by Dave Zeckhausen
If you have taken your car to the track on weekends, changing between racing brake pads and street pads, you've probably experienced brake judder. Brake judder, also known as "brake shimmy," is the feedback through the steering wheel and suspension when you apply the brakes at certain speeds and pressures. The steering wheel can shake back and forth, ranging from a faint, barely noticeable vibration to a violent shudder that feels like it could rip itself from your hands.
Judder can occur on the street or track and, in both cases, is caused by uneven build-up of pad material on the surfaces of the rotors. It is aggravated by the high temperatures experienced at the track and by switching back and forth between two incompatible friction materials.
Despite the popular myth, brake judder is not caused by warped rotors. Judder is the result of a thickness variation in pad buildup on the rotors' surfaces.
To understand why this is happening, you first need to understand the concept of bedding brakes. Click here for bedding instructions. In order to maximize the performance of your brakes, you must transfer a thin layer of pad material onto the surfaces of your brake rotors. This "transfer layer" forms molecular bonds to the pads when you apply the brakes. As the rotors rotate underneath the pads, these bonds are torn apart. The resistance of the bonds to being broken yields additional friction above and beyond the abrasive action of the pads and rotors scraping against each other. This additional friction is called "adherent mode." You must bed your brakes in order to benefit from adherent mode friction.
When you install race pads, you would like to remove the street pad transfer layer from your rotors before you bed them in. Conversely, when your track event is over and you reinstall your street pads, you would like to remove the race pad transfer layer before bedding in your street pads. How do you do this?
Some drivers swap pads and rotors at the same time, keeping one set of rotors dedicated for the track and the other set dedicated for the street. This works by keeping the transfer layer pure and not piling up incompatible layers of pad material on top of each other. But it's a lot of extra work. The last thing you feel like doing, after a long track event, is to swap hot rotors. There is a better way.
Street pads exhibit a mixture of abrasive and adherent friction almost immediately without extensive warm up. Race pads don't get into their adherent mode unless they are several hundred degrees, a temperature rarely seen on the street. Under normal street driving conditions, a race pad will operate almost purely in abrasive mode.
Use Race Pads as a Tool to Remove Deposits from Rotors
If you install race pads before your track event and drive around normally, the race pads will polish away the transfer layer left behind by your street pads. This leaves you with nice, clean rotors which may be bedded with your race pads when you are at or near the track.
After the track event is over, you should not switch back to street pads right away. Instead, drive home from the event using your race pads. As they cool down, the race pads will once again go into pure abrasive mode and polish away their own transfer layer from your race weekend. Cold race pads will even cure judder problems from pad deposits accumulated during the track event. By the time you get home, the rotors are polished clean and ready for you to install and bed your street pads.
In essence, you are using your race pads as portable brake lathes. The race pads are a tool for removing unwanted transfer layer before and after your track event.
Avoid Pad Imprinting on your Rotors
After a fast lap on the race track or a high speed stop from over 120 mph, your brake rotors may be literally glowing red hot. If you keep your foot planted firmly on the brake pedal after coming to a complete stop, you may find your brake pads bonded firmly to the rotors. The extreme heat melts the surface of the pads and forms an imprint on the rotors. This imprint is a few ten thousandths of an inch thick and can result in a noticeable shimmy. If it happens on unbedded rotors, you will also end up with a significant difference in the coefficient of friction between the imprint and the rest of the rotor, further aggravating the situation. In addition, the heat transfer through your brake pads can boil your brake fluid, resulting in a spongy pedal feel.
To avoid this problem, try to take advantage of the cool down lap before entering the pits at the end of your track session. Use the brakes as little as possible during your last lap. If you must enter the pits directly after a hot lap, make sure to roll to a stop and place the car in gear (or park) to keep it in place, rather than leaving your foot on the brake pedal. If your parking brake is incorporated into the rear calipers (e.g., many VW and Ford models), then avoid using it. Use a wheel chock, if necessary, to keep the car from rolling.
In the excitement of a track event, it's easy to forget all of this. As adrenalin courses through your body, your hands will be firmly clamped to the steering wheel and your foot on the brake pedal. Try to relax and keep that foot off the brake pedal! By following these techniques, you can forever banish the brake judder demons from your car.