Issue link: http://read.uberflip.com/i/634989
STORY BY BRET T BECKER PHOTOS AND ILLUSTRATIONS COURTESY OF WILWOOD T E C H M A T T E R S GT ASYMMETRICAL GROOVE PATTERN ROTOR HAT CALIPER ROTOR MOUNTING BOLT KIT LEFT HAND SIDE SHOWN BRAKE PADS SRP DRILLED / SLOTTED PATTERN ROTOR EXISTING UPRIGHT / HUB ASSEMBLY CALIPER BRACKET MOUNTING BOLT KIT CALIPER MOUNTING BOLT KIT CALIPER MOUNTING BRACKET WHAT IT IS, HOW IT HAPPENS — AND WHAT TO DO ABOUT IT. W e've all seen the driver ahead of us dab the brakes before a big braking zone, but why, you might ask? It's to remedy a condition called pad knock-back. Pad knock-back occurs when pads lose contact with the rotors, and it can be accompanied by excessive caliper piston retraction. There are a number of reasons why it happens, which we'll get into a bit later, but that quick flash of brake lights indicates the driver is doing a couple of things. One, he's finding out what kind of pedal he's got. When knock-back occurs, the master cylinder has to push the caliper piston out farther than usual, which results not only in a spongy brake pedal but a longer than usual stroke. A second stroke is often required to begin slowing the car. In addition to testing the pedal, that little dab of the brakes pushes the caliper pistons out and puts the pads back into contact with the rotors, giving the driver a proper pedal when he applies the brakes fully. "Pad knockback is one aspect, and sometimes it can be the result of excessive piston retraction," said Carl Bush, product applications and customer technical support representative for Wilwood Engineering. "It's sometimes hard to differentiate between the two, but they end up being the same result, which requires that extra long pump by the driver to finally get the pads engaged against the rotor." Knock-back can result from loose or worn parts, such as a hub bearing, a rotor that has become distorted, run- out or anything that would cause the rotor to vacillate and cause them to push the pads farther away and the caliper pistons further into their bores. Bush points out that excessive retraction also can take place due to centrifugal force in a hard turn, or driving the car over bumpy sections of track or the "gatored" curbing at some racetracks. "In a real hard corner, if there's room available, the parts are trying to move toward the outside of the turn," Bush said. "Another good way to get things moving around is a good trip over the rumble strips. All of a sudden things get bounced around and the next corner after the rumble strips you've got a long pedal stroke. And sometimes that pump of the pedal is the only fix." Bush said Wilwood has discovered a number of things that can help dampen and minimize knock-back and excessive piston retraction. One fix the company has used on its high-end calipers for oval track and road racing is to put damping springs behind the pistons in the caliper bore. According to Bush, damping springs won't prevent knock- back due to worn bearings or other causes for the rotor moving back and forth, but they will stop some of the knock-back that occurs due to a bumpy racing surface, and PAD KNOCK-BACK Ensuring that components such as rotors and hub bearings are straight and true, and free of play can reduce instances of pad knock-back. 20