Join Date: Oct 2004
Location: DC / Maryland suburbs
Bikes: Homebuilt tourer/commuter, modified-beyond-recognition 1990 Trek 1100, reasonably stock 2002-ish Gary Fisher Hoo Koo E Koo
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Originally Posted by donrhummy
It was like this:
The chain on top of the chainring stayed on the chainring and underneath the chainring the chain got sucked in between the frame and the chainring.
I see two possibilities now: bent chainring, or slack chain.
If the chainring is bent, the chain may slip off the bottom of the chainring, while the correctly-adjusted front derailer prevents the chain from falling off the top. Check if there's more than one or two mm of wobble in that chainring (you may be able to bend it back, see Sheldon's site for tips).
The other possibility is that the chain has too much slack in it (perhaps it is too long, or the rear derailer does not have enough capacity to take up the slack). The part of the chain between the bottom of the chainring and the bottom pulley of the rear derailer is not under stress from pedaling, so if the derailer spring doesn't pull on it enough, the chain may go slack and it can simply "bounce" off of the bottom of the chainring. To check this, try shifting between the gear combination where you had a problem, and a larger chainring. Feel the tension in the bottom of the chain by hand. Does the rear derailer maintain adequate tension even with the small chainring?