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I've had my Trek 3900 for about a year now, and I replaced the chain about 3 months ago, but otherwise it's running with the same parts it came with (not counting tubes and tires). I had to replace my chain because it had gotten really rusty from riding in rain/mud/water on trails, and had started skipping.

My problem is, whenever I ride, if I get above or below a very light pressure when pedaling, the chain skips. I was wondering if this could be caused by my chain, which has gotten a little rusty but not as stretched as the other one was, or if I might need a new cassette?

I just readjusted my derailer, but it still skips.

Does anybody know what could be causing this?

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3 Answers

up vote 3 down vote accepted

When your chain "stretches" beyond a certain point, typically .75mm per 4 inches links, it will begin to reshape the teeth on your cassette and chain rings.

At that point, replacing your chain will not help unless you also replace your cassette, and possibly your chain rings.

Check the answers on this question to see how to know when your chain is worn.

If your chain measures beyond 1.0mm per 4 inches, then you must replace chain, cassette and chain rings, or it will skip as you are describing.

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How many miles do you suppose are on the bike?

First off, your chain should not be rusty. If you frequently ride in wet conditions you should use a "wet" chain oil on it, and clean it at least a 2-3 times a season.

The skipping could be due to a rusty chain with frozen links, could be due to a poorly adjusted derailer, or could be due to a worn out cluster. Though there's a fairly wide variation, a cluster on average is good for about 5K miles, and having a badly stretched chain wears the cluster (and chain rings) out faster.

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@zenbike is right on this - it is a common problem. What happened was that the old, 'stretched' chain damaged the rear sprockets. Look closely and you will see that the shape made by the sprockets is an elongated 'u' shape, particularly so on the sprockets prone to skipping (the smaller sprockets).

There is the option to replace the sprockets, however, you will need a special tool to remove the sprockets and you may not have the money for the tools + sprockets right now.

In the past, when confronted with this situation, I have continued to ride and, after not too long a time, the new chain 'stretches' to suit the sprockets and no longer skip. It need not stretch to the length of your old chain, just 'grow' the tolerances required to work with the setup that you have. It is up to you whether you follow this 'cheapskate' option, however, you will need to remember not to ride out of the saddle on the gears where skipping is a problem.

Since you may have already damaged the new chain anyway and it will not be suited to a brand new sprocket set, you may want to see how you get on running 'cheapskate'. However, I cannot stress enough the importance: do remember to ride in the saddle as a jumping chain when out of the saddle is likely to cause you an accident when you least need it.

After a short period, surprisingly short, the tolerances should be good and you'll get a bit more life out of your sprocket set. Eventually though you will have to replace sprocket+chain. Don't wait for the gears to start jumping again, check the wear of the chain. Too much wear will damage the front chainset and that will cost real money.

There are tools for measuring chain wear, personally I prefer to just put a screwdriver under the most forward part of the chainset when the chain is on the outer ring and see how much you can pull it forward. If this gets to more than 5mm or so you know it is a badly worn chain. That is pure rule of thumb requiring experience, but that has worked for me in the past and I do not personally own a chain wear tool.

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