· Hugo · Maintenance · 3 min read
What Is a Bicycle Cassette, Sprocket & Bottom Bracket?
The cassette, sprockets, and bottom bracket are core drivetrain parts. Learn what each does, how they differ from a freewheel, and when to replace them.
The drivetrain has a lot of jargon. Cassette, sprocket, and bottom bracket are three terms that come up often.
The short answer: The cassette is the set of rear gears (cogs) that mount on the freehub. A sprocket or cog is a single gear tooth—often used for the rear cogs. The bottom bracket is the bearing assembly that connects the cranks to the frame. Modern bikes use cassettes (not freewheels). Bottom brackets come in BSA, press-fit, and other standards.
Here’s a clearer breakdown.
What Is a Cassette?
The cassette is the cluster of rear gears (cogs) that mounts on the freehub—the part of the rear hub that allows coasting. The cassette slides onto the freehub’s splines and is held by a lockring.
Cassette vs freewheel: Older bikes used a freewheel—a single unit that combines the cogs and the coasting mechanism, threading onto the hub. Modern bikes use a cassette—the cogs are separate from the freehub. The freehub is part of the wheel; the cassette is a replaceable part. If your bike has a cassette, the cogs slide off as one unit. If it has a freewheel, the whole unit unscrews.
Cassette sizing: Cassettes are described by tooth count, e.g. 11-28 (smallest to largest cog). An 11-speed cassette has 11 cogs. Compatibility depends on the hub (freehub body type) and the drivetrain (Shimano, SRAM, Campagnolo have different spacing). For how gears work in practice, see How Do Bicycle Gears Work.
What Is a Sprocket?
Sprocket and cog are often used interchangeably for the rear gears. Technically, a sprocket is any toothed wheel that engages a chain. In cycling:
- Rear sprockets/cogs — The individual gears on the cassette.
- Chainring — The front gears attached to the crank. Sometimes called “chainring” to distinguish from rear cogs.
So “sprocket” can mean a rear cog. In practice, “cog” is more common for rear gears, and “chainring” for front gears.
What Is a Bottom Bracket?
The bottom bracket is the bearing assembly inside the bottom bracket shell—the part of the frame where the cranks attach. It allows the cranks to spin smoothly.
Bottom bracket types:
| Type | Description | Common on |
|---|---|---|
| BSA (English) | Threaded shell, cups screw in | Many road and MTBs |
| Press-fit | Cups or bearings press into the shell | Modern road, some MTBs |
| BB30, PF30 | Larger diameter, press-fit | Performance road |
| T47 | Threaded, larger diameter | Some modern frames |
Shell width varies—68 mm, 73 mm, 83 mm for MTBs; 68 mm, 86.5 mm for road. Compatibility depends on frame shell and crank spindle type (e.g. 24 mm, 30 mm).
When to replace: Creaking, grinding, or play when rocking the cranks often indicate a worn bottom bracket. Replacement requires the correct tool for your BB type.
Cassette, Sprocket & Bottom Bracket FAQs
What's the difference between a cassette and a freewheel?
A cassette is a set of cogs that slides onto a freehub (the coasting mechanism is in the hub). A freewheel combines the cogs and coasting mechanism in one unit that threads onto the hub. Modern bikes use cassettes.
What is a BSA bottom bracket?
BSA (British Standard) is a threaded bottom bracket. The shell is threaded, and the cups screw in. It's common and easy to service. Shell width is often 68 mm (road) or 73 mm (MTB).
How do I know my bottom bracket type?
Check the frame's bottom bracket shell: threaded (BSA) or press-fit. Measure shell width (68, 73, 86.5 mm, etc.) and note the crank spindle diameter (24 mm, 30 mm). Your crank manufacturer's specs or a bike shop can confirm compatibility.
Does 'sprocket' mean the same as 'cog'?
In cycling, they're often used interchangeably for the rear gears. Technically a sprocket is any toothed wheel that engages a chain. 'Cog' is more common for rear cassette gears; 'chainring' is used for the front gears.
Summary
The cassette is the set of rear cogs on the freehub. Sprocket and cog both refer to those rear gears. The bottom bracket is the bearing assembly that connects the cranks to the frame. Modern bikes use cassettes, not freewheels. Bottom brackets come in BSA (threaded), press-fit, and other standards—check your frame and crank for compatibility.