Join Date: Apr 2002
Location: Montréal (Québec)
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If I understand correctly, the derailleur hasn't been moved and doesn't move by itself. It's the chain that moves on the cogs when you backpedal. So there is no relation to a bent or unbent derailleur hanger.
When one backpedals, it's the upper section of the chain that shifts. Basically it happens because at some point, the chain is oblique to the cogs. Reasons:
– Oblique rear wheel. Especially possible on an older bike with horizontal dropouts. If the wheel is not installed properly, it will be oblique and the freewheel or cassette will be oblique. On newer bikes with vertical dropouts, it's almost impossible to do.
– Oblique chainline. If the cogs are too much inboard or the crankset too much outboard, then the chain is oblique. No problem when you pedal forward, because the derailleur keeps the chain in place. But when you backpedal, the oblique chain tries to shift and may succeed when it reaches the "ramp" which is precisely designed to help indexed shifting. the only solution might be to put the crankset more inboard or outboard, probably with a washer on the bottom bracket or with a spindle of a different length.
- Bent cog. If you have a bent cog, it may help derail the chain. Slowly pedal forward or backward and look at the chain and/or cog to see if it wobbles in one point. If that's the problem, the only solution is to change the cog or cassette.
In all those situations, a clean chain might help.