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Abnormal pad wear with AP 6-pots

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Last year I switched from the Brembo Pro Race 4-pots to the new AP 6-pots with the 25mm pads. Since I haven't really stressed the brakes much with the tracks I have been running everything looked pretty much like new up to now. But after doing a brake bleed before my next event I noticed something disturbing. Looking more closely at the Ferodo pads I noticed severely uneven pad wear that I didn't notice earlier this year. It looks like the thickness is about 23mm at the top of the caliper where the smaller pistons are and about 17 mm where the largest piston sits at the bottom. I didn't take pics but the pad surface appears perfectly normal with no cracks or discoloration. It was like that on all 4 pads. I do remember the brake "feel" was different the last two TNiA events and the pedal seemed a bit longer and grab was slightly less forceful.

Could there be air in the calipers not allowing even pressure? I use a suction bleeder with my air source which always seems to do well. Never saw this before.

I reversed the pads to see if I can get them back to a more even wear pattern but that's not fixing the problem. Any ideas?
 
1,163
2,121
Exp. Type
Time Attack
Exp. Level
5-10 Years
SoCal
I’ve read several places that 6 pot calipers can wear a wedge shaped pattern on the pads, which rotating the pads as you described is an acceptable remedy to prolong pad life. I believe it’s more common in 6 pot calipers that have equal sized pistons than in ones that have the 3 different size pistons from big to small. I’m not familiar with the AP calipers specifically, but is it possible they’re installed on opposite sides of the car, which may be counteracting the staggered piston size? The PP Brembos have both bleeder screws on top, so really easy to tell that they’re on the correct side. I can’t recall if it’s that obvious on the AP’s. Just a SWAG.
 

Norm Peterson

Corner Barstool Sitter
939
712
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
That's the way I'd read it. Because there is a fraction of an inch distance separating the pad friction surface from the backing plate (where it bears against the caliper proper) a moment is generated that would be resisted by slightly more unit pad contact force at the leading edge,somewhat less at the trailing edge, and some sort of variation in between. Wear rates could certainly differ, leading edge to trailing, as a result.

I'm assuming that their definition of 'leading in the disc rotation' means where the rotor rotates into the caliper.


Norm
 
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1,246
1,243
In the V6L
@Norm Peterson AND @67GTA BOTH called it. I've done lots of brake installs, but when I read the original post, I didn't notice the problem, but it's pretty clear what's wrong. Here's a piece cut out of the installation drawing for the AP Racing CP9665 six piston racing caliper. It shows the direction of rotation of the disk: the small piston leads and the large piston trails, which, on the front of a Mustang, means that the small piston is at the bottom and the big piston at the top. In the original post, the words "the thickness is about 23mm at the top of the caliper where the smaller pistons are and about 17 mm where the largest piston sits at the bottom" pretty much tell the tale - the calipers are upside down. Swap them side to side (I believe they're made with bleed screw holes up and down so you can do this) and all will be well. You'll need new pads, but at least they'll wear evenly.

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Thanks for the replies. I will have to go back and re-check the caliper mounting. I did have the instructions in front of me when I was mounting them and thought I had them correct. That could explain it. Then again, it would not be the first time I had instructions and still did it wrong.:rolleyes:
 

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