They don't. The vibration you feel in the pedal that everyone explains to you is a "warped" rotor is actually a thickness variation in the rotor. If a rotor was warped, it would simply wobble slightly side to side and the caliper (which SHOULD slide freely) will follow it back and forth and no problem will ever be noticed. Most brake vibration problems are caused by rust or dirt build up or a slight runout in the hub. The outside diameter of the hub is maybe 2 or 2.5 inches from the centerline whereas the outside diameter of the rotor is between 5 and 7 inches from the centerline, depending on the car, and possibly more or less, depending on the brake system. This means that .001" or .002" of runout at the hub will translate into .006" or more of runout at the OD of the rotor. This indicates a slight rotor friction surface "wobble". As the rotor wobbles, it will touch the pads slightly at one point on the inboard surface and at another point on the outboard surface. The pads will eventually wear the rotor slightly at these two points causing a thickness variation in the rotor. Instead of floating the caliper back and forth, a thickness variation will force the piston back into the caliper and then let it back out slightly, multiple times throughout one revolution of the rotor. As the piston moves into and out of its bore in the caliper, it moves the fluid into and out of the master cylinder, which is mechanically connected to the brake pedal. This is why a vibration is felt in the pedal. There is also another reason for the cause of brake vibrations...but for this one, I will have to give credit to the rocket scientist STEVE RUIZ of STOPTECH, our sister company for his exemplery explanation..check out this page
