Disc Brake Tutorial - Page 4
Brake System Mechanical Components - Rotors
The vibration from the warped disc will show up more when the brakes are applied. This is because the pistons behind each pad are now rigidly extended because of the hydraulic pressure generated in the master cylinder. They can no longer slide back and forth inside the caliper. As a result the caliper itself must now try to take up the motion.
In the rear brakes this bracket is attached to the rear suspension which is incapable of moving back and forth to the left and right as the front wheels do. In this case the caliper cannot move. Instead the disc slows even faster as the warp goes through the caliper because it has to squeeze past the pad now. This tends to be felt throughout the car. As you roll almost to a stop the last few revolutions of the rear wheels can be felt as a pulse in the stopping motion of the car as the warp tries to force past the caliper.
In the rear brakes this bracket is attached to the rear suspension which is incapable of moving back and forth to the left and right as the front wheels do. In this case the caliper cannot move. Instead the disc slows even faster as the warp goes through the caliper because it has to squeeze past the pad now. This tends to be felt throughout the car. As you roll almost to a stop the last few revolutions of the rear wheels can be felt as a pulse in the stopping motion of the car as the warp tries to force past the caliper.
In some cases the warp is very slight and is not noticeable when the pads are worn down and the pistons are quite extended out of the caliper. But when new pads are installed the pistons are forced all the way back into the caliper. Now the slight warp, which had previously been dampened by the pistons can no longer be dampened and the caliper is once again forced to take up the vibration. Since the entire caliper and its attaching bracket are rigidly fastened to your car the once unnoticeable vibration is felt distinctly. The only cure is to replace the rotors as well as the pads.
Below is a diagram showing the caliper bracket (lower left) and an animation showing how the entire caliper and bracket are forced to move left and right as the warp passes through the caliper (lower right).
The calipers of the front brakes are attached by a bracket to your steering assembly. When the caliper begins the back and forth motion the vibration is passed through the caliper bracket into the steering system and shakes the steering wheel back and forth while you apply the brakes.
Once the rotor wears below a minimum thickness the disc will no longer absorb the heat created by friction during braking and the rotors become warped. This leads to vibrations while braking, felt in the steering wheel if the front discs are warped and through the body of the car if it is the rear discs. The more warped the rotors become the more violent the vibration is.
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