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Old 08-02-08, 05:30 PM   #1
big scholar
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Chains keeps getting derailed -- Nexus 3 speed

Hi guys. Wondering if anyone here could help me out: the chain on my wife's bike got derailed twice in the span of a six mile roundtrip to the market this afternoon. Her bike has a Nexus 3 speed internal gear hub with a coaster brake. Anyone know what the problem might be? Simply a matter of the chain having gotten stretched?

Any input is appreciated. Thanks!
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Old 08-02-08, 05:53 PM   #2
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Is there a chainguard? Is the coaster-brake arm firmly bolted to the chainstay? Most likely the rear-wheel needs to be slid back in the dropouts to tighten the chain so that there's no slack for it to hop off the chainring.
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Old 08-02-08, 06:08 PM   #3
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Thanks for the reply, DannoXYZ. There is a chainguard, and the coaster brake seems to be firmly bolted. There is about a 1/4 inch of between the hub and the dropout, and I have noticed that the chain seems pretty slack. I'll slide it back. Thanks for the help!
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Old 08-02-08, 06:18 PM   #4
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Originally Posted by big scholar View Post
Hi guys. Wondering if anyone here could help me out: the chain on my wife's bike got derailed twice in the span of a six mile roundtrip to the market this afternoon. Her bike has a Nexus 3 speed internal gear hub with a coaster brake. Anyone know what the problem might be? Simply a matter of the chain having gotten stretched?

Any input is appreciated. Thanks!
Since there is no actual derailleur involved, this means that something is out of alignment. In addition to what has already been offered, I would add that two other possibilities could be a warped/dented chainring (usually due to the chainring getting a good whack, at some point) and a bad bend in the chain itself (usually caused by having the chain derail and then pedalling hard after the chain gets wedged). Both of these can be sussed out by cranking the drivetrain and watching the chainring carefully. Any hops/twists/bends will be pretty obvious.
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Old 08-02-08, 06:29 PM   #5
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Thanks for the reply, DannoXYZ. There is a chainguard, and the coaster brake seems to be firmly bolted. There is about a 1/4 inch of between the hub and the dropout, and I have noticed that the chain seems pretty slack. I'll slide it back. Thanks for the help!

Due to the fact that chainrings are almost never perfectly round, a properly tensioned chain (on a fixie or single speed) will feel tight for part of the revolution of the chainring and then slightly loose for another part. As you crank the drivetrain, you should see the chain alternately droop a bit and then get tight and straight. Tensioning a chain sounds pretty easy, but, in fact, it's not. Pull the chain tight on the drive side by pulling back on the axle and then tighten the drive side axle bolt just a bit. Then center your wheel with the non drive side of the axle and tighten that bolt a bit. Now check for proper tensioning as I described above. If it's good then tighten each side a bit at a time and alternate to the other side, or, if it's bad, go back and start over. Even after a LOT of practice at this process, I rarely get it correct on the first try. Have patience and assume it's not going to cooperate on the first shot.

This is exactly why good quality BMX bikes have chain tensioners on the drive side dropout. Something anyone considering a BMX purchase should be looking for.
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Old 08-03-08, 06:09 PM   #6
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Thanks for the input, guys. Tried pushing the wheel further back in the dropout, but the chain derailed again while we were out riding. Finally just a mechanic friend check it out. Even with the wheel adjusted, there was too much slack, and when we removed a link from the chain, it was too short. We just ended up installing a new chain!
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Old 08-03-08, 07:08 PM   #7
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Thanks for the input, guys. Tried pushing the wheel further back in the dropout, but the chain derailed again while we were out riding. Finally just a mechanic friend check it out. Even with the wheel adjusted, there was too much slack, and when we removed a link from the chain, it was too short. We just ended up installing a new chain!
You can also put in what's called a half-link.
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