The Complete Handbook of Bicycle Cassettes & Freewheels

Chapter 1: Understanding Bicycle Cassettes & Freewheels

To understand the bicycle cassette and freewheel, you have to see them not as a simple collection of gears, but as the second half of a two-part precision interface. The first half is the chainring at the crank. The second half, the subject of this book, is the cluster of gears mounted on the rear wheel. Every pedal stroke you make transfers energy from the crank, through the chain, and to this rear cluster, which then drives the wheel. The cassette or freewheel is the final mechanical gateway in that transfer. If it is inefficient, poorly maintained, or incorrectly specified, all the power in the world won't translate into useful forward motion.

The terms "cassette" and "freewheel" are often used interchangeably by casual cyclists, but they refer to two distinct mechanical systems with a crucial difference. A freewheel is a single unit that derives its name from its function: it is both a set of gears and a mechanism that allows the wheel to keep rolling when you stop pedaling. The entire gear cluster and the internal ratcheting mechanism is threaded as one piece onto the hub. When you pedal, the body of the freewheel drives the wheel. When you coast, the internal pawls disengage, allowing the wheel to spin freely while the gear cluster remains stationary.

A cassette, on the other hand, is a stack of individual gears, or sprockets, that slides onto a splined cylinder on the hub, called a freehub