Shimanos Secret Bottom Bracket Cartridge
Look Ma, there are no threads
Shimanos’ press-in bearing assembly is paired with an external-type, threaded bearing cup. The nylon housing conceals the same angular contact bearing but can be installed in the same way as a headset into the unthreaded bottom bracket shell.
Chris Cocalis is the founder of Titus, now Pivot Cycles. He has collaborated with Shimano in developing a threadless bottom bracket cartridge. It simply fits into the frame just like any other standard bearing. This concept is not new. In fact, Gary Klein and Tom Ritchey were among the first to popularize the system before seal bottom bracket technology (and mass marketing) became mainstream.
Today’s top road bike manufacturers use the BB 76cm threadless bottom bracket standard. We expect mountain bike designers to follow their lead. Suspension designers have never had to install bearings directly into frames. It makes sense, therefore, that the same technology be used for crankset bearings.
WHAT’S WRONG with THREADS?
Threaded bottom brackets were once common because the balls ran directly onto the bearing cups and crank axles. Race mechanics would often disassemble the bottom bracket assembly and rebuild it under a tent after each event because they were not well sealed. The fact that the threaded bearing cups were easily accessible with a pair large stamped-metal wrenches was appreciated by mechanics. Modern cartridge and external bottom brackets are extremely reliable, easy-to-replace parts that don’t require any maintenance.
The problem with threads is that they need to be precisely machined and can easily be damaged. Bearing cups made of soft aluminum can be damaged by hand pressure if tiny threads are not aligned correctly in the frame or cross-thread. The process is not possible with modern materials such as titanium and carbon fiber. Carbon threads are weak and prone to abrasion. Titanium can also self-destruct, or weld to threading dies, unless the conditions are ideal. These are the reasons threaded headsets vanished forever. It is also a reason to remove threads from the bottom bracket.
Similar, but different: The standard X-type press-in cartridge has the same width. Bike makers will need to increase the frame’s bottom bracket shell by 73 millimeters (pictured), in order to go threadless.
EXTERNAL TO INTERNAL
Mountain biking’s destructive nature forced crank manufacturers to increase the diameter of their crank axle. This reduced the amount of space that could be used to place screw-in cups or ball-bearings within the frame’s bottom bracket shell. Because they were not within the frame’s confines, external bearing cups allowed for larger bearings.
Shimano and Cocalis, smart frame and component designers, finally decided enough was enough. Shimano designed a double-sealed nylon cart that uses the same bearings as the standard external bottom brackets. It is sealed in a way that keeps the bearings in line and presses into the shell of a tubular bottom bracket shell. There are no threads to worry about – the cartridge simply presses into the bottom bracket shell and is held in place by internal friction and crankset spacing.
In action: The Pivot Mach 4 was first bicycle to utilize Shimanos press in system. The 1.5-millimeter long flange, which sits against the frame’s face, is the only visible part of the cartridge. For XTR cranksets, the wavy aluminum portion between the crankarm & bottom bracket can be used to adjust bearing end play.
WHAT ARE THE AVANTAGES?
Because both bearings are located at the inside diameter of bottom bracket shells, alignment is guaranteed. This has been done in one machine operation. This inner bore is used to align and index the frame as it is constructed. A press-in bottom bracket will therefore always line up with the frame’s centerline.
Threaded cups are based on the precision of the outer faces of the frame and also the accuracy of two internal threads. Because each of these interfaces is made separately, small deviations could result in noticeable misalignments. This margin of error is magnified when you consider that many bike shops will reface or rethread frames to ensure that critical operations are done correctly.
Chris Cocalis acknowledges that bikes built with press-in bottom brackets require a level of accuracy that is currently out of reach for TIG-and–toss, low- and mid-price frame contractors. However, his impressive list of benefits is remarkable.
1. The frame’s bottom bracket shell can be made wider than 73. This is the industry standard. This makes the crank stronger and allows for more linkage.
2. There is no need to reface the bottom bracket shell in any way or chase threads with expensive taps.
3. Because there is no variance in frame width due to post-machining at the factory, or at the bike shop, the chain line is always precise. External spacers are not necessary for the same reasons.
4. Over conventional external designs, a weight loss of 99 grams is achieved.
5. Installation and removal are simple with a press-in/press out.
FUTURE BENEFITS
The obvious decrease in manufacturing complexity for component and bike makers is not the only benefit of Shimanos threadless top bracket system. It has the potential to set a new industry standard similar to the threadless headset. Cocalis uses Shimanos wider bottom bracket shell to allow for more space for lower-spaced linkage bearings in the Pivot Mach 4 or 5 dwlink rear suspension.
In the 29er market, the bottom bracket shell is required to accommodate a wider rear tire that encroaches on the chainstay junctions. The chainstays in the 29er model could be extended outboard to allow for larger tires. Shimano could design a press-in bearing that fits flush in a shell of standard width, 73 millimeters. This would make it more convenient for single-speed riders as well as cross-country racers using narrow-drivetrain roadbikes.
Carbon fiber frames should use a titanium or threaded aluminum insert to interface with a traditional bottom bracket. Shimanos’ press-in system for carbon frames is an easy one, as a tubular, precise bottom bracket shell can easily be incorporated into the mold.
Making space: The Mach 4 makes use of the 92 millimeter bottom bracket shell in order to give the suspension’s lower linkage a wider stance. This increases rigidity and decreases the load on the linkage bearings.
WILL EVERYBODY USE IT?
No. No, at least not for a while. Like any new concept in the bicycle industry there will be a waiting period during which major brands and crank makers will debate the merits. Although many were disappointed when threadless headsets first appeared, they quickly became a popular choice for mountain bikes and then dominated road bike manufacturers in a matter of ten years. For a while you won’t see Shimanos threadless bottom bars in your local dealer’s glass case, but keep an eye out for them on top-level mountain bikes.
progresses.
