I had been trying to diagnose a squealing and sticking problem with my Giant MPH3 front disk brake and finally (after 6 damned months)have discovered what was wrong.

I’m not looking for any assistance here but I think it’s worth noting my observation nonetheless.

Over six months my front brake had been a mess (squealing, piston sticking in caliper, pad not retreating from disk) and I had tried everything to settle it down. This fault probably applies to most types of disk brake.

During overhaul (caliper off, pads out, caliper disassembled, pistons removed, seals replaced, system bled etc) I noted faint rings on the side of the piston. I also noted marking on the inner face of the caliper in which the piston sits. I went back to the pads to find that the friction surface was ‘sloping’. What I now think is that when the brake is applied, the piston forces the pad onto the disk, but because the friction surface is not level, the pad effectively tries to force the piston to move ‘out of true’. What I mean to say here is that instead of moving in and out at 90 degrees to the plane of the disc (as per manufacture), the piston is being forced to move at – say – 85 degrees. This is why it is catching the caliper housing and causing the damage mentioned earlier. It is almost certainly where the squealing has come from.

I have bought several sets of MPH3 pads before which have not been level and now wonder what on earth is going on here.??? I never really gave it any thought until today. Surely all disk pads should have the same thickness throughout ?

The pads material was wedge shaped on purchase? V. odd.

they wear down crooked if not aligned correctly…

More likely the pad has been sticking in the calliper and tipping causing it to wear into a wedge I would have thought

Hi, yes pads slightly “trapeezoid” from purchase. Never really gave it much thought until today.

It seems so obvious now, but I just didn’t think.

More likely the pad has been sticking in the calliper

do you mean “piston” ?

… or the drop-outs/wheel, caliper or caliper mount aren’t perfectly true to one another causing uneven wear on the pad.

No idea what the tolerances between the piston bore and the piston itself is, but I doubt if you could achieve 5-degrees of misalignment – if all the pad material was worn away on one edge and as-new on the other (i.e., as bad as it could be), a fag-packet calc says that’s around 5-degrees at the absolute limit.

<shrugs> Dunno – I’ve had pads catching the caliper body as the brake operates due to burrs on the backing plate cause a similar thing, as TJ suggests

btw, I could accept misalignment as the cause if the friction material had worn in a way which is consistent with that theory. But (having retrieved said pads from workshop bin : real time blog) I can say that if you put the pads together as they sit in the caliper then they are fat-to-fat and thin-to-thin. I would expect a fat-thin and thin-fat combo if misalignment was the case. Sorry if that makes no sense. 🙂

Perhaps the metal backing of the pad has been jamming in the caliper causing the piston to push on it unevenly?

No SFB – I mean the pad sticking in the calliper as I said. I have seen this with cheapo pads – they are not free to move in the caliper body as they need to be able to do

Formula brakes seem to be very prone to this. The fit of the piston in the bore of the caliper is very sloppy allowing the pistons to “find their own way” Got some in the workshop now that are down to the metal on the inside edge to 1mm of pad at the outside edge.

never had a problem with formulas me self, they stop me all too well.

Don’t know if this applies to your exact brakes but some calipers are designed to do this. M965 XTR calipers apply more pressure to the rear (non leading) edge of the pad to avoid squealing. If you look at one of those calipers you see the piston does not press in the center of the pad, its towards the non leading edge. The same applies to the old type Hope M4’s one piston is smaller than the other. My M965 pads regardless of manufacturer always wore unevenly

to ben1979 … these are not “compound pot ” calipers … they are single pot (one on each side) simple. I believe they are a licensed copy of the hope c2.

to TrevorB : that makes sense. So I guess these calipers are sensitive to imperfections in the plane surface of all the other components in the chain … i.e. “weakest link in the chain” (well sort of)… Well at least I now know… after 6 months of squealing !!