Feature: Everything You Need to Know About Bottom Brackets
Square-tapered bottom brackets started it all. At the time, they worked well, delivering a reliable system with plenty of durability. But, as frame designs became more advanced, with huge leaps in weight savings and stiffness, the old square-tapered system became the weak link. Shimano hoped to rectify this with their Octalink system, which used a larger-diameter spindle to drop weight and increase stiffness. It worked, but it forced the designers to use smaller bearings to fit everything inside the bottom-bracket shell. The tiny bearings suffered durability issues, forcing many riders to go through multiple bottom brackets per year. This proved to be the death of the design. ISIS bottom brackets, which were developed by Shimano’s competition as a way around the Octalink patent, suffered from the same issues.
THE OUTBOARD REVOLUTION
The constraints of internal bearings forced companies to think outside the box, or, more precisely, outside the bottom bracket shell. The first outboard bearing systems, also known as X-Types, were the catalysts for the current standards in bottom-bracket design. They essentially moved the bearings to external cups, allowing the bearing size to be increased and leaving more real estate inside the shell for the spindle. This allowed designs to be stiff and durable–a revolutionary concept. There were others who offered similar designs, but Shimano was the first to successfully market the large-bearing and large-diameter-spindle idea to the mainstream.
THE CURRENT STANDARDS
With durability and stiffness issues all but solved, where was there to go? The current crop of press-fit bottom brackets are the most technologically advanced yet. They maintain the durable large bearings, the large-diameter spindle, and fit everything inside of the frame. How did they do it? By eliminating the final weak link in bottom-bracket design: the threads. Increasing the bottom bracket shell diameter and simultaneously increasing the shell width allows the pressed in bearings to be supported by the frame instead of traditional cups. This not only yields weight savings and improved performance, but also opens the door for improvements in the frame by using larger diameter tubes and different materials. Examples of this design include BB30, SRAM’s PressFit 30 and Shimano’s BB92.
This is the press-fit bottom bracket that started it all. It was originally designed and launched on Cannondale bikes in 2000 and referred to as the “Cannondale SI BB.” It’s an open and free standard that any manufacturer can use. This design has a 30-millimeter crank spindle, 42-millimeter shell diameter, and bearings that are directly pressed into the frame without any cups. It features all the benefits of the larger dimensions. The only problem is that durability and noise problems can arise due to tolerance issues between the frame and bearings. Contaminants are also a concern, one that mountain bikers deal with constantly. Road bike designers continue to use this standard, while many mountain bike designers have found other solutions.
The BB92 standard was developed by Shimano in collaboration with Pivot Cycles. It uses a 92-millimeter-wide shell and Shimano’s Hollowtech 2 cranks, which have a 24-millimeter crank spindle and 41-millimeter shell diameter. By using the already standard spindle width, Shimano eliminates the need for a new spindle size and still reaps the benefits of oversized, supported bearings. The design also uses pressed in cups, eliminating the inconsistencies of a direct bearing-to-frame interface and minimizing the noise and durability issues.
SRAM PRESSFIT 30
The newest design in bottom brackets, the PressFit 30 features all the best from BB30 and other press-fit designs in one package. It uses a 30-millimeter spindle, 46-millimeter shell diameter and 73-millimeter shell width. These bottom brackets offer greater clearance for chainrings and ankles with additional weight savings over other designs. They also are incredibly stiff because of the oversize spindle and use bearings housed in nylon cups to eliminate frame interface issues.
MORE TO COME
The mountain bike is a constantly evolving animal. There are other standards and improvements being made. While many bike designers complain about new standards, the MBA wrecking crew prefers to embrace change, especially when the innovations make your bike faster, lighter, stiffer, and more fun to ride. We’re all for that.
Reprinted from our March 2012 issue. Like us on Facebook