Square taper

The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. 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.

The term "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames which also form the shell that contains the spindle and bearings; the terms is since 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 axle/spindle is stationary, as that in a hub, or rotates, as that in a bottom bracket. American bicycle mechanic and author 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, alternatively termed one-piece cranks.

In typical modern bikes, the bottom bracket spindle is separate from the cranks. This is known as a three-piece crankset. The cranks attach to the spindle via a common square taper, a cotter or via a variety of splined interfaces.

Earlier three-piece cranks consist of a spindle incorporating bearing cones (facing out), a fixed cup on the drive side, an adjustable cup on the non-drive side, and loose bearings. Overhauling requires removing at least one cup, cleaning or replacing the bearings, reinstalling the spindle, and adjusting the cups.

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Wikipedia