Bottom bracket – Simply put, it's an axle mounted on two bearings in the bicycle frame, onto which the crank arms are mounted. The bottom bracket is screwed into the frame's bottom bracket shell.
Currently, there are several bottom bracket standards. The basic division includes classic ball bearing bottom brackets and machined bottom brackets, commonly called compact bottom brackets. Bottom brackets also differ in their mounting method, as they can be pressed or screwed in. With screw-in bottom brackets, it's important to be aware of several thread standards: Italian (ITAL), English (BSA), and French. Bottom brackets also differ in the size of the shell and the direction in which the cups are screwed in. In the past, wedge-type bottom brackets were installed on bicycles. Currently, square-type bottom brackets are most common. Based on their shape, we can distinguish two standards: ISO and JIS. The successor to square-type bottom brackets were the Octalink V1 and V2, ISIS standards. Instead of a square end, the axle had a spline cut out. A newer bottom bracket standard is the one with external bearings. In these cases, the axle is not permanently connected to the bearings, but to the right crank arm. Depending on the type of bottom bracket shell on the bike and the manufacturer of the crank, the following varieties are currently available: Shimano hollowtech, GXP, PressFit and PressFit30, T47.
