If you need to change the chain of your bicycle, you must know its length to replace it with a new one of the same size. You can compare the original with the replacement and match the pins. If you don't have the original chain or the one you have is not the correct size, use the largest cassette and the largest chainring to discover where the new chain should be cut. If you want to find out the proper measure without having to manipulate the string, use a mathematical equation. By counting the number of teeth on the chainring and cassette and measuring the length of the lower chainstay, you can determine how long the chain should be.
Steps
Method 1 of 3: Cut the Original Chain
Step 1. Lay the original chain and the replacement chain side by side
If the original chain is the size you want to replicate on the replacement chain, place them side by side so that the plates are side by side. Make sure the plates on each chain are aligned. If you are using a master link, insert it so that the measurement is accurate.
For example, align an outer plate with another outer plate or an inner plate with another inner plate
Step 2. Line up both chains
Insert a metal rivet or long pin through the end of the chains that you won't cut. The pin will hold the chains together and keep the links aligned while you prepare your measurements and cuts.
Step 3. Match the chain pins
The original chain has likely been stretched over time of use, so try to bring the pins together in such a way that they directly match those of the new chain.
Step 4. Determine and mark where you will cut the new chain
Once you've matched the pins, you can see exactly where the original chain ends. Arrange the new chain so that the plates are facing you and make a mark on the pin to be cut. This should match the last pin on the original chain.
Once you've cut your new chain, it's ready for you to install on your bike
Method 2 of 3: Use the Largest Cassette and Plate
Step 1. Shift the gears and fit the new chain around the rear cassette
Shift the front derailleur to the largest chainring and shift the rear derailleur to the smallest cassette. Place the new chain around the larger rear cassette.
- If you're using a connecting chain, move the end with the outer plate toward the front of the chainring. You should continue threading the chain until the end is at the 5 o'clock position (alluding to the hands of the clock).
- If you are using a master link chain, place the link half on the board.
Step 2. Pull the bottom of the chain toward the front of the chainring
Gently pull the lowest part of the chain up to the front chainring. Hook the chain onto the front chainring so that the link coming from below the chain meets the end of the chain coming from above.
With this method you won't have to pull the chain through the rear derailleur
Step 3. Find the cut point on the chain
Locate the outer plate closest to the inner plate where the chain meets. Use a marker to circle the pin that will join the two chains. Count 2 pins in the excess of the chain. You will need to cut the chain at this point, so consider drawing a line with a marker. Use the chain cutter to cut the chain to the proper length.
The 2 links are roughly equivalent to 2.5 cm (1 inch), which is enough to adjust the rear derailleur
Method 3 of 3: Calculate the length of the chain
Step 1. Write the number of teeth on the largest plate
Look at the largest plate and find its measurements. The number of teeth is designated by a number with a "T" below. Use the first number on the largest front plate. For example, if the largest chainring says 52 / 36T, it means it has 52 teeth.
The complete equation to calculate chain length is as follows: (lower chainstay x 2) + (chainring / 4) + (rear cassette / 4) = correct chain length
Step 2. Write down the number of teeth that the largest rear cassette has
Look at the second number on the cassette to find out how many teeth it has. For example, if you see 11 / 28T it means it has 28 teeth.
Step 3. Measure the length of the bottom sheath and multiply by 2
Extend a tape measure from the center of the rear axle to the center of the crank bolt. Write the length to the nearest eighth of an inch, this way you will have the length of the bottom sheath. Multiply this number by 2 and write it down.
- For example, if the bottom pod is 16.25 inches (41 cm), multiply by 2 and you get 32.5 inches (82 cm).
- You will have to write the measurements in inches so that you can use the equation. Once you get the result, you can convert it to the metric system if you want.
Step 4. Divide the number of teeth on the largest plate by 4
Check the number of teeth you have written. For example, if it has 52 teeth, divide it by 4 and you will get 13. Write this number.
Depending on the cassette or platter, you may get an odd number to divide by 4. The calculation will work despite having decimals
Step 5. Divide the number of teeth in the largest posterior cassette by 4
Look at the number of teeth you have written in reference to the rear cassette. If you have 28 teeth, divided by 4, you will get 7 as a result, write it down.
You will probably get a decimal result if the number of teeth on the cassette is odd
Step 6. Add the modified numbers plus 1 inch (2.5 cm) to get the length of the chain
Add the multiplied length of the lower chainstay, the divided number of teeth on the front chainring and the rear cassette and add 1 (or 2.5 cm). The result is equal to the length that the string must be.