Why Does My Bike Chain Keep Falling Off?

Having a chain fall off while you are riding your bicycle is annoying, if not dangerous! However it also can be extremely difficult for a beginner to be able to figure out why in the world it is doing this.

If your chain has started to come off frequently when it didn’t in the past, something in your bike setup has changed.

The first thing to do is check that all of your drivetrain bolts are tight, that nothing has moved or got whacked, and to make sure your rear wheel is correctly seated in the dropouts.

To know more about this, we will discuss all  the reasons leading to chain drop along with a few ways on how you can fix it.

Why does my bike chain keeps falling off?…

Most of the time, bike chain keeps falling off can be caused by a the chain itself or bad adjusted on the drivetrain system, after long riding mileage, the chain will get stretched causing damage to both chainrings and cassette teeth, therefore it loses its good mechanical contact causing the chain to jump from the gear to fall out.

Table of Contents

Reasons your bike chain keeps coming off (and how to fix them!)

Reason 1: Drivetrain is worn out

How long has it been since your drivetrain was replaced? If you’ve put more than a thousand miles on the drivetrain and the chain consistently slips off the back, then it very well could be time to look into replacing the drivetrain.

Inspect the teeth of the cogs in the drivetrain. If the teeth have become hooked and shark-tooth looking, then it’s probably time to replace them.

This is usually more noticeable in the gears you use the most.

While it’s sometimes possible to just replace the individual cogs, it’s usually easier and more practical to replace the entire cassette.

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Reason 2: Chain Stretch

Technically, chains don’t stretch but they do elongate! This happens over time as the internal components of the chain start to wear down.

If you’ve got a chain checker , this is really easy to figure out. Most tools have two sides, one marked with a “0.5” and the other with “0.75”. These labels indicate if the chain has stretched 0.5% and 0.75% from its original length.

Using the 0.5 side, install the hook end into a link with inner plates to ensure that you’re able to get an accurate measurement against the roller. Allow the other end of the tool to fall downwards and see if it’s able to fall into the gap of the next roller.

If the chain check doesn’t fit in – that’s a good sign! It means that your chain has stretched less than 0.5% at this point and does not need to be replaced. If it does fit in, then it has stretched 0.5% or more.

Flip the tool over and repeat the test using the 0.75 side. If it’s able to slip in, then it means your chain is at least 0.75% longer. The decision to replace the chain for numbers at or below 0.75 will be dependent upon the number of rear gears.

  • 0.5% – Replace the chain if you have 11+ rear gears
  • .75% – Replace the chain if you have 10 or fewer gears
  • 1.0% and up – Replace the chain if you have a single speed or two-sprocket bike

Reason 3: Chain is clogged

If your chain is downright filthy then it’s not going to operate well. Think about how poorly your car would run if you NEVER changed the oil! Well, cleaning and oiling your chain is your bike’s equivalent of a car’s oil change.

Dirt and grit can work their way into the chain components and cause it to not move smoothly over the cogs. Because they don’t articulate as smoothly, it’s much easier for the chain to drop off during bumpy trails sections.

Depending on just how dirty your chain is, you may need to run through a full cleaning session with a degreaser before re-oiling the chain. However, you can also try using chain oil and a few clean rags to spin it clean as a shortcut.

Note that you’ll probably have to repeat this process a few times if the chain is particularly dirty.

Take a look at my in-depth guide on chain cleaning for more specific details on this process.

Even if you’re doing a good job keeping up with your chain maintenance, sometimes all it takes is a single stiff link to pop the chain off. A stiff link will have a hard time bending around the chainrings and jockey wheel, leading to mishaps.

If oiling the chain doesn’t fix it, slowly backpedal the chain while watching the rear derailleur. Carefully watch for a link that jumps and clicks as it passes over the jockey wheel.

If you’ve got a chain tool, you can use it to help work the link loose.

Otherwise, use your hands to fold the stiff link and try bending it side to side until it’s able to move smoothly again (in the correct direction). Gloves are recommended for this approach.

Reason 5: Excessive Chain Angle

Did your chain drop when you were riding cross-chained? This happens when you’re using the largest ring in the front and back, or vice versa.

Doing so creates an excessively slanted chain angle that stresses the hardware and causes chain rub, eventually leading to a dropped chain.

The good news is that fixing this issue only requires you to be more cognizant of your gear usage during rides.

Rather than using the extremes of your gears, stick to more intermediate combinations to prevent excessive chain angle and give yourself more options for shifting (up or down).

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Reason 5: Wrong Size of Chain

Did you just replace your chain? Are you sure that the new one is the right size? With a chain that’s new or very lightly used, chain stretch can’t be the issue since it hasn’t had enough use yet.

If you’ve still got the old chain, you can compare the length of the two. Because the old one will have stretched, your new one should be a little bit shorter.

If the chain is too long and has too much slack, you will probably need to remove a couple of links to obtain the proper length.

A chain that is at the proper length should be able to reach the large-large gear combination, and not have the rear derailleur overlapping itself when in the small-small gear combination.

Reason 7: New chain is incompatible with drivetrain

Assuming that you’re not rocking a single-speed mountain bike, then your bike is using a derailleur bike chain. Derailleur bike chains are designed to allow the chain to transition from one sprocket to the other during shifts.

As the number of gears in the cassette increases i.e. 10 to 12, the spacing between the cogs often gets smaller. To allow for proper shifting, chains follow this trend and also get narrower to accommodate the narrower spacing.

For this reason, chains are manufactured for use with a specific speed drivetrain.

  • Double-check that your chain speed is compatible with the drivetrain.
  • Verify that your drivetrain doesn’t require use with a specific chain. For example, most 11-speed drivetrains are not cross-compatible with chains from other manufacturers. Stick with all Shimano, SRAM, Campagnolo, etc…

Reason 8: Bent Chainring or Chainring tooth

While most cases are nowhere near as severe as the one pictured here, a bent chainring or even chainring tooth can be your source of dropped chains.

Accidents are the most common cause, however, a bike that simply topples and strikes a hard surface could be enough to bend a chainring tooth.

Inspect the chainring by backpedaling and seeing the distance between the chainring and frame changes. If the eye test is hard to judge, brace your hand against the frame and try gently pushing something into the chainring while it’s rotating.

If you feel a sudden change in resistance pushing back against your hand it means that the ring is bent. Badly bent chainrings will have a very apparent wobble to them while turning.

Because many modern mountains have only 1-2 chainrings, it’s not likely that you don’t use them. You can try to use a wrench to gently bend the chain ring back into place, or they also make specialized chain ring straightener tools as well.

If you don’t want to go to the hassle or the chain ring is really in terrible shape, then just go ahead and replace it.

Reason 9: Rear/Front derailleur is not aligned or they are bent

Not only can poor alignment of the rear derailleur lead to noisy shifts, but it can also lead to chain drops in severe cases of misalignment.

Limit screws control how far the derailleur can travel inward and outward. They prevent the chain from going into the spokes or frame.

Definitely not something that you want to happen while mountain biking!

You should also inspect your rear derailleur hanger to see if it has been bent. It should be perfectly parallel to the cogs, and not bent inwards or outwards.

Use a wrench or another tool to manually bend it back into alignment.

Be aware that this process could cause the hanger to break, so it helps to keep a spare on hand.

Reason 10: Chain tension

If your chain came off after a big impact it may just be that you need to install a chain guide or chain tension tensioner to help it stay in place.

This is particularly useful during large drops or extremely rough downhill sections to avoid chain slap.

The quickest fix is to add on a chain guide. MRP makes cage-like one that resembles a front derailleur with a bottom bar to prevent the chain from flopping too far downwards.

Bike Chain Falling Off Frequently Asked Questions

What does it mean if your bike chain keeps falling off?

As mentioned above, there are many reasons why your bike chain keeps falling off, including worn out chain, loose chain, stiff or bent links, or has become clogged up with dirt.
A visual inspection while turning the cranks should reveal if there’s a problem here. The chainring, or a chainring tooth, could be bent.

How do I know if my chain is worn?

To know if your chain is worn, you have to  measure it with a ruler. By simply lifting the chain off of the front chainrings to measure the distance between two rivets on your chain.
But the easiest and most accurate way to determine chain wear is by using a tool such as the CC-3.2 Chain Checker or the CC-2 Chain Checker.

Conclusion

There’s nothing more frustrating than a mountain bike chain that keeps coming off! In some cases, a dropped chain is a one-off occurrence, but if it continues to happen then there’s likely a good reason for it.

The most popular reasons, and easiest to fix, range from the chain being worn out, or that one of your derailleurs just isn’t as accurate as you would like.

A bike with a chain keeps falling off is not pleasant or even safe, thus, fixing this problem as soon as possible is highly recommended.

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By Paul P.

Paul Panha is an avid cyclist and sports performance enthusiast. Regularly on the road testing (and buying) new bikes. Paul is a co-owner of GetBike.fit and is the head resident author here.