Splined Bottom Brackets
By Brad Quartuccio
For decades the square taper bottom bracket axle was the defacto standard with nearly every high quality bicycle up through the late 1990’s having the same basic crank arm interface. These days the market supports a number of splined bottom bracket designs, none of which are compatible with one another. At the high end of the market, external bearing designs with oversized spindles fused to one crank arm or the other have all but taken over, again with incompatibilities across brands. How did we get here, and now what?
The 90’s saw an explosion of changes in the cycling world, a perfect storm of manufacturing technology and riding styles each pushing the limits of design. From the mountain bike side, square taper axles were proving to be undersized for the increasingly aggressive riding that some were pushing, with racers on both road and dirt desiring an overall stiffer and stronger system. It’s easy to point the finger at Shimano for the explosion of bottom bracket “standards” that have appeared since, all beginning with their introduction of the Octalink V1 spline back in 1997. While not truly the first splined system, it was the first commercially successful version with eight splines and a larger and stiffer axle as compared to square taper designs, yet still fitting within the same roughly 35mm diameter bottom bracket shell, the lower most portion of the frame that houses the bearing assembly. Within a few years of its introduction, a similar Octalink V2 was introduced with the same pattern, but deeper splines. The Octalink design is protected by a number of patents, and while Shimano has allowed some crank manufacturers to license the interface, aftermarket bottom bracket makers were shut out.
In response to Shimano’s proprietary Octalink system, Chris King, Truvativ (pre SRAM ownership) and Race Face joined together and created a competing 10-splined interface that resided in the public domain for any manufacturer to use without fees. The International Splined Interface Standard (ISIS) quickly gained wide acceptance, and a bad reputation for a short bearing life span. Since the larger axle and bearings were still completely contained within the same bottom bracket shell as before, the bearings themselves were made smaller in diameter with a subsequent increase in wear. Perhaps because of this design shortcoming, Chris King never introduced an ISIS bottom bracket and riders everywhere wished for longer lasting bearings. The Octalink design was susceptible to similar bearing wear problems albeit less pronounced, with the bearing lifespan issues arguably leading to the introduction of external bottom bracket designs. Truvativ has since created the low-end Power Spline design as a lower cost alternative which is in essence a square taper spindle with 12-splines instead of the usual 2° taper. Campagnolo wisely skipped this phase, sticking with square taper until making the jump to an external bearing cup.
While bearing wear issues contributed to the development of external bearing systems, the desire for even stiffer bottom bracket and crank arm combinations was also a