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Diagnosing why a chain falls off

Old 04-16-16, 10:06 PM
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Diagnosing why a chain falls off

A friend of mine had her chain fall off while biking. I know some basic bike repair, but I'm not the best at diagnosing. Her chain seemed to be on the second of the three front sprockets when it decided to jump off. I'm not sure if she was in the middle of shifting or not.

Is this likely a limit screw problem?

One thing I noticed was that her rear derailleur did not seem to have much slack. There was not much of an "S" shape to the chain. Could it be that she needs to add a couple links to her chain to make it longer? Thanks.
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Old 04-16-16, 10:19 PM
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Anything happening while in the middle ring of a triple will not be related to front derailleur limit screws.

Some quick, easy things to check are chain and chain rings. On the chain you're looking for twisted or maybe stiff links. Chain rings might toss a chain if they have bent teeth or a fairly large lateral bend in the chain ring.

Pictures, or at least a better description of the bike might help further diagnose.
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Old 04-16-16, 10:35 PM
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I have my doubts that a too-short chain could be the problem, either. That would be more likely to hold it in place on the chainring, rather than letting it bounce off like a chain that's slack because there's too much of it.

I think taking it to the shop is the best course of action here.
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Old 04-16-16, 11:03 PM
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The best advice I can give you is to not start grabbing a straws and making theories until you know more. Not about bikes as much as about the details of the conditions under which the chain came off.

Even a bicycle expert with years of experience can't come up with any meaningful diagnosis based on the skimpy information you offered.

Anyway a single event doesn't mean anything, she might have shifted under load, or any number or things. For the moment, chalk it up to an isolated event and wait and see if it happens again. If so, try to note any details such as which gear she was in (both front and rear), whether she was shifting at the time, was the chain under load, such as first starting to climb, and shifting late, etc.

Finally, if you do ask for help, don't clutter it up with your theories, that can make hings harder by misdirecting people and having them look one place rather than stepping back and starting fresh.
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Old 04-16-16, 11:19 PM
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Thanks everyone. I'll see if I can get any more info from her.
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Old 04-17-16, 05:25 AM
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Can you take it for a test ride and run through all the gears ?
Firsthand info is much better, and will cut down on wild guesses.
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Old 04-17-16, 02:08 PM
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Originally Posted by Homebrew01
Can you take it for a test ride and run through all the gears ?
Firsthand info is much better, and will cut down on wild guesses.
+1. Andy
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