The maddening thing is that the original shard is usually a bit of aluminum that came off the rim itself, and then it increases in size as it accumulates more aluminum that the first bit of aluminum scrapes off the rim. So if you continue to use that brake, the rim effectively destroys itself.
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My C&V Bikes:
1972 Bottecchia Professional, 1972 Legnano Olympiade Record,
1982 Colnago Super, 1987 Bottecchia Team C-Record,
1988 Pinarello Montello, 1990 Masi Nuova Strada Super Record,
1995 Bianchi Campione d'Italia, 1995 DeBernardi Thron