Car is MY01 996TT.
Symptoms: strong shuddering in steering wheel and brake pedal.

The car has sat a while in the garage, but I've made sure that I moved it back and forth once a month. The other day I equalized the tire pressures in all 4 wheels, and decided to go around the block to warm things up a little.

When I accelerate hard, the car tracks straight, and there is no vibration. I only could take it up to 50 mph on public street. When I brake VERY hard, the brake pedal and steering wheel starts to shudder.

My thoughts are that it could be a warped rotor or something, but I'm not sure. I drove the car hard two months ago, and didn't have this problem then. It's been in the garage since. I don't think it's a flat spot on the tire, because it would make the car shudder during driving AND braking, right?

Any thoughts?

 

I had problems with the stock contis which caused a somewhat similar problem. In my situation, it turned out to be the tires. I'm not sure if it was a flat spot or perhaps cord shift. I replaced with new tires and everything is fine. That was at about 4,000 miles.
Rob
I re-read the original post. I was wrong, I thought you had the prolem with straight line driving. I realize now that it is only with hard braking. I agree it doesn't sound like the tires. I hope the rust on the rotors turns out to be the problem and not warped rotors.

 

Ya, the tires are practically brand new. I've only had them for about 400 miles.

 

I'll bet you washed the car or it was wet right before storing it for a while?

Most likely rust or pad material on the brake rotors. The turbo brakes seem to be reasonably immune to warping, although that is possible also.
I'd try several hard stops from high speed (using appropriate caution, of course) to try and cleanup the rotors. A weekend at DE would be effective also!

 

My first thought would be warped rotors,but like someone said this is actually kinda rare...
I think what your feeling is actually the ABS system taking over. You mentioned you have to brake very hard for it to happen and you feel it in the pedal. Try going over a different road surface perhaps one with a less than ideal surface. Hit the brakes again,you should feel the same "shuddering" but with much less pedal effort.
I was surprised the first time I drove a Porsche how easy it was to start to lock up the tires(prevented of course by ABS)

 

WCE,

You know you may be right. I did wash the car before storing it. I do have a track event coming up, about 5 hours drive from my house, so I will be testing the brakes on the way down. I sure hope you're right about the imbalance due to rust/pad material. As I mentioned before, everything was fine and dandy up until now.

Pyroproblem:
I really don't think it's the ABS. ABS pulses are a lot faster, and you can actually hear the rapid-fire staccato sound, and I've used it several times already. In this case, I didn't hear it. Wouldn't it also trigger the PSM light too? I also noted the road surface at the time; it is quite smooth black tarmac.

At any rate, if it is the rotors, it won't be cheap! :P

 

I think you have a high spot of material on the rotor. Either pad material or rust from sitting. Change the front pads and break them in. Certain materials will actually skip on the rotor causing a shimmy on heavy braking. I only suggest this since you indicate it was not an issue until sitting. My car did it for a short while until I replaced the pads. Don't kid yourself though, these rotors can and will warp if not taken care of.

 

Its just pad material imbedded on the rotors, it will go away. carl