Join Date: Apr 2002
Location: Montréal (Québec)
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There may or may not be a solution to your "problem".
The may-be solution:
Park the bike and look carefully at your chain when you backpedal. Does it fall down from largest to second largest cog when the cog is always in the same spot? A good place for the fall would be at one of the ramps used to help the chain go up when you shift. If that's the case, it's most likely normal, as the end of the ramp will also help the chain to shift down when you backpedal. That unfortunate turn of even happens especially when the chain is very oblique, which is why it happens mostly on the outside rings.
But here is another catch: look carefully at the offending cog and check for straightness. Large 8- and 9-speed cogs are fairly easily bent, and a crooked cog will allow the chain to fall down. If that's the case, you have two options:
– accept the problem and learn NOT to backpedal;
– change the offending cog or the whole cassette. If you do so, you should also replace the chain, unless it's a very new one.
BTW, as it happens when you backpedal, the derailleur has nothing to do with it. The derailleur allows you to shift (by moving right and left) and helps to keep the chain in check (by being aligned with the cog) as long as you pedal forward. But when you pedal backwards, the chain isn't guided by the derailleur.