BB - common acronym for Bottom Bracket.
WORKSHOP TIP: The spindle is often incorrectly called an axle. As it revolves with the cranks however, it is a spindle. An axle, for example a wheel axle, revolves with the wheel.
The BB and cranks provide the platform for transferring power to the rear wheel. It is a part of the drivetrain system.
Bearings in the BB mean that a low friction transmission is possible as force is transferred from the pedals to the rear sprocket(s).The bearings in the BB must take the weight of the rider, as well as the force applied to turn the pedals. This means they are under the most severe and changing loads in the bicycle. Inevitably, the bearings wear out relatively quickly.
Assessment of the BB is necessary to determine whether the bearings in a BB (or indeed the entire BB unit) will need to be refurbished or replaced. Part of a bike’s annual service should always include this careful assessment.
