Originally Posted by majohnst
... The problem only happens when the chain is in the larger cogs on the rear cassette. I have a triple, and the gear jump problem only happens on the 52 and 42 front cogs. So I am assuming it has something to do with the angle of the chain. Right before the jump happens, the front cog doesn't catch the chain correctly and snags. This causes the rear hanger to move forward. On a side note, the bottom of my hanger seems to be tilted slightly towards the wheel, so it is not perfectly vertical. Would that be a part of the problem?
The chains on many (most?) bikes will grind or shift toward the center of the cogset when you backpedal in a cross- or near-cross-chained combination, such as large-large or small-small. You should be able to backpedal pretty freely in most of the other gear combinations.
That derailleur hanger needs to be straightened (slowly and gently) so that the entire assembly hangs vertically.
The only derailleurs which are SUPPOSED to shift while backpedaling are Tullio Campagnolo's Cambio Corsa and Paris-Roubaix.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069