So you're having a lovely time out on the trails or in the hills, when suddenly you're pedalling really fast and going nowhere. You've got a broken chain. Hurray!
Before you touch anything, find out what caused the chain to break. Has a stick gone into the rear derailleur? (And, if so, has the derailleur and/or hanger also been destroyed? If so, you'd better start walking.)
You'll need a joiner link and a good-quality chain-breaker tool. (Modern chains are pretty hard and will often destroy a cheap tool, or vice versa.)
There are many brands of joiner links these days, so work out which fits your chain and keep a couple in your tool kit (...er... unless someone from Shimano, SRAM or Campag is reading this, in which case, only ever use the joining system approved by the chain manufacturer). *cough*
Now, back to it. Simply push out the pins of the damaged link, then fit the joiner.
If several links need to be removed, you'll need to remember that you've shortened the chain when you get back on the bike. Avoid any gear combinations that may over-stretch the derailleur cage (such as the big chain ring/low gear combination). If you're not careful, it may lock up your gears or, worse, rip off your rear derailleur.
It's always a good idea to carry a few spare links of the same chain you use, plus two joiners.
This enables you to replace whole sections of chain without losing overall chain length.
For a home user, we generally don't recommend trying to use genuine Shimano joining pins: they require a very high-quality chain tool and can damage the chain's outer plates if fitted incorrectly, leading to another broken chain further on down the trail. With all repairs in the wild, keep it as simple and easy as possible.
If this is the cause of the chain break and the wheel has been buckled, straighten the wheel by loosening the neighbouring spokes.
If the derailleur and hanger have also been damaged, try to bend them straight-ish with your hands. (It might be scary, but if you're miles from anywhere and you've got to get home; what else are you going to do?)
If your gear cable has broken during the incident, just leave it in the smallest rear gear; you've still got one, two or three chain rings to get home with.
In extreme circumstances, you may need to bypass the derailleur altogether.
To do this, run the chain on the inner chain ring (if you have a 2x system)/middle chain ring (if you have a 3x)/the only chain ring (if you're running 1x) plus a middle rear cog which allows you to achieve decent chain tension. (You'll obviously need to shorten the chain considerably.) You can now discover the wonderful joys of a single-speeding on your return trip!
If you have a dual suspension bike, lock out the rear shock if possible (otherwise pre-load the shock as much as possible), leave a little slack in the chain (not too much) and pedal gently.
Now try to get home 'cause your day is unfortunately officially over.
Once you're safe and sound, click here and we'll get the damage sorted out for you.