Disc Brakes

A hydraulic caliper squeezes brake pads against both faces of a rotating rotor. The friction between pad material and rotor converts the vehicle's kinetic energy — its energy of motion — into heat. A vehicle traveling at 60 mph has an enormous amount of kinetic energy. The brake system must convert all of that energy into heat and dissipate it fast enough to stop the vehicle safely. That is why brake components get extremely hot during normal operation.

Two main caliper designs exist. A floating caliper slides on guide pins and uses a piston on the inboard side only. When hydraulic pressure pushes the inboard piston against the inboard pad, the caliper body slides inward on its pins, pulling the outboard pad against the rotor from the other side. A fixed caliper is bolted rigidly in place and uses pistons on both the inboard and outboard sides. Fixed calipers provide more even pad wear and better pedal feel but cost more. Either way, when the driver releases the pedal, the caliper piston seal flexes back slightly, pulling the piston away from the pad just enough to release clamping force. The pads do not fully retract — they ride just off the rotor surface.

Every rotor has a minimum thickness specification cast into the rotor itself and listed in manufacturer service data. As pads wear, they gradually remove material from the rotor surface. A rotor worn or machined below minimum thickness is too thin to safely absorb braking heat. It overheats, warps, and can crack. Measure the rotor thickness with a micrometer before machining it. If the rotor is at or near minimum before the cut, it cannot be turned — replace it.

Lateral runout is a side-to-side wobble in the rotor as it spins. Thickness variation is a difference in rotor thickness at different points around its surface. Both produce pedal pulsation during braking. Measure runout with a dial indicator mounted to the caliper bracket — specification is typically less than 0.002 inches. Measure thickness at eight evenly spaced points with a micrometer — variation above 0.0005 inches causes noticeable pulsation. Runout often comes from debris trapped between the rotor hat and the hub flange during installation. Always clean both mating surfaces thoroughly before installing a rotor.

Brake pads come in three main friction material types. Organic pads are soft and quiet but wear faster and produce more dust. Semi-metallic pads contain metal fibers, handle heat better, last longer, but generate more noise. Ceramic pads offer the best balance of low noise, low dust, and good performance for most street applications. Always match the pad type to the vehicle application and driving conditions. Performance vehicles and towing applications need semi-metallic. Daily drivers do well with ceramic. Never mix pad types side to side on the same axle.