Modern automobiles use disc brakes, which consist of a caliper and a rotor assembly. A caliper acts very much like a hand in the way that it grips the rotor, which causes the braking action. The caliper is attached to a bracket that holds it securely on top of the rotor. The rotor is a flat circular disc mounted over the wheel studs. This disc spins with the wheel, while the caliper is held immobile with the rotor passing through it. The caliper has two brake pads mounted on it that come in contact with the rotor. When the brakes are applied, the caliper acts like a hand and squeezes the rotor, using the resulting friction to slow the rotation of the rotor and the car.
Vibration Causes
The rotors are the major culprit for causing vibrations felt in the car and in the brake pedal. Rotors are very precise in that they cannot be out of round even two-thousandths of an inch or they will cause a vibration. When they are out of round, they tend to wobble as they are rotated. As they pass through the caliper, this wobble tends to move the pad in and out.
The cause for this phenomena is a caliper not sliding freely enough or not releasing all the pressure the way it should or an overly rapid cooling of the rotor after extensive use of the brakes or a hard stop. Rotors tend to heat up considerably when used extensively, and running through a puddle of water, which quickly cools the rotors, tends to warp the rotor and take out some of the temper. There is no way for a person to avoid this; it is just one of those service items that are uncontrollable.
To eliminate the vibrations, have the rotors turned on a brake lathe. This cuts just a little metal off the rotor to make it true again. Every rotor has a minimum thickness embossed on the front or rear of the rotor. As the rotor is used, it tends to wear down and get thinner. The thinner rotors get, the more susceptible to warping they become. This is the reason for the minimum thickness on the rotors. Turning a rotor that is already thin is not a good idea because it will warp much more easily. Once it gets too thin, it is better to replace the rotor.
