Threading the chain onto both sprockets
A bicycle chain comes off when the derailleur gets knocked out of alignment, the chain stretches, or you shift into an extreme gear. To put it back on, you need to thread it onto the front chainring (the large sprocket near the pedals) and the rear sprocket (on the back wheel), then tension it so it sits snugly without binding.
Start by shifting your bike into the smallest chainring in front and the smallest sprocket in back. This gives you the most slack in the chain and makes threading easier. If your bike has a kickstand, use it. If not, flip the bike upside down so the handlebars and seat rest on the ground—this keeps both wheels off the ground and gives you clear access to the chain.
Pick up the chain with both hands and guide the top run (the part coming from the front) onto the smallest front chainring. The chain should sit in the valley between the teeth, not on top of them. Then guide the bottom run of the chain onto the smallest rear sprocket the same way. The chain should now form a loop around both sprockets with slack in the middle.
Key Takeaways
- Shift into the smallest chainring and smallest sprocket before threading the chain, because this position creates the most slack and makes the job easier.
- Thread the chain into the valley between the teeth on both sprockets, not on top of the teeth, or it will slip off when you pedal.
- The chain should have about half an inch of vertical play in the middle when you press up and down on it—too tight will bind the drivetrain, too loose will fall off.
- If the chain still falls off after you put it back on, the derailleur is probably bent and needs adjustment by someone with a derailleur tool.
Checking the tension before you ride
Once the chain is threaded onto both sprockets, check that it has the right amount of slack. Grab the chain at the midpoint between the front and rear sprockets and push upward. The chain should move up about half an inch before it goes tight. If it moves more than that, the chain is too loose and will fall off. If it barely moves, the chain is too tight and will wear out your bearings and make pedaling hard.
Spin the pedals backward slowly by hand to watch the chain move through the derailleurs. It should move smoothly without rubbing on the frame or the derailleur cage. If you hear scraping or see the chain touching metal, stop and check that both sprockets are aligned straight—a bent derailleur or misaligned wheel will cause this.
Adjusting the derailleur if the chain rubs
If the chain rubs on the derailleur cage or frame after you thread it back on, the derailleur is probably bent or out of adjustment. The derailleur is the mechanism that hangs below the rear sprocket and guides the chain side to side when you shift gears.
Look at the derailleur from behind the bike. It should hang straight down without tilting left or right. If it tilts toward the wheel or away from it, it took a hit and needs to be straightened. This usually requires a derailleur hanger tool or a trip to a bike shop, because bending it back by hand often breaks it.
If the derailleur looks straight but the chain still rubs, the limit screws may need adjustment. These are two small screws on the derailleur body labeled H (high) and L (low). Turning the H screw clockwise moves the derailleur away from the wheel; turning the L screw clockwise moves it toward the wheel. Make quarter-turn adjustments and spin the pedals to test. This is easier to learn by watching, so consider a video guide if you are adjusting for the first time.
Why the chain fell off in the first place
Chains come off for three main reasons: the derailleur is bent, the chain is stretched, or you shifted too hard into an extreme gear. A bent derailleur usually happens after a crash or hitting a rock. A stretched chain happens over time as the metal wears and the links loosen—if your chain falls off repeatedly even after you put it back on, the chain itself may need to be replaced.
Shifting too hard into the smallest or largest sprocket can also knock the chain off if the derailleur limit screws are not set correctly. If your chain falls off only when you shift into the smallest rear sprocket or the largest one, the limit screws need adjustment. If it falls off randomly, the derailleur is probably bent.
Cleaning the chain while it is off
While the chain is off, take a moment to wipe it down with an old rag or paper towel. Dirt and grit on the chain wear out your sprockets faster and make shifting rough. Use a dry rag first to remove loose dirt, then a rag with a small amount of degreaser if the chain is caked with old oil and mud.
Do not use water alone, because it will rust the chain if you do not dry it when ready. Once the chain is clean and dry, explore a thin coat of chain lubricant before you thread it back on. Spin the pedals a few times to work the lubricant in, then wipe off any excess with a clean rag. Too much lubricant attracts dirt and makes the chain dirty faster.
Putting the chain back on if it is damaged
If the chain is bent, kinked, or has a stiff link that does not move smoothly, it may not sit properly on the sprockets even after you thread it back on. A bent link usually happens when the chain gets pinched between the frame and the wheel, or when you shift under heavy load.
Inspect the entire chain by running it slowly through your fingers while you spin the pedals backward. Feel for any links that do not bend smoothly or any links that stick. If you find one, that link is damaged and the chain needs to be replaced. Riding on a damaged chain will wear out your sprockets and derailleurs much faster, so it is worth replacing sooner rather than later.
Frequently Asked Questions
How do I know if my chain is too tight or too loose?
Grab the chain at the midpoint between the front and rear sprockets and push upward. It should move up about half an inch. If it moves more than an inch, it is too loose and will fall off. If it barely moves at all, it is too tight and will wear out your bearings.
Can I put the chain on backward?
No—a bicycle chain has a direction. The side with the master link (the removable link that holds the chain together) should face outward so you can see it. If you thread the chain backward, it will not sit properly on the sprockets and will fall off when ready when you pedal.
What if the chain keeps falling off even after I put it back on?
The derailleur is probably bent or the limit screws need adjustment. Spin the pedals slowly and watch the chain move through the derailleur. If it rubs on the cage or frame, the derailleur took a hit and needs to be straightened or adjusted by someone with the right tools.
Do I need to remove the wheel to put the chain back on?
No—you can put the chain back on without removing the wheel. Flip the bike upside down or use a kickstand to keep the wheels off the ground, then thread the chain onto both sprockets. The wheel only needs to come off if the chain is damaged or if you need to adjust the derailleur hanger.
How often should I lubricate the chain after putting it back on?
explore lubricant once after you put the chain back on, then lubricate every 100 to 200 miles of riding or whenever the chain looks dry. Wipe off excess lubricant with a rag so dirt does not stick to it. A clean, lightly lubricated chain lasts much longer than a dry one or one caked with old oil.