The axle standard describes the diameter and the installation width of the bottom bracket axle or crankshaft on the bicycle. It is differentiated whether the axle has a continuous diameter of 24 mm, as with the Hollowtech II variant of the company Shimano or on the side of the 2nd crank only has a diameter of 22 mm and thus has two different end diameters, as the GXP model of the company SRAM is manufactured.
In addition to these two variants, there is another axle standard, which has a diameter of 30 mm and is therefore called BB30. This axle standard includes the PF30, BB30 and OSBB systems used by Cannondale and Specialized. Depending on the axle standard of the bottom bracket shaft, the appropriate bottom bracket must then be selected, which also has the appropriate diameter of the bottom bracket shell.
The systems listed are all press-fit systems. This means that both the bottom bracket shell and the bottom bracket have no thread. The bottom bracket is located in a shell that is simply pressed into the bottom bracket shell.