Bicycle bottom bracket evolution
developments that might improve performance. Self-evidently, high-performance bikes need to be light, but they also need to be stiff to ensure that as little as possible of the rider’s effort is lost to flex. A nearly flex-free transmission is critical for efficient power transfer, and the bottom bracket system
– axle, bearings and shell or housing – is at the heart of the transmission. It affects performance for reasons related to axle stiffness, to bearing life, to the security of the crank-to-axle interface and to the extent to which the bracket shell can contribute to overall frame stiffness.
The typical pedalling downstroke applies force to the crank from various directions as the pedal travels through its arc. While all cyclists to some extent apply radial force, which bows and either extends or compresses the crank, the force that actually turns it is applied at a tangent to the pedal circle.
This tangential force deforms the crank by twisting and bending along the axis between pedal and bottom bracket axles and applies a torque or turning force to the crank.
When it is applied to the right hand crank, its attachment to the frame need only constrain the crank to rotate in a precise arc around the axis. Given that the right-hand crank is generally integral with the spider, it is in practice rigidly attached to the axle.
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