Bottom bracket

The bottom bracket on a bicycle contains the parts to which the cranks and chainrings and pedals are attached. The cranks are connected to a bottom bracket spindle, rotating on bearings inside the bottom bracket shell. (The shell is part of the bicycle frame).

Older bicycles use loose bearings, with separate cups and spindle; overhauling requires removing at least one cup, installing new ball bearings, replacing the spindle and adjusting the cups. Most modern bicycles use a one-piece cartridge bottom bracket instead, with the spindle and bearings sold as a single unit. This makes servicing the bottom bracket a simple matter of removing the old cartridge from the bottom bracket shell, and installing a new one in its place.