Look Ma, no threads
Simple is better: Shimano’s press-in bearing assembly alongside an external-type threaded bearing cup. The nylon housing hides the same angular contact bearing, but is installed directly into the frame’s unthreaded bottom bracket shell in the same manner as a headset.
Chris Cocalis, the founder of Titus and now Pivot Cycles, has worked with Shimano to develop a threadless bottom bracket cartridge that simply presses into the frame like any standard bearing. The concept isn’t new; in fact, Gary Klein, Tom Ritchey, and other notable pioneer mountain bike builders popularized the system in the early days before sealed bottom bracket technology (and mass-marketing) caught up.
Today, top road bike makers are already using the “BB 30” threadless bottom bracket standard, and we expect that mountain bike designers will follow suit. Installing bearings directly into the frame has posed no problems for suspension designers, so it makes sense—for reasons of simplicity and flexibility of design—that the same technology should be adopted for the crankset’s bearings.
WHAT’S WRONG WITH THREADS?
At one time, threaded bottom brackets made sense because the balls ran directly on the crank axles and bearing cups. The bearings were poorly sealed, and race mechanics often disassembled and rebuilt the bottom bracket assembly under a tent after every event. Mechanics appreciated the fact that threaded bearing cups could be easily serviced with a pair of big stamped-metal wrenches. Modern cartridge and external bottom brackets, however, are ultra-reliable, remove-and-replace items that require no maintenance at all.
Threads themselves are a nightmare, however, because they require precise machining to ensure proper alignment and are easily damaged. Bearing cups are soft aluminum and can be destroyed with hand pressure if the tiny threads miss their que in the frame and cross-thread. Modern materials like titanium and carbon fiber do not lend themselves to the process. Carbon threads do not have any strength or abrasion resistance, and titanium tends to self-destruct or weld itself to threading dies unless conditions are perfect. These are the reasons that threaded headsets disappeared forever—and reason enough to eliminate threads from the bottom bracket, too.
Same, but different: The press-in cartridge is the same width as the conventional X-type system. To go threadless, however, bike makers must widen the frame’s bottom bracket shell from 73 millimeters (pictured) to 92 millimeters.
EXTERNAL TO INTERNALThe destructive nature of mountain biking forced crank makers to enlarge the diameter of the crank axle, which in turn shrank the space available to put screw-in cups and ball-bearings inside the frame’s bottom bracket shell. External bearing cups were a last-ditch effort to salvage the European road-bike threaded bottom bracket standard because, being outside the confines of the frame, they allowed for a larger-diameter bearing.
Smart frame and component designers (like Shimano and Cocalis) finally said enough is enough. Using the same sized bearings as the new-standard external bottom brackets, Shimano engineered a double-sealed nylon cartridge that holds the bearings in alignment and presses directly into a simple, tubular bottom bracket shell. No threads, no worries—the cartridge just presses in and is retained by internal friction and the crankset spacing.
In action: The Pivot Mach 4 was the first production bicycle to use Shimano’s press-in system. The only part of the cartridge that is visible is the 1.5-millimeter flange that sits against the face of the frame. The wavy aluminum part between the crankarm and bottom bracket is used to adjust bearing end-play on XTR cranksets.WHAT ARE THE ADVANTAGES?
Alignment is assured, because both bearings are located by the inside diameter of the bottom bracket shell, which has been bored in a single machine operation. This internal bore is used to index and align the frame while it is being constructed, so a press-in bottom bracket will always line up true to the centerline of the frame.
Threaded cups rely on the accuracy of the machined outer faces of the frame—and also, the accuracy of two opposing internal threads. Each of those four interfaces are made in separate operations, so minute deviations can add up to noticeable misalignments. The margin for error is compounded when one considers that many bike shops will reface and rethread frames “just to be sure” that these critical operations were done correctly.
Chris Cocalis admits that building bikes with press-in bottom brackets requires a degree of accuracy that is presently out of reach of TIG-and-toss, mid- and low-priced frame contractors, but his laundry list of benefits is impressive:
1. The frame’s bottom bracket shell can be made wider, 92-millimeters instead of 73, which is the widest industry standard. This supports the crank more rigidly and makes room for linkage and larger, stiffer downtubes.
2. No need for precise refacing of the bottom bracket shell or chasing threads with expensive taps.
3. The chain line is always exact, because there is no variation of frame width due to post machining at the factory or bike shop. There’s no need for external spacers for the same reasons.
4. A 3.5-ounce weight savings is realized over conventional external designs.
5. Easy press-in/press-out installation and removal.
FUTURE BENEFITS
Beyond the obvious reduction in manufacturing complexity for both bike and component makers, Shimano’s threadless bottom bracket system has the potential to create a new user-friendly standard for the industry, similar to the threadless headset. Cocalis uses the wider bottom bracket shell of Shimano’s press-in bearing assembly to make room for wider-spaced lower linkage bearings in his Pivot Mach 4 and 5 dw-link rear suspension.
There is a demand for a wider bottom bracket shell in the 29er market where the rear tire encroaches into the chainstay junctions. In the 29er application, the chainstays could attach farther outboard and thus make room for wider tires. Should Shimano design a press-in bearing to fit flush in a standard-width, 73-millimeter shell, the reduced crank width (Q-factor) would be more ergonomic for single-speed riders and cross-country racers who train on narrow-drivetrain road bikes.
Finally, carbon fiber frames must use a threaded aluminum or titanium insert to interface with a conventional bottom bracket. Shimano’s press-in system is a slam-dunk for carbon frames, because a precise, tubular bottom bracket shell can be designed into the mold.
Making room: The Mach 4 takes advantage of the 92-millimeter bottom bracket shell to give the suspension’s lower linkage a wider stance. This adds rigidity and reduces the loads on the linkage bearings. WILL EVERYBODY BE USING IT?
No. At least not for a while. As with any new concept that is injected into the bicycle industry, there will be a wait-and-see period where big-brands and major crank makers will argue the merits of the threadless system. Many holdouts wept when threadless headsets appeared, but the concept swept the mountain bike industry and then overwhelmed road bike makers in less than a decade. Don’t expect to see Shimano’s threadless bottom brackets in the glass case at your local dealer for a while, but watch for them on elite-level mountain bikes as the season
progresses.