Using a scrap of cable to set the derailleur in a convenient sprocket
Gear cables are specifically designed to pull against the resistance of a spring inside the derailleur. When the cable is released, the derailleur springs back to its default position. This is usually the smallest sprocket or chainring, though Shimano briefly produced a type of rear derailleur known as Rapid-Rise, or lownormal, which defaulted to the largest sprocket. If you do snap a gear cable, one option is to allow the chain to return to its default position and ride home in that gear, which still gives you the flexibility to use the other derailleur normally.
If the cable is broken and trailing wires, get rid of them so they don’t catch on anything. Either undo the pinchbolt that holds the cable on and remove it completely, or coil up the dangling wire and tape it to your frame. If you’re removing the cable, coil it up and take it home – don’t discard it on the trail.
Broken rear gear cables occur relatively frequently because the cable is long and passes through several angles, especially with dual suspension bikes. Occasionally the cable frays and breaks; more often the outer casing splits and gives way. Both breaks have the same effect: without the pull of the cable, the spring in the derailleur pulls standard derailleurs to the smallest sprocket, and rapid-rise ones to the largest. Broken front gear cables seem to happen less often, maybe because they don’t work as hard as rear ones. The breakdown is still irritating because the spring in the derailleur will pull the chain to the smallest chainring, leaving your legs spinning furiously without making much progress.
Broken rear derailleur cable
Gear cables usually fray long before they break. It’s worth getting into the habit of checking them whenever you clean your bike so that you can replace them at the first signs of wear and tear. But sometimes they catch you out and break unexpectedly. Without the balancing tension of the cable, the spring in your derailleur will pull the chain into its neutral position – the smallest sprocket (highest gear) for normal derailleurs, or the largest sprocket (lowest gear) for rapid-rise derailleurs. This may not be the most convenient gear for you to limp home in, so try these methods of temporarily locking your chain into a more useful sprocket.
If you’d like an easier gear than the smallest sprocket at the back, you can use the end-stop screws to reset the derailleur. Get someone to lift up the saddle to get the back wheel off the ground. Turn the pedals slowly with your right hand and use a Phillips screwdriver to screw in (clockwise) the high ‘H’ end-stop screw. As you turn the end-stop screw, the chain will gradually change gear from the smallest sprocket to the next one. It might even make it to the third sprocket. When the screw has gone in as far as possible, back it off (undo it, anticlockwise) so that the chain runs easily in the chosen gear, without clicking or trying to drop into another gear.
A spare scrap of cable can be used temporarily to set the chain to run in a lower gear. A spare brake cable will work if you have one; if not, remove the broken cable from the shifter and use the end with a nipple still attached. Feed the cable through the barrel-adjuster on the rear derailleur so the nipple sits in or over the barrel-adjuster. Next, push the rear derailleur across by hand so that it sits under a more convenient sprocket, and clamp the cable in the usual place under the pinch bolt. Use the barreladjuster to fine-tune the position of the derailleur so the chain sits directly under a sprocket without rattling. Coil up spare cable so it doesn’t get caught in the chain or back wheel.