If you’ve ever driven on an autocross or road course, you’ve most likely been a victim of brake pad knockback. It’s probably one of the scariest things you can experience on a track or even while driving on the street, for that matter. You go to hit the brakes and … nothing. The brake pedal goes to the floor and the car continues on its merry way. You panic, let the pedal raise, which feels like forever at that point, and hit the brakes for a second time when—thankfully—the brake pads squeeze the rotors and you are overcome with a feeling of relief as the car slows down as originally planned ... hopefully before hitting anything or missing a corner. But like all of us, we are accustomed to and expect the brakes to work on the first hit.
Due to the development of wider and stickier tires over the past decade or so, our muscle cars are pulling lateral g’s like never before … on street tires. Add in stronger and more functional suspension components that allow aggressive cornering along with larger diameter brake discs and you have a recipe for a big bowl of brake pad knockback.
So what exactly is brake pad knockback?
Bret Voelkel of RideTech explains: “Brake knockback happens when the spindle [or rear axleshaft] flexes and allows the caliper to flex with it. When the caliper moves in relation to the rotor, the brake pads are then pushed back into their bore, creating clearance between the pad and the rotor. When you hit the brakes the next time a large part of the pedal travel is then required to push the pads back against the rotor to begin the braking activity. While the flex and resulting clearances are small (a few thousandths of an inch), the result is a momentary butt-puckering loss of braking action. On a racetrack at over 100 mph with a turn looming in your future, this is heart stopping!”
Bret goes on, “There are several band-aids for this issue depending on severity, and a couple of real solutions. In mild cases, a simple RPV (residual pressure valve) can be installed between the master cylinder and the caliper. These typically come in 2-pound and 10-pound calibrations. The 2-pound is usually appropriate. Another band-aid is simply to get in the habit of double pumping the pedal in preparation for a turn. This can work OK on a road course where you may have some time to accomplish this, but it’s a terrible solution on an autocross where you have but a fraction of a second between turning events.
“There are some real solutions, as well. Some brake companies can incorporate ‘knockback’ springs that go into the piston and keep a small amount of pressure on the pad. Stronger spindles for the front may be required to minimize spindle flex and unwanted caliper movement. On the rear, a common solution is a floating brake system. This is where the axle is divorced from the wheel hub and ‘floats’ independently of any lateral load imposed through the wheel hub.
“We found that as soon as we started using a sticky tire, the lateral g loads increased to the point where we experienced brake knockback. As the cars got faster, the g loads increased and the problem got worse. At this point, our fast track cars use our recently developed Track 1 billet modular spindle that incorporates a C7 bearing hub and floating rear brakes. The street cars still use a pin style spindle and non floating rear brakes, even if they see some limited autocross use.”
Matt Jones, lead mechanical engineer at Art Morrison, tells Chevy High Performance, “We try to tackle the problem on the front end. We have the customer’s wheel/tire data, weights, wheel specs, etc., all up front. In the design stage, we look at what we can do to center the wheel bearing in the tire’s contact patch to reduce these issues.” Matt goes on, “For the more extreme loads (which isn’t too common, but more so than it used to be) we resort to a unit bearing up front as spindle pins will bend inevitably.
“The press-on bearing used on 9-inch axles do present some issues. A couple helpful ideas we have found are: matching bearing cup and races, thicker bearing retainers (the piece that bolts to the housing end that retains the bearing) of a quarter-inch or more and are a one-piece construction, full floater, and going with one of our independent rearend suspension systems.
“Obviously, full floaters and an IRS cure the issue, but minimizing loads on the bearing in the first place is key.”
Some of us just deal with pad knockback and have grown accustomed to tapping the brakes twice prior to approaching a hard corner while on an autocross or road course, but having to do the “double tap” only adds unnecessary time to our course laps. Adding a mere tenth of a second to your lap time can be the difference between winning and losing, and qualifying or sitting out the next round. Trust me, it’s not fun watching from the sidelines as the person that beat you by 0.01 of a second continues to race.
If you’ve ever experienced brake pad knockback for the first time, then you know it’s just about the scariest thing that can happen behind the wheel of your car. While autocrossing Orange Krate, our second-gen Camaro project, we were experiencing horrible pad knockback. Doing the double pump on the brake pedal was the norm and we just dealt with, but we knew it was killing our lap times, so it was time to make a change.
We went to Baer and looked into installing their Tracker full floating axle conversion system so we could fix the problem once and for all. Now, this isn’t your typical driveway installation piece, so we packed up our rearend and new parts then headed over to Currie Enterprises in Corona, California, to have them tackle the job for us. Their staff of professionals have done tons of floater rearends for customers in the street/track performance and off-road industry and have been in the rearend business since 1959 when Frank Currie started building differentials in his home garage. Needless to say, we were in good hands.
Follow along as the crew at Currie Enterprises shows what’s involved in taking our rearend to the next level of performance braking while knocking out knockback. CHP
Photography by Nick Licata