Choosing a Rear Derailleur for Road, MTB, or Hybrid Bikes

Choosing a Rear Derailleur Is the Same for Any Type of Bicycle

Whether you’re replacing the rear derailleur on a road bike, MTB, or cross bike, the basic selection criteria for rear derailleurs are the same.

1. It must be from the same manufacturer as the shift lever.
2. It must match the number of speeds (gears).
3. It must be compatible with the sprocket capacity.

As long as you follow these three points, you can choose the right derailleur for any bike.

Same Manufacturer as the Shift Lever

The rear derailleur works in coordination with the shift lever. In mechanical systems, the shift wire moves the derailleur, while in electronic systems, wireless or wired electrical signals determine its position.

Most current shifting systems use an “indexed” system, where the number of clicks determines the shift. In indexed systems, the shift lever defines how far the rear derailleur moves (or how much wire is pulled in mechanical systems).

Since the “movement distance” varies slightly between manufacturers, you must match the brand: Shimano derailleurs require Shimano shifters, Campagnolo derailleurs need Campagnolo shifters, and so on.

While some SRAM components are compatible with Shimano, this only applies to certain models. From 11-speed and up, SRAM uses a proprietary system, making them incompatible with Shimano.

Thus, it is essential to match the rear derailleur and shifter from the same manufacturer.

Matching the Number of Speeds

Rear derailleurs are designed to match specific numbers of speeds.

For example, a Shimano rear derailleur for an 11-speed drivetrain is officially only compatible with 11-speed systems. This is the manufacturer’s stance.

However, for 8 to 10-speed setups, the amount of wire pulled by the STI lever is relatively similar, so you can often find online reports claiming “it worked!” when mixing components. Still, since this is not a manufacturer-recommended combination, shifting performance issues may occur.

Matching Sprocket Capacity

A common mistake when replacing a rear derailleur is overlooking the sprocket capacity.

Manufacturers define the sprocket capacity for each derailleur model.

Sprocket capacity is based on tooth count (expressed as “T”) and includes three main points:

1. Maximum and minimum teeth for the low gear
2. Maximum and minimum teeth for the high gear
3. Total capacity

If the capacity does not match, a different rear derailleur will be needed.

For Shimano, typically, derailleurs with a max low gear of around 28T–30T are classified as “SS” (short cage), while those handling around 30T–32T are “GS” (mid cage). However, this can vary depending on the product, so always check the sprocket capacity of the derailleur you plan to use on the manufacturer’s site beforehand.

For more about the difference between SS and GS rear derailleurs, check the article below:

What Are the Differences Between Grades for the Same Number of Speeds?

When choosing a rear derailleur, one final point of confusion is the difference between component grades.

For example, in Shimano’s 11-speed road bike components:

1. DURA-ACE: RD-R9100, RD-9000
2. ULTEGRA: RD-R8000, RD-6800
3. 105: RD-R7000, RD-5800

These six rear derailleurs are all compatible with 11-speed systems according to Shimano’s official compatibility information. You can use any of them within an 11-speed drivetrain setup.

Lighter Shifting Operation

In Shimano’s latest high-end road rear derailleurs, additional guide parts are incorporated to reduce shift resistance. As a result, the friction in the shift cable decreases, making shifting operations feel lighter.

Smoother Chain Movement

Starting from the ULTEGRA grade and higher, Shimano rear derailleurs feature sealed bearings in the pulleys, making the chain movement through the derailleur much smoother.

In terms of converting pedaling power efficiently into propulsion, even minor resistance in the drivetrain—such as from the chain, sprockets, or pulleys—can lead to power loss. Therefore, if you seek maximum efficiency, upgrading to a higher-grade rear derailleur is worth considering.

We’ve covered how to choose a rear derailleur for road bikes, mountain bikes, and cross bikes.

Once you understand the system, customizing isn’t particularly difficult. However, it’s surprisingly common to accidentally purchase an incompatible product when replacing a rear derailleur yourself.

Manufacturer websites provide detailed specifications, so be sure to double-check them before making your purchase.