment set and then slowly and steadily the toe goes out (great for auto-x, bad for tire wear). The front rubber bushings have been replaced with Vette Brakes Poly bushings and have been in the car for several track events (road track) and 25k street miles. The car has a total of 110k miles. The bushings look good. There is no play in the front wheels. Wheel bearings and ball joints are good. About 40k miles ago, I installed Vette Brakes front outer tie rod ends. The big problem is I am now out of adjustment. I just can't figure out why the front wheel toe keeps creeping out.
Just offering .02
Check the torque on the a-arms and x-member etc. nuts and bolts of all the front suspension.
Perhaps a steering rack issue?
Perhaps Voodoo?
I don't have as extensive data as you do, but 1.5 seasons of autoxing threw my alignment specs out of whack. I found that the alignment that was done when the car had about 5k miles was already out of spec by the time I had it aligned at a little over 8k miles. The out-of-spec is quite staggering, the car was actually moving sideways. I also had abnormal tire wear in the front before the last alignment, which I heavily suspect, due to the out-of-spec alignment.
And, no, I didn't hit potholes big enough or run the car into a curb or any immovable objects between the two alignments. I highly doubt that an autox cone would cause it.
Kinda glad to find out I'm not the only one, kinda sad because now I realize that it wasn't just my imagination or a bad alignment job. And I don't know what to think other than the possibility that the alignment spec won't last long. Is it really that easy ?