Sizing a Derailleur Chain to Length

Chains for derailleur bicycles should always be of adequate length. A chain that is too long or too short can cause shifting problems. However, the rear derailleur cage should be long enough to take up slack as the chain is moved between the different gear combinations. This is referred to as a derailleur’s total capacity, and is discussed below.

This article will describe two methods to achieve adequate chain length. Both methods tend to yield adequate chain lengths. It is not necessary to use both methods, as one method may appeal to you and your particular situation better. The first system described is the EXISTING CHAIN method. It assumes the bike already has a chain installed. The second system is LARGEST COG-TO-LARGEST CHAINRING method, and does not require that the bicycle have a pre-existing chain installed.

NOTE ON REAR SUSPENSION: If the bike has a rear suspension linkage, it is usually necessary to account for the movement of the rear hub from the bottom bracket. If the rear hub moves away from the bottom bracket, set chain length is the longest position. Consult the bike manufacturer for correct chain length if in doubt.

DERAILLEUR CAPACITY: The derailleur capacity refers to the derailleur’s ability to take up chain slack as the derailleur shifts between different gear combinations. Some bicycles may have gear combinations that do not allow the derailleur to take up the chain slack. In this case, the gearing on the bike exceeds the capacity of the derailleur. If the derailleur capacity does not match the gear sizes on the bike, the chain may appear to fail either the “too long” test or the “too short” test. A chain length cannot be found that will pass both tests. In this case it is better to size a chain that will pass the too long rather than the too short test. The EXISTING CHAIN method below will safely size a chain when a bike is violating the derailleur capacity. It may be necessary to avoid gear combinations that cause problems in pedaling or shifting on these bikes.

Method #1: Existing Chain

Before removing the old chain, check the bike for acceptable length. Cut the new chain relative to the old chain length. Shift bike to smallest chainring in front and smallest cog in back. Inspect the section of chain between lower derailleur pulley wheel and bottom of smallest chainring. There should not be an obvious sag in the chain. Check also that the lower section of chain does not rub at the derailleur’s upper pulley. It is normal for there to be low chain tension in this position, but the chain should not sag. Sagging in this position indicates a chain that is too long.

Too long a chain length sags in the smallest chainring at the front and the smallest cog at the rear. Note the chain-to-chain contact at the derailleur’s upper pulley.

Adequate chain length will not sag.

If the chain is too long, find the chain rivet contacting the bottom of the chainring, and mark the chainring at this point. Count two chain rivets toward the derailleur, lift this rivet and move to position of mark. The chain is now effectively shorter. Check for sag again. Repeat counting two rivets and moving to the chainring mark. The number of rivets necessary to shorten the chain must always be an even number (2, 4, 6, etc.) Repeat until the chain has no visible sag.

To determine if the chain is too short, shift chain to largest chainring and second largest rear cog. Chain will appear tighter in this position. Inspect chain for “S” bend as it passes through pulley wheels. Shift slowly and carefully to largest rear cog. If chain appears to jam, it is too short. If chain does shift, but there appears to be no double bend of the chain at the pulley wheels, it is too short. You will need to add two rivet lengths to new chain compared to the old chain.

NOTE: Never attempt to lengthen an old chain by adding new links.

Adequate chain length seen above in largest sprockets front and back. Note the chain bends at both pulleys.

Chain length is too short in the two images above. Note the lack of an obvious bend at the pulleys.

If the current chain length passes one of the tests above, but fails the other, it is likely that the dereailleur capacity does not match the gearing on the bike. Set the chain length so it passes the large ring to largest rear sprocket test. It will then be the responsibility of the user to avoid shifting to gear combinations that cause chain slack.

If a new chain is being installed and the old chain is the correct length, the new one may be shortened to the old length before being installed.

1. Remove the old chain and lay it on a flat surface with the rollers aligned vertically. Pull the chain straight.
2. Lay the new chain next to the old chain in the same fashion. Make sure the ends of the two chains match, with either a rivet or no rivet at each end. The new chain will appear shorter, so push the links of the old chain back to match up with the new chain.
3. Locate the matching end rivet on the new chain with the rivet on the old chain.
4. Add or subtract chain links as necessary from inspection results stated above and cut the chain at this point.

Method #2: Largest Cog-To-Largest Chainring

An alternative method for determining chain length for new chains is to use the largest size sprockets on the bike. It is easiest to size the chain without threading it through the derailleur.

1. Remove the old chain.
2. Shift the front derailleur over the largest chainring, and the rear derailleur on the smallest cog.
3. Thread the new chain through the front derailleur. It is not necessary to thread the chain through the rear derailleur at this point. Simply wrap the chain around the largest front chainring and around the largest rear cog.
4. Pull the chain tight, and note the closest rivet where the two could be joined. Keep in mind a chain can only be joined by mating inner and outer plates.
5. From the closet rivet, lengthen the chain by counting over an additional two rivets (two links), which is a distance of one-inch. Cut the chain at this point.
6. Remove the chain from the bike and thread it through both derailleurs and join the ends.

MASTERLINK NOTE: If the bike chain uses a “master link”, it is necessary to account for the link. Install one-half of the master link on one side of the chain. Size the chain by cutting the other end of the chain.