Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 57 Post(s)
Tagged: 1 Thread(s)
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Chain suck has a variety of causes but basically boils down to the forces (friction) holding the chain on a sprocket being greater than the forces pulling it off the bottom (chain's weight and RD cage spring tension). It's very analogous to how fishing line overruns onto the spool when cast bait hits the water and the reel keeps spinning.
You have to attack it at the cause, address anything that might contribute to added friction; dirt, bent sprocket teeth, twisted links (especially inner links) and friction within the chain. You can work it from the other end by increasing tension in the lower loop, by adjusting the cage spring if possible) or using a shorter cage RD. Shortening the chain doesn't help because the change in cage tension is too small.
Once you understand the basic cause, you can see why chain suck happens more under certain conditions. Slammer sprockets, mean lower cage tension so it's more common to have chain suck when on the inner ring. Often a borderline bike will have chain suck only when shifting to the inner ring. As the shift happens the lower loop has a moment of zero tension when the chain comes off the outer chainring because the RD cage cannot pull the extra chain back fast enough.
Preventing suck depends on keeping everything well lubed and free of binding grit. But once you have chainsuck, the suck itself can bend teeth, or twist chain links making it progressively worse.
To solve your problem, start by checking for any bent teeth or twisted links. One way is to backpedal at medium speed while pushing the lower RD pulley forward to slightly slacken the lower loop. Loop for any pulses or flicks in the chain. I do this running crossed over(big/big & small)small) to have the most side pressure on the chain, but you should do it in a variety of gears. Once you have a spot, check the teeth and links involved for any hint of damage. If you don't see anything mark the areas and repeat the process until you decide it's something specific, or random.
For random chainsuck, remove the chain, wash and rinse it very thoroughly in mineral spirits to ensure that any grit trapped within the plates is flushed and the solvent stays clear. You can save the used solvent for reuse since the grit will settle out over time. Oil your clean chain with a good chain oil (I have a bias so consider the following accordingly). A good wet lube does a better job preventing chain suck because it makes the chain more supple than many other choices, and also because it does a better job preventing dirt from getting between the plates, especially if you ride in the wet. As I said that's only my opinion, so try whatever you prefer first, and switch if the problem recurs.
An ounce of diagnosis is worth a pound of cure.
“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin
“One accurate measurement is worth a thousand expert opinions”
- Adm Grace Murray Hopper - USN
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Last edited by FBinNY; 08-30-12 at 11:03 AM.