# Do disk brakes resist being pulled out?

1. Oct 5, 2004

### carlfogel

When two pads squeeze a moving rotor with thousands of pounds of force in a disk brake, the friction resists the rotation of the disk enough to slow a car.

How much resistance (if any) does the squeezing of the pads provide if you try to pull the moving rotor out at roughly 90 degrees?

Imagine a rotor that is spinning steadily and powerfully on an axle in front of you, while a pair of pads on its far side steadily squeeze and brake it.

How hard do you have to pull the axle toward you to pull the rotor out from between the pads?

Will a tiny tug gradually pull the rotor out from between the pads?

If so, will it take an appreciably long time for the minute force to pull the rotor free?

Or is a powerful pull required, a force above some lower limit?

Or does it depend on how powerfully the pads are squeezing the rotor? That is, can the rotor be easy to pull out under light braking, but much harder under heavy braking?

Please feel free to use whatever figures are most convenient.

(The question came up in a weird disk brake thread in a bicycle forum. The well-meant replies did not inspire confidence.)

Thanks,

Carl Fogel

2. Oct 5, 2004

### Machinus

The force of friction between the disc and the pad will increase, linearly as the speed, as the pads are moved toward the outside of the disc. If the wheel is driven and moving at a constant angular velocity, then it sort of depends on where the pads are relative to the force being used to pull the wheel. The pads will not just slow down the wheel, but exert a vector force on the chassis that they are attached to. So the answer is that yes they might, depending on their position.