Added sources and updated by the Pedalwise team

The cassette is the cluster of rear sprockets (cogs) that the chain runs on. On a cassette hub, the cogs slide onto a splined freehub body and a lockring holds them in place (Park Tool cassette guide).

What it's used for

The cassette gives you your gear range. It wears with the chain. Park Tool explains that a worn chain wears the cogs faster, and that a new cassette costs much more than a new chain (Park Tool chain wear guide). If a new chain skips, the cassette may be worn too (Park Tool chain replacement).

What to look for

• Cassette or freewheel. Park Tool notes many e-bikes may use a freewheel, which threads onto the hub instead of sliding onto a freehub (Park Tool). The two are not interchangeable, so check which you have.
• The same speed count and range as your shifter and derailleur. Your bike's manual or the drivetrain maker's site lists them.
• The right lockring tool for removal (Cassette lockring tool).

Using it safely

• Note the order and orientation of the cogs and spacers as they come off (Park Tool).
• Line up the wide spline when installing, grease the lockring threads and tighten to at least 40 Nm or the maker's value (Park Tool).
• Check your rear derailleur adjustment after fitting a new cassette or wheel (Park Tool).
• On a hub motor wheel, removal can involve motor wiring, so follow your maker's manual or ask a shop.

Videos

Guides that use it

• Check and replace a chain
• Adjust a rear derailleur

Sources

• Cassette removal and installation, Park Tool
• When to replace a chain on a bicycle, Park Tool
• Chain replacement: derailleur bikes, Park Tool