What is Chain Suck? What causes it? and How do you stop it?
Who ever coined the phrase “Chain Suck” needs a pat on the back. Not only is the phrase very descriptive, it just sounds miserable . . . and so it is with the described affliction . . . because it can halt your ride pretty suddenly.
Here are a few details they took out with the edited version:
What “Chain Suck” is NOT !
The phrase “Chain Suck” is commonly miss-assigned to all sorts of chain related disasters. A Google Search will quickly show several misleading examples. Here are some that are certainly problems, but are NOT chain suck:
Web Definitions: “Chain Suck: The annoying little habit your chain has of going too far. Your chain has slipped off its gears and you can no longer pedal. This can happen to front, rear, or both. Generally caused by over shifting and improperly adjusted derailleur. Generally causes frustration and a few choice words.” (http://www.doodahroadclub.org/Definitions.htm link dead)
This would be “Over-Shifting” … They do have the cause and cure correct.
This little video on YouTube: http://www.youtube.com/watch?v=SGLDgKN_Vms
This fun video shows the inertial effects of high-speed chain motion when one element (the crank) stops suddenly. Theoretically the chain can be caught by the tire and pulled to the inside of the chain stay doing damage, but that’s not common. It’s a pretty cool video, but not “Chain Suck”. Does it really create a problem? Not really, because most people can’t pedal that fast — and don’t normally stop pedaling that fast. Interesting video, nonetheless.
An interesting problem description wrongly ascribed as “Chain Suck” (from Bike Forums): “… Suddenly the crank arms won’t move. The first time I was able to get the chain unstuck from between the smallest chainring and the bottom bracket …”
This problem is related to over-shifting (described above) where, during a shift, the chain over-shot the smaller ring going beyond and off the small ring. Unfortunately, the geometry on his bike guided the chain into a place where it jammed between the ring and the BB. That’s a bad problem for sure … and understandable why they called it “chain suck” but that’s not really a correct description. Over-shifting is the root, and jamming is the problem. Derailleur adjustment is likely the answer.
Enough on mislabeled examples.
The Mechanics of Chain Suck:
Chain Suck — when the bicycle chain fails to disengage from the teeth of a chainring (usually during a shift), sticking to the tooth (or teeth) as the chainring continues to turn, wrapping back up and around the chainring. A pretty classic example is shown in the image at the top of this page. An end view of a different example is shown in the image below.
This image shows how the chain on the chainring teeth — looking directly down through the chain at the ends of the chainring teeth. If you look close, you can see how the chain is wedged around 3 teeth. The main 2 are in the middle of the image poking up through the chain. The third is in the lower left contacting the chain on the outside as the chain is derailing from the chainring. When there is force on the chain during a downshift, the chain can be pulled tight in this configuration such that the chain is asked to “bend” tighter than it is able — thus pulling into this condition where it binds on the teeth.
This may happen more often than you know, but if it comes off easily, then you’d never know it happened. However, if the chain is particularly dry, or if there is any damage on the chainring teeth, or if there is grit or dirt it can exacerbate the problem and sometimes end up leading to the dreaded chain suck. Best to just leave torque out of the equation while you’re shifting.
What Causes “Chain Suck”?
Read all about the causes of Chain Suck, see the photos, and discover solutions to avoid this infection in the full article at MTBR.com.
If this topic is of particular interest, more has been written. In particular, Jonathan Levy has a detailed page of Suck info. and a Field Test to see if your bike is likely to get chains suck.