How to Measure Mountain Bike Chain Length Correctly

Getting Your Mountain Bike Chain Length Just Right

Ensuring your mountain bike chain has the correct length is key for smooth rides and a happy drivetrain. A chain that’s too long can cause sloppy shifting and even damage your frame from chain slap. Too short, and you risk premature wear on your chain and other components.

We’ve found that most folks get it right by simply measuring their current chain. This is often the easiest starting point. Let’s look at how you can do this yourself.

Methods for Measuring Your Mountain Bike Chain

The Simple Link Count Method

This is the most straightforward and commonly used method. You’ll need a way to physically count the links on your existing chain.

What You’ll Need

You don’t need fancy tools for this. A pair of gloves can keep your hands clean. A rag is also handy.

Counting the Links

You’ll want to count the inner plates and outer plates. A chain consists of alternating inner and outer links. Most mountain bike chains are measured in half-inch increments. This refers to the distance between each pin. A full link is typically considered one inner plate plus one outer plate. You’ll want to count to the nearest full link.

For example, if your chain is connected, you can often find the master link (if it has one). If not, you might need to break the chain with a chain tool. Once broken, lay the chain flat. Count from one pin to the corresponding pin on the other side. Many people count pairs of links (one inner and one outer plate) for simplicity. Just be consistent.

Using Your Existing Chain as a Guide

If you’re replacing an old chain with a new one of the same brand and type, you can often just match the length. The new chain will usually come with extra links.

Transferring the Length

You can lay the new chain next to your old one. Align the master links or the break points. Then, you can trim the new chain to match the length of the old one. Make sure you’re accounting for the same number of inner and outer plates. This ensures you’re comparing apples to apples.

The “Big-Big” Method (For More Precision)

This method is especially useful if you’re unsure about your current chain’s length or if you’ve changed your drivetrain components, like the cassette or chainrings.

What You’ll Need

• A chain tool (to break and connect the chain)
• Gloves and a rag
• Your new chain

Steps for the “Big-Big” Method

First, make sure your rear derailleur is in good working order. You’ll route the new chain. Do NOT shift into the smallest cog on the rear and smallest chainring on the front. This is for measuring only.

Route the new chain over the largest chainring in the front. Then, route it around the largest cog on your cassette. You do this without going through the rear derailleur pulleys. Bring the two ends of the chain together. They should meet around the large chainring and cog.

Now, add two full links (one inner and one outer plate) to this length. This accounts for the take-up by the rear derailleur. This is the general guideline many mechanics follow (Park Tool). Some manufacturers may suggest slightly different amounts. Always check your new chain’s specific instructions.

After you’ve determined the length, use your chain tool to connect the ends. Make sure to use the correct connector (master link or connecting pin).

When Might You Need a Different Length Chain?

Most often, you’ll want your new chain to be the exact same length as your old one. However, there are a few situations where you might need to adjust.

• Changing Cassette Size: If you’re upgrading to a significantly larger cassette (e.g., going from an 11-34t to an 11-46t), your chain will likely need to be longer.
• Changing Chainring Size: Similarly, installing larger chainrings up front might require a longer chain.
• New Drivetrain Components: If you’re installing a completely new set of drivetrain components (cassette, chainrings, derailleur), it’s wise to measure using the “Big-Big” method to be sure.

We found that these component changes are the main drivers for needing a different chain length. Always double-check your drivetrain’s specifications for maximum cog size compatibility.

Common Mistakes to Avoid

Getting the chain length wrong can lead to headaches. Here are some common pitfalls to steer clear of.

• Assuming all chains are the same: While many chains are similar, subtle differences can exist. Always measure or compare to your existing chain.
• Not accounting for derailleur wrap: When using the “Big-Big” method, remember to add those extra links for the derailleur to function correctly.
• Cutting too short: It’s much easier to shorten a chain than to lengthen one. If you’re unsure, err on the side of slightly too long, then trim down.

Quick Checklist for Chain Length Measurement

Here’s a handy checklist to make sure you’ve got your chain length covered:

• Count links on your old chain for a baseline.
• Use the “Big-Big” method for critical setups.
• Add two full links when using the “Big-Big” method.
• Ensure the new chain matches the old if no changes were made.
• Double-check with a master link or connecting pin.
• Test shifting through all gears to confirm proper length.