[BYprodriver]

Originally Posted by jayspeed999

Was this problem ever solved? I just picked up a 2001 "S" and experienced the same thing. It does not happen every time I go over 75 mph. However, this is the same situation that occurred with one of my prior cars, a BMW 740 Xi. I learned what caused the vibration with that car: Cold, slightly squared tires.

Specifically, when I would park in my office's parking garage and leave it there for 8 hours or so, the tires would cool down a lot as it was colder in the garage than outside. Most of my drive home (around 17 minutes), I would experience the vibration.

However, if I parked outside my house in my driveway, then drive the car, there was no vibration at all above 75 MPH. I experienced the identical thing today with my Boxster: Yesterday after parking in the parking garage, I had vibration. But this morning, after sitting in my driveway with sun, there was no vibration detected above 75 MPH.

YMMV, but this has now been consistent and I think that in my case, this is the cause of the vibration. I believe that when the tires get cold, they retain the slightly flat or "square" shape where the tires contact the cold concrete. I believe the "squared" tire is the cause of the vibration. That is why the vibration does not occur when sitting in my driveway in the sun: the tires do not get cold enough to "square."

Thanks for searching ! Sounds like you are on to the problem. Most ultra-high performance tire can "flatspot" when cold as you have discovered. If the tire uses nylon for tread belts it is more likely since nylon expands & contracts greatly with temperature extremes. Read your sidewalls it should list tire construction materials for tread & sidewalls.