Pad knockback?

[Brad Roberts]

Watch the RSR's and Cup cars closely during a pro race. You will see what we see in the data. We call it "The Pro Tap". They will reset the pads with a tap right before they actually need to use the brakes. Pat Long does it, Johannes, Jorg, Stanton, Pobst. The cars with floating rotors do tend to do it less, but they still do it. You will see it less in a sprint race and more in the 2:45 or longer races. I beleive it became more common when everyone went to the quick disconnect no fluid loss couplers a few years back. Instead of changing pads, the whole caliper is replaced during the pit stops for the enduro's.

This guy's problem sounds like multiple issues occuring at once. I'll consult the Brembo race engineer I work with and the Performance Friction guy's I deal with and see what they have to say about.


B

[Walt Fricke]

After years of good service, I started having what might be the opposite problem - pads which were hanging up or not retracting, leading to unpredictable pulling. Inspecting the calipers, I found wear on the rears at the bottom of the pads.



Temporarily I smoothed that out with a file, but long term it seems I should do what Wilwood does: has a stainless insert in the aluminum caliper to take the sliding action.

The pistons, after 15 or so years of hard use, were a bit mungy (though they never have leaked):



But they polished up pretty well:



I have taken to doing some prep work on the sliding edges of new pads:



Following a suggestion from Wilwood, I also made up "spacers" to use when pads are half worn or so:



It seems advantageous to keep a minimum of piston outside of its caliper cylinder. Less chance of anything binding, and less sideways leverage on the pistons in their travels.

Still, absent warped rotors, or something which otherwise has them not coplanar with the caliper, I can see why this is so frustrating. Speaking of being coplanar, we think of that as a rotor which is warped, and thus alternately too wide and too narrow with respect to being squeezed between pads, so when released the pistons are forced too far back on both sides. But, among other things, one ought to be able to feel pulsations with warped rotors, no? What if the caliper is mounted at an angle, either fore and aft, or up/down, relative to the rotor?

Is it possible that differential heating of the caliper (or its mounting) could cock the caliper?

What about warping or flexing of the brake pad backing plates?

I assume that the brakes on the Nissan use differential sized pistons for the leading and trailing edges of the pads? Do the pads show taper wear, either top to bottom or inside / outside? Is there anything of interest in the wear marks the pistons make on the back of the pad backing plates?

In the old days, before I had the heat under control, I'd do that little left foot tap before a heavy braking zone. But it was mainly to see if the brakes were there, so I could start the mad pumping early enough. Tended to upset following cars to see the brake lights where they didn't belong, though.

[KTL]

Walt,

Nice documentation of your own small problems. I agree pad prep should be something people always look at. Depending on who makes the pads, one may find a build-up of paint or a poorly stamped pad backing plate. This definitely causes poor pad movement.

Over the winter I did some brake work on my brother's 2001 Silverado HD. One thing I noticed was how poorly the pads fit in the caliper mounting saddle. Despite cleaning & filing the contact surfaces on the saddle, the Delco pads still would not fit w/out binding. I filed the edges of the pad backing plates to the point where I got good clearance to allow the pad to freely move in the saddle's pad guide slots. While not a track/racing application, I think the situation I described still applies to anyone, whether one has the crappy sliding calipers or fixed calipers. Still need to ensure the guides or a sloppy pad tracing isn't hanging up the pads.