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I have a 7 speed Jamis Boss Cruiser, 7 gears in back and one up front. I've been using it fine for several months now. Lately, the chain is coming off of the single front gear, usually after stopping at an intersection when I start back up again.

I took it to a bike shop and the guy claims the front gear is not the right type of gear, its the type of gear that's meant to work with a derailleur. He says that's why the chain comes off. I'm disinclined to trust him since it's worked without problems for almost 8 months. If it was the wrong gear, I would have been having problem before now, right? Is it true that there is a type of front gear for use with derailleurs and a type of front gear meant to be used as a single front gear?

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How many miles have you put on it? It's possible the rear sprockets are hooked and are causing the chain to "jump". A new chain would likely clear it up. –  Daniel R Hicks Feb 13 '13 at 0:19
    
Maybe 300 miles so far? Is the rear sprocket problem you mention something that requires cleaning or replacement? –  Nick Feb 13 '13 at 3:36
    
300 miles is nothing, especially if you got the bike from new. You shouldn't be looking at replacing any parts just yet, for sure. –  PeteH Feb 13 '13 at 12:42
    
Yep, 300 miles is no big deal. Somewhere between 2000 and 5000 miles is where wear would begin to be a problem. But the rear derailer should be cleaned and checked to be sure it's not bent, and the "freehub" should be checked to be sure it's not binding. –  Daniel R Hicks Feb 13 '13 at 12:55
    
So the rear derailleur can cause the chain to come of the front gear? –  Nick Feb 13 '13 at 18:24
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2 Answers

up vote 10 down vote accepted

Yes, there is a difference between front chainrings for derailleur equipped bikes compared to bikes without a derailleur.

Basically a derailleur suitable chainring "wants" to fall off. It's designed so that the chain is happy to climb onto the gear and also fall off the gear. There are various ramps for the chain to engage into. Ramps and Pins

Non-derailleur chainrings are much simpler and are just the straight teeth. Single speed chainring

To solve your problem, you could:

  • Check your chainring and potentially replace it with a single speed chainring
  • Get a chain keeper like this one
  • Or both!

More information is on these questions:

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Ramps and pins shouldn't ever derail the chain off of a chainring. If they did, the chain would constantly drop off on doubles and triples and cause it to skip. Once you shift to a chainring, a well maintained drivetrain will stay on that chainring (regardless of whether the rings are ramped and pinned!) –  WTHarper Feb 13 '13 at 5:48
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Much more significant than ramps and pins on the side (which is to say maybe actually significant) is the profiling of the teeth themselves. If you look in the above pictures you'll see that some teeth are sharp and some have flattened tops, and, of the sharp teeth, some tilt one direction and some the other. These are designed to give the chain a "kick" at the right point in a shift. –  Daniel R Hicks Feb 19 '13 at 19:35
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Although this has been answered I thought I'd chuck another suggestion in there. As Mac said there is a difference between chainrings meant for deraileurs and those meant for single-speeds. That's not to say you can't use the former without a deraileur, but as it is easier to derail the chain you should check your chainline is well set up. If your front ring is aligned too much to either side of your rear cassette, when you shift to a gear on the opposite side (e.g. the highest or lowest gear depending on the misalignment) it could cause the chain to jump off the front ring. This is because the chain is being pulled to one side.

Ideally your front ring should be more-or-less in line with the middle cog on your rear cassette.

You say the problem usually happens when you stop at intersections. This could be because you are shifting into a low gear to ready yourself for setting off again and it's this situation that causes the most lateral force on the chain. Combined with high force on the crank from accelerating and possible wear on the front ring increases the chances of derailment.

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