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factor. As both road and mountain bikes were pushed further both in use and in the design studio, it was clear that a larger bottom spindle was necessary to increase stiffness and that without a sea-change in the size of bottom bracket shells across all frame manufacturers at roughly the same time that the bearings themselves would be forced outside the confines of the frame. Most of the major players have since introduced external bottom bracket systems where the drive side crank arm is permanently attached to an oversized bottom bracket axle which then fits into the non-drive arm. Shimano Hollowtech II, FSA MegaExo and RaceFace X-type systems are cross-compatible, meaning bearings from one manufacturer can be used with the cranks of another, while the Truvativ GXP and Howitzer systems stand alone. The Campagnolo Ultra-Torque design is similar, but has each crank arm permanently fused to one half of the bottom bracket axle which then joins in the middle with a Hirth joint. These external systems each have their quirks of setup, with compatibility issues best addressed by the manufacturer’s fine print.

As this bottom bracket style has matured frame manufacturers have begun to incorporate larger bottom bracket shells into the design, allowing the bearings to go back inside the frame due to a now larger diameter shell. Some frame makers have created their own “standard” for this larger shell, others have adopted a newly coined BB30 public domain standard that also addresses concerns about the added width between the pedals and excess weight of external systems. Of course, the BB30 standard comes with its own cranks and bearings that press directly into the frame. As of now, this design is fairly limited to the extremely high end of bicycles currently on the market and is likely a long way from your commuter, if it ever gets there.

In the evolution of splined bottom brackets there were a few stumps on the evolutionary tree, not that they didn’t make their mark in some way. The Bullseye, Magic Motorcycle (eventually purchased by Cannondale) and Sweet Wings mountain cranks of the early 1990’s were well ahead of their time with oversized splined spindles and bearings at least partially housed outside of the shell, but never sold enough to push the designs of the more mainstream makers. Largely skipped for the purposes of this article, BMX cranks have been splined for some time yet for one reason or another have had relatively little influence on the development of road and mountain systems.

While square taper is likely to remain serviceable for some time, the future of given splined systems is up in the air. The confusion between bottom bracket designs is not likely to go away any time soon, as there are now countless bikes out there with each competing design and demand for ongoing replacement parts for each. As these designs trickle down market and into the used sector I’d expect even further confusion, and unfortunately obsolescence as replacement parts for each are eventually taken out of production and even newer designs inevitably come to be.


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