Bicycle bottom bracket evolution

Bicycle bottom bracket evolution


and axle has nevertheless influenced their development. Before there was any point in lightening the near-solid steel axle, something had to be done about the traditional “square-taper” interface.


Introduced over half a century ago to replace the then-archaic cottered crank, square-taper is simple and provides a secure connection in all planes between crank and axle. However, it is, of necessity, of sturdy build. The wedge action of the taper generates severe tensile stresses in the crank head, as do the flats of the axle as it tries to rotate inside the crank. Add that the square orifice incorporates sharp corners susceptible to crack initiation and this part of the crank itself must be of substantial build to ensure durability.


The answer is some sort of splined interface, which can be designed to avoid the wedging effect and which, because it transfers torque without generating excessive radial stresses, requires less material to be used in the crank head.


Because deeper splines are better in terms of their durability, both Shimano’s Octalink concept and the 10-spline “open standard” ISIS Drive® design it inspired had axles of larger diameter than seen with square-taper. The more recent Hollowtech II system revolutionised crankset design by making the oversized axle a permanent, rigidly-attached part of the right-hand crank assembly, removing the need for a separable, heavy and potentially flexible interface. The left-


hand crank slides over 38 fine splines, two of them double-width to ensure crank alignment, on a non-tapered axle. The large number is need to compensate for the fragility of the shallow splines dictated by the thin axle walls.


Once snug against the bearing face, it is secured using bolts that pinch the crank around the axle. Alternative approaches using the same fixed axle layout include Truvativ’s GXP. Campagnolo’s Ultra-Torque system, which employs a half-axle embedded in each crank, may be

BB30 and KCNC axles side by side



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