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Old 09-08-11, 12:13 PM   #1
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Help with jumping chain!

Hey guys, I need your help with my bike as it is just marginally ridable now and I'm out of ideas on how to fix this problem.

The bike is a Trek 5200 (Ultegra 5600 triple up front), with about 2200 miles on it. What I'm seeing now is that anytime I put power down, i.e. starting from a stop or going from coasting to full sprint, my chain snaps off of the middle cog in front. It either ends up on either the small cog or completely off on the inside. This only happens on the middle cog up front, bigger and smaller ones I can't reproduce it. I do about 50% of my riding on the middle cog, 40% on the large cog and about 10% on the small cog.

The only thing I can think of right now is that I changed the chain recently without changing the cogs, but I'm skeptical because I was able to do 3-4 long rides without this issue. Also when it started, it was fine in the morning when I commuted to work, then it started doing it on the way home about 2 lights from home (its done it on every start since then).

I also confirmed its not a front DR issue. I tightened the cable and it still does it and I also tried overshifting (so the chain *almost* wants to jump to the large cog) and it still is easily reproducible.

Any ideas?
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Old 09-09-11, 06:13 AM   #2
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This is a classic worn "favourite" front chain-ring issue that is highlighted after changing the chain. I'm 99% sure that replacing the middle and larger chain-rings will solve your problem if you haven't done many miles with them and your new chain.

The fact that it's slipping indicates that you haven't done any damage to your new chain (it will wear faster with worn chain-rings) so I'd suggest replacing them sooner rather than later.

Is your rear cassette in OK condition too? It's worth getting your whole chain-set sorted so you can have maintenance free riding for another 2-3000 miles
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Old 09-09-11, 01:56 PM   #3
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What brand/series chain is on your bike now?...and who installed it?

I'm wondering if an improperly (i.e., reversed) installed asymmetric chain (CN-5700/6700/7900) could cause or contribute to the problem you're experiencing.
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Old 09-09-11, 09:26 PM   #4
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It could be a new chain/work sprocket issue, but it could also be more simple, like chainring deflection under load. before spending dough chasing this problem do this simple test to eliminate this possibility

Put your front wheel against a wall so the bike can't roll, set the right crank at three o'clock and put some load into it while watching for deflection of the ring using the FD cage as a reference point. All bikes will move a bit, but if it moves enough for the chain to touch the FD cage, that might be whats causing the problem. Now you only need discover why, which could be a loose BB.
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