We’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 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 hardline 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.”