| Generically, this refers to any
instance of the chain getting pulled in between the crank and the right
chainstay. It has two main forms, with different descriptions,
causes, and remedies. Derailment Chain Suck: After an inward derailment, the chain can be snagged by the small chainring mounts, and pulled along until it wraps up and jams between the crank and the frame--usually the forward region of the right chainstay. This happens at a small radius from the BB axle, so the force of pedaling can bring a great deal of leverage to bear on the chain. With so much force driving the chain, it may be very difficult to extract, and it may even do serious damage to the frame. The cause of derailment chain suck is contact between the chain and some part of the crank that can snag it. Derailment is not really the cause of it, and derailment doesn't always result in it, but I call it derailment chain suck because derailment is always a necessary precursor event. The chain has to be where it is not supposed to be in order for it to happen. There are basically two remedies. 1) Make it less likely for some part of the crank to catch the chain after it derails (known examples include extended mounting tabs on the inner chainring, and a washer-like shield mounted under the BB fixed cup flange) . Or 2) Eliminate all inward derailments. Release-Failure Chain Suck: This happens because the chain fails to disengage and spool off the bottom of the chainring as it should--instead, wrapping up until it jams between the chainring and the chainstay. From the right side of the bike, this may look a lot like derailment chain suck, but there is no derailment involved here. The chain is still threaded on the chainring right around to where it is jammed. The force driving the chain is not quite as great as in derailment chain suck, and the impact with the right chainstay occurs further back, but with repeated hits, this type of chainsuck can be very destructive as well. Release-failure chain suck can have a variety of causes, but the most common root factors are dimpling, hooking or burring. Dimpling is essentially a pressure dent in the chainring teeth where the chain rollers bear against them. This tends to cause chainsuck most under a heavy pedal load, and the smallest gear is most vulnerable. Strong, heavy riders can dimple small aluminum chainrings in a single ride. Hooking is like dimpling except caused by wear, so it takes longer to form. Hooking will sometimes afflict the middle ring more than the others, because of the amount of time spent there. Burring (or mushrooming--where ring material has been pushed out to the side) causes trouble almost irrespective of load--though possibly more often during shifting. There is a detailed examination of chain-suck that is caused by deformations in the teeth or chain here. For most tooth wear or tooth-deformation problems, the preferred remedy is a harder and stronger material for the chainring, such as stainless steel or titanium. Stainless is usually cheaper, and titanium usually wears longer, but either will outperform aluminum in most every respect except weight. Makers of such rings include Action-Tec, Blackspire, and Cycle Dynamics. Other possible factors for release-failure chain suck include loss of lubrication, bent or impacted teeth, foreign matter in the drivetrain, and resuming full pedal pressure immediately after initiating a downshift, before the chain can engage the smaller ring. Failing prevention, there are basically two strategies for dealing with a stuck link that won't let go. Knock it loose or pull it loose. The original knock plate was the Ringle Anti Chain Suck Thing and there are stamped steel versions by Tektro and Minoura. Because of their position, a stuck link will hit them in a roughly tangential direction, but extracting a stuck link requires force in a roughly radial direction, so a very large impact force may yield only a small component of useable extraction force. The impact will also contribute to pressure dimpling in the chainring teeth, increasing the frequency of subsequent chain suck events. As a remedy for chain suck, knock plates are not so good. Their first, best use appears to be protection from serious frame damage. Strategies to pull the chain loose include boosting/assisting the rear derailleur tension spring or adding another pulley to pull the lower chain up. Neither strategy adds much pulling force to the chain, and both tend to degrade shift performance seriously. These are probably best regarded as options of last resort. |