The bottom bracket is inserted into the frame and connects it with the drivetrain (cranks-freewheel-pedals). It absorbs large loads, that’s why it’s made from steel or titanium, not aluminium.

Key points:

· Type: it can be pressed into the frame (AS30, BB23, etc.) or threaded (standard 1.37” x 24tpi thread)

· Cranks interface: the most popular is the ISIS drive (10 slots), but there are many others: JIS (square taper), Shimano HG, etc.

· Axle length: from 122.5mm, but it’s recommended to choose a BB of at least 127mm to prevent the cranks from rubbing the stays.