Vibrations at speed on A&C

[John Poling]

This isn't aimed just at A&C owners but a general question.

I have just completed my build and am in the find and fix the bugs stage. Really I guess I am pretty lucky as the only bugs I have are cooling at long idle periods, which the cure is just an additional fan (puller) instead of just the dual pushers.

The other is vibration at speed. I seem to have 3 different vibrations.

One is at low speed (around 30-40 mph) and is felt in the steering wheel. I think I have figured this one out that I have what I believe to be excessive run out (between .004"-.007") on the machined surface that the front wheels mount to on the rotors which equates to around .080"-.100" at the road contact part of the tire.

The second transitions in above the 40 mph mark and up to at least 70 doesn't diminish, it is felt more in the chassis. I have an idea of what this is but am not sure. I have not done any measurements yet so these measurements are just a guess. My drive shaft angle is going uphill from the tail shaft of the transmission to the pinion of the rear end, this angle would be somewhere around the 10 degree mark as measured from the output shaft. This seems very excessive to me and could be the culprit as I think that this may put a heck of a bind on the rear u-joint. I also only notice this vibe when I have the drive train "loaded" and not under a deceleration condition, as soon as I let off the accelerator it goes away for the most part.

The third is at highway speed of 60 mph and up and is felt in the steering wheel as a shimmy, almost like the whole steering column wants to get out and dance a jig, this may be related to the first one I described here.

I have had it to an alignment shop and it was set up to an early Fox chassis specification and balanced the wheels off the car and then fine tuned the fronts on the car.

I have contacted Advanced Chassis in Antwerp, Ohio (an NHRA certified chassis design shop) and will be taking the car to them soon, but I would like to get some ideas from anyone that may have had some of these same issues on theirs and what they found to be the problem.

Any input would be greatly appreciated.

Thank you.
John