Once upon a time, all bikes were made of steel. And the bottom bracket axle was a solid steel rod. There were cups in the bearings into which loose ball bearings fitted, with the axles having a curved surface which pushed against the ball bearings. A threaded bottom bracket shell used to be the norm when bikes were made of steel. There were just two main standards: BSA threaded, where the shell was 68mm side to side and 33mm in diameter; or Italian threaded, where the shell width was 70mm and diameter 34mm.
Road bike bottom bracket standards: everything you wanted to know but were afraid to ask
Bottom Bracket Standards and Terminology | Park Tool
By Benedict Pfender. This comprehensive guide tells you everything you need to know about bottom brackets — from a breakdown of all the bottom bracket systems available and their compatibility with one another to how a bottom bracket works and how to stop a bottom bracket creaking, this guide has it all. Below we rundown each of the major bottom bracket systems currently available, along with advantages, disadvantages, notes on compatibility, and some input from their proponents on why they exist:. Modern, external cup threaded bottom brackets are among the most common of standards. Square taper bottom brackets also fit in most shells designed for external cup bottom brackets, but to keep things simple, we will only cover the modern standard here. The idea is simple, by moving the bearings outboard of the shell, you can then use a much larger spindle. In fact, companies such as Santa Cruz continue to stand by it despite the plethora of other options now available.
Over the years, the bicycle industry has created numerous bottom bracket standards. Unfortunately, the industry has not been clear and consistent when creating terms and names for them, sometimes creating several names for the same bottom bracket standard. This article will review the many bottom bracket standards on bicycles. The bottom bracket is held inside the part of the frame called the bottom bracket shell. With use, the bearings will wear out and require replacement.
The bottom bracket on a bicycle connects the crankset chainset to the bicycle and allows the crankset to rotate freely. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks. The bottom bracket fits inside the bottom bracket shell , which connects the seat tube , down tube and chain stays as part of the bicycle frame. The term "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames  which also form the shell that contains the spindle and bearings; the term is now used for all frames, bracketed or not.