You’re riding hard in the big chainring toward a steep climb. You push to keep your momentum going as long as possible, but finally you’re forced to shift to the small ring. Oops! The chain overshoots and falls onto the frame. You’re dead in the water, wildly turning the cranks with no resistance.
Later, the opposite happens. You’re spinning in the small chainring and need a bigger gear. But your shift to the big ring sends the chain over the top. Now it’s flopping around the crankarm.
Throwing a chain is usually the result of a badly adjusted front derailleur. That’s the first thing to correct if it’s happening frequently. But even properly adjusted equipment can goof if you’re pedaling too hard or too fast while shifting.
The chain can skip off the inside ring or you can experience the opposite problem — it won’t move off the big ring at all. That happens when there is so much tension on the chain that the derailleur can’t pull it from the teeth. The solution is simple: Reduce pedal pressure a bit just as you make the shift.
What To Do If You Throw a Chain
Shift the chain back on. Gently! No matter what causes the chain to fall off, you may not have to stop and get your hands greasy putting it back on. While still rolling, turn the crank easily and shift the front derailleur in the appropriate direction. The chainring teeth should catch the chain and set it back in place. Stop pedaling instantly, though, if the chain tangles or binds. Any force at this point can damage the chain, the chainrings or the derailleur. If you have enough momentum, you can backpedal to free the chain and then try shifting it again.
Set the chain back on. If nothing works, click out and stop before you teeter over. You’ll have to re-rail the chain by hand. When it has fallen to the inside, sometimes you can do it by picking up the rear of the bike so that the chain drapes over the small ring. Then turn the crank by hand so it catches the teeth. Otherwise, look for a stick or piece of litter so you can pull the chain up and on without soiling your hands. Another trick is to use one of the tire levers you should be carrying in your seat bag. Or the spare rag you might have wrapped something in.
Use a Chain Catcher. There are several commercially available chain catchers. They work by physically preventing the chain from falling off inside the small chain ring. In effect, they “redirect” the chain back onto the small ring.