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I've got a mountain bike which occasionally drops the chain off the lowest gear, and then the chain gets stuck in the bottom bracket. This happens while pedalling.

Once the chain is there, it takes either a lot of pulling (potentially bending the chain), or removing the crank to get the chain out.

Either way, it's a pain when it happens on the trail.

Note: I'm not talking about chain suck.

And I've seen products like the Deda dog fang chain watcher and Jump Stop, anyone have any experience with these?

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1  
I can't work out why the gap is designed just right to jam the chain. –  Tom Hawtin - tackline Sep 1 '10 at 16:50
    
@TomHawtin Murphy's law. –  Trey Jackson Sep 1 '10 at 18:33

2 Answers 2

up vote 3 down vote accepted

There is a bottom limiter screw on the front derailleur which may be of some help. Tension it up till it just goes far enough to shift. It's also possible that your rear derailleur isn't taking up the chain slack properly and is letting it bounce as you go over bumps. This would cause the bottom of the chain to slap as it goes forward to the front ring and could cause it to come off. In that case you might see about replacing the rear derailleur with one on which the tension springs are newer and reduce the slap.

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Most derailleurs have a screw to increase the chain tension -- somthing to try before replacing it. –  darkcanuck Aug 31 '10 at 23:44
    
true, true forgot to mention that. –  curtismchale Sep 1 '10 at 16:44

First off, is it falling off when you shift, or is it more of a "big jump knocks chain off chainring" type issue?

If it's falling off when you shift, you may want to adjust the front derailer's low-gear limit stop, which is probably a small bolt/screw head of some sort on the front derailer (but which one will depend on model/age...)

If it's falling off otherwise, there's several options out there for chain watchers. Here's 3: http://www.sheldonbrown.com/harris/derailers-front.html#deflectors

See also: http://www.sheldonbrown.com/derailer-adjustment.html#front

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