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Old 08-29-12, 10:18 PM   #1
karinbur
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Chain suck: Can anyone diagnose this?

My mechanic has given up; but I'm hoping someone out there has some idea what happened here:

1. Replaced the SRAM chain on my bike. SRAM Red front derailleur, XX rear. 34/50 on front, 11/32 cog in the back. There were NO problems shifting, but the chain measurements were indicating some stretch had occurred.

2. I began to experienced the chain "locking up" (I guess this is called "chain suck") at least once on every ride. I examined the chain, wiggled every link to check for a stiff one (didn't really find anything), and lubed it. Everything was great for 1 ride, then the problem returned.

3. Installed a chain guard to prevent the chain from coming off on the inside. This made no difference. The chain still locked up. Slow back-pedaling would unstick it.

4. My mechanic shortened the chain by one link and installed a new 34 chain ring.

5. Now, in addition to the chain lock-up, the chain falls off on the OUTSIDE, usually once on every ride. Usually I'm just on a straightaway, somewhere in the middle on the rear cassette, and shifting from the small to the big ring in the front. Not putting much pressure on at all.

Are there any hypotheses on this? Any input is appreciated. My mechanic is stumped; now wants to replace everything: chain rings, cassettes, and the new chain itself. I hesitate to do so unless I know what's wrong.

Thanks for your ideas...
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Old 08-30-12, 05:07 AM   #2
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First lets get the terminology correct.

'Chain suck' When the fails to disengage from a front chainring resulting in the chain pulled up and inot the chain stay. 'Chian suck' is strictly pertains to the underside of the front chainrings and can be caused by an excess of grime/crud/buildup on the teeth which leads to a poor release of the chain. Another cause is deformed chainring teeth, deformation doesnt allow the chain to release.
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Old 08-30-12, 06:57 AM   #3
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You need a new mechanic. Your mechanic wanting to replace the cassette shows him/her clueless and grasping at straws. The chain falling off to the outside is a misadjustment of the high limit screw on the front derailleur. Chain suck makes me wander if the new chain is correct. There are subtle differences in width between 7/8 speed chain, 9, 10, 11. Narrower chain might be the answer. OK to run a 10 spd chain on 8/9 speed, other way around is not so good, IMHO.

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Old 08-30-12, 10:52 AM   #4
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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.
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Old 08-30-12, 10:57 AM   #5
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When you make a change in equipment and a problem immediately follows that change the first thing to focus on would be the item that changed. If truly nothing else was changed (front derailleur position, chainring, etc) the first thing I would try is to replace the chain again.

p.s. You don't say whether the new chain is a different brand or speed -that may be important.
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Old 08-30-12, 12:00 PM   #6
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A photo of your drivetrain would help. What you are experiencing is not technically chain suck, almost without exception chain such occurs when you down shift; what your are probably experiences is chain bridging, where the chain is locked between the big and small ring. Saying this I'm sure you're thinking "I don't car what you call it, I just it to stop". Since this is occurring when you shift from small to big, it means the small chainring is not properly releasing the chain. My first question would be what make chain did you replace and what make chain did you replace it with. Further do you recall how badly the old chain was worn? Another thing I would check is to see if the small chain ring is correctly oriented. The small chainring has a small tad on the inside if ring opposite the teeth. This tab should oriented with the crank arm.

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