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Good morning, happy Friday to everyone.

My '87 S4 auto has a shake that develops at 40 mph and then goes away at 60 mph.

I have turbo twist front rims, but the tires our new and only have around 500 miles on them. The tires were professionally installed and balanced.

The motor mounts on the car are also new (Rogers's), and while needed, did not cure the 40-60 mph vibration. I put a new steering rack bushings at the same time as I did the motor mounts, they were also needed but appear not to have impacted the vibration at all.

I haven't been able to get the car on a steep enough he'll to have it accelerate it neutral from just below 40 mph to 60 mph to see if it has anything to do with being under power when the vibration happens.

The vibration feels like it's coming from the front end of the car.

I'm inclined to think that either I have a problem with a rim or a bad balance, but it seems weird that it would smooth out again once I get through that speed range.

Are there any weird vibration/harmonics that other people have dealt with at lower speeds that I should be aware of? Ideas?

Thank you in advance for the help