Mountain Bike Chain Length Basics

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Introduction

Having the proper chain length is extremely important for your bike. A chain that is too long will lead to sloppy shifting, dropped chains, and that awful slap against your frame. Yet, sizing a chain too short will lead to broken chains, binding, improper shifting and can even damage your rear derailleur. Many riders never realize that most bikes that come from the factory or bike shop floor are equipped with oversize chains that are too long for the bike which causes shifting problems soon after they begin riding. While there are ways to calculate chain length, a practical approach to sizing is easier, and will allow you to properly size your chain and position your rear derailleur easily.

The tools you need to size a chain properly:

  • Chain breaker tool
  • flat and Phillips head screwdriver
  • Tools to remove the pressure or spring from your rear shock (if applicable)

Full Suspension Bikes

When sizing a chain, it is important to remember that if all else is identical the chain on a full suspension bike must be slightly longer than that of a hard tail. The reason for this is that full suspension bikes are subject to chain growth. Chain growth refers to the fact that the distance between the bottom bracket and the rear axle increases as the rear suspension compresses on your bike. This means that if you don't have some slack in the chain to accommodate for this that the chain will be put under stress as the rear end of the bikes moves up and down. This will inhibit the function of your rear suspension and could break or damage your chain.

What this means is when we size a chain, any full suspension bike must be set to its lowest point of travel. This means that you will have to release the air from your rear shock, or remove the spring from your shock. Don't remove the shock itself as that will give you too much length in your chain. Once the bike is fully lowered make sure that it stays this way until you finish sizing the chain, I find it is best to zip tie the rear end of your bike to make sure it does not move around.

Derailleur with a too short chain
Derailleur with a too short chain | Source
Derailleur with a proper length chain
Derailleur with a proper length chain | Source
Derailleur with a chain that is too long
Derailleur with a chain that is too long | Source

Sizing the Chain

The easiest way to size your chain is to use the rear derailleur as a guide. The tension of the rear derailleur will allow you to easily visually asses the length of your chain on the bike. To size your chain correctly first make sure that your derailleur aligns the chain properly in the highest, and lowest gear settings on your bike. Make sure the derailleur is in proper working condition and the springs have adequate tension.

First will will examine the bike using the largest rear cog and the largest chainring in the front that you will be running. Some mountain bikers use the large ring as a built in bash guard, and never shift into it regardless make sure you use the largest cog and chainring that your chain could possible be shifted into.

  • Break the chain if it is not already. If it has a powerlink or masterlink please disassemble this.
  • Run the chain around the largest gears of the bike, so that the two ends dangle down near the pedals.
  • Pull the rear derailleur side to achieve the appropriate tension. See the photos provided to find what is adequate tension, and what is too much or too little.
  • Mark the link that you will need to break and then using the chain tool remove any excess chain. Remember that if you have a master link you will have one extra link of space so mark appropriately.
  • Assemble the chain on your bike, and voila, we have an appropriately sized chain.

Now, shift the bike into the smallest cogs you will be running. This should set the derailleur back to its most relaxed position. The rear derailleur however, should not be completely slack. If their is no tension in the lowest setting then the chain can easily be dropped and will slap the frame as you ride.

One way to deal with this issue is to tension the rear derailleur out from the cassette using the the position screw, however, you do not want to do this too much as it will create less contact with the cogs on the rear cassette. If there is slack consider removing a single link from the chain at this point, knowing that shifting into the largest cog on both ends is unlikely to happen. If you are uncomfortable with this option, it becomes a bit more complicated.

Drivetrain Adjustments

If you cannot achieve chain length that suits the gearing of your bike then it may be appropriate to adjust the gearing. I find that in a double ring setup that adjusting the size of the smallest front chainring is the easiest way to alter chain slack in the lowest settings. Simply sizing up two teeth can have a positive affect on getting a correct chain length and facilitate shifting and improve drive train function.

You can also change the gearing of your rear cassette although I find that this can often lead to shifting problems and gear skipping if you are altering the smaller cogs. Instead, you may want to consider running a 1X setup for your bike which in the long run does not eliminate as much gear range as you would expect. You can see the details in my Hub: The Advantages of Changing Your Mountain Bike to a 1 X 9 Drivetrain. Using a single ring is a growing trend in my experience and eliminates a host of issues you would other wise have.

In my experience, a bit of slack is a bit better than a chain that is just too small. It is more likely to break a chain when hammering down a trail in setup with too much chain tension, than it is you will lose a chain going slowly up a climb. Some riders run their chain purposely short and never shift into the extreme gearing used to size the chain. Using such a setup requires a comfort level with your gears and bike setup that many riders do not have. Being in the wrong gear could easily lead to a broken chain. Once your chain is appropriately sized, you should notice smoother shifting, less chain slap and a quieter drivetrain on the trail.

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