The bottom bracket on a bicycle contains an axle to which the crankset is attached and the bearings that allow the axle and cranks to rotate. (The chainrings and pedals are attached to the cranks.) The bottom bracket fits inside the bottom bracket shell, which is part of the bicycle frame.

Bottom bracket types

Three-piece

In typical modern bikes, the bottom bracket axle is separate from the crank arms. This is known as a three-piece crank. The crank arms attach to the axle via a common square taper or via a variety of splined interfaces.

Earlier three-piece cranks consist of an axle (known as the spindle) with bearing cones (facing out), a fixed cup on the drive side, an adjustable cup on the non-drive side, and loose bearings. Overhauling requires removing at least one cup, installing new ball bearings, replacing the spindle and adjusting the cups. Most modern bicycles use a one-piece cartridge bottom bracket instead, with the spindle and bearings sold as a single unit. This makes servicing the bottom bracket a simple matter of removing the old cartridge from the bottom bracket shell, and installing a new one in its place.

Eccentric

An eccentric bottom bracket is a cylindrical plug that fits in an enlarged bottom bracket shell. The plug is machined to accept a typical bottom bracket. The bottom bracket is offset in the plug, so by rotating the plug, the location of the bottom bracket (and hence the chain tension) may be adjusted. The plug is then fixed in place by a pair of set screws.

Eccentric bottom brackets are usually found on tandems, where they are used to adjust the chain that connects the stoker's and captain's cranks. They may also be employed in bicycles designed without a derailleur but with vertical dropouts, such as single-speeds, fixed-gears, or bicycles with internally-geared hubs.

Ashtabula

With an Ashtabula crank and bottom bracket, the axle and crank arms are a single piece. The bottom bracket shell is large to accomodate removal of this S-shaped crank. Bearing cups are pressed into the bottom bracket shell. The crank holds the cones, facing in; adjustment is made via the left-threaded non-drive-side cone.

Ashtabula cranks are easily maintained and reliable, but heavy. They are found on BMX bikes as well as older low-end road and mountain bikes.

Ashtabula cranks are also known as "one-piece" cranks.

Thompson

The Thompson bottom bracket uses adjustable spindle cones and cups pressed into the bottom bracket shell like the Ashtabula bottom bracket. Unlike the Ashtabula crank, the non-drive side crank is removable, allowing for a smaller bottom bracket shell. Frames with either Italian or English bottom bracket shell diameters (independent of threading) may be fitted with Thompson bottom brackets. This having been said, the Thompson bottom bracket is rare.

Other

In 2003, Shimano introduced a two-piece bottom bracket in their high-end XTR line. The drive-side crank and bottom bracket axle are an integrated unit and the bearings are placed outside the bottom bracket shell.

Schlumpf makes a bottom bracket that incorporates a two-speed transmission.

Bottom bracket height

The height of the bottom bracket is of concern when designing the frame. The height of the bottom bracket is the baseline for the rider's height while riding. Combined with the length of the cranks, it determines the bicycle's ground clearance.

A higher bottom bracket is useful for mountain bikes. In a fixed-gear, the bottom bracket should be high enough to prevent the pedals from coming in contact with the ground while cornering.

For touring bikes, a lower bottom bracket creates a lower center of gravity and allows for a larger frame without creating an uncomfortable standover height.