You don’t realize how annoying chainsuck is until your bike is infected. Then you can’t stop talking about it until it’s cured – which can take patience as chainsuck has several different causes and is sometimes caused by a combination of factors.
What is chainsuck?
Normally as you pedal, you push down on the cranks and the teeth on the chainrings mesh with the links on the chain, dragging it forward. The links on the chain also mesh with the teeth on the sprockets, dragging them around, which makes the wheel go around. As each section of chain is dragged around the chainring, it drops off the bottom and is pulled back around again.
Chainsuck happens when the chain fails to drop off the bottom of the chainset and, instead of heading backwards toward the cassette, it stays stuck to the chainring, getting dragged up and around it as you pedal. It rapidly gets jammed in the gap between chainring and frame, the entire drivetrain locks and you fall off your bike. To add insult to injury, the chain often takes chunks out of the chainstay. The problem usually lies with the smallest chainring, but it can also happen in the middle chainring and sometimes even in the big one.
Worn chain, chainring, or both
When both the chain and chainring are new, the distance between each link in the chain is the same as the distance between each tooth on the chainring. When you put pressure on the pedals, dragging the chain around the chainring, the pressure is taken up by only the top few teeth. The pressure gradually reduces in each link as the teeth progress round the chainring, until at the bottom of the ring the links are released entirely and drop off freely, as intended. The pressure from the pedals is spread over just those few teeth at the top of the chainring, reducing the amount each chainring wears.
Once either chainring or chain is worn, nothing will work as well as it has been. A worn chain on a fresh chainring engages only a single tooth at the top of the chainring, which will accelerate chainring wear. The chain links at the bottom of the chain get caught too far back in the valley of the bottom chainring tooth and are dragged upwards. As the chain wears and stretches, the distance between each link expands. As the chainring wears, the valleys between the teeth get wider and deeper, which allows the chain to slip back in to each valley under pressure.
Chain and chainring damage
There is only enough room between the sprockets for a straight chain link. A twisted one always catches on the neighbouring sprocket and causes the gears to slip or catch. A twisted link is also a weak point, so sort it out before it busts and strands you in the middle of nowhere. With care, you can spot a twisted link by looking along the chain from behind while backpedalling gently. Check also for stiff links, which can easily cause your chain to slip under pressure, even when everything is adjusted correctly and none of the drivetrain is worn. To check for a stiff link in the chain, turn the pedals slowly backwards, while looking at the chain. Squat beside the bike and hold the lower part of the chain between two fingers. Roll your hand so that one finger is slightly higher than the other. Pedal backwards, so that the chain is dragged between your fingers. You feel stiff links as they pass between your fingertips. Use a goodquality chain tool to spread the outer plates that restrict movement and cause stiff links; see page 108.
To get a good look, remove the chainset from the bike and the rings from the chainset. Check particularly for bent or damaged teeth. You can pick out the areas on each ring where the chain is having problems, as the adjacent chainring will be scarred. Twisted teeth are irritating – they can be tricky to spot, but will hook onto the chain and lift it up and around, rather than releasing it as they should. Bent teeth can be eased back straight with care. File off any tooth or part of tooth that protrudes sideways. Each tooth has a pulling surface, at the front of the tooth at the top of the chainring. When the chain is under pressure, this area of tooth can get splayed so a lip forms on one or both sides. As a temporary fix, these can be filed off, but it’s usually a sign that the chainring is worn and needs replacing.
Lubrication and cleaning
First, try cleaning everything – the cheapest option, but one that may make things worse if it proves you have at least one seriously worn component, held together only by its own dirt. Or try an ACSD (AntiChainSuckDevice). This is an ugly aluminium plate that bolts to the bottom of the chainstay tube behind the chainset. Steps on it are shaped to fit the steps around the three chainrings, but without quite touching. The chain gets sucked up behind the chainring and hits a step on the acsd instead of carving chunks out of your chainstay. ACSDs are a mixed blessing. Sometimes they help, however, sometimes the chain jams in them, and makes everything worse. We recommend finding the source of the chainsuck and sorting it out, rather than hoping the acsd will bail you out. The only exception is with carbon frames, which should be permanently fitted with protection under the chainstay; chains make grated cheese from expensive carbon chainstays in seconds.
Source : BIKE MAINTENANCE TIPS, TRICKS & TECHNIQUES
See also bike maintenance tips, tricks and techniques “Replacing chainrings”