A loose bike chain can cause poor shifting, chain skipping, chain dropping, or accelerated drivetrain wear. Luckily it is usually an easy fix!

To find out whether your chain is too long, take a look at it from the side, with your eyes level with the chain. A loose chain will sag close to or below the chainstay as the chain hangs between the rear cogs and chainring. The chain might skip as you pedal or even drop off the gears. On derailleur-equipped drivetrains, the drivetrain might shift poorly as well.

How Tight Should My Bike Chain Be?

On a derailleurless drivetrain, you should be able to lift the chain about 1/2 inch above its resting position at the middle of the chain above the gears. If the chain is too tight, you might feel and hear binding in the drivetrain as you pedal. If it is too loose, the chain could skip or drop, and you can see it sag.

On bikes with derailleurs a loose chain will sag when the drivetrain is in the “small-small” gear combination, and a tight chain will not be able to shift into the “big-big” gear combination (even though these are not considered usable gear combinations for riding). A correctly sized chain will be able to shift into both of these gear combinations without sagging or getting stuck, although they should still be avoided while riding. The rear derailleur on these bikes serves two purposes: to allow shifting across the cassette or freewheel, and to tension the chain as the amount of chain wrapped around the gears changes as you shift.

Is A Loose Bike Chain Bad For Your Bike?

A loose chain can cause accelerated wear of your drivetrain components, leading to premature cassette or freewheel or cog replacement, chainring replacement, or chain replacement. 

Additionally, the chain could unexpectedly drop off the gears, causing you to lose your balance as you pedal leading to a crash, or it could become stuck in other parts of the bike and cause damage as it wedges itself between the crankset and frame, small cog and frame, or large cog and the wheel spokes behind it.

Here’s a helpful article on Bikepush if your chain keeps falling off. It explains in detail what the possible causes are and how to fix the problem.

Tools you might need to adjust the chain tension:

• 15 mm wrench or adjustable wrench
• Hex Wrenches
• Chain Tool
• Master link pliers

Size and adjust Chain Tension On A Bike With Derailleur

On a derailleur equipped bicycle, the chain should have just enough links to be long enough to shift into the “big-big” combination, but also not sag in “small-small” 

Note that for bikes with rear suspension the chain may sag in “small-small” as the chain often needs to have a few extra links so that the rear triangle can lengthen as the suspension compresses. You need to check your bike’s documentation for correct chain sizing information.  

In most cases, to correctly size a chain you should do the following steps:

• wrap the chain around the largest cog in the rear and the largest chainring in the front, and bypass both derailleurs;
• while holding one end against the teeth of the largest chainring, pull the other end of the chain and wrap it around the largest cog on the cassette or freewheel;
• line up the end of the chain against an opposite link (inner and outer links), and then
• add two rivets of chain length.
• Using a chain tool, break the chain at this rivet and attach using a connecting pin or connecting link.

After correctly routing this length of chain through the derailleurs and connecting the ends, this chain should have adequate tension even in the unusable “small-small” and “big-big” combinations (source).

If there is still sag in the chain, the rear derailleur itself may be damaged and unable to properly tension the chain, requiring repair or replacement. The spring that pulls the derailleur cage back may be worn out, and this is not something that is usually serviceable, but a new derailleur will get you back shifting.

Fix Loose Bike Chain On A Single Speed / Fixie / Internal Gear Hub

A derailleurless drivetrain will have an alternative mechanism to tension the chain. It may have horizontal dropouts, sliding dropouts an eccentric bottom bracket, or a dedicated chain tensioner. 

To correctly size the chain, adjust your tensioning mechanism so that it is in its “slackest” position (where the chainring and the rear cogs are closest to each other), and follow the same sizing method as in the derailleur drivetrain.

• while holding one end against the teeth of the chainring, pull the other end of the chain and wrap it around the cog;
• line up the end of the chain against an opposite link (inner and outer links), and then
• add two rivets of chain length.
• Using a chain tool, break the chain at this rivet and attach using a connecting pin or connecting link.

Here, the extra two rivets of chain will allow you to more easily remove your rear wheel. Once a correct length of chain is installed, the chain must be manually tensioned as there is no derailleur. 

For a bike with horizontal dropouts, pull the wheel back and lock the axle nuts in place at a position where the wheel is centred in the frame and the chain is adequately tensioned (you should be able to lift the chain about 1/2 inch upwards at the middle of the chain above the gears).

Some bikes with this style of dropout will be equipped with chain tensioners such as the Surly Tuggnut. These devices allow for fine tuning of the chain tension with the screw sticking out of the back before the axle is secured. If you are unable to tension the chain without the axle slipping out of dropout (rear facing dropouts) or reaching the end of the dropout (forwards facing dropout), your chain may be too long. 

For sliding dropouts, loosen the dropouts, slide them back until the chain is adequately tensioned and secure in place, similar to the horizontal dropouts. If you are unable to tension the chain without the dropouts sliding to their farthest position, your chain may be too long.

For eccentric bottom bracket systems, rotate both sides of the eccentric bottom bracket until the chain tension is adequate. If you are unable to rotate into a position where the chain is adequately tensioned, it may be too long.

Bikes without horizontal dropouts, sliding dropouts, or eccentric bottom brackets will need another device to tension the chain. Usually, these bikes have standard vertical dropouts and were designed with derailleur drivetrains in mind. Chain tensioners are available such as the Surly Chain Tensioner which attach to the derailleur hanger of the bike frame, and function similarly to the cage of a derailleur which applies tension to the chain.

Conclusion

A visit to your local bike shop will always solve the doubts you may have, but if you want to do your own bike maintenance or you’re just curious whether your chain is tensioned correctly, there’s a lot that you can do yourself, even if you don’t consider yourself a DIY expert.

Happy pedaling!