Chain suck is something that happens mostly on mountain bikes that are ridden a lot, especially in muddy conditions. When it occurs, usually the pedals suddenly jam with an accompanying grinding sensation. What's happened is that the chainring (one of the sprockets on the crankset), has grabbed the chain (the bottom portion) and pulled it up, jamming it into the underside of the right chainstay. In bad cases, the chain can get forced between the chainring and the chainstay becoming seriously stuck. And, even when it's just a hiccup in your pedaling, the chain strikes the stay and may damage the paint.
The most common cause of chain suck is a worn chainring. As the teeth wear, they develop a hook shape that makes them more likely to snag and suck the chain. You can actually see the difference by looking closely at the teeth to compare them with the teeth on the large chainring (which wears at a slower rate because it has more teeth). Worn teeth will look like a shark's dorsal fin, whereas the good teeth have a symmetrical shape with an even pitch on both sides.
So, the best cure is usually to replace the chainring. The exception is when you experience chain suck on a new chainring, which can happen. In this case, the problem is usually caused by mud or grime build-up on the chainrings. If there's a build-up on the rings, it can jam and suck the chain, the same way worn and hooked teeth can. The solution for this is to clean the rings. Sometimes, this is as easy as stopping at a creek crossing and rinsing the rings to clear the mud. In bad cases, you might have to remove the crankset at home to thoroughly scrub the rings. Remember to lubricate the chain and chainrings after cleaning, because a dry drivetrain can bind and suck, too.
Another thing that can cause chain suck is shifting while you're pedaling too powerfully. What happens here is that the chain is trying to move sideways to complete the shift but you're applying excessive forward pressure. This combination of forces can cause the chain to jam, get pulled up by the chainring and get sucked into the chainstay. To prevent this aggravation, shift only when you can ease the pedaling pressure. This means that if you're on a steep section, you should have shifted before the steep part when there was an opportunity to take some pressure off the pedals and complete a smooth shift. Besides preventing chain suck, this technique helps your drivetrain components last longer and guarantees that all your shifts are trouble free.
If these suggestions don't end your chain-suck problems, bring your bike in and let us have a look. We'll find out what's going wrong and offer a cure for this annoying glitch.