What it Does
The brake rotor spins with the wheel. When you apply the brakes, the brake pads squeeze against the rotor, and the friction between the pads and rotor stops the vehicle.
Why This Matters
The brake rotor is crucial to being able to stop the vehicle. Rotor failure will severely inhibit the ability to stop the vehicle, and even more severely impact the ability of the brake system to respond quickly and properly. Brake rotors problems can also result in excessive heat build-up, which raises the possibility of boiling brake fluid. Brake rotors can also cause mechanical trouble due if they cause excessive vibration due to thickness variation.
Possible Problems
Low thickness (rotor worn down):
A thin rotor will run excessively hot due to limited material for heat dissipation. If the rotor is severely abused, it will loose material to the point of structural failure.
Separation/structural failure:
Well-built rotors practically never do this unless a workmanship error is made during brake work, resulting in air being trapped in slider pin bores and thus brake pad application being stuck “on.” Rust seizure in slider pin bores may cause a similar problem.
“Warped” rotor:
A commonly misunderstood term; please see thickness variation below.
Thickness variation:
This concern occurs when there are uneven deposits of pad friction material on the rotor. This results in high and low areas on the rotor that cause the caliper to move rapidly back and forth as the pads travel over these high and low areas. Too much of this sideways movement causes vibration to transmit through the steering linkage and up through the steering wheel, causing it to vibrate or shake. In severe cases, there will be significant steering wheel movement and possibly a heavy feeling of shuddering through the vehicle’s body. (This vibration is commonly misunderstood as being caused by a warped rotor, which is a very rare occurrence, if even that.)
Thickness variation is commonly caused due to the nature of brake (adherent-type) pads, which are designed to give up a little friction material to coat the brake rotor. Lots of heat encourages these pads to give up more material, and if this occurs while the vehicle is stopped, this can “grow” a high spot on the rotor.
Please note that vibration caused by thickness variation is more likely to be noticed at higher speeds and after the brakes have warmed.
Runout:
A rotor with installed runout does not rotate evenly and would appear (under magnification) to wobble. This results in one part of the rotor making harder contact with the brake pad. This encourages adherent pads to give up excess material, which grows a high spot on the rotor, which will lead to vibration.
Recommended Parts Brands
- OE grade
- Centric
- Brembo
Repair Notes
Runout measurements should be taken during rotor installation. What commonly happens is that the rotor can be installed in a position that minimizes install runout, which lowers the risk of vibration developing due to thickness variation.