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Specifically when changing gears from middle chain ring to smallest, the chain will sometimes get 'sucked' up between the inner chainring and chain stay. It seems to happen more often in muddy conditions. I keep my drive train clean and lube my chain often.

I bought the bike new and maybe 1200-1500 miles on the original chain/derailers. I have swapped out the original (aluminum) inner chainring with a new steel chainring and still get chain suck.

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So it's falling off the smallest ring? –  ChrisW Nov 30 '11 at 16:06
    
No, it stays on the smallest ring but 'sticks' to the ring basically fully wrapping the ring –  Glenn Gervais Nov 30 '11 at 16:29
    
If you're still getting stuck with a new ring then it's probably a (rear) derailer problem. Or, by any chance, could you have simply installed a too-narrow chain on the bike? –  Daniel R Hicks Dec 2 '11 at 13:28
    
Drive train (except for my new inner ring) is all original. Bought bike new Jan 2010 - 9 speed, Shimano XTR Derailleurs, Deore XT cassette, KMC super narrow chain. I am going to switch to a Shimano chain. Thanks for the great advice! –  Glenn Gervais Dec 2 '11 at 15:53
    
BTW, if you got 1500 miles on your original chain then your rear cluster is probably badly hooked. –  Daniel R Hicks Dec 5 '11 at 16:45
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4 Answers

up vote 3 down vote accepted

The problem you describe is caused by either a badly worn cog, a rear derailer with insufficient "tooth capacity" (given the gear combo you're using), a seriously deficient rear derailer tension spring, or a chain that is simply too long. A worn chain will tend to exacerbate things, as will a chain that's "sticky" from grease or mud.

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While it is logical to expect the RD spring tension to be a factor in chain suck (since that spring works to "peel" the chain of the chainring), its role does not seem to be supported by practical observations. When shifting-related chain suck occurs, the "holding" force present at the point where the chain departs the larger chainring toward the smaller one is often so strong, that no reasonable RD spring has any chance of overcoming it. The chain is basically jammed on the larger chainring. –  AndreyT Jan 7 at 5:08
    
@AndreyT - Generally that's true. But you can experience chain suck with a reasonably good chain and cogs if the derailer tension is insufficient. –  Daniel R Hicks Jan 7 at 12:18
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If a chainring needed replacement (these usually last longer than the cogs), it is quite likely the cassette and chain did too.

(Drivetrain) maintenance is something you should get in the habit of doing if you want to get maximum mileage and enjoyment out of your bike in general, especially if you are riding in dirt and mud.

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It is a bit unusual for the small ring on a triple to be the one to wear out first, though. That's the one odd thing about Glenn's problem. –  Daniel R Hicks Dec 2 '11 at 13:26
    
It actually wasn't worn out, I read somewhere that an aluminum inner chainring could cause chain suck. I still get it with the steel ring. –  Glenn Gervais Dec 2 '11 at 15:55
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You seem to have answered yourself, but there are a few causes:

  1. worn chain / cogs.
  2. rusty cogs, especially toward lower gears.
  3. dirty cogs.
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There's no definitive answer about the root causes of chain suck. Or, more precisely, the definitive answer is probably the one that says "there is more than one contributing factor".

The two obvious cases that come to mind are: 1) extremely worn chainring that "hooks" the chain, 2) new chain on an old chairing (even if the latter is not extremely worn). In the latter case the most load is transferred by the bottom teeth of the chainring greatly increasing friction at the bottom point.

But unfortunately chain suck happens even with perfectly new chainrings and chains. You can find quite a few theories floating around the Net. It is clear that chain suck is caused by excessive "friction" between the chain and the larger chairing, which prevents the chain from detaching from the larger chainring at the final phases of shifting.

As for what causes that excessive friction... There are just too many factors involved here. How clean the chainring is. How clean the chain is. How long the chainstays are (which affects the angle that the chain sweeps during the shift). How resistive your chain is to any lateral displacements. How sharp is the angle at which it bends during the shift. How well the plane of the chainring is aligned with the current cog. And so on and so forth.

You can consider yourself lucky if in your case the chain gets stuck between the chainstay and the ring. In my case it doesn't. As it wraps around the chainring, it travels all the way around to the front derailleur and slams into the derailleur cage. The derailleur gets disfigured beyond recognition.

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