So what really happens when the rotor is “warped”?

When you’re on the brakes and there’s a pulsing sensation or vibration, we typical call this situation warped rotors. Surprisingly, I personally have never seen a rotor that actually has physical distortion. I have even placed a dial indicator on so-called warped rotors to find that they have no run-out what-so-ever.

What does happen is that the layer of pad material on the rotor surface builds up unevenly, and the metallurgy of the rotor can change states. The layer of pad material on the rotors surface, if it is unevenly distributed, will create hot spots. If these spots get hot enough, it can form cementite in the rotors metal – a rough iron carbide formation that creates a lot of friction, but is terrible at dissipating heat. The cementite formation can get so bad and cause so much friction that even when you are off the brake pedal completely, because your pads are always in contact with the rotor ever so slightly, it can create a vibration when driving normally. I have even mistaken this vibration as my tires being out of balance.