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My bike chain has been "skipping", so I've gone through the obvious:

  • I replaced the chain
  • I replaced the smallest cassette cog, which was showing significant wear

After my replacements I noticed what the real problem was. Every once in a while, rather than the chain being pulled into the valeys of the rear cog, a chain link land on the tooth, and will not roll into the cog's valley and engage. Each subsequent link of the chain will land on a tooth. When the last engaged link begins to leave the valley, all the disengaged links will be snapped back into engagement by the rear derailleur.

What's going on here? Is my rear derailleur's tensioner spring just too old?

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It sounds like your drive train was very heavily worn, and you need to consider replacing all the toothed components. Particularly the derailleur pulleys, as that is where you seem to have an immediate issue. –  zenbike Aug 16 '13 at 6:57
    
I did spend some time cleaning up the pulleys, and they appear to be in good order. Could the skipping be a symptom of worn out pulleys? –  Jonathan Mee Aug 16 '13 at 11:48
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It is a symptom of worn teeth on the drivetrain, bad chainline on the bike, or bent or broken teeth, or links in the chain. But given where you are describing an issue, and what you are describing, I would say that the pulleys are likely causing this particular issue. You should replace them. But you should also replace the rest of the cassette, and the chainrings. –  zenbike Aug 17 '13 at 3:08
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4 Answers

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The tensioner spring is designed to keep the entire chain under tension, even after it passes the crank. So, the symptom of a too-weak tensioner is slack in the chain between the crank and tensioner.

It sounds like the distance between chain links is not quite the same as the distance between valleys in your sprockets. This is not unusual if you replaced a very worn chain (because the sprockets would have worn to match the old chain). It's also not impossible that the sprockets are worn not quite enough to match the old, worn out chain, yet still worn too much to match the brand new chain. You may need to replace the rest of the sprockets in the cassette.

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So... my chain is skipping on the new rear cog. but it sounds like my actual cog and chain may be incompatible? –  Jonathan Mee Aug 16 '13 at 11:45
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Essentially, yes. All bike chains are 1" pitch (center to center of the pins), but the chain stretches out with use, and the cogs also wear. So after a while, neither is what it is supposed to be. Often a chain and cogs will wear together, so the old chain will fit the cog better than the new chain. Sometimes replacing an old chain also forces you to also replace both the cogs and the chain rings. –  nick g Aug 17 '13 at 5:36
    
Thanks for all the answers, I have selected this one as I feel it was closest to my actual issue. As I describe in my comments on my question: Best Cassette Cog Material the issue was that I was actually trying to replace a cassette cog with a single speed cog. –  Jonathan Mee Sep 16 '13 at 11:47
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Depending on the condition of the entire drivetrain and how many miles/how much use you've put on it, it could be a couple things. As mentioned, it could be a case of worn parts not matching new parts, but I doubt that. More likely is the rear derailleur needing adjustment, as I just had a similar problem.

Try adjusting the limit screws on the rear derailleur to make sure that it's shifting all the way to the center of each gear. Also check to make sure that the cable tension to the shifter is correct so that it's pulling the chain all the way over and not only half way when you shift. I can't say I've ever heard of a spring getting worn out, but I know they can either break or come loose. If that's the case you can try and repair it but you're probably better off replacing it.

Check out this excellent article from BikeRadar about transimission wear as it might save you some frustration: Workshop: Transimission Wear

Or this one for rear derailleur adjustment: Park Tool: Rear Derailler Adjustments (derailleur)

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It sounds like your drivetrain is worn out. You may need to replace the gear changer. In the short term you can experiment with a shorter chain. Have you got the right size chain? Is your cassette 7,8,9 speed? because each takes a different size chain.

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What you have described is a limit screw or cable tension adjustment issue. If the chain is being guided so that the plates are landing on the cog teeth instead of the rollers then you need to adjust one or the other or both.

The replaced sprocket may have caused it to start happening. On the other hand, is it possible that you bent your derailleur hanger? This will knock the aforementioned adjustments out of whack.

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