Bottom brackets might not be the most exciting part of a bike, but without them you'd be stuck pushing around a high tech scooter. In this edition of To The Point we sort through an alphabet soup of acronyms and abbreviations to take a brief look at the numerous 'standards' on the market today.
What exactly is a bottom bracket?
A threaded bottom bracket shell. What are the main types of bottom brackets currently being used for mountain bikes?
A bottom bracket is the assembly that allows a bicycle's crank arms to rotate. The bottom bracket sits in the part of a frame called the bottom bracket shell, which is located where the down tube, seat tube and chain stays meet. Depending on the configuration, a bottom bracket can either be a self contained unit made up of a spindle that rotates on a set of bearings, or if the spindle is affixed to one of the crank arms, the bottom bracket would consist only of a set of bearings. The bottom bracket is either threaded or pressed into the frame depending on the standard that is used.
These days the most common options are the traditional threaded style, with either external or internal bearings, and two variations on the press fit theme, the PF30 and the BB92. The BB30 and Trek's proprietary BB90 system are also used, but not as extensively as the other styles. The BB30 and BB90 use bearings that sit directly in the bottom bracket shell, while the PF30 and BB92's bearing are housed in a retainer that is pressed into the shell.Many downhill bikes use a threaded 83mm bottom bracket shell combined with 150mm rear hub spacing, although both SRAM and Shimano now offer press fit options that can be used with the wider rear hub spacing. What's the difference between a BB30 and a PF30?
Both standards are designed to use a crank arm spindle with a 30mm diameter (hence the '30' in the name), but with a BB30 set up the bearings sit directly in the frame, and with a PF30 the bearings are housed in a nylon or aluminum retainer that gets pressed into the frame. Tolerances need to be tighter with a BB30, making this a more expensive system to manufacture. BB30 was originally developed by Cannondale in 2000, but they have since made the dimensions and technical drawing available to any company who would like to use the BB30 standard. To make things more confusing, several companies are now offering cranks with 30mm spindles that work with the BB92 standard – by using a bearing without a retaining cup, an inner race that's wide enough to accommodate the larger spindle can be used. Race Face's Next SL cranks use a 30mm spindle diameter, but the company offers a bearing that allows them to be compatible with a BB92 bottom bracket shell. So then what's a BB92?
The BB92 standard, also known as Shimano Press Fit, uses a bearing housed in a nylon or aluminum cup that is pressed into a bottom bracket shell that measures 92mm wide. This standard is designed around the 24mm spindle diameter used by Shimano. Because the bearings are housed inside the shell, the spacing remains the same as it would be with an external bearing configuration, which means a wider spindle length is not necessary for riders switching from a 68/73mm threaded shell to a BB92 set up. What are the advantages of a press fit bottom bracket?
What are the disadvantages of a press fit bottom bracket?
For the consumer, the benefits aren't immediately apparent, and some manufacturers like Santa Cruz have resisted the press-fit trend, stating that the disadvantages outweigh the advantages. For manufacturers, doing away with a threaded shell saves time and money, especially when constructing a frame out of carbon fiber. It is much easier to create a smooth, circular opening rather than cutting threads or installing a threaded sleeve into a frame. Using a wider bottom bracket shell also allows for more real estate to work with when trying to stiffen up the rear end of a bike. For instance, Kona's new Process
series of bikes uses a BB92 in order to gain the clearance necessary for the bike's wide chain stays.
What does the future hold? Why isn't there one standard?
The most common complaint regarding press fit bottom brackets has to do with noise. If the tolerances are not tight enough, hard pedaling can cause the cups to shift slightly, causing an annoying creak with each pedal stroke, especially if contaminants have managed to make their way between the cups and the bottom bracket shell. Careful installation
and the use of Loctite or a sleeve retaining agent helps to mitigate any problems, but this issue does seem to be more common with press fit style bottom brackets compared to the traditional threaded style. Frame damage can also occur if a bearing is pressed in improperly - again, using the correct tools during bearing intsallation and removal is the key to avoiding any issues.
It's unlikely there will ever be one standard, but press fit does seem to be gaining traction, especially as carbon fiber frames grow in popularity. The major drivetrain players, Shimano and SRAM, are each pursuing their own standard, with Shimano supporting the BB92 and SRAM the PF30. Only time will tell if one will prevail, but for the near future there will continue to be multiple 'standards.'