A bicycle chain is enormously strong and relatively lightweight. But unfortunately, it is not trouble-free and sometimes slips off the sprockets. Oddly enough, this is not necessarily a sign that the bike is in bad shape--sometimes a chain slips off for no apparent reason. But an experienced rider can fix this problem in seconds, and a novice can learn to do it in a few minutes. A chain that repeatedly jumps off is a sign that something is out of line. Start by checking the rear wheel alignment. The wheel should spin freely and not wobble from side to side. Additional checks, most likely with the help of a bike shop, include making sure the derailleur is properly aligned and none of the sprockets are worn or bent.
Instructions
1. Quit pedaling as soon as you realize the chain has slipped out of place. Further pedaling can jam the chain between the sprockets and the frame.
2. Stop the bike and get off.
3. Free the chain if it has jammed. Forget about keeping your fingers clean.
For a chain that has slipped off the front sprockets
4. Lay a section of the chain across the top of any of the front sprockets (see A). You do not need to wrap the chain all the way around the sprocket.
5. Lift the rear tire of the bike slightly off the ground. Slowly rotate the pedals. The chain should fall into place on the sprocket and spin freely.
6. If the chain immediately slips off the front sprockets again, adjust the position of the front shift lever and try again. You will not damage the bike by moving the shift levers while the pedals are stationary.
7. If the chain stays on the sprockets but continually makes a rubbing or clicking noise, adjust the position of the front shift until the noise disappears. If the noise does not disappear, you will need to adjust the derailleur alignment (see Fix a Poorly Shifting Bicycle).
For a chain that has slipped off the rear sprockets
8. If the chain has slipped off the largest rear sprocket, check to see that the rear derailleur is not in danger of jamming in the rear wheel spokes. Lift the rear of the bike and spin the wheel. Look to see if there is clearance between the derailleur and spokes. If there isn't, do not use the largest rear sprocket until you have a chance to adjust the derailleur alignment (see Fix a Poorly Shifting Bicycle). Otherwise, you can ruin the wheel and the derailleur, and possibly crash.
9. Lay the chain across the top of any of the rear sprockets. Don't worry if it crosses over several sprockets at first.
10. Make sure the chain travels through the rear derailleur freely. It should follow both guide wheels in the rear derailleur (see B).
11. Lift the rear tire of the bike slightly off the ground. Slowly rotate the pedals until the chain falls into place. If the chain slips or jams, adjust the position of the rear shift lever and start over.
12. If the chain continues to fall off, avoid setting the shift lever at the extreme positions (very high or very low) until you have a chance to check the derailleur alignment (see Fix a Poorly Shifting Bicycle).
Tips Warnings
A bicycle with only a rear coaster brake (a brake that is activated by the pedals) has no brakes if the chain falls off. Do not ride this type of bike if you suspect there are problems with the chain.
Keep your fingers clear of the sprockets when turning the pedals.
