Bottom brackets rarely get the love or respect they deserve; until they go wrong. When they do fail or wear out, often riders know little about what bottom bracket their bike uses and consequently what replacement to buy or ask for. A quick word on threaded bottom bracket shell widths.
The bottom bracket, or 'BB' as it often gets shortened to, is the humble bearing which supports the rotation of the cranks. This out of the way components tucked mostly out of sight between the crank arms has morphed to include nearly a dozen different styles and sizes. Even so, the traditional threaded BSA threaded version remains the most popular, and for good reason, it works and is easy to fit so this guide concentrates on this type.
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.
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The evolution of road bikes may have provided cyclists with frames that are lighter, stiffer and more aerodynamic, but there has also been a substantial increase in the number of bottom bracket and crank axle designs. The once ubiquitous threaded bottom bracket shell has given way to larger threadless designs while the diameter of crank axles has also grown. Both may have helped elevate the performance of contemporary road bikes but consumers have been left to contend with myriad options and some frustrating incompatibilities.
Square taper bottom brackets may seem like anachronisms dating from the last century, and yet they remain the best option for metal cranks. Here is why our Rene Herse cranks use square tapers and will continue to do so in the future. Modern bottom brackets have larger spindles, so they can use thinner walls.
There are now several different crank bearing systems and standards. The different standards do not allow interchangeability between bearings or cranksets. Having some knowledge of the standard will help the mechanic be aware of service options. Threaded bottom bracket shells have internal threading.