Originally Posted by thewritingwriter
Do a google search on "rotor runout" that's what gives you the pulsation in the pedal. Rotors don't actually warp, it's when the rotor wobbles past it's maximum tolerance (measured in thousandths of an inch) and the pad will actually wear low spots in the rotor causing the "warped" rotor feel.
Originally Posted by chuckywang
That Stoptech article says that vibrations during braking are not caused by warping. Instead, it gives the reason as "friction pad material transferred unevenly to the surface of the disc". My question now becomes: What caues the friction pad material to transfer unevenly to the surface of the disc? I couldn't find the answer to that anywhere.
Another question I had regarding that article is this: Does the article imply that warping (ie misshapen rotors due to excessive heat) doesn't exist, or does it imply that it exists, but it just doesn't cause vibrations during braking.
The Stoptech article, and chuckywang, are closer to correct than thewriting. Brake rotors can and do physically deform in response to thermal stresses though it is MUCH more common for them to deform in response to uneven lug nut torque. Additionally, normal service conditions don't produce thermal stresses severe enough to produce physical deformation. Getting a rotor cherry red from racing and then quenching half of it with water, for example, is the level of thermal extremes we're talking about here. Even once the rotor has deformed, most deformations can't actually alter the braking force developed.
Consider runout. When the "high" portion of the runout slides through the caliper, the caliper simply floats over to follow the rotor. In the case of an opposed piston design, one piston extends while the other retracts. The centerline of the caliper follows the centerline of the rotor in either case.
Now, consider uneven deposition of pad material to the rotor. At places where minimal transfer has occurred, the rotor is physically thinner than average. At places where excessive pad material has been deposited, the rotor is physically thicker than average. When transitioning between relatively thinner and thicker portions of the rotor, the effective clamping force changes. This modulates the resultant braking force and produces the pulsing you experience.