Bicycle bottom bracket evolution

Bicycle bottom bracket evolution

Square taper axle crank interface


formats who followed – was to surmount the limitations imposed by a bottom bracket shell diameter chosen some 100 years earlier. This, the original “standard” on which more recent designs attempt to






























improve, is either 34.9mm or 36mm, depending on whether it is BSC or Italian threaded. 68mm wide for road bikes, the conventional bracket shell is big enough to house a solid steel axle rigid enough for racing, turning in acceptably durable




ball bearings. Even with a small hole bored through the middle, the axle is more than adequate to the task; until very recently, top track sprinters still favoured the anvil reliability of the old-school square taper steel axle when subjected to their enormous power and explosive delivery. Used as part of a machine that was relatively flexible and heavy everywhere, the traditional steel axle played its part well. But, as bicycle engineers began with the advent of new materials to apply fresh thinking to the racing bicycle, the whole bottom bracket area became something of a stumbling block to progress.


Put simply, the standard layout limited designers’ scope to play with technical

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