“Presuming that the hub and wheel flange are flat and in good condition and that the wheel bolts or hat mounting hardware is in good condition, installed correctly, and tightened uniformly and in the correct order to the recommended torque specification, in more than 40 years of professional racing…I have never seen a warped brake disc,” said Smith, who authored StopTech’s white paper on the topic. “I have seen lots of cracked discs, discs that had turned into shallow cones at operating temperature because they were mounted rigidly to their attachment bells or top hats, a few where the friction surface had collapsed in the area between straight radial interior vanes, and an untold number of discs with pad material unevenly deposited on the friction surfaces — sometimes visible and more often not.”
Smith says every vibration issue he’s seen attributed to a warped brake disc has actually turned out to be friction pad material that was transferred unevenly to the surface of the rotor. This uneven build-up results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures.
Smith explains how the problem occurs by taking a more in-depth look at the nature of braking friction in his white paper. The bottom line, though, is you must properly break in your brake pads and rotors to prevent materials from transferring between surfaces in a random fashion. This allows the bonding resins to be burned off slowly to prevent uneven deposits and fade. The right break-in procedure will also relieve any thermal stresses on new brake rotors and will transfer a smooth layer of pad material onto the disc.
It’s also important to choose the right brake pad materials for your driving style. According to Smith, there is no such thing as an ideal all-around brake pad. For example, a friction material that is quiet and functions at low temperatures around town won’t stop a car that’s being driven hard. A true racing pad, used under normal conditions, will be noisy and not work well at low temperatures around town.
So what pad should be used in high performance street cars?
“The answer is a high performance street pad with good low temperature characteristics,” Smith stated. “The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid, the condition comes on gradually enough to allow us to simply modify our driving style to compensate. On the other hand, should an emergency occur when the brakes are cold, the high temperature pad is simply not going to stop the car.”
Again, the right pads, mounting procedures, and pad/rotor break-in process are key to avoiding dreaded brake judder.
Myth #2: A Soft Pedal is the Result of Pad Fade
A mushy brake pedal is caused by overheated brake fluid, not overheated pads. According to Smith, there are two causes of brake fade:
- The temperature at the interface between the pad and the rotor exceeds the thermal capacity of the pad. This causes the pad to lose friction capability due largely to binding agent outgassing in the pad compound. In this situation, the brake pedal remains firm and solid, but the car won’t stop. The first indication of this condition is a distinctive, unpleasant smell.
- Boiling brake fluid in the calipers causes air bubbles to form. Since air is compressible, the brake pedal becomes soft and “mushy” and pedal travel increases. You can usually still stop the car by pumping the pedal, but efficient modulation is gone.
Myth #3: The Brake Fluid Reservoir Should Be Topped Off
In modern passenger cars, the brake fluid reservoir is designed with a specific volume and equipped with an internal float. The volume corresponds to the amount of fluid displaced when the pads have worn to the replacement point.
Then, the descending float completes an electrical circuit and a dash light appears warning the driver that the pads should be replaced.
“If the brake fluid is topped up, the first warning of worn-out pads will be the screech of steel backing plate against iron disc,” Smith said.