Bottom brackets

Simply put, the bottom bracket on a bicycle is the component around which the crankset rotates. The bottom bracket contains some sort of axel and bearing system which fits inside the bottom bracket shell of the bike frame.

Bottom bracket types

Bike bottom brackets from in a variety of different types and sizes, often determined by the type of bicycle in question.

Three piece

In typical modern bikes, the bottom bracket is a separate component to the chainset and cranks, making it a three piece system.

Earlier examples of three piece systems consisted of a spindle with separate bearing cones. However, most modern bikes now come with sealed one piece cartridge bottom brackets with the spindle and bearings coming in a single sealed unit. These modern bottom brackets are now known as standard bottom brackets, with the phrase Three Piece referring to BMX style bottom brackets, which still come with separate bearing cups.

Eccentric

Eccentric bottom brackets are usually found on tandems, where they are used to adjust the chain that connects the stroker's and the captain's chain. They may also be occasionally used on single speed and fixed gear bikes with horizontal drop outs.

An eccentric bottom bracket is a cylindrical plug that fits into an enlarged bottom bracket shell, allowing the bottom bracket to be offset in the plug, adjusting the location of the bottom bracket.

Ashtabula

Ashtabula bottom brackets are more commonly referred to ask one-piece cranks. Ashtabula cranks and spindle are a single piece, coming in a roughly s-shaped design. Bear cups are pressed into the frame, with the cranks holding in the bearings using bearing cones.

Ashtabula cranks are simple and easily maintained, however they are heavy, but offer a good cost effective option for entry level BMX bikes as well as older, lower end road bikes.

Thompson

The Thompson bottom bracket employs adjustable spindle cones and cuts pressed directly into the frame, similar to those used by Ashtabula systems. However, unlike Ashtabula cranks, Thompson bottom brackets offer a two piece system, with the non-drive crank being removable. This allows for a smaller bottom bracket shell than that required for the lesser Ashtabula system.

External bearing

Many current designs are choosing to use an integrated bottom bracket with external bearings. Intended to solve the problems caused by relatively small diameter shells, external bearing systems offer two alternatives; larger bearings and a thinner spindle, which lack stiffness, or smaller bearings and a thicker spindle, offering more stiffness but less durability.

Bottom bracket interfaces

In addition to the various different ways to fit a bottom bracket within the frame's bottom bracket shell, there are also a variety of ways to connect the crank arms to the spindle.

Cottered

Now almost entirely obsolete, Cottered bottom brackets have a spindle with a slightly tapered cylinder with flattened ends. Today, only a few manufacturers produce spares as bikes have long come fitted with modern equivalents, such as Square Tapered systems.

Square Taper

Cottered axles' successor, Square Taper is currently the most popular design. Based around a square tapered spindle, a corresponding square tapered hole in the crank arms, the Square Taper system is relatively simple and efficient. Perhaps the best known example of a Square Taper system is the Japanese Industrial Standard made famous by Shimano, despite the fact that Shimano have since moved on to develop more advanced systems.

Octalink

Originally designed by Shimano to address the problems with the Square Taper system and its lack of standardisation, Octalink provided a stiffer interface between the bottom bracket and the crank arms. Today, as a result of patents and license fees, relatively few companies other than Shimano produce Octalink systems.

Octalink exists in two variants; Octalink v.1 and Octalink v.2. Shimano Ultegra 6500 and Dura Ace 7700 cranksets use version 1 with more recent mountain bike designs tending to use the deeper grooved version 2.

ISIS Drive

Created in response to Shimano's Octalink, King Cycle Group, TruVativ and Race Face set about designing the ISIS Drive. Standing for International Splined Interface Standard, ISIS Drive systems operate using an open standard splined specification interface, putting splined interfaces in the public domain.