Solving Pad Knockback

[silver69camaro]

As our R&D Camaro has progressed, we've been increasing front and rear grip by a fair amount. Late last year, I've been battling some pad knock-back that's been driving me nuts. This car uses modified C6 front suspension with a 9" axle and tapered bearings. Knock-back is more noticeable if you have manual brakes, because there is more pedal travel needed to overcome the pad-to-rotor gap. If you are experiencing a soft brake pedal after a hard corner, follow this to hopefully solve your problem.

First, find out which side is having the problem. Does the brake go to the floor immediately after a hard left turn, or right?

Second, find out if it's the front or rear. I do this by turning hard, then shutting the engine off while in gear, coasting to a stop. Jack the car up, remove the wheels, and you should see daylight between the pads on the offending corner. Typically you'll see .010" gap or so - it doesn't take much. Obviously this requires an area you can safely do this in. A guy could find a parking lot at night and run a tight circle to accomplish the same thing.

Now that you know if it's front or rear, you can start to diagnose the problem. If it's a front wheel, replace the hub assembly if you have C6 hubs. If you have a spindle pin & bearing, the bearings and races probably need to be replaced.
The rear is a bit more difficult. Possible causes are:

1. Axle flex: Happens typically if R-compound tires are used on stock axles with heavy cars. Aftermarket axles tend to have more material that negate this problem.

2. Improper bearing preload: If the bearing isn't held tight enough in the housing end (press-on bearings only), it can move around causing the rotor to kick the pads into the calipers. Try adding some 12-bolt carrier shims between the bearing and housing end.

3. Excessive c-clip clearance: For guys that run c-clips, you may need to run thicker clips to keep the axles walking laterally. I've heard of people making a thick clip, then shaving it down a couple thou at a time until it fits snug.

4. Axle retainer flex: For axles with press-on bearings, check the axle retainer plate. This was the problem I was having, which the retainer would flex enough to allow the axle to move. The Wilwood retainers are pretty thin and are prone to flexing. Mine was pretty bad, and replaced it with a 1/4" retainer plate. Problem solved! Currie makes a nice thick retainer for about $16, but is a full-circle type retainer so you have to put it on before the bearings are installed.



Here's the set that I made. It ended up working very well and I didn't have to remove my bearings.

Brakes in question


Old, Bent Retainer Plate


New Retainer




Hopefully this helps. It's nice to fix this problem without resorting to piston springs or residual valves.