The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset attaches, and the bearings that allow the spindle and crankset to rotate. The chainrings and pedals attach to the cranks. Bottom bracket bearings fit inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.
The term "bottom bracket" appeared around the 1890s, when some bicycle designs included "brackets" bolted to the bottom of the frame, which held the bearings and the pedal spindle. In America, similar brackets were often called "hangers" because of the way they hung from the bottom of the frame. Later, the "bracket" was thought to refer to the tube fittings that are used to hold frame tubes together in lugged steel frames which also form the shell that contains the spindle and bearings. The term "bottom bracket" is now used for the bearing-spindle component of all frame types, regardless of construction details.
There is some disagreement as to whether the word axle or spindle should be used in particular contexts. The distinction is based on whether the unit is stationary, as in a hub, or rotates, as in a bottom bracket. American bicycle mechanic and author Sheldon Brown uses axle once and spindle four times in his bottom bracket glossary entry. This article uses spindle throughout for consistency.
Bottom bracket assemblies are available in several types, and can be split into whether they are assembled and disassembled with screw threads or whether they are pressed into the bottom bracket. Since the 2000s and especially the 2010s, a lack of standardization, or rather the constant introduction of new standards that disappear after relatively short periods, has been described as a complex topic to deal with for those who want to buy bicycle components or maintain bicycles. Many bicycle brands have introduced their own dimensions for bottom bracket bearings, and the different use of terminology by the various manufacturers has been described as confusing.
Bottom bracket types
Three-piece In typical modern utility bikes, the spindle is separate from the cranks. This is known as a three-piece crankset (spindle, left crank-arm and right crank-arm). The cranks attach to the spindle via a common square taper, cotter, or splined interface.
Loose bearing (adjustable cup and cone) Earlier three-piece cranks consist of a spindle incorporating bearing cones (facing out), a fixed cup on the drive side (with a cone), an adjustable cup on the non-drive side (also with a cone), and loose bearing balls (or held by a cage). Overhauling requires removing at least one cup, cleaning the cups, cleaning or (more usually) replacing the bearing balls, reinstalling the spindle, and adjusting the cups. The design is nowadays mostly found on affordable bikes due to its simple and affordable design, and is also an easily serviceable design.
Bayliss Wiley unit bottom bracket
The Bayliss Wiley unit bottom bracket is a self-contained unit that fits into a plain, slightly larger-than-usual bottom bracket in a bicycle frame. It comprises a standard spindle and bearings in a steel cylinder with a slightly-modified bearing cup at each end. The cylinder, bearing, and spindle are placed in the shell and held in place by the bearing cups, each of which has a narrow flange that bears against the edge of the shell. Royal Enfield Revelation (retrieved 26 August 2013).
The Bayliss-Wiley Unit Bottom Bracket was introduced in the mid-1940s. It was fitted to various English lightweights through the 1950s and was used by Royal Enfield on its 'Revelation' small wheeler in the mid-1960s. However, the unit bottom bracket was never popular and it had a reputation for being troublesome. A lack of positive location allowed it to rotate within the frame, loosening the bearing cups. Contemporary users overcome the problem by fixing the unit in the frame using adhesive or a screw.
Cartridge bearing Many modern bicycles use what is called a "cartridge" bottom bracket instead. "Cartridge" here refers to the bottom bracket unit being a pre-mounted assembly containing the spindle (or axle) as well as its bearings, as opposed to them being separate parts which must be mounted separately. Cartridge hence refers to the functional assembly unit, and does not indicate what type of bearings the cartridge utilizes. Either loose ball bearings (adjustable cup and cones), sealed bearings or other types of bearings may be used in a cartridge assembly. Independent of the type of bearing used, the cartridge often has some sealing to protect the assembly from the environment.
Sealed cartridge bottom brackets are normally two pieces, a unit holding the spindle and bearings that screws into the bottom bracket from the drive side and a screw-in support cup (often made of light alloy or plastic) that supports the spindle and bearing assembly on the non-drive side. Other designs are three piece, the spindle is separate, but the bearing cups incorporate cheaply replaceable pairs of standard industrial sealed bearings. Either arrangement 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. Cartridge bottom brackets generally have seals to prevent the ingress of water and dirt. The early Shimano LP bottom brackets from the 1990s had the support cup on the drive side and used loose bearings inside; they could be dismantled and serviced much like adjustable cup and cone bearings.
In general use, the term 'three piece' refers to the former design, with sealed bottom brackets being seen as the 'standard'. Designs utilizing separate bearings are usually found on low end bikes, due to the low cost.
One-piece (Ashtabula)
With a one-piece (also called Ashtabula) crank and bottom bracket, the spindle and crank arms are a single piece. The bottom bracket is large to accommodate removal of this S-shaped crank. Bearing cups are pressed into the bottom bracket. The crank holds the cones facing in; adjustment is made via the left-threaded non-drive side cone.
One-piece cranks are easily maintained and reliable, but heavy. They are found on BMX bikes as well as children's bicycles and low-end road and mountain bikes. They fit only frames with American sized (also known as "Pro size") bottom brackets.
The bearings are normally open to the elements and easily contaminated, although this rarely causes failure. Ball retainers (caged bearings) are used to facilitate assembly and to reduce the number of balls required.
Thompson The Thompson bottom bracket uses adjustable spindle cones and cups pressed into the bottom bracket like the Ashtabula bottom bracket. Unlike the Ashtabula crank, the non-drive side crank is removable, allowing for a smaller bottom bracket. Frames with either Italian or English bottom bracket diameters (independent of threading) may be fitted with Thompson bottom brackets. Thompson bottom brackets are rare. The design is similar to a typical hub bearing and theoretically supports the load better but is hard to seal effectively against dirt and water.
External bearings Since around the late 2000s, several designs with integrated bottom brackets with outboard bearings have emerged. The sales pitch of these systems have been to enable reduced weight and increased stiffness compared to internal bottom brackets. Because of the relatively small 1.37″ (34.9 mm for ISO, or 36 mm for shells threaded to the Italian standard) diameter shell, designs that place the bearings inside the shell can either have large bearings and a thinner spindle, which lacks stiffness, or smaller bearings and a thicker spindle (such as the original Shimano Octalink), which lacks durability. External bearings allow for a large diameter (hence stiff) and hollow (hence light) spindle. They also offer more distance between the two bearing surfaces, which contributes to stiffness while allowing lighter components (but also may increase the Q factor).
A different approach than to move to threaded external bearings could be to standardize on one of the larger diameter press-fit BMX shell standards for all bicycles, or the press-fit BB30 standard originally introduced by Cannondale.
Several implementations of external bearings have been brought to market.
X-type and Hollowtech II In one design, the driveside (right) crankarm and the bottom bracket spindle are an integrated unit and the bearing cups are placed outside of the bottom bracket shell, threaded into the bottom bracket shell. There are a number of versions of this design available: Shimano's Hollowtech II, RaceFace's X-type, FSA's MegaExo. The terms 'X-Type' and 'Hollowtech II' are both used to refer to any design of this type, but are in fact trademarks of the corporations marketing the systems. These external bearings are compatible with those from other manufacturers. With this new standard have come several cranksets designed to use the external bearings of other manufacturers, such as DMR's "Ex type" and Charge Bikes "Regular" cranks.
In the early 1990s at Magic Motorcycle, a small USA component manufacturer later purchased by Cannondale, and re-formed into Cannondale's CODA brand (Coda Magic 900 cranks), made a proprietary external bearing bottom bracket, oversized spindle and crank system. The design resembles the external bottom bracket designs marketed by FSA, RaceFace and Shimano. The modern versions used the same sealed bearing size (6805-RS, 25 mm inner diameter, 37 mm outer diameter) and the original mounting tool fits. The crank had intricately CNC machined wide hollow crank arms made of two halves glued together. However, Cannondale moved on from that system and developed the SI cranks and the new BB30 unthreaded press-fit bottom bracket standard. BB30 requires special frames which have a 42 mm diameter unthreaded bottom bracket shell (which is larger than the 34.9 mm threaded ISO standard threads) allowing use of internal sealed bearings while their top level SI crankarms are still two machined aluminum halves glued together.
Another precursor of the current external bearings/through spindle design was developed by Sweet Parts, a micro-manufacturer of high end cranks and stems. Their Sweet Wings cranks from the early 1990s incorporated the through spindle concept by attaching the two half pipes coming off each crank arm and held together with a single bolt that resided within the cavity of the spindle itself. Their bottom bracket bearing arrangement was a hybrid internal/external bottom bracket, with the right-side bearing being internal inside the bottom bracket shell, and the left-side bearing being external (and having the 6805-RS sealed bearing, too).
Giga-X-Pipe
Giga-X-Pipe was Truvativ's (later bought by SRAM in 2004) approach, and is an evolution of the ISIS Drive bottom bracket, but with a longer spindle and the bearings outside the bottom bracket shell. The spindle is permanently pressed into the right crank. The left side spline interface looks similar, but is different so as to prevent installation of older ISIS Drive crankarms—which are no longer compatible because Q-factor and chain line cannot be maintained using these older cranks with an external bearing bottom bracket. Truvativ refer to this design as 'Giga-X-Pipe' or 'GXP.' They also make a heavier duty external bearing bottom bracket called 'Howitzer.' The Howitzer BB is more like a traditional bottom bracket in that the spindle is not permanently pressed into the right crank. Again, the Howitzer spline looks similar to the ISIS Drive standard spline but is actually different, so as to prevent the usage of ISIS Drive cranks on the external bearing bottom bracket, which would affect chainline and Q-factor.
Ultra-Torque