The part of the frame around which the pedal cranks revolve, also the bearings and axle assembly that runs through the bottom bracket shell of the frame. A cartridge bottom bracket. The splined tool is shown above the bottom bracket. The crank would need to be removed to actually use the tool. This is usually used in connection with one-piece cranks .
Bottom brackets come in different sizes, according to the nationality of the frame.
The overwhelming majority of bicycles in current production. British and I.S.O. are interchangeable. 70mm Italian and some high-end French bicycles. Prone to problems due to the right threaded fixed cup, which tends to unscrew itself in use. 68mm Obsolete, used on older French bicycles.