bmussatti

I found this interesting post today and thought I would share with this forum. It is a good question. Here is the question and answer:


QUESTION:

I recently had the good fortune to make multiple business commutes between Albquerque and Colorado Springs; about 400 miles of mostly wide open and lightly traveled interstate. Consequently I was able to explore some triple digit runs over extensive miles. I noticed that the car experienced a consistant vibration throughout the entire chassis between 90MPH and 100MPH. Above that and up to it's limit it was very smooth, as it was below 90MPH. The best way to describe it would be a one second shimmy, then 1 second smooth; one second shimmy, 1 second smooth etc. etc.; very periodic in nature. While running at the affected speeds I could lift off the throttle and coast, even put the car in nuetral with engine at idle and it would continue the shimmy until the road speed dropped below 90MPH; so I'm thinking its not related to the engine. Is this just a phnomenon of high speed driving, perhaps related to the aerodynamics of the car?


ANSWER

This is do to your tires. The oscillation that you describe is a harmonic vibration going back and forth in the car. Your tires are either mismatched front to rear by brand, type or speed rating. Another possibility is they are worn low in the rear or one of the rear tires is defective.
Scott Slauson - PCA WebSite - 12/2/2005