The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It contains a spindle that the crankset attaches to, and the bearings that allow the spindle and cranks to rotate. The chainrings and pedals attach to the cranks. The bottom bracket fits inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.
The term “bracket” refers 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 terms is since used for all frames, bracketed or not.
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 axle/spindle is stationary, as that in a hub, or rotates, as that 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.
An old American term for bottom bracket is hanger. This is usually used in connection with Ashtabula cranks, alternatively termed one-piece cranks.
In typical modern bikes, the bottom bracket spindle is separate from the cranks. This is known as a three-piece crankset. The cranks attach to the spindle via a common square taper, a cotter or via a variety of splined interfaces.
Earlier three-piece cranks consist of a spindle incorporating 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, cleaning or replacing the bearings, reinstalling the spindle, and adjusting the cups.
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 shell 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.
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.
Many modern bicycles use what is called a “cartridge” bottom bracket instead. Sealed cartridge bottom brackets are normally two pieces, a unit holding the spindle and bearings that screws into the bottom bracket shell from the drive side and a support cup (often made of light alloy or plastic) that supports the spindle/bearing assembly on the non-drive side. Other designs are three piece, the spindle is separate, but the bearing cups incorporate cheaply replaceable standard cartridge bearing units. 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 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.
With a one-piece (also called Ashtabula) crank and bottom bracket, the spindle and crank arms are a single piece. The bottom bracket shell is large to accommodate 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.
One-piece cranks are easily maintained and reliable, but heavy. They are found on BMX bikes as well as 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.
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.
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.
Many current designs are now using an integrated bottom bracket with outboard bearings. This is an attempt to address several issues associated with weight and stiffness. Because of the relatively small 1.37″ (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. A different approach would be to standardise on the larger BMX shell for all bicycles, or the 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 bearings are placed outside of 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.
Magic Motorcycle, a small USA component manufacturer that was 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 in the early 1990s. This design is similar to the external Bottom Bracket designs that are currently being marketed by FSA, RaceFace and Shimano. The modern versions are using the same bearing size (6805-RS) and even the original mounting tool fits but the bearings are sitting closer to the frame now. The crank had intricately CNC machined wide hollow crank arms made of two halves glued together. However, Cannondale moved on and developed the SI cranks and the new BB30 bottom bracket standard. Their special frames have a larger bottom bracket shell allowing the bearings to be inside again while their top level SI crankarms are still two machined aluminum halves glued together.
Another precursor to 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” – the right side was internal while the left side was external (and had the 6805-RS sealed bearing, too).
Truvativ’s approach is an evolution of the ISIS Drive bottom bracket. The spindle is made longer, and the bearings sit 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 chainline can not be maintained using these older cranks with an external bearing BB. They 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 BB 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 BB, which would affect chainline and Q-factor.
In late 2006, Campagnolo introduced an outboard bearing design called Ultra-TorqueTM, which has both crank arms permanently attached to halves of the spindle (called semi-axles), which then join in the middle of the bottom bracket with a Hirth joint and a bolt.
Pressed Bearing Standards
Bicycle frames utilizing pressed bearing standards do not have any threads inside the bottom bracket shell. The bottom bracket is pressed directly into the frame. Using pressed in standards allows frame manufacturers greater flexibility in frame design that many times offers greater stiffness and weight reductions. The current pressed-bearing standards (and developed by) are: BB30 (Cannondale), PF30 (SRAM), BB90 & BB95 (Trek), BB86 & BB92 (Shimano), BB79 (Cervelo’s BBRight), and BB386EVO (FSA and BH bicycles). In BB30, BB90 & BB95 systems: the bearings are pressed directly into the frame. For PF30, BB86 & BB92, BB79, and BB386EVO; the bearing is housed in a nylon plastic cup that is pressed into the frame’s bottom bracket shell. Pressed in standards usually require two-piece cranksets where the spindle is attached to one of the crankarms. Due to fixed spindle length and diameter, cranksets designed for one pressed standard may not be compatible with another. For example, a crankset made specifically for BB30 will not fit in a BB86 bottom bracket and frame. There are other instances where adapters can be used to fit a crankset made for one standard into another. For example, a Shimano (two piece Hollowtech II 24mm OD spindle) road crankset can fit into a BB30 bottom bracket shell (42mm ID) using aftermarket adapters.
Lightning Cycle Dynamics, Inc. offers a carbon crank bottom bracket assembly with semi-axles that connect in the middle via a hirth-like joint to form the spindle.
Schlumpf makes a bottom bracket that incorporates a two-speed epicyclic transmission.