From Wikipedia>
Warping is often caused by excessive heat. When the disc's friction area is at a substantially higher temperature than the inner portion (hat) the thermal expansion of the friction area is greater than the inner portion and warping occurs. This can be minimized by using "floating" rotors which decouple the friction area from the inner portion and allow thermal expansion to occur at different rates.
Primary causes of overheating include undersized or excessively machined brake discs, excessive braking (racing, descending hills/mountains), "riding" the brakes, or a "stuck" brake pad (pad contacts the disc at all times).
Measuring warping is accomplished using a dial indicator on a fixed rigid base, with the tip perpendicular to the brake rotor's face. It is typically measured about 1/2" (12 mm) from the outside diameter of the rotor. The rotor is spun. The difference between minimum and maximum value on the dial is called lateral runout. Typical hub/rotor assembly runout specifications for passenger vehicles are around 0.0020" or 50 micrometres. Runout can be caused either by deformation of the disc itself or by runout in the underlying wheel hub face or by contamination between the rotor surface and the underlying hub mounting surface. Determining the root cause of the indicator displacement (lateral runout) requires disassembly of the rotor from the hub. Rotor face runout due to hub face runout or contamination will typically have a period of 1 minimum and 1 maximum per revolution of the brake rotor.
Another cause of warping is when the disc is overheated and the vehicle is stopped with the brakes continuously applied. In such a case, the area where the pads are in contact with the disc will cause uneven cooling and lead to warping.
Incorrect fitting also leads to many cases of warping; the disc's retaining bolts (or the wheel/lug nuts, if the disc is simply sandwiched in place by the wheel, as on many cars) must be tightened progressively and evenly. The use of
air tools to fasten lug nuts is extremely bad practice, unless a torque tube is also used. The vehicle manual will indicate the proper pattern for tightening as well as a torque rating for the bolts. Lug nuts should never be tightened in a circle. Some vehicles are sensitive to the force the bolts apply and tightening should be done with a
torque wrench.
Several methods can be used to avoid overheating brake discs. Use of a lower gear when descending steep grades to obtain
engine braking will reduce the brake loading. Also, operating the brakes intermittently - braking to slower speed for a brief time then coasting will allow the brake material to cool between applications. Riding the brakes lightly will generate a great amount of heat with little braking effect and should be avoided. High temperature conditions as found in automobile
racing can be dealt with by proper pad selection, but at the tradeoff of everyday driveability. Pads that can take high heat usually do best when hot and will have reduced braking force when cold. Also, high heat pads typically have more aggressive compounds and will wear discs down more quickly. Brake ducting that forces air directly onto the brake discs, common in motorsports, is highly effective at preventing brake overheating. This is also useful for cars that are driven both in motorsports and on the street, as it has no negative effect on driveability. A further extension of this method is to install a system which mists the discs with water. Jaguar has reported great reductions in disc temperatures with such a system.
Warping will often lead to a thickness variation of the disc. If it has runout, a thin spot will develop by the repetitive contact of the pad against the high spot as the disc turns. When the thin section of the disc passes under the pads, the pads move together and the brake pedal will drop slightly. When the thicker section of the disc passes between the pads, the pads will move apart and the brake pedal will raise slightly; this is pedal pulsation. The thickness variation can be felt by the driver when it is approximately 0.17 mm or greater (on automobile rotors).
Not all pedal pulsation is due to warped discs. Brake pad material operating outside of its designed temperature range can leave a thicker than normal deposit in one area of the disc surface, creating run-out due to a "sticky" or "hotspot" that will grab with every revolution of the disc.
[7] Grease or other foreign materials can create a slippery spot on the disc, also creating pulsation.
Rotors can be machined to eliminate thickness variation and lateral runout. Machining can be done in-situ (on-car) or off-car (bench lathe). Both methods will eliminate thickness variation. Machining on-car with proper equipment can also eliminate lateral runout due to hub-face non-perpendicularity.
