A Guide to
There are two main types of bicycle bottom bracket, namely those that thread into your frame and those that are pressed in.
Threaded Bottom Brackets
Threaded bottom brackets come in number of different versions. Cheaper and older designs use bearings that sit inside the frame with a captive (fixed) axle that the crank arms are tightened onto. There are a multitude of standards for the captive axle. The most common and cheapest is a square interface (square taper), but Shimano offer a multi splined (Octalink) interface and there is also a standard called ISIS, though these latter two are more rare, having been superseded by other designs.
Most modern threaded designs now use a three piece design that has a pair of larger diameter bearings that sit outboard of the bottom bracket shell, which an axle that is permanently fixed to one of the crank arms threads through. This design has a large advantage in that the bearings sit outside the bottom bracket shell and can be of a much larger diameter, making them much more hard wearing. The axle can also be made thinner and lighter. These designs include Shimano Hollowtech II and SRAM GXP , although these two systems aren’t compatible.
Press-Fit Bottom Brackets
Press-fit bottom brackets that don’t require a threaded shell on the bike frame are becoming more popular. They allow designers to make composite frames that don’t require tough inserts for the bearings to sit in. There are a number of standards here too. Shimano’s Press-Fit 92 uses the same spacing as their external, threaded Hollowtech II system and is cross-compatible with those cranks using the right 24mm spindle. There is also BB30, which uses even larger diameter bearings for a stronger, stiffer and lighter axle, but this requires a dedicated crank or the use of a converter such as the Wheels Manufacturing adaptor.
The bearings are highly important, and their quality and dictates how much energy is lost as friction and how long they’ll last. Fully sealed cartridge bearings are preferable unless you simply cannot afford any better. They are non-adjustable, fit and forget units that don’t need any maintenance but have to be thrown away when they are worn out. More expensive units use bearings that have much higher tolerances, resulting in less friction and longer life. Mountain bike bottom brackets often have a larger amount of sealing for the increased demands of off-road use, which increases friction as a side effect. More lightly sealed, lower friction ‘Road’ versions are also common.