The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It is not a bracket as such. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks. The bottom bracket fits inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.
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 axle/spindle is stationary, as that in a hub, or rotates, as that in a bottom bracket. Sheldon Brown uses axle once and spindle four times in his bottom bracket glossary entry. This article uses spindle throughout for consistency.
An old American term for bottom bracket is hanger. This is usually used in connection with Ashtabula cranks, alternately termed one-piece cranks.
Read more about Bottom Bracket: Interface Types, Sizes, Bottom Bracket Height, Eccentric Bottom Brackets
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