Monday, December 16, 2013
Do You Have to Resurface the Rotors Drums on a Brake Job
As thick and sturdy as they are, youd think that brake rotors would outlast the entire car. However, those rotors are thick for a reason: They are subject to some of the harshest and most demanding conditions imaginable. Operating under extreme temperatures, mechanical stress and even chemical assault, even the best rotor will eventually score, warp or crack and require some sort of machine work or replacement.
Normal Stresses
Rotors and drums exist to act as a friction surface for the pads to press against, meaning that at some point theyll have to turn several thousand pounds of forward momentum into thermal energy. The pads need to be almost as strong as the rotors to deal with the thermal and shearing stresses involved with braking, which is no mean feat when the rotors are made of hardened steel. Over time, the hard pad material will begin to eat the softer parts of the rotor surface, sometimes wearing it down evenly and other times leaving grooves on the surface.
Warpage
Warped rotors and drums are a result not of rapid heating, as conventional wisdom may have it, but of uneven cooling. Overusing the brakes will overheat the rotors, possibly to the point of glowing red or white hot. At these temperatures, metal acts more like a very slowly flowing liquid than a solid, so it may not assume the same shape once it cools down. Certain parts of the rotor will inevitably cool and contract a little faster than others, pulling some parts of the material in while other parts remain stationary. This causes warpage or "runout," which is a waviness in the rotor that causes brake pedal pulsation afterward.
Normal Resurfacing Conditions
Resurfacing is usually done with a lathe. The rotor goes on a lathe, and a cutting head passes over it to eliminate ridges and waviness. You dont necessarily have to resurface rotors during a pad change, since the rotors are far harder and should, by nature, outlive the pads. Rotors vary in terms of allowance for groove depth, as some rotor faces might be thicker than others. The only way to know for sure is to check your manufacturer recommendations regarding acceptable runout and groove depth. General Motors, for instance, allows for a fairly deep scoring of 1.5 mm before requiring machine work (AA1 Car Library, Understanding Brake Rotor Service). The rule of thumb is that grooves deep enough to catch a fingernail warrant some resurfacing.
Special Conditions
Oddly enough, there is some logic in putting off machine work until the manufacturer requires it. One of the oldest tricks in racing is to use a very thick rotor or drum, and then intentionally score it on a lathe to create a sharply grooves surface. Those deep grooves will increase the rotors surface area; once the pads bed in and conform to the rotor, they can provide significantly more clamping force than they would with a perfectly flat surface.
While no ones suggesting that you engage in any such idiocy with your street car and stock-thickness rotors, its something to bear in mind when considering a pad change without resurfacing. Until the pads bed in and conform to the shape of an un-machined rotor, their reduced contact area guarantees that the new pads will provide less initial stopping power than old pads with the same rotors.
