Expert: Roland Finston - 10/6/2011

Question
I have a problem similar to your repley to a 4/7 letter...
At 25-30mph and 45-50mph there is a distinct vibration in the car. I have had it in to the tire dealer twice. He has checked balance and road test the tires twice. He feels it, amd has no explanation. Neither he or I can tell if it is from the front or the rear.

Not the brakes (not hot), not the bushings, (tire dealer checked while car was up on the lift). Nothing to do with acceleration or deceleration/braking.

any additional ideas?

Answer
Hi Herman,
I can't find the 4/7 letter.  But if these are new tires on the vehicle it is possible that as mounted on the wheel one or more of the tires may be out of round (radial runout) by more than .04" which can cause what you describe. Even though they are balanced that could be the case. I would jack up each tire and then with a steady tool positioned close to the center of each tread rotate the wheels to see how much the maximum variation in the radius is. If it is more than .04" it may be that by also checking the radius of the wheel with the tire removed for similar variation you can re-position the tire on the wheel so as to cancel out the variation enough to get the overall variance to less than .04".
That is called 'match mounting'. If the dealer didn't check for out-of-roundness then this is your next step.
Another possibility is that the position of the engine laterally in the vehicle is out of spec such that there is an issue with the fit of the half-shafts. But that would be the next thing to check.
Roland