Bike cassettes provide a range of gear options that your chain can run on. On a larger sprocket it’s easier to pedal, so you can climb a steep slope. Smaller sprockets require more pedalling effort so are used for fast riding on flat roads. Most modern bikes will use 9, 10 or 11 speed cassettes. The number of teeth on each sprocket determines how high or low the gear is; typical road cassettes have 12-28 teeth. The closer the spread of gears on the cassette, the smoother the jump between gears. Not all cassettes are compatible with all freehubs so check before you buy.