Bicycle drive-train systems are composed of dozens of engineered components, including derailleurs, chains, hubs, bottom brackets and cassettes. Cassettes are thus an integral part of the drivetrain of a multispeed bicycle. Understanding cassettes can improve your knowledge of options for better bike performance.
Cassettes are sets of sprockets that attach to the rear hub of a bicycle. Each cassette contains multiple sprockets that correlate with different gears. A cassette is designed so that a biker can easily switch between different sets of gear ratios or replace worn sprockets, simply by switching out cassettes. Older cassette sets also included a freewheel, a mechanism that allows you to coast. Modern cassette sets only contain the sprockets, which wear out more quickly than the freewheel assembly.
Cassette sets are designed to allow for easy switching between sets, but not all brands are compatible. Shimano is the most common maker of both hubs and cassettes. As such, all Shimano freewheel hubs should work with Shimano cassettes. Other brands such as SRAM and Sunrace have decent compatibility, but not with every cassette. Campagnolo is the most incompatible manufacturer of cassette hubs and freewheels. Before buying a cassette, check that it is compatible with your hub.
Cassette sizes are determined by their gearing. For example, common sizes of cassettes are five, seven, nine and and 10 speeds. The speed of a bicycle is found by multiplying the number of sprockets in front, where the pedals are, by the number of sprockets in the rear. Larger cassettes will have more sprockets on them. Within cassette sizes, consideration is made for the number of teeth each sprocket has. The more teeth, the larger the sprocket.
Road vs. Mountain
In general, cassettes can be classified as meant for either road or mountain bike use. Mountain bikes require larger gaps between gear ratios because of the vastly different terrain they face. Mountain bike cassettes have a larger range of sprocket sizes when compared to road bike cassettes. Road bike cassettes are designed with smaller differences between sprockets. This allows road bikers to make minor gear changes that optimize high-speed cycling sessions.