Chain-Suck: Why it Sucks and What You Can Do About it.

You’ve just finished pushing your bike through the swampwhack from Hell and now the sadistic course-setter has presented you with the mother of all climbs. Gearing down to your granny, you start painfully spinning your way up the hill when with a sickening and expensive-sounding “crrrrrrrr-chunk” your drivetrain locks up completely. You lose what little momentum you had and slowly, almost gracefully, fall over onto some sharp and pointy rocks. Naturally you aren’t able to unclip because both shoes and pedals are gummed up with muck from your recent “ride” across the swamp. Your team-mates stop, dismount and fall down laughing as you struggle to disentangle yourself from the bike and assess the situation.

Figure 1: D'Oh!!! *$^(@@#$^%$

Figure 1: D'Oh!!! *$^(@@#$^%$

Well, isn’t this romantic: your chain and chain-ring have decided that they are madly in love and never want to be parted. The sad reality is that bike frame geometry simply won’t allow for such a relationship, meaning that the chain has been sucked up the back of the chainring and jammed against the frame, completely locking up your drive-train. Welcome to the wonderful world of chain-suck, the most perfect of all bike-related terms (although endo-faceplant is pretty good too). Read on to find out what it is, why it happens and, most importantly, what you can do about it.

What is it?

Technically speaking, chain-suck is when the chain fails to disengage from the bottom teeth of a front chain-ring. The teeth snag the chain and carry it up and around the back of the ring, winding it back onto itself, and jamming it between the chain-rings and chain-stay. This is a 100% effective way of stopping you dead in your tracks. It looks something like this:

!! IMPORTANT !!

Stop pedaling immediately! If you try to force the cranks forward you will make this problem much worse and more expen$ive. The chain can be permanently twisted, chain-ring teeth can be damaged, and aluminium or carbon-fibre frames can be gouged or shredded. Get off the bike and try to un-wedge the chain by spinning the cranks backwards while trying to pull the chain out by hand.

Why is this Happening?

The short answer is a single word: friction. The friction between the chain rollers and the load-bearing faces of the teeth on the chain-ring becomes greater than the weight of the chain and the pull of the rear derailleur. This friction keeps the chain from disengaging from the bottom of the chain-ring and voila: le chain-suck.

That’s fascinating Dr. Pete, but what causes this increased friction? Well, there are four main reasons why the friction between your chain and chain-rings will increase:

1. Increased forces between the teeth and chain.

This will happen when you have to pedal hard (i.e. climbing), when you are in a smaller chain-ring (again, while climbing), and especially if you are doing both (yes…climbing). As you can see, chain suck is designed to happen at the least opportune times: when you are killing yourself trying to climb a mountain in your granny gear.

2. Worn chain-ring teeth.

Chain-rings wear out. This is a sad and painful truth, so just accept it and move on. What happens is that the load-bearing faces of the teeth get worn, deform, and no longer mesh smoothly with the chain rollers. This happens most readily on your granny gear, since it has fewer teeth to distribute the pedaling forces onto.

3. A new chain on a worn chain-ring.

Chains also wear out, and they do so much faster than chain-rings. Chains will “stretch” as they wear out, and need to be replaced before this becomes excessive. A stretched chain will greatly increase the wear on the chain-rings. If you then swap in a new chain onto a worn chain-ring, you have a nice scenario for chain suck since the chain rollers and teeth won’t be well matched in shape.

4. Mud and/or grit.

Mud and grit will strip away lubricants and then proceed to gum up the chain links so that they become stiff. This will increase the friction and can result in chain suck, even with a completely new chain and drive-train.

Please Make it Stop - An Ounce of Prevention

Your best, least frustrating and least expensive strategy is to try to prevent chain suck. Probably the single most useful thing you can do is to monitor and swap out your chain before it stretches to the point that it starts to deform your chain-ring teeth. A stretched chain will quickly wear all other parts of the drive-train. If you ride for too long with a stretched chain then you’ll likely need to replace some sprockets or your whole cassette in addition to getting a new chain.

A simple measurement of your chain with a ruler or measuring tape will tell you all you need to know. Remove your chain from the bike and measure out a 12-inch section. Start measuring from the front edge of a pin to the front edge of a pin 12 inches away. If the chain shows less than about 1/16th of an inch of stretch, then you’re ok. Otherwise, it’s time to put on a new chain.

While you’re checking your chain you might as well monitor the chain-rings for tooth wear, especially the granny, and replace them as needed. Remember that if you let your chain-ring wear down, you’ll have problems when you finally put on a new chain.

Please Make it Stop - A Pound of Cure

Installing a new chain-ring will usually fix most chain suck issues. Remember to swap out the chain as well if it has stretched out significantly. If your existing chain-ring is symmetrical, then you can extend its useful life by flipping it over and using the unworn faces of the teeth. A final chain-ring option is to rotate the ring by one or more bolt holes. This will alter which teeth receive the chain during the high-load portions of the pedal stroke, and might help.

If it's sucking because of mud or dirt then it can be helped by spraying the chain with water, or stopping and submerging the drive train in a river or pond.. This actually happened during every lap of the 2004 World Mountain Bike Championships, which shows that even brand new high-end drive-trains can suck if conditions are bad enough. In consistently mucky conditions you will need to do this every 5 minutes, so unless you’re riding a waterfront trail or you conveniently need to take a leak, it’s on to plan B.

Plan B involves turning yourself into Jan Ullrich. Get out of the granny and push a (relatively) huge gear at about 12 rpm. The larger chain-ring will reduce the load, and therefore the friction, on the chain-ring teeth. The obvious downside is trying to ride hilly, technical and mucky terrain in a massive gear. You will exhaust your legs and most likely be unable to ride steep or technical bits due to rear wheel slippage. The upside is that, even though you’ll be hike-a-biking almost as much, you won’t be risking destruction of your drive-train or frame.

You are now an official expert on chain-suck: I dub thee “sucker”. Go forth and ride in the mud!