The inner bearing is one of the central elements of every bicyle. This bearing assimilates the cranks and looks back at a long development. The result is a multiplicity of different „standards“ which create a lot of confusion today.
Before you buy an inner bearing you have to pay attention to this two points:
Connection with the frame
Depending on the frame there are seceral types of inner bearings. Thereby you can roughly seperate between the ones that must be screwed in and the ones that must be pressed in.
English winding (BSA)
This bearings were recently the „standard“ of mountain bikes and many racing bikes. The winding size is 1,37“ x 24 tpi (threads per inch) in which the right bearing has a left winding and the left bearing has a right winding. The body housing width of the frame is 68mm or 73mm
Italian winding (ITA)
Originally the „standard“ of racing bikes in Italy, therefrom the name. The bearings always have a right winding with the size 36mm x 24.
The body housing width of the frame is 70mm
BB30 / Pressfit 30
A „standard“ created by Cannondale for bearings that must be pressed in the frame. he body housing width of the frame is 68mm or 73mm, the outer diameter of the bearings is 42mm. Pressfit 30 has the same sizes. This ist the name the company SRAM or rather the subsidiary Truvativ gave for this „standard“.
Shimano has several variants but they all have in common that they have a construction for a 24mm crankshaftand the outer diameter of the bearing is 41mm.
Different are the varying body housing widths of the frames of 86,5mm or 92mm
Connectio to the crank
Even the axles that conncet both of the crank arms have differences. In the past patron bearings with BSA or ITA winning that were firmly connected with the axle were often used. The cranks were screwed on both sides.
The oldest form of axles with a conical square on every ending on which the crank arms are pressed using crank guides. Disadvantage is the the wear of the intake and the cumbersome dismantling via extractor.
Fixed gear axle
The further development of the square axle is the fixed gear axle. The Shimano „standard“ Oktalink and the free „standard“ ISIS are still established. But they are not compatible among each other. An exception is the slim Oktalink gearing at Shimano Dura-Ace cranks.
On fixed gear axles both crank arms are also pressed up via screws and can only be removed with an extractor.
Axles on two-part cranks
Currently crank systems have established themselves in which the axles are not connected with the inner bearing but with the right crank arm. The bearings are screwed or pressed in the frame. The axles are just put through and screwed with the left crank arm. Be careful that the inner bearing diameter and the diameter of the crank axle accord to each other. The assembly width of the bearing and the crank have to match, too. The advatages of this system are a larger span, an extended bearing age and an easier montage and removal.
The frame should always be prepared for the assembly of an inner bearing. Thereto mainly belongs the recut of the winding and the face milling of the body housing. Only then the sides are parallel to each other and the bearings work without deforming. Otherwise it would lead to early bearing wear.