What causes rotor warping?

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Many people complain about brake pulsation and warped rotors. This guide is meant to help understand why you get brake judder and what you can do to prevent it. 

Brake judder is primarily caused by the formation of hard nodules in the iron. Engineers call these nodules heat spots. When I measure warped rotors, I find that the radial runout as referenced to the wheel mounting surface is in spec. So the term "warped" rotor may not be an accurate description of the problem.  However, my dial indicator shows elevated bumps from heat spots on the rotor surface. As the rotor passes under the pad, these spots cause brake judder, i.e. pulsating brakes. It only takes a spot .001 inches above the rotor surface to feel brake judder. 

Pulsating brakes start with a localized zone of high temperature in the rotor. The most common cause is uneven friction deposition on the rotor. At colder operating temperatures (less than 400 degrees), brake pads work with abrasive friction, just like sandpaper on wood.  At higher temperatures, brake pads transfer a thin layer of material to the iron rotor surface. As the rotor heats up, the primary friction force changes from abrasive friction to adherent friction, i.e. pad material is applied to the rotor and simultaneously sheared and the broken chemical bonds cause a resistant friction force. Most racing pads work using adherent friction, which explains why racing pads don't really stop well under cold conditions or normal street driving. 

An example of an uneven friction deposit is pad resin glazing. If you get your rotors hot, the resin from the pad will liquify and glaze on the rotor surface. As the pads rub on the elevated glazed surface, it gets a lot hotter, and the heat is localized in just one area of the rotor. When the temperature exceeds 1150F, the cell structure of the iron changes into hard brittle spots called cementite. These heat spots don't wear down like the normal iron material surrounding it. So the spots become elevated above the rotor surface, and you start to feel brake pulsation. You can try to turn down the rotor, but the cementite nodule is often deeper than the cut, so the brake pulsation just comes back after a few weeks. Here are some ways to prevent the problem:

  1. Use drilled rotors to keep the temperature below the point where heat spots start to form. Drilled rotors also prevent pad glazing as shown in SAE paper 2006-01-0691. 
  2. Don't clamp down on the brakes after a hot stop. If the rotor is hot and you clamp down on the brake, you will deposit friction from the pad to the rotor in just one spot.  The elevated friction area will get hotter than other parts of the rotor and start the cycle that produces cementite nodules.  I try to roll the car a little after a hot stop so the pad does not rest in just one place.
  3. Break in your new pads properly to initiate a uniform friction layer on the rotors. Some people install new drilled rotors and pads and immediately cook the brakes to see if it works.  Not only can this cause extreme thermal shock to propagate cracks (new rotors are susceptible to thermal shock), it also can cause "warped" rotors.  It only takes a few minutes to break in the pad properly.  I recommend 5 stops from 45-50 mph at moderately high pedal pressure, but don't come to a complete stop between each application.  Then drive around and let the brakes cool down and do 3 stops at light to moderate pedal pressure, letting the brakes cool between each application.
  4. Use high quality rotors that conform to SAE specs for chemistry and hardness and don't use rotors that contain secondary scrap. Secondary scrap contains rogue elements such as tungsten, vanadium, etc. that contribute to carbide formations in the rotor.  Carbides weaken the rotor and can cause stress risers and eventual cracking. 

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