Warped discs?

For the past few weeks I've been feeling a vibration on the steering wheel when braking. FYI, I've never tracked my car but I drive it pretty briskly, i.e. goes frequently to mountain or twisty roads. I do some stretches where I brake very hard but not more than 4 or 5 hard stops before I ease down for some corners. My CS has 10k miles on the clock. Also, I always let the car cool down before stopping and no thermal shock as occurred. The only situation I can remember was an emergency braking (precisely on a Porsche club day!) where the leading car had to stamp on the brakes and then all others also until full-stop. What I feel now is somewhat hard to pin down but just today I drove my CS and after I did a couple of high speed/hard braking corners on the next braking area I approached the steering started to vibrate very noticeably. I think it is more pronounced with mid pressure braking and less on hard braking. I don't think there's any significant vibration form the pedal itself, mainly steering. Also when I'm driving very slowly, coming to a stop, with windows opened, I can hear a "swoosh" "swoosh" (faint) noise that seems like pads touching discs, doesn't worry me very much but now I'm thinking it might be something to do with this. What could this be? I would be somewhat bummed if the brakes can't handle spirited road driving!

It's pretty hard to warp these rotors, unless you get them very, very hot, and then clamp down on the brakes and leave them clamped-on for like, 5 minutes. Even in that case, they'll likely not warp. but transfer pad material in lumps to the rotor. Likely, you've somehow transferred some pad material unevenly onto the rotor. Find a spot where you have some room, and do 4-5 hard stops from about 50 mph (80 km/h). Squeeze the brakes on quickly - don't slam them on, and don't get into the ABS. Give yourself about 30 seconds between stops - then let them cool - don;t set your parking brake. That should clean off the old, and evenly transfer some new pad material onto the rotors. brad

Sounds more like the ABS the is kicking in.

Its seems that you are driving them a bit too hard. Consider inexpensive GT3 brake ducts. Also, how worn are your pads? They will heat up more quickly if quite worn. I think you are pushing the stock pads to the point where they start leaving considerable amounts of deposits on your rotors. They do that when they get bit too hot. I assume that after few minutes of normal driving the vibration goes away? That indicates that the pad deposits were polished off and you are good as new. I do not think you need to do anything, just understand what is causing the vibration and that it will go away. You may want to consider upgrading a different pad at next change. EBC Yellowstuff or Pagid Sport may do the trick. I am sure that someone locally will also recommend a different pad.

Thanks for the input. It's definitely not ABS. I would have to be an utter muppet to be activating ABS that frequently and violently ABS very rarely intervenes and it's usually very briefly and subtle, I can tell when it happens. Besides its un unmistakable pedal pulsating feel, this is good'ol vibration on the steering and (maybe) brake pedal. @Tomasz , I think you're right, just got back from another drive and it I definitely starts vibrating strongly after the first couple of hard stops, so I presume it happens when the discs/pads get hot. When it cools down it does stop vibrating, or at least much less. But this happens precisely after a couple hard stops so the prescribed treatment some suggested appears not to be working? I'm doing this from about 100 to say 40. I do drive it hard but it's nowhere near as hard as track driving which as much more prolonged periods of hard braking so is this normal? I look at the pads regularly and it seems to be lots of material in (1 cm?). So either people here who say they regularly track the car don't brake hard enough or I have some defective pads/discs? I might take it to the dealer to see if it's something wrong but by past experience I'm foreseeing a blame game if there's something to fix...

use a mike and run out gauge. have the front tires been removed lately ?

"use a mike and run out gauge" you lost me there? No the tires are still the same, no wheel removed either.

You may not have the type of feeler gauge and bracket needed to measure runout. But, it is a simple procedure, and the dealer can do it quickly. Anything is possible, including a warped rotor, but it is unusual. Even the ones diagnosed as warped, often are not. But, uneven deposits produce the same result, so take a set of free rotors if offered. If the wheels were installed with uneven torque values, a rotor could be warped, but these are relatively robust rotors, and I expect it would be difficult to bend one in that manner. But, incompetence is unlimited I suppose. The OE pads are good, but you might be heating them beyond their efficient working range. If so, you just need to use a more aggressive pad.

I would be hard pressed to believe or guess that its the rotors!!! Maybe... weights came off your wheels and tires aren't balanced?

This is where I would start too - given all of the above. I would find a shop with a (Hunter brand if available) road-force type tire balancing machine, that knows how to use it properly. (It places a roller against the tire at force and measures variations, so it measures balance and roundness of the wheel/tire assembly). This may eliminate the problem and will give you more good info if not.

I also think it would be strange, besides my rotors look very clean visually, no identations or cracks, smooth surface and still little wear but you never know. You also got me thinking that the other common denominator other than heat is speed. After all the vibration is much more pronounced on high speed braking which equals higher loads on the front tires and may exacerbate any wheel unbalancement? I'm going to take it to the dealer, I hope they do a good job and find out what it is . I will report after that so that this thread can be of even more use to anybody with a similar problem. Fingers crossed...

Just a quick related question, do you car's brakes start making a "rumble" noise after a couple of high speed hard braking? Stopping power and pedal travel is unaffected but they do make the "whoooo" noise.

You should not hear the rumble sound you mention. Your problem is brake related, not an unbalanced or imperfect tire. The symptoms appear only with braking, and include noises. A tire problem could have certain speeds at which the vibration is more pronounced, but it would not be as specifically and directly related to braking, nor would it create the kinds of sounds you describe.

Ok, went to leave my car at the dealer this morning and the bad news was that the discs were indeed caput. I don't know what's the term for this in English but the service manager said they were not warped (they tested them) but had suffered some kind of thermal shock and were, err, "un-tempered"? When cold the vibration wasn't felt or was very mild but as soon as the brakes started heating the vibration was on and pretty severely. In real terms the result is the same I guess. The good news was that they immediattly replaced the rotors with new ones and delivered the car to me a few hours later, so a good job and me as a happier customer after a previous service blunder. Now I'm left scratching my head what could cause this. I didn't do an emergency stop in rain to avoid a water puddle and diving into it. I always let the brakes cool down after hard driving. I take care to not clamp on the brakes if I have to come to a stop after hard braking... Could it be from taking the car to wash? I almost always hand wash my car but sometimes I take it for a quick jet wash rinse (destilled water only program). Even so I drive the car normally and usually wait some minutes before applying the water stream to the rims. Or did they wilt from the workout they get when driving through the twisties? People here say their brakes aren't noisy but mine always made the "rumble" noise after a set of hard braking corners. When I say hard braking I mean really hard braking, not ABS intervening zone, but close to. Even so nowhere near as hard as on track where they would be pounded lap after lap on the limit. I must say my confidence is somewhat dented on the brakes resistance but I'm really trying hard to understand what could have caused this and if I was to blame, how!

It's nothing you did. Sounds like these were rotors that were not properly heat treated in the manufacturing process, thus too soft.

My guess is they were somehow defective as well. I agree, it's not water puddles or car washes - you can do those with complete confidence.

Look for smearing on the rotor surface, about 4+ inches long - I'd bet pad transfer.