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> I think the reason for grit in brake pads is not clearly understood. The grit in the brake pad is
> primarily aluminum and aluminum oxide. I have not found rock particles in these agglomerations.
> Brake pads that generate hot-spots from inhomogeneity can cause excess local heating from which
> minute bits of metal transfer to the pad. Once started, the 'tool bit' continues to cut more metal
> and it piles up in the pad, gradually destroying the rim while reducing braking.
The worst pads I've seen with this behavior (admittedly, I haven't used a lot of different brands)
were the Shimano "M Series" black pads. Being somewhat of a pack rat, I have a dozen or so of these
(used), looking at them, embedded specs of aluminum are quite visible, usually associated with a
scored track. When I dig some of the larger ones out, and forcefully rub the contents on a sheet of
paper, there is eventually left a hard piece of grit, which does not mark the paper or
crumble/abrade easily. It may be aluminum oxide or road grit, I can't tell.
> Kool-Stop is not entirely immune to abrasive lumps but even then, the obstruction can be removed
> by pumping the brake hard. THis is sometimes required on wet dirt roads. I have not had to clean a
> brake pad manually since switching the Kool-Stop Salmon pads years ago.
I just returned from a 3-day (~300 mile) tour of rural New England roads. The conditions were wet,
and the roads very gritty. Coincidentally, I had just replaced the front wheel rim before I left. I
just scrutinized the pads and rim, and found not even the slightest evidence of rim scoring or bits
of metal in the pad. My bike has Kool-Stop "salmons" on cantilever brakes.
I ride weekly during the winter, over the same route, with generally the same group of riders. It
may be my imagination, but on wet rides, when the roads are sanded, especially, when you can hear
the grinding sound of grit on the pads, it seems like my brakes stop making the sound quickly, while
the others (not Kool-Stop) persist.