Body Shake

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

[RandallNeighbour]

I think what we're all learning is that our cars require good quality high speed tires, and that the tires we choose must be balanced properly and replaced more often than our passenger cars. Porsches give the driver lots of road feedback and without good tires and proper balancing, we'll experience far more vibration from the front and rear.

When I bought my boxster, I assumed I'd be buying new tires every 18 months. I'm ready for a set now, and I'm replacing the 17's with 18's while I'm at it. I'm prepared for a less comfortable ride on 18's though, but more precision handling.

The day I get my new rims and tires, and they're properly balanced at a speed shop, you can be sure I will post what it's like to drive the car up to 120 mph with that special kind of feeling one has with a brand new Porsche. Well, at least that's my hope!