Sprocket, chainring, derailleur hanger, bottom bracket… A road bike drivetrain is made up of around ten components whose names may sound technical at first, but whose functions are easy to understand once you see how they work together. Whether you're trying to understand a technical term on a product page or identify the component responsible for a drivetrain issue, this glossary provides a clear explanation of each part.
Each entry includes links to our in-depth guides for further reading, covering road bike cassettes, road bike chainrings, and the complete range of drivetrain components.
1. Sprockets and the cassette
A sprocket is an individual toothed cog mounted on the rear wheel around which the chain runs. Stacked together from the smallest to the largest, these sprockets form the cassette. The larger the sprocket, the easier it is to climb steep hills; the smaller the sprocket, the higher the speed you can achieve on flat roads.
To learn everything about selecting, gearing and replacing this component, read our complete guide to the road bike cassette. The number of sprockets must also match your derailleur and chain. For more information, see our guide on cassette, derailleur and chain compatibility.
2. Chainring and crankset
Chainring
The chainring is the front equivalent of a sprocket: it is the toothed ring mounted on the crankset around which the chain wraps before engaging with the cassette. A road bike may feature either a single or double chainring setup, depending on its intended use. To learn more about chainring sizes, standards and manufacturers, see our guide to the road bike chainring.
Crankset
The crankset is the complete assembly consisting of the crank arms, one or more chainrings, and the spindle that connects them to the bottom bracket. It is the component that transfers your pedalling power to the drivetrain. Browse our selection of road bike cranksets to learn more.
Crank arm
The crank arm is the metal arm that connects the crank spindle to the pedal. Its length, typically between 165 and 175 mm, directly influences pedalling comfort and power transfer according to the rider's physiology and riding style.
Bottom bracket
The bottom bracket is the assembly housed inside the frame that contains the bearings allowing the crank spindle to rotate smoothly. Several standards exist, including threaded and PressFit systems, and they are not all interchangeable between different frame designs and manufacturers.
3. Chain
The chain transfers power from the crankset to the cassette by running around the chainring and then the selected sprocket. Its width depends on the number of gears in the drivetrain: the more sprockets the cassette has, the narrower the chain must be. A worn chain gradually wears down both the cassette and chainring teeth, which is why replacing it regularly is one of the most important drivetrain maintenance tasks. Our article How to Replace a Road Bike Cassette explains in detail how to replace the cassette and chain together.
4. Derailleur and its components
Front derailleur and rear derailleur
The front derailleur moves the chain between the chainrings on bikes equipped with multiple chainrings. The rear derailleur shifts the chain across the cassette sprockets while maintaining chain tension using its guide and tension pulleys. This component must be compatible with both the number of gears and the cassette's gear range.
Derailleur hanger
The derailleur hanger is a small component, usually made of aluminium, that attaches the rear derailleur to the bike frame. It acts as a mechanical "fuse": in the event of a crash or impact, it is designed to bend or break first, protecting the much more expensive frame and derailleur. On most modern bikes, the derailleur hanger is removable and replaceable, with each frame model requiring its own specific design. To learn more about derailleur compatibility, see our guide on derailleur cage capacity and compatibility.
Good to know: If your derailleur suddenly stops shifting correctly after a crash, inspect the derailleur hanger before assuming the derailleur itself is damaged. Replacing a hanger is usually a much less expensive repair.
5. Freehub body and compatibility
The freehub body is the splined part of the rear hub onto which the cassette is mounted. Several standards are available, including HG, XD/XDR and N3W, and they are not interchangeable. Because the freehub body determines which cassettes can be installed, it is one of the most common sources of compatibility issues when upgrading a drivetrain. To learn more, read our complete guide to road bike freehub bodies.
6. Summary table: the role of each component
| Component | Function | Related Guide |
|---|---|---|
| Sprocket | An individual rear cog around which the chain runs. | Road bike cassette |
| Cassette | The complete set of rear sprockets mounted on the freehub body. | Road bike cassette |
| Chainring | The front toothed ring mounted on the crankset. | Road bike chainring |
| Crankset | The complete assembly consisting of crank arms, chainring(s) and spindle. | Road bike crankset |
| Crank Arm | The arm that connects the crank spindle to the pedal. | Road bike crankset |
| Bottom Bracket | The bearing assembly housed inside the frame that allows the crank spindle to rotate. | Road bike crankset |
| Chain | Transfers pedalling power from the crankset to the cassette. | Replacing your cassette and chain |
| Front Derailleur | Moves the chain between the chainrings. | Derailleur compatibility |
| Rear Derailleur | Moves the chain across the cassette while maintaining chain tension. | Derailleur compatibility |
| Derailleur Hanger | Secures the rear derailleur to the frame and protects it by bending or breaking in the event of an impact. | Derailleur compatibility |
| Freehub Body | The splined interface on the rear hub onto which the cassette is fitted. | Road bike freehub body |