Issue link: http://read.uberflip.com/i/634989
help control pad retraction. Another component Wilwood has developed to minimize knock-back is an inline check valve, a residual pressure valve, which helps stop individual line pressures from bleeding back. You can install it anywhere in the brake lines, but it's usually best at the end of the chassis hard line where the flexible brake hose attaches. "I actually have race teams that are using both damping springs and residual pressure valves," Bush said. "If nothing else, even if we don't get excessive knock-back, it keeps the pedal nice and tall for the driver, so it retains a real tall, consistent height." There are also other sources for knock-back on live-axle cars. If you have a car that's using production axles and spindles, they just might not be made for the lateral side-loading forces in racing, especially if you are using a larger-diameter rotor, but you're still using the production spindle and hub. The larger that rotor is in diameter, if it moves back and forth, it's going to move farther than a smaller-diameter rotor because it's farther off centerline. Cars with live axles can get knock-back on rear brakes, too. GM 10- and 12-bolt rear ends, as well as Ford 8.8-inch axles all have flanged axles and C clips to hold the axles in place. Bush recommends eliminating the C clips and going to full- floating, double-splined axles. "If you have a good sticky tire that's really gripping the race track, and you have a fairly heavy racecar that's really trying to push sideways on the front spindles and the rear axles, things move," he said. For racecars that use OEM calipers, the good news is that the slider-pin setups tend to be a little more forgiving because it can move if you do have a small amount of runout in the rotor or bearing play. OEM hardware such as the springs and clips don't do much to prevent knock-back, Bush said. Those parts are more for preventing squeaks and pad rattle in road cars. If you're getting knock-back with an OEM setup, Bush recommends using less curbing if possible, or the well-known method of dabbing the brakes before you enter a braking zone. "It becomes very difficult sometimes when the driver is having that experience, whether it be excessive piston retraction or pad knock-back, to identify exactly what the culprit is," Bush said. "One thing they can be sure of, though, if the pads are getting knocked back, it's typically not a problem with the caliper. It's typically what's causing movement to take place between the caliper, however it's mounted, and the brake rotor, however it's mounted. If that rotor or that caliper is moving side to side, it's moving those pads back, requiring that extra long stroke of pedal to reload the pads again on the next pump of the pedal." SN Under the right conditions, aftermarket and OEM calipers alike can be subject to knock- back. Pad knock-back occurs when pads lose contact with the rotors, and can be accompanied by excessive caliper piston retraction. RESOURCES: www.wilwood.com 21