If you buy a new bike chain, it is most likely longer than needed for most bicycles. This is because chain manufacturers sell chains longer than is necessary to accommodate a wide range of bike types.
Having the correct chain length on your bike is important because the incorrect length may cause shifting and riding problems. For instance, if the chain is too short, it will be impossible to shift into some gears. Or if the chain is too long, shifting will become sluggish or the chain will start to come off.
So you’ll need to make sure you’re using the correct chain length for your bike, which means the new chain will need to be sized to your particular bike and gear combination. Therefore, the new chain needs to be “cut” (links separated or removed) to fit each bike.
So how do you know what is the correct chain length for a bike? A general rule for chain length is that there should be enough chain to allow shifting onto the largest cassette cog while also on the largest chainring. If you are putting on a new chain, you will need to determine how many links you’ll need to get the correct bike chain length by measuring it in one of three ways:
- Compare it with your old chain, assuming your old chain was the correct length
- Bike chain sizing using the largest chainring to the largest cog method
- Chain sizing using a mathematical equation
Below we will lay out step-by-step instructions for each of the three methods to properly size the chain length and discuss ways to detect if the chain is too long or too short. So if you want to know how to size a bike chain like a professional cyclist, then keep reading!
Chain Sizing Using The Old Chain
If a new chain is being installed and the old chain is the correct length, the new chain may be shortened to match. Here’s how to do it correctly:
- Test the length of the old chain by shifting to both the largest front and rear sprockets and then the smallest front and rear sprockets. If the chain passes both tests it is an acceptable length.
- Remove the old chain and lay it on a flat surface with the plates aligned vertically. Pull the chain straight.
- Lay the new chain next to the old chain in the same fashion, with inner plates of both chains at one end. Keep in mind that the new chain may not exactly match rivet to rivet toward the end of the chains.
- Push the links of the old worn chain together to match up with pins or rivets of the new chain. Make sure you account for any master link by placing it at one end of the new chain.
- Locate the matching end rivet on the new chain with the rivet on the old chain and cut the new chain.
Chain Sizing Using The Largest Chainring to Largest Cog Method
The procedure below will help you size the bike chain to permit the chain to be shifted to the largest front chainring and the largest rear sprocket. Although most cyclists may not use the large front and large rear combination, we need to assume that shifting to this gear combination might happen. And if it does happen, we want to be assured that the chain will not be too short to cause a jam and potentially cause damage to the drivetrain.
- Shift the front derailleur over the largest chainring and the rear derailleur under the smallest sprocket
- Route the new chain through the front derailleur, but do not thread the chain through the rear derailleur
- Wrap the chain around the largest front chainring and around the largest rear sprocket
- Bring both ends of the chain together and note the rivet closest to where the two ends could be joined.
- From the closest rivet where the two ends could be joined, count over an additional two rivets, which should add about 1 inch to the length
- On the end of the chain with extra links, pinch the link with your thumb and forefinger where you need to remove any extra links. This will help you remember where to break it.
Chain Sizing Using A Mathematical Equation
A simpler way to size a bike chain without removing the old chain from the bike or even taking the new chain out of the box is using this simple equation:
L = 2 (C) + (F/4 + R/4 + 1)
L = Chain Length in inches
C = Chainstay Length in inches
F = Number of teeth on the largest front chainring
R = Number of teeth on the largest rear cog
Obviously, before you can use the equation to get the chain length, you will need to gather some information about your drivetrain system so you can plug them into the equation. Here is what you’ll need:
- Count the number of teeth on the largest front chainring. The number of teeth may also be printed or etched on the sides of the sprocket. Plug the number into the “F” variable in the equation.
- Count the number of teeth on the largest rear sprocket. This may also be printed or etched on the sides of the sprocket. Plug this number into the “R” variable in the equation.
- Measure the chainstay length to the nearest 1/8 inch and convert it into decimals. This is the distance between the middle of the crank and the rear axle. Plug this number into the “C” variable in the equation.
- Solve the equation to get the chain length. Keep in mind that bike chains are joined together by linking the inner plate with the outer plate, therefore, round it to the nearest inch.
- Measure and cut the new chain at the length determined in step 4
How Do I Know If My Bike Chain Is Too Long?
When the bike is in the smallest rear sprocket and smallest front chainring, a chain that is too long will sag between the derailleur and the chainrings. Although the chain may have little tension in this position, it should not droop or sag much between the front and the rear sprockets.
Another sign that the chain is too long is when the chain comes into contact with itself as it passes by the upper derailleur pulley while riding in the smallest rear sprocket and the smallest front chainring.
How Do I Know If My Bike Chain Is Too Short?
Conversely, you may also experience problems when the chain is too short. To detect a short chain, you must set the chain to its tightest chain tension. Chain tension is normally the tightest when it is positioned at the largest chainring and the second-largest rear sprocket.
While shifting slowly and carefully to the largest rear sprocket, inspect the chain for a double bend (“S” bend) as it passes through the pulley wheels. If you lose the double bend at the pulley wheels, your chain is too short. Also, if the chain appears to jam, it is too short.
When the chain is too short, attempting to change to the larger rear sprocket and the largest front chainring combination may damage the derailleur and/or derailleur hanger.
Chain slack is generally taken up by the rear derailleur cage as the chain moves between the various front and rear sprocket combinations.
However, there are some bicycles that have sprocket combinations and derailleur models that prevent the derailleur from wrapping up chain slack in every conceivable gear combination. In this case, the sprocket selections exceed the chain wrap capacity of the derailleur.
And when this happens, the chain length will appear either too long in the smallest sprocket to the smallest chainring combination or too short in the largest sprocket to the largest chainring combination. This is often seen when a short cage derailleur is used on a bike with a wide gear range.
If your bike has a derailleur that does not meet the gearing capacity, then it will be necessary to avoid certain gear combinations that cause problems with pedaling or shifting.
Learning how to size a bike chain is an important aspect of caring for your bicycle. The last thing you want when riding your bicycle is an incorrectly sized bike chain which can cause all sorts of problems with shifting and pedaling efficiency.
This article has given you three ways to properly measure your bike chain, and we’ve also explained what happens if the chain length isn’t correct. So we hope that this article has been helpful in finding the correct bike chain length.
Please feel free to comment below. We would love to hear which method you chose to measure your bike chain length.