Nothing to understand...

Hello folks..., I ''dewinterized'' my car at the beginning of April and the first thing i did was to get a new wheel alignment. Now i will explain the problem Today i took the car to go shopping with my wife. So we went to the market place located about 12 miles from where we live.We went there with no problem. As we return on the Highway suddenly at around 70mph the car begin to rumble and shaking for no reason. I dropped the speed from 70kmh to 40mph and the car stopped to rumble and shaking.Then i return to 70 and 80mph and no problem at all. The car begin this problem a few weeks ago and do that problem almost each time i take the car out but as i stated it made it one time and that's it....I can drive the car for 2 hours and it will never do it again. Please help for one of the first time i do not understand what can cause this. I bought 2 new front tires (not installed yet) and the ball joint of the front upper right control arm is going south... The car is equipped with 5mm wheel spacers but never had any problem with them last year. During winter time i replaced the outer front cv joint wheel bearing Thanks Robert 2000 C4

Check your wheel nuts - long shot but had a very similar problem once, cost an Audi dealer a new wheel, tire, hub

Wow lug he heat generated by a bad bearing should transfer to the wheel since it would be acting like a heat sink since its bolted onto the hub. Now I'll do some wild reaching for answers like possibly the ABS burped due to computer or diry sensors caused a missread. Or a bent rotor sue to over tightened wheel lug nuts which I'm not sure if 911's are prone to that problem but its a good idea to check and never use a miracle impact wrench on wheels. Brakes might of gotten hot enough to slightly warp the rotor then that would cause the brakes to be applied on an uneven rotor which would translate into a feeling of shaking. Then the old mud or dirt was on the wheels some place and it fell off after your scare. It would be good to give the whole wheel a cleaning to make sure there isn't any build up on your rims. Hopefully the tires are compatible with the electronic monitoring your car might be doing. Don't forget it could of been a mouse hitching a ride under your car and he jumped onto the inside of you rim throwing the balance off till he puked his brains out then sumersalted down the road behind you. Oh yah, wheel weights sometimes aren't the greatest for staying on and you might of lost some new ones or old ones a guy desided to leave. Have fun finding whatever it might of been. Best when checking for the dirt clod problem to visually check and see if you see any place it left dirty.

remove the wheel spacers.

I agree, wheel spacers feathered a set of tires, never again, I sold them.

OK folks i finally found the problem.... First after a ride i decided to have a look at all the wheel to know if one of them was hotter then the other ones....And yes....the rear left wheel was really hot to the touch. At first i thought the bearing was shot but i was sure the wheel bearing was OK...... so i decided to just turn the wheel by hand....impossible to turn it and the hand brake was not on so...the problem was only the hand brake adjustment... I adjusted the hand brake and voila...everything back to normal.... By the way i also replaced the upper front right control arm and tomorrow i will have 2 new front tires. Bye Bye Robert 2000 C4

Good find Robert.

Yep heat tends to make rotors act strange and deform a bit then the brakes dagging highlight the deformity in the form of grabbing that makes thing shake and such. Moisture in brake lines expands causing the brakes to be applied slightly or all the way depending on how much water and heat. Crusing at speed you wont notice the rotors heating up from the dragging brakes till the warping the heat makes either shakes the car or in extreme cases the car ends up stopping. Had a 71 SS Camaro that had extreme water in the brakes. The full brake system master to the pads cost less than 300 a few years back. I'd hate to think of the 911 cost. That just feeling the wheel near the hub sure gets you in the ball park. I feel mine from time to time as a force of habit.