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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

3 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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up vote 2 down vote

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
up vote 1 down vote

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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