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The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The chainrings and pedals attach to the cranks. Bottom bracket bearings fit inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.

Aerozine ceramic bottom bracket bearing cups BSC/ISO

The term "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames[1] which also form the shell that contains the spindle and bearings; the term is now used for all frames, bracketed or not.

There is some disagreement as to whether the word axle or spindle should be used in particular contexts. The distinction is based on whether the unit is stationary, as in a hub, or rotates, as in a bottom bracket.[2] American bicycle mechanic and author Sheldon Brown uses axle once and spindle four times in his bottom bracket glossary entry.[3] This article uses spindle throughout for consistency.

An old American term for the bottom bracket is hanger. This is usually used in connection with Ashtabula cranks, alternatively termed one-piece cranks.