Advantages, Disadvantages, and Precautions of Upgrading to a Large Pulley for Rear Derailleurs

What Is a Big Pulley?

A big pulley refers to an oversized gear (pulley) in the rear derailleur, larger than standard models. While not common on road bikes or MTBs, it has gained quiet popularity as a performance-enhancing customization.

Here are the key differences between a standard pulley and a big pulley:

Installing a big pulley typically increases the number of pulley teeth by 3–4. This enlarges the radius of the chain’s path around the pulley, resulting in a gentler curve as the chain moves through the derailleur.

Benefits of Installing a Big Pulley

The main benefits of installing a big pulley are reduced friction and less strain on the chain, which can lead to smoother pedaling.

The gentler curve reduces the friction generated as the chain passes over the pulley, helping minimize power loss during pedaling. However, since the power loss from chain friction is relatively minor in the overall bike system, the improvement in pedaling efficiency is often subtle.

As for the second benefit—reduced chain strain—this is also due to the gentler curve, which puts less stress on the chain’s linkages compared to a standard pulley.

Disadvantages of Installing a Big Pulley

The main drawbacks of installing a big pulley on your rear derailleur are the high cost of parts and the potential loss in shifting performance.

Big pulleys are available at a wide range of prices, but those from major brands can cost over $200, making this a very expensive customization. Some cyclists argue that, rather than installing a big pulley with uncertain benefits, it might be more effective to upgrade to higher-grade bearing pulleys.

Another important concern is the possible decline in shifting performance. Since manufacturers design their derailleurs to work optimally with stock pulley sizes, altering this can compromise shift quality. While in some cases, installing a big pulley may not cause any noticeable issues, the potential risk should not be ignored.

Precautions When Installing a Big Pulley

While installing a big pulley—either by replacing the pulleys alone or the entire pulley cage—is a relatively simple customization, there are several important considerations.

Pulley, Cage, and Derailleur Compatibility

Each derailleur brand has its own specifications for pulley cage attachment. If you’re replacing the entire cage, make sure to choose a model that is compatible with your specific rear derailleur.

Even when replacing only the pulleys, keep in mind that the diameter of the pulley bolts may vary by manufacturer. Be sure to select pulleys that match your derailleur’s specifications.

Check for Chainstay Clearance

Because a big pulley or pulley cage is physically larger, it may come into contact with the chainstay of your bike frame depending on the derailleur’s movement range. Always check clearance between the pulley cage and the chainstay when installing.

There have been some reported cases where, on carbon road bikes, a big pulley cage has struck the chainstay and cracked the carbon frame. If you’re using a carbon frame, pay extra attention to this risk.

Shifting Compatibility and Adjustment

Manufacturers typically do not design their derailleurs to work with third-party pulleys, which means shifting performance may be compromised.

Additionally, because installing a big pulley reduces the clearance between the pulley and the cassette, you will need to adjust the B-tension screw to maintain proper function.

In most cases, adjusting the B-tension screw is enough to maintain shifting functionality, but it may still affect shifting speed or smoothness. If you’re unsure about making these adjustments, it’s best to consult or have the installation performed by a professional bike shop.