Drivetrain squeak

[cycleguy55]

Originally Posted by Gaz64

Rotor runout should be zero, any runout is NOT what pulls the piston back.

The deformation of the square section caliper bore seal is what brings the piston back. The pads only move in thousands.

If you have runout pushing the pads back, you have brake shudder, and a long brake pedal, severity of either depends on the runout value.

Gary

The runout I'm referring to is normal, and at levels insufficient to be felt in the brake pedal. It's probably the extremely rare brake rotor that has zero runout at any given temperature, and even rarer for zero runout at all temperatures.

For example, here are excerpts from a Factory Service Manual for runout and rotor thickness variations:
Runout limit: 0.035 mm (0.0014 in) or less
Maximum uneven wear (measured at 8 positions): 0.015 mm (0.0006 in)

You'll note the maximum for uneven wear or runout isn't zero. IOW, some runout or rotor thickness variation is acceptable within limits.

While I've found references that would indicate each of us is correct, this would indicate it may be a bit of both: https://auto.howstuffworks.com/auto-...isc-brake2.htm

"the rubber piston seal and any wobble in the rotor may actually pull the pads a small distance away from the rotor"