Bottom Bracket Tech Breakdown
Ever get confused by all the bottom bracket “standards” out there? English thread vs. Italian thread. BB30, BB86 and BB90?
No worries. Here’s your primer on all things bottom bracket, from it’s history and humble beginnings all the way up to today’s emerging technology. It’s everything you need to know about bottom brackets…
The bottom bracket is a critical component to every drive train. Connecting the left and right cranks, this is one of those important elements on the bike that directly effects power transfer between the rider’s legs and the bike’s drivetrain.
While its concept is simple enough to understand, when it comes to bottom bracket technology, it’s easy to get confused. Incremental changes almost every year coupled with a slew of proprietary technologies from major component companies have flooded the market with more options than ever before, and it can be very confusing for the average cyclist. We broke down every type of bottom bracket you are likely to encounter and present them in a simple, easy to understand format. From mountain to road, threaded to pressed, and integrated to external, it’s all here…
Why The Variety?
Bottom bracket compatibility is one of the most confusing topics for amateur and seasoned cyclists alike. Years ago, it was simple. There was one bottom bracket design, and it was strong, durable, and easy to service. Commonly referred to as “one-piece” Ashtabula cranks, these are made of one solid metal piece, meaning the left crank, the axle, and the right crank are all one piece, held in place by cups pressed into the bottom bracket shell. If you still have an older bike or even a newer children’s or department store bike, chances are you will be able to find one of these on there.
Durable and simple as they may be, these cranks are heavy and inefficient. Component manufacturers found that they could shave significant weight and improve stiffness by manufacturing parts from lighter materials, but in order to maintain the integrity of the set ups they would have to change the way bottom brackets were assembled. The innovation race took off in the cycling industry, but companies failed to agree upon a standard development platform, and rapid development quickly made new technologies obsolete.
Loose Bearing Bottom Brackets
Before the 1990’s, loose bearing bottom brackets were the standard on almost any bike purchased outside a department store, and still are for some lower to mid-range bikes. The system consists of a solid metal spindle or axle, with loose, individual bearings that must be packed into retainer rings during installation or after cleaning/inspecting. With this design came the birth of the “square taper bottom bracket,” which gets its name because of the squared ends of the spindle, over which the crank arms are pressed, secured by a bolt. We will talk about other types of bottom bracket-crank arm interfaces later in the article.
The Birth of the Cartridge
Recently, the loose-bearing system is largely being phased out in favor of another system, known as the cartridge bottom bracket. Much less complicated and with a significantly longer lifespan, cartridge bottom brackets basically encase the mechanical workings of the loose bearing bottom bracket into a much simpler, two piece system. A cartridge bottom bracket consists of the cartridge (duh), which screws (or is sometimes pressed, more on that later) into the drive side of the bottom bracket shell and contains the bearings and spindle, and a lockring, which supports the bottom bracket on the non-drive side.
This type of bearing assembly is largely the standard for today’s bikes, although there are changes on the horizon, namely in the ultra high-end markets. With a simplified design, overhauls no longer involve tedious inspections of individual bearings, and riders can instead easily and cheaply replace or upgrade the cartridges which hold the bearings.
Threadings and Sizes
While a cartridge bottom bracket may seem straightforward, bottom brackets are not one-size fits all. In order for a standard cartridge bottom bracket to be installed on a frame, the two pieces must be threaded into the bottom bracket shell of the frame. This is where choosing a threaded bottom bracket starts to get tricky, because different frames from different manufacturers have different bottom bracket shell sizes, thread sizes and TPIs (threads per inch.) Most frame manufacturers stick to a few more common threadings, such as British, Italian, French, and Swiss, each of which has their own threading and sizing specs. A majority of frames use English threading (1.37″ X 24 TPI) but Italian branded frames (Colnago, Pinarello, etc.) use a slightly different Italian threading pattern (36mm X 24TPI.) French threading is measured at 35mm X 25.4 TPI. The widths of bottom bracket shells also differ; a 68mm shell width is standard for British and French road frames, and 73mm is standard for British and French mountain frames. Italian frames use a unique 70mm wide bottom bracket shell. In order for a bottom bracket to screw into a frame’s shell, its threading must match perfectly with the shell, so check the threading of your frame before ordering a new bottom bracket. You local bike shop will be able to tell you what type of threading and bottom bracket shell diameter your frame has if you don’t know.
Once the bottom bracket is installed, it’s time to attach the cranks. There are several different interfaces, and crankset interfaces must be the exact same as the bottom bracket to attach properly. The square taper spindle was the original interface, where cranks have a square hole and are slid onto the squared ends of the spindle which get thicker as you get closer to the center. Bolts then press the arms further onto the spindle.
ISO vs. JIS
European and Asian manufacturers split when developing their own square taper interfaces, resulting in two standards that survived in the international cycling community. Europe gave birth to the ISO square taper bottom bracket, and Japan developed the JIS, or Japanese Industrial Standard. The two interfaces are remarkably close in dimensions, but the ISO design tapers down to a slightly smaller end. Logically enough, most European brands (Campagnolo, etc) adapted the ISO interface and Asian brands (Shimano, Sugino, etc) stuck with the JIS interface. One interesting note; the Japanese NJS parts highly prized by the fixed gear crowd and used for elite Keirin track races use a European ISO interface, and the parts are threaded to European standards. NJS standards use the same threading as Campagnolo track parts.
Moving Past the Square Taper
This was just the beginning of the confusion, because manufacturers eventually began to abandon the square taper interface all together, and proprietary interfaces began to surface from some major component companies as the innovation and marketing bugs began to spread. First onto the market was Shimano with their Octalink Interface. Shimano developed a system which would attach the cranks via a splined interface, with eight grooved splines that would line up with eight inverse splines on the crank, replacing the square taper interface.
The Octalink system had several advantages; it created a stiffer interface, giving the rider more power transfer from their legs into the mechanical system of the bike, much more than a square taper bottom bracket. While Shimano did have a major improvement in bottom bracket technology, they quickly patented it, and charged a licensing fee to any company wishing to use the technology in their products, making it inaccessible and expensive to their competitors and other component manufacturers. Keep in mind that cranksets must be manufactured with an interface to match the bottom bracket, so crankset manufacturers had to pay Shimano if they wanted to make Octalink-compatible cranks.
Some companies jumped on board and paid, while others decided to counter Shimano’s stronghold with the development of an alternative to the Octalink system which could be widely used across the industry as an open development standard, meaning any company would be free to develop products using the technology, thus creating an internationally accepted and accessible standard. So major industry brands King Cycle Group, Truvativ, and Race Face got together to develop the International Splined Interface Standard or ISIS, a system which was similar to Shimano’s but used ten splines instead of the Octalink’s eight. Both interfaces are still widely in use today, but ISIS is generally more popular, because it is more accessible.
By this time, the race was on to develop stiffer, lighter bottom bracket and crank systems. The focus moved away from development of the interface and began to shift toward structurally superior configurations. As innovation continued, engineers soon found they were limited by the diameter of the bottom bracket shell. To improve power, manufacturers were looking to beef up the diameter of their bottom bracket spindles (axles), but in order to do so, the bearing sizes had to be smaller in order to fit the entire configuration inside a standard bottom bracket shell.
Smaller bearings mean less durability, something they weren’t willing to sacrifice, so the spindle size had effectively been maxed out. This problem gave birth to the external bottom bracket system, which moves the bearings of the bottom bracket into cups which sit outside the bottom bracket shell of the frame, allowing for large bearings and a thick spindle within the traditional bottom bracket shell dimensions. Remember that up until this point, in traditional square taper, Octalink, and ISIS set-ups, the spindle and bearings were all housed WITHIN the bottom bracket shell of the frame. Moving the bearings outside the shell created more room for a larger spindle inside, so external bearing bottom bracket systems can have both larger spindles and bearings than their traditional counterparts.
Spindle diameters could suddenly go up to 24mm, which made them stronger and stiffer than ever before. This was a MAJOR improvement, and created a much stiffer crankset/bottom bracket set-up, which is being widely used today. Almost every manufacturer has brought their own brand of external bottom bracket systems to the market; popular models include Shimano Hollowtech II, Race Face X-type, FSA MegaEXO, and Truvativ/SRAM’s Giga X Pipe (GXP). These external bottom brackets are probably the most common type of bottom bracket found on bikes in today’s market. Each brand’s external bottom bracket system is fairly similar, and most have also designed exclusive crank/bottom bracket product lines based on external bearing bottom bracket designs.
It should be noted that Campagnolo’s Ultra Torque crankset/bottom bracket system uses external bearings, but each crank arm is permanently attached to one half of the spindle, and they are joined in the middle of the bottom bracket shell, the only bottom bracket to do so.
Thought that was it? While external bottom bracket systems are still relatively new to the market, there is still more change on the horizon with the BB30 standard. Debuted by Cannondale at the 2000 Tour de France, the BB30 system starts from the frame up, using a larger-diameter bottom bracket shell sizing to create more space for an even larger 30mm diameter axle, 6mm thicker than the 24mm steel axle used on external bearing systems. (The traditional inside shell diameter was 34mm, BB30 uses a 42mm inside diameter.)
The BB30 system is also unthreaded; the bearings are pressed directly into the shell instead of sitting inside of cups threaded into the frame. This saves weight, because it eliminates the need for some parts, like the threaded cups. Even the threads that they require to screw into the shell add some weight.
Similar to the ISIS concept, BB30 is an open standard. Cannondale initially developed it as a proprietary technology, but saw the potential for the set up and wanted to avoid some of the mistakes made by Shimano with their Octalink system. Thus, it was published as an international standard, and is now free for any manufacturer to use and develop.
Many experts in the industry see a bright future for BB30 systems, as they are the stiffest and lightest to hit the market thus far, and perhaps more importantly, the technology is equally available to every company, making it a good contender for worldwide acceptance. As for now, this system is mostly reserved for higher-end bikes. In addition to a new bottom bracket shell, cranks must be manufactured to work with the BB30 system, so options are still limited. SRAM and FSA are some of the early adoptors of this technology. SRAM offers their Force, Red, and XX cranksets with a BB30 option and FSA Gossamer, Afterburner, SL-K and K-Force cranksets also have a BB30 option. While it does provide a significant performance increase, frame and crank manufacturers must design new frames around the larger shell size of the BB30 standard, and it will most likely be a few years before the larger shell specs trickles down frame manufacturers’ product lines.
Some frame manufacturers like Scott, Trek, and Giant have tried to take the BB30 system even further, creating the “integrated bottom bracket.” This system requires yet another new bottom bracket shell sizing, designed around the diameter of bearings in existing external cup bottom brackets. The technology behind BB86/90 systems is similar to BB30, but it is designed to be used with existing external cup, two piece cranksets. Just like BB30, bearings are pressed into the shell, eliminating the need for cups and threads, and again, eliminating more weight. BB86 and BB90 get their names from the width of the shell (86mm and 90mm wide), while BB30 got its name from the 30mm spindle diameter. Shimano’s own adaptation and proprietary interpretation of this technology is called the “Shimano Press Fit” bearing system. Shimano has not yet adapted any of their products to the BB30 system, and is instead investing its resources into its incompatible Press Fit systems.
Lastly come the eccentric bottom brackets, which are used almost exclusively on single speeds, fixed gears, and tandems. These setups allow the bottom bracket to slightly rotate in order to adjust chain tension without moving the rear axle.
Get Your Bearings
Ceramic bearings have made quite a splash in the high-end market recently as a lighter, stronger, and smoother alternative to steel ball bearings. While they do tack on a few extra bucks, most agree that there is a performance increase in ceramic bearings. Some high end models come with ceramic bearings, but aftermarket ceramic bearing cartridges can be purchased in a variety of sizes from several manufacturers.
Bottom bracket technology will be a constantly evolving area, and we are most likely rather far from developing a lasting standard. Much like the headset, constant and quick innovation will drive introductions of newer, better standards as engineers develop better products and materials. The bottom bracket wars are far from over, and it seems to be only a matter of time before the current technologies become largely obsolete, giving way to stiffer, lighter, stronger designs.
Take a deep breath. For the moment, anyway, you can consider yourself up to date.