Originally Posted by thewritingwriter
^^^ Yes you are also correct. But for the most part, like 95% of the time on the street it's caused by something other than thermal stress. Most people don't get their rotors cherry red on the street. Racing is a completely different story.
Getting (good quality) rotors cherry red or even white hot is not the root cause for many rotor issues. It's what people do with them after they are hot
. Pros know that a cool-down lap is required to bring temps down to just smokin' hot before heading to the paddock. Newbies sometimes bring their car to a stop with hot rotors and sit with their foot on the brake pedal. This is a sure fire way to cause brake judder from one or both of two phenomenon:
- Pad print off. This is easy to spot as the pad leaves a witness mark on the rotor. That spot now has more pad transfer film thickness than the rest of the rotor, meaning the fiction coefficient is slightly different. From this point forward, every time that section passes between the pads, they grab just a little more --> brake judder.
- Formation of cementite. If the rotors are extremely hot and the brakes are applied at rest, carbon from the pad compound gets pulled into the iron rotor surface. With enough carbon, the rotor at that point can setup a very hard crystalline structure -- cementite. Since this spot is now quite a bit harder than the rest of the rotor, it won't wear nearly as much. While the rest of the rotor wears per normal, it becomes thinner everywhere but this harder spot. Now the tech will measure the rotors with a rotor mic and say they are "warped" --> brake judder.
There are other causes for brake pedal pulsation or judder, but these two are the most likely when dealing with rotors that have been to extreme temperatures. So, yes, most street driving won't cause these sorts of problems, unless a caliper is sticking or a rubber brake line has collapsed internally.