Chain suck is the term for the situation where a chain ring doesn’t let go of the chain in time, leading at its worst to the chain getting wrapped around the bottom bracket area.
Chain suck happens for several reasons, but it is one instance where the fix has to happen before the diagnosis.
First, deal with the immediate problem of removing the chain from around the bottom bracket. You may be able to pull it loose, but on expensive frames this is going to at least damage the paint if not the frame material. You will probably also damage the chain. The safest course of action is to remove the drive side crank arm (and thus the chain rings). See the section on removing crank arms.
Once you have removed the chain and reassembled the crank arm (if necessary), check to find out what caused the problem.
- Worn chain ring teeth will “hook” a chain. Check that your chain ring is in good shape by comparing the one which sucks to the other ones. Frequently your middle chain ring wears out fastest because you use it the most.
- Old chains stretch. This in itself can cause chain suck, but stretched chains also wear out chain rings. Twelve inside and outside link pairs will measure exactly twelve inches from rivet to rivet on a new chain. Anything over twelve and one eighth inches is worn out.
- Dry and rusty chains don’t flex. This almost guarantees chain suck. Get a new chain and keep it lubed.
- Chains with a stuck link can suck. See the diagnosis and fix.
- Running a 9-speed chain on an 8-speed or 7-speed system leaves too little tolerance. Chain suck is probably the least of your issues in this situation.
- Constantly cross-chaining (little chain ring to little cog, big to big) increases your risk of chain suck. Only use sensible gears.
- Too little chain tension can suck. Did you replace the chain without making the new one the right length? The correct length is found by making a loop from big chain ring to biggest cog without passing through the rear derailleur, then add two links. If you are using a medium-length derailleur cage rather than a long one, it may not be able to take up the slack in certain gears.
- Replacing a particularly worn chain with a new one can also cause chain suck. Your old chain made the equivalent of a butt dent in a couch seat – the chain ring and cassette wore out over time to fit with the chain – and so the new chain is now rattling around on the ground down stubs of the teeth. If you ever let your chain get to this point, you will be paying for new chain rings and a cassette as well.