We were recently schooled on warped rotors from some friends over at Baer Brakes (
baer.com). First off, it’s exceedingly rare to actually warp a rotor. Instead, the cause of the shudder you feel through the pedal is an uneven buildup of brake pad material on the rotor itself.
Here’s how it works. Modern asbestos-free and nonmetallic brake pads contain different resins in addition to the actual brake material itself. Some of these resins and materials transfer themselves to the rotor face so that there will be adherent (sticky) braking in addition to abrasive braking. In a nutshell, when a brake system is properly bedded in, there’s a thin layer of pad material on the rotors. When you use the brakes hard, such as when coming down a steep grade, the heat buildup causes these resins to loosen. When you stop the vehicle completely, such as at a red light at the bottom of an off-ramp after that steep hill, you actually make an imprint of the brake pad on the rotor, creating a high spot. By the time you get gas or stop for lunch (or whatever) the rotors have had a chance to cool off, but that high spot is still there.
As little as 0.0004 inch can be felt through the pedal, and 0.001 inch can be downright annoying. Repeated use just leaves more material on the high spots, making the shudder worse.
Now that you understand what’s going on, here’s what you can do to help combat it. If caught early, you can often eliminate the shudder by rebedding the brakes. To do this, find a safe area where you can make a series of hard slowdowns from 50 mph to about 10 mph. Don’t come to a full stop. Doing this several times will heat up the material on the rotors and help smear it evenly on the face of the rotors again. Then drive around for a little while without stopping much to let the brakes cool off.