What bicycle part is the central bearing assembly that the crankset rotates on?

The bottom bracket is the bicycle’s central bearing assembly that the crankset rotates on. It sits in the frame’s bottom-bracket shell and contains bearings plus a spindle or axle to which the crank arms attach.

When a rider pushes the pedals, the crank arms turn the bottom-bracket spindle. The chainrings mounted to the crankset then drive the chain, transferring power to the rear wheel. In modern bicycles, the bearings may be housed in a sealed cartridge, placed in external cups, or integrated into a press-fit system.

The name comes from early bicycle construction: brackets mounted beneath the frame held the pedal spindle and its bearings. The bottom-bracket shell is formed where the seat tube, down tube, and chainstays meet, making it a structural center of the frame as well as a rotating interface.

It is often confused with the headset, which lets the fork and handlebars turn, or the freehub, which allows the rear wheel to coast while the cassette remains attached. Neither supports crank rotation.