A bottom bracket axle is a component most often found on a bicycle. It consists of bearings that are threaded into the lowest part of the bicycle frame, as well as an axle or spindle that presses against the bearings. The bottom bracket axle extends outward from the bottom bracket shell, which is the round, threaded part of the frame in which the bottom bracket is mounted. Crank arms are mounted to the spindle, and pedals to the crank arms so that the bicycle can be propelled forward.
Several types of bottom bracket axle assemblies exist. The most common types are three piece cranks with a press-in bottom bracket, a cartridge-style bottom bracket, and outboard bearing bottom brackets. Press fit bottom brackets require bearing cups to be pressed carefully into the bottom bracket shell. This is usually done using a clamp or vise, or a specially designed bottom bracket tool. The bearing cup is then packed with grease, and ball bearings are placed in the grease. The bottom bracket axle will sit between the bearing cups, and a bearing cone will press against the other side of the bearings, allowing the spindle to rotate freely. The entire bottom bracket axle assembly is secured in place using lock rings.
Cartridge-style bottom brackets are far easier to install and uninstall, though the bearings usually cannot be removed, cleaned, and repacked. Instead, the entire cartridge is thrown away when it is worn out, and it is replaced with a new one. The cartridge threads into the bottom bracket shell, which must be machined to include threads. One side of the cartridge is called the fixed cup, while the other side is known as the adjustment cup; the fixed cup is installed first, and then the adjustment cup is used to adjust the tightness of the bottom bracket axle assembly.
Outboard bearing bottom brackets feature larger bearings that are less likely to wear out quickly. To accommodate the extra bulk of the bearings, the bearing cups are placed on the outside of the bottom bracket shell. The cups are secured into place by threading into the existing threads of the bottom bracket shell. One of the crank arms will feature a fixed hollow spindle that slides through the bottom bracket; it is secured in place by attaching the opposite arm and cinching down securing bolts. This is usually a lighter system than other types of bottom brackets, and it tends to last a fairly long time.