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Chain Suck

There is a nasty ride halting condition that occurs once in a while — it’s a condition where the chain does not want to “let go” of the chainring — instead, wanting to “Stick” or “Suck” to the chainring and wrap around again. This is usually initiated during a shift — and most often when shifting under power. It’s manifest when the bottom run of the chain does not release from the chainring and gets carried up and around the ring until something binds — usually as it wedges between the chainrings and the frame chainstay.

Now you know “What” it is, here are some typical causes and fixes.

Chain Suck Causes:

Chain suck is usually “Caused” by a combination of factors — not a single item. Contributers are most often (listed in order of importance):

Damaged parts. Certain damage like bent, blunted or gouged chainring teeth can cause chain suck. Also bent or scarred chains.
Shifting while applying power. The energy to bind the chain onto the chainring teeth comes from the forces applied at the crank.
Warn parts. Either the chain (stretch), or the chainrings (shark tooth or other wear), or both.
Environmental conditions such as mud or grit, or the opposite, dry and dusty, will contribute.
Situational conditions like front shifting while in a cross chaining configuration can contribute.
Needing Maintenance. Though lower on the causes list, things like lack of lube can contribute.

Avoiding the Problem Altogether:

There is no bullet proof way to completely avoid chain suck. Even single front ring systems can have it if they mushroom a tooth by using it as a bash ring. However, to avoid it as much as possible:

Keep the chain and gears clean and well lubed.
Inspect the drivetrain frequently and replace worn or damaged parts.
Avoid equipment that can be more susceptible to issues — things like lightweight chains with holes in the side plates that can hold the mud and grit.
Avoid situations that “gunk up” the drivetrain — things like avoiding mud and sand (especially muddy sand) where possible.

Solutions: (If it occurs.)

For whatever reason, it seems like once it occurs, it starts to happen again and again. One time may be an anomaly, but it it occurs more, it usually means something needs to be fixed. So, if you experience chain suck:

First, carefully examine the chain and the chainrings looking for damage like a bent tooth tip or scar in the side of a tooth, or a bent or damaged chain link. This is the most sure way to have chain suck. If found, correct it — carefully smooth the area or replace the damaged part.
Second, shift under less power. Shifting under power can be fun, but it is not generally a good idea. When shifting under load, chain tension is carried through the shift, and can twist the chain around a gear tooth — binding the chain, or wedging a tooth in a link. This condition of forced chain suck is not the same as a dirty condition chain suck, but the result is similar.
Third, check all the drivetrain components for wear. Check the chain wear (stretch). Check for wear on the chainring teeth — especially those that are involved with the “suck”.
Fourth, make sure the drivetrain is clean, well lubed and in good condition. Chains, in particular, need to be cleaned on the inside as well as the outside.
Fifth, shifting under load — especially the drivetrain is becoming warn — can contribute to chain suck.
Finally, shifting in a cross chaining situation (see below) is known to contribute to chain suck — especially under power — because it puts additional side loads on the chain and chainring teeth. Avoid that, especially when conditions are dirty, muddy, or extra dry. Also see our page about “Cross Chaining”.

Want to know more?

Chain Suck is a complex and involved subject of study. The complexity comes in the many ways that what we call “Chain Suck” manifests itself on the bike, but are caused by several of the above, interacting factors. If you’re interested in reading more, check out this much longer explanation at: “http://www.fagan.co.za/Bikes/Csuck/”.