Chain Length Sizing
Replacement chains for bicycles are always longer than you need. This repair help article will take you step by step through the different methods of sizing a bike chain.
Before we begin the chain sizing process, let’s first define the two types of multi-speed bicycle chain. There are chains that use a master link and chains that use a special connecting rivet chains. Master link chains use two removable outer plates to connect the chain. Connecting rivet chains use a special rivet to connect the chain.
Check & Remove Chain
Before removing the old chain, verify that it is the correct length.
Shift the bike into the largest chainring and rear cog. The chain should be long enough to make this shift with two slight bends at each pulley of the rear derailleur.
Next, shift to the smallest chainring and sprocket. There should be no slack in the chain, and the derailleur should not pull so far back that the chain contacts itself.
Next, remove the rear wheel. This will take tension off the chain and give you better access to the chain. Inspect the chain for a master link.
Master Link Chains
If a master link is present, use a master link pliers such as the Park MLP-1.2 to disengage the link. If you have a chain tool and your chain is worn out, you can ignore the master link and simply cut the chain using a chain tool.
Connection Rivet Chains
Connecting rivets will appear visually different from other rivets. When selecting a rivet to break, be sure that it is at least a couple of links away from any connecting rivet already installed in the chain. Install the chain tool and bring the driving pin of the chain tool into contact with the connecting rivet. Ensure the chain tool pin is driving in a straight line into the chain rivet. Turn the handle with force and drive out the rivet, then remove the chain.
Sizing to Original Chain
If the old chain is an acceptable length, lay it next to the new chain. Always line up outer plates with outer plates or inner plates with inner plates. This example is incorrect because we have an end with outer plates being compared to an end with inner plates. Insert the master link to get a true side by side comparison. Take care to match them rivet by rivet noting that old chains will lengthen as they wear.
Largest Cog & Largest Chainring Method
If you don’t have an old chain to compare length with, you can use this method to determine the correct chain length for your bicycle. First, shift the front derailleur over the largest chainring and the rear derailleur to the smallest cog. Begin by wrapping the chain around the largest rear cog. If the new chain has one end with an outer plate, it should be routed toward the front chainring. Pass the chain end through the front derailleur cage onto the largest front chainring and hold it at about the five o’clock position.
If the chain uses a master link, install half of the master link. This will account for the extra half inch the master link provides. With this plate installed, the rest of the process is the same for master link and connecting rivet chains. Pull the lower section of chain snug and engage it on the front chainring.
Pay special attention that the chain is fully engaged on the largest rear cog for accurate sizing. You will bypass the rear derailleur entirely for this step and extra length will be added later to account for this. Find the closest rivet where the two chain ends could be joined. You will match inner plates to outer plates in order to join the chain. The rivet closest to where the chain can be joined is the reference rivet. From the rivet we add two additional rivets.
This is the cutting point for your chain. It can occur that, when we pull the lower section snug, an outer plate meets and outer plate. This cannot be the reference rivet because the chain cannot be joined here. Add one rivet. This becomes the reference rivet and from here we add two additional rivets, cut the chain with a chain tool, and the chain is sized.
Exceptions & Considerations
If the bike uses a chain guide, make sure it is routed through the system before determining chain length. Add the same two rivets before to establish the cutting point.
The amount of rivets added is different if you are using the following component configurations:
A SRAM® derailleur with an eleven or twelve speed cassette and a single front chainring with no rear suspension. With this system, add four rivets from the reference rivet.
Another consideration deals with rear suspension. The distance between the rear cogs and front chainrings will change as the suspension compresses and moves for bumps. To account for that, disconnect the shock, and compress the linkage.
This is the maximum possible distance between the front and rear sprockets. With the suspension held in place, we use the same technique as before, adding two rivets for the cutting point. This is for all bikes with rear suspension including SRAM® 11- and 12-speed 1x systems.
Chain Sizing by Equation
Bicycle chains consist of inner and outer plates. It is only possible to join inner plates to outer plates. Because of this, chains can only be connected at whole (integral) inch increments. For example, some derailleur bike chain could only be 51, 52, 53 inches, etc. Chains cannot be cut to 52-1/4″, 53-1/8″, or even 52-1/2″. It is possible to determine chain length from industrial drive train equations, and then cut the chain before installing it on the bike.
Begin by counting the number of teeth on the largest front sprocket and largest rear. These numbers are often printed right on the sprockets and cogs. Next, measure the distance between the middle of the crank bolt to the rear axle. This is also the chain stay length. Measure to the closest 1/8″, and convert this to decimal form. A simple chart below will assist this conversion.
Fractional conversion to decimal for 1/8″ measurements:
- 1/8″ = 0.125″
- 1/4″ = 0.25″
- 3/8″ = 0.375″
- 1/2″ = 0.5″
- 5/8″ = 0.625″
- 3/4″ = 0.75″
- 7/8″ = 0.875″
For most bicycles, a relatively simple chain length equations may be used. For bicycles with extreme differences in chainring sizes and short chain stays, the more rigorous and complete equation may be used. This is discussed later in this article.
Simple Equation: L = 2 (C) + (F/4 + R/4 + 1)
L = Chain length in inches. Round the final result to closest whole inch figure. C = Chain stay length in inches, measure to closest 1/8″. Use chart above to find decimal measurement. F= Number of teeth on largest front chainring. R= Number of teeth on largest rear cog.
Example: A bike has a 42-32-22 front chainring set up. Use only the 42 for the equation. The rear cog set has 32 teeth on the largest cog. The bike measures 16-3/8″ from the center of the rear axle to the center of the crank bolt. The decimal equivalent for 16-3/8″ is 16.375 inches.
L = 2 (16.375) + (42/4 + 32/4 + 1)
In the example above, this becomes 32.75 + 10.5 + 8 + 1 = 52.25 inches. Chains cannot be joined at 52.25 inches, so this length is rounded to 52 inches.
For this system, round up from 0.5. For example, a chain length figured to be 55.5 would be rounded to 56 inches.
To measure the new chain, lay it on a flat surface with the rollers and plates aligned vertically. Pull on each end to straighten out the chain. Measure from either end. Remember, you can only shorten the chain at whole inch increments. If the chain uses a master link, install it on one of the chain for purposes of measuring, and measure including the master link.
Examples of bikes using the rigorous equation would be a track bike with a large front ring, perhaps a 55 tooth, and a small rear cog, such as a 11 tooth. Additionally, to require the rigorous equation, the bike would need a very short chain stay, such as 15 inches or under in length. As the front and rear sprockets differ more in size, the chain must diverge more off a line represented by the chain stay. This is the hypotenuse of a triangle, which can add to the chain length. Again, it is only a concern in very extreme cases.
For an example of the rigorous equation, assume a one speed bike has a 15 inch chain stay, a 58 tooth front ring and an 11 tooth rear sprocket. In this there is no derailleur, so the extra one-inch is not added into the length. The extra inch is added for derailleur bikes only.
The short equation in our extreme track bikes yields an answer of 47.25 inches, which should be rounded to 47 inches. The rigorous equation yields an answer of 48.17 inches, which is rounded to 48 inches. Again, in the example only, the extra inch is not added in for the derailleur cage, as there is none. The simplified short equation would be an inch too short. This again is an extreme situation. Even in this one-speed example, if the chainring is reduced to a 55t, the two equations come up with the same answer.