Bicycles may seem like simple machines, but their design is changing constantly. New tube profiles, frame materials and levels of integration appear at a staggering pace and, while that’s all well and good, it is often the less glamorous elements of innovation that can have the biggest impact on the bikes that we ride. While most of us will have a cursory knowledge of a bottom bracket and how it functions, it’s easy to disregard as a realm for nerds and mechanics, so we thought it was time to demystify the humble BB and explain what all the fuss is about.
The primary function of a bottom bracket is to facilitate the rotation of the cranks, which transfers power from your pedals to your drivetrain. To that end, bearings are needed to allow the cranks to move freely. While that is the basic principle, the factors affecting bottom brackets have become increasingly complicated over the years in the quest for peak performance and efficiency. Nowadays, it’s not only the bearings but also the shape, structure and size of the bottom bracket area – known as the bottom bracket junction – that are subject to endless tweaks and refinements.
Factors at play
If the key purpose of the bottom bracket is simply to allow the cranks to rotate, the sheer number of different BB
