What Does Minimum Thickness on a Rotor Mean?
All disc brakes use a rotor and pads. A rotor is a flat, round piece of steel, roughly shaped like a pancake and located directly behind the...
All disc brakes use a rotor and pads. A rotor is a flat, round piece of steel, roughly shaped like a pancake and located directly behind the wheel. When you depress the brake pedal, the pads squeeze the rotor, so the friction stops the wheel from turning. Engineers understand that pads wear out, so they design the system so pads are easily replaced. However, engineers further understand that rotors wear out, too. Because of the wear factor, rotors are manufactured with extra steel on the contact surfaces, so they can be repaired.
Anticipating the Future
Engineers and designers anticipate the future. They understand pads and rotors wear out. Theoretically, rotors should last a long time. In fact, many do. If you change your brake pads as soon as a squeaking is heard, the pad won't wear completely through. If a pad wears through, the metal backing contacts the rotor, leading to scoring and deep gouging of the rotor surface.
Scoring Repair
The only way to repair a scored rotor is by lathe turning. In this process, the rotor is mounted on a special lathe. The rotor is turned and a tool removes metal accurately. The scoring is removed and the rotor is replaced, provided the maximum standard for material removal isn't violated.
Rotor Warpage Repair
Sometimes, even if a car owner faithfully replaces his brake pads, rotors warp due to excessive heat and rapid cooling. Suppose you do a lot of stop-and-go driving. The rotor heats up due to pad friction. On the way home, you hit a puddle that splashes cold water onto the hot rotor. The sudden shock of cold water hitting very hot steel warps the rotor. This is a common reason brake shops have to lathe turn warped rotors. Lathe turning a warped rotor also removes metal from the rotor faces and shops can't remove more than what's allowed.
Minimum Thickness Specified
in the process of rotor repair, material must be removed from the rotor faces. Engineers studied how much material is removed and how much is left on. In the process of material removal, the rotor is made thinner. Safety limits are set for how thin a brake rotor can be before failure. This area is so important that laws have been passed prohibiting a shop from turning a rotor past the manufacturer's limit. For example, the State of Washington has legislation pertaining to brake rotor minimum thickness. (see reference 1, point 4). The state of Connecticut's inspection report for public service vehicles requires discarding a rotor that is past the manufacturer's minimum thickness specification(see reference 2). A brake shop can't lathe turn the rotor past this minimum thickness dimension. If it does, the rotor will be too thin, leading to brake failure. If the rotor is thinner than the minimum thickness limit, it has to be replaced.
