|Friction, Brake Rotors, and You-|
To get an idea of how the rotors and brake pads work on your vehicle, grab a paper plate and spin it on your finger. As it spins grab it with your other hand. It stops instantly. The spinning plate is your rotor and your fingers and thumb are the brake pads. If the plate was spinning an awful lot faster and you tried to grab it, your hand and the plate would get hot in the process from the friction created. The energy of the spinning paper plate is converted into heat by the process of the hand grab. Not that you could actually spin the plate that fast, but you get the idea.
You may, of course, be asking yourself what all this "Ask Dr. Science" stuff has to do with driving down the road and worn out brake rotors. The answer is—a lot. Every time you hit the brakes, all that energy you built up by mashing on the throttle is converted into friction and heat and—presto—you slow down and come to a stop. This process is repeated hundreds of times without much thought, be it on a run to the supermarket or a weekend road trip. Over time the friction and heat created by the brake pads clamping down on the rotors actually wear down the metal surface of the rotor, and problems arise.
When the rotor gets too thin it loses its ability to channel heat away from the braking process and it overheats. The combination of too thin and too hot is a bad one for the brake rotor. Overheating brakes can cause a myriad of mayhem with your braking system. Pads get too hot and produce gases that cause brake fade. Rotors will warp and discolor as they overheat. Worst of all, brake fluid can boil and total loss of braking can occur. Warped rotors can be felt as a wobble or pulsation when you hit the brakes. This pulsation is not to be confused with the action of ABS brakes, and is usually felt on light application of the brake pedal.
Scoring and Warping
Imagine again the paper plate spinning around. In a perfect world the brake rotor is smooth and flat like the plate. When a rotor becomes warped it looks more like a potato chip than the plate. The measure of this flatness is called run-out, and a difference as small as .001 of an inch off center can be felt in the brake pedal. Beyond warping, rotors can become scored with deep scars if overheated. A rotor should have a relatively smooth surface for optimum braking. If you can feel deep scores in the rotor surface with your fingernail then it's probably time for rotor service or replacement. Do this test only when your brakes are cool.
Remove and Replace
The first thing to measure when considering replacement rotors is the rotor minimum thickness. The next is run-out. If the rotor is within specifications it may be resurfaced and then reinstalled on the vehicle. If any of the measurements are beyond specifications, the rotor is dangerous junk and must be replaced. Remember that skimping or cutting corners on brakes may be the worst and last decision you will ever make. When in doubt, the safest strategy is replacement. Another benefit of replacement is saving time. Removing the rotor and getting them checked and turned can chew up precious hours.