Originally Posted by Bbenavitz
It is interesting all these sources tell you it is nothing more than a build up or deposit of pad material, but if you have your rotors turned to true them the lathe operator will show you the metal being removed to bring the run out down to acceptable levels. It is definately not pad material being removed, because if it was your rotor thickness would not be sustantially reduced they way it is to get a true rotor. Since I first started seriously tracking cars in 1989 using stock rotors and race pads I have found that every car since then (Mazda RX2 race car, Acura Integra R, C5 Corvette, Mitsubishi Evo 8, C6 Z06, and now the M3) all will produce a warped rotor after 4 to 6 one day track events, assuming you are pushing hard enough to get a lot of heat into the braking system. In my opinion rotors are consumable items when tracking your M3 just like the pads, brake fluid, and tires are.
I use to have issues with cementite when using heavy metallic pads but never with the newer type pads. and when you try to cut them with a brake lathe the bit will start to chatter and will not cut properly and then the best solution is to replace the disc.90 % of the time rebedding the brakes will clean up the pad deposits.On my previous E92 M3 ran 3 years with about 40 track days on the original standard rotors with no issues.
[quote]The only fix for extensive uneven deposits involves dismounting the discs and having them Blanchard ground - not expensive, but inconvenient at best. A newly ground disc will require the same sort of bedding in process as a new disc. The trouble with this procedure is that if the grinding does not remove all of the cementite inclusions, as the disc wears the hard cementite will stand proud of the relatively soft disc and the thermal spiral starts over again. Unfortunately, the cementite is invisible to the naked eye./QUOTE]