Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems utilize a disc (rotor) with brake pads positioned on either side of it. Braking is achieved in a manner similar to the way many bicycle brakes work, by squeezing a disc that is spinning between two pads. The disc (rotor) is a casting equipped with cooling fins located between the two braking surfaces. This enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade. Dirt and water do not affect braking action since contaminants are thrown off by the centrifugal action of the rotor or scraped off by the pads. Also, the equal clamping action of the two brake pads tends to ensure uniform, straight line stops. Disc brakes are inherently self-adjusting.
The trucks covered by this guide utilize a sliding caliper design in which the pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, bringing the outside pad into contact with the rotor. Sliding calipers are usually secured by mounting bolts/pins equipped with a sliding surface to allow caliper movement.