The bottom bracket on a bicycle connects the crankset (chainset) to the bicycle and allows the crankset to rotate freely. It is not a bracket as such. 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.
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. 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, alternately termed one-piece cranks.
Bottom bracket types 
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.
Loose bearing 
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.
Cartridge bearing 
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.
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 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.
External bearings 
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 is stiff but less durable. External bearings allow for a large diameter (hence stiff) and hollow (hence light) bottom bracket 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 their ISIS Drive standard 'Giga Pipe' 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 in fact different 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 it is in fact different to prevent the usage of ISIS Drive cranks on the external bearing BB, as doing so would cause incorrect chainline and Q-factors.
In late 2006, Campagnolo introduced an outboard bearing design called Ultra-TorqueTM, which has each crank arm permanently attached to halves of the axle (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 inside diameter (ID) and overall width of the bottom bracket shell is necessary to determine which of these standards is compatible in the frame. 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. If the user is unsure what BB standard they have on their bicycle or what is compatible, it is advised to seek help at a local bike shop or with a licensed mechanic.
Schlumpf makes a bottom bracket that incorporates a two-speed epicyclic transmission.
Interface types 
As well as the different means to fit the bottom bracket into the frame, there are a number of ways of connecting the crank arms to the bottom bracket spindle. Shimano introduced a proprietary splined interface named "Octalink". Several other manufacturers (King Cycle Group, Truvativ, and Race Face) created a competing open standard called "ISIS Drive" or simply "ISIS", for International Splined Interface Standard. The goal of ISIS was to increase interoperability of bottom brackets and cranksets. Previously, it was more difficult to match two components (from different manufacturers) to fit. Also, ISIS was designed to be stronger than the traditional square taper interface.
One of the earliest standards of crank interface, cottered cranks are now considered obsolete in Western countries, but are still in common use in China, India and other developing nations. The spindle is a cylinder, and a flat region across it (a land). The crank has a hole through it to fit onto the spindle, with a transverse hole for the cotter pin. The cotter pin is cylindrical with one side flattened at an angle to form a wedge to match the spindle land. When tightened, this produces a simple and effective interface. The problem is that normally the interface cannot be tightened enough without a cotter press, a highly specialized tool, though still produced and for sale. Cotters can also be installed with an improvised tool designed for another purpose, such as a ball joint splitter or hammer, with mixed results.
Because all the load is on one very small area of the cotter pin and the crank land the cotter pin deforms plastically under normal use and so must be replaced regularly. If this is not done the crank and the spindle wear and must be replaced. There is warning because of a characteristic creak sound that aging pins cause the cranks to make.
Square taper 
Often referred to as 'cotterless', since this was the design that was introduced after cottered spindles, square taper was once the most popular (and only) style 'cotterless' crank. This interface consists of a spindle with square tapered ends that fit into square tapered holes in each crank. Tightening the two together creates a relatively efficient and simple interface. Although it has been replaced at the high-end of many product lines with the introduction of other styles such as Octalink and Ultra-Torque, it is still manufactured in great numbers for less-expensive bicycles and maintains popularity for such applications as single-speed and fixed-gear bicycles.
Not all square taper crank and bottom bracket combinations are compatible. Although nearly all spindles use a 2 degree taper, there are two competing standards, defined by the measurement across the flat at the end of the spindle. The JIS size is used by Shimano and most other Asian manufacturers. The ISO size is primarily used by Campagnolo and other European manufacturers, in addition to cranks that adhere to the NJS standards (Sugino 75). Some manufacturers make cranks and bottom brackets to both specifications. The overall length of the spindle has no bearing on crank compatibility but does affect frame clearance, chainline, and Q factor.
Lower-quality square tapered bottom brackets are threaded at the ends and use nuts. Higher-quality square tapered bottom brackets are hollow and crank bolts thread into the ends. This allows alloy crank bolts to be used, but only after steel crank bolts are first installed and removed, to securely tighten the crankarm onto the spindle.
Titanium has been used in an effort to make bottom brackets lighter, but early attempts were not entirely successful. Several manufacturers have built bottom brackets with titanium spindles and alloy cups but their durability was lower than that of steel. Early Campagnolo Super Record titanium spindles (which were hollow) were replaced by a later version that used solid, nutted spindles in the lower-quality pattern for improved reliability. However unlike these early titanium spindles, which used a grade of titanium less strong than that available today, current titanium bottom brackets are as reliable as steel designs whilst being lighter across the spindle, but are more expensive and less stiff in general.
Another method of reducing weight is to increase the diameter of a hollow spindle. But because the diameter of the bottom bracket shell of the frame is fixed, a larger diameter spindle would need to have the bearings and crank interface moved outside the bottom bracket shell, then a hollow steel bottom bracket could be built at a lower cost and with a reduced weight.
In recent years Shimano has migrated much of their product line away from square taper to a spline attachment called Octalink and to external bearing types. In late 2006, Campagnolo announced that it was abandoning the square taper interface for double chainsets in favor of an outboard bearing design called Ultra-Torque, which uses a splined interface between spindle halves.
Greasing of tapered cranks 
Most manufacturers (Specialites TA and White Industries being notable exceptions) recommend that square-taper cranks be fitted to the bottom bracket "dry", with no grease or other lubricant. See for example Campagnolo Crankset manual http://www.campagnolo.com/repository/documenti/en/7225254_Crankset-07-05.pdf where on page 28 it says "degrease axle and crankset square heads thoroughly". The validity of this is hotly disputed among cyclists, and a source of frequent "holy wars" on Internet discussion groups. Cranks can occasionally seize onto the spindle sufficiently to prevent their removal by a conventional puller, and grease or anti-seize compound at the interface can help to prevent this. One argument against greasing is that the crank may slide too far onto the spindle, reducing the designers' intended chainline and potentially cracking the crank. Another is that a press fit should not be lubricated.
Conventional mechanical engineering practice across many industries whenever it involves mating of tapered assemblies requires lubrication to ensure that sufficient "preload" is achieved between the mating parts to prevent them working loose under cyclic loading; this includes lubricating the threaded fastener to ensure sufficient preload is developed in the fastener(s).
This system, designed by Shimano, was the first successful alternative to the older square taper. The Octalink system uses a larger and stiffer spindle with eight splines. The splines provide a greater contact area between crank and spindle for a stiffer interface. Octalink exists in the marketplace in two variants, Octalink v1 and Octalink v2. The difference between the two can be seen by the depth of mounting grooves on the bottom bracket spindle. XTR, 105, Ultegra 6500 and Dura Ace 7700 cranksets mate to version one bottom brackets, while later mountain bike designs use the deeper-grooved version two. The system is proprietary and protected by Shimano patents and licence fees, thus few third-party manufacturers (such as Blackspire) produce Octalink cranksets. While some competitors to Shimano (especially Campagnolo) continued to use the older square taper, others worked together to create a similar ISIS design as a free alternative. Although Octalink is a successful and reliable three-piece system, Shimano's newer two-piece systems integrate the spindle onto the right crank arm.
ISIS Drive 
ISIS Drive, the International Splined Interface Standard, is an open standard splined specification for the interface between a bicycle crankset and the bottom bracket spindle. It was created by King Cycle Group, Truvativ, and Race Face in response to the proprietary Shimano Octalink splined bottom bracket standard. Because the Shimano splined interface is covered by patents, the ISIS Group created the standard and put it in the public domain so that other companies could make interoperable components. As the standards are separate, parts made for one are incompatible with those made from the other; an Octalink-standard bottom bracket (8 spline) cannot connect to an ISIS crankset (10 spline) and vice versa. One shortcoming in the design of the ISIS bottom bracket is the decreased bearing life compared to square taper bottom brackets. This is because it uses a larger diameter spindle in the same sized shell, so the bearings are smaller. Arguably, it was this shortcoming that led to the development of external bearing designs.
Other designs 
BMX 3-Piece bottom brackets typically use a spindle either 19(.05)mm (3/4") or 22 mm in diameter. In some cases these are splined and the number of splines depend on the manufacturer/model of the crankset, or in other cases the spindle is specific to the crankset.
There are other designs in use that have varying degrees of popularity. One is Truvativ's Power Spline interface. It is a 12 spline spindle proprietary to Truvativ offered as a lower cost alternative to other spline designs. It is essentially a beefed up square taper spindle with splines instead of tapers.
Phil Wood uses a similar splined design to the Shimano bottom bracket. The difference is a 18-tooth versus a 20-tooth as per the Shimano design.
Bottom brackets have several key size parameters: spindle length, shell width, and shell diameter.
Shell width and spindle length 
There are a few standard shell widths (68, 70, 73, 83 or 100 mm). Road bikes usually use 68 mm; Italian road bikes use 70 mm; Early model mountain bikes use 73 mm. Later models (1995 and newer) use 68 mm more commonly. Some downhill bikes even use a 83 mm bottom bracket shell. Snow bikes use a 100mm shell.
Spindles come in a wider range of lengths (102 – 140 mm), and are sized to match not only the shell width, but also the type of crankset it will support (longer for triple, shorter for single, etc.). Spindle length, along with cranks shape, determines the Q factor or tread.
Shell diameters and threading 
There are a few standard shell diameters (34.798 – 36 mm) with associated thread pitches (24 - 28 tpi).
Most (except for Italian and obsolete French) designs use right-hand (normal) threading for the left side and left-hand (reverse) threading for the right (drive) side. This is opposite of most pedal threading and is done for the same reason: to keep the bottom bracket cup from backing out of the bottom bracket shell due to a process known as precession.
With the development of external bearing designs, the standard shell diameter has become a considerable constraint, limiting both the diameter of the bottom bracket spindle and the size of the actual bearing balls in the races. Consequently, these external bearing designs can be less durable than older internal bearing designs. To address this problem several designers have recently promoted proposed standards for a larger diameter bottom bracket shell, the best known of which is the BB30 standard promoted by Cannondale. The name BB30 refers to the 30mm diameter of the spindle, not of the shell.
|Bottom Bracket Thread Name||Nominal Thread Description||Cup Outside Diameter||Shell width||Shell Inside Diameter|
|ISO/English or BSA||1.37 in x 24 TPI||34.6-34.9 mm
Left-hand thread drive side
|68 mm (73 mm Oversize)(83 mm Some Downhill & FR bikes)(100 mm "Fat bikes")||33.6-33.9 mm|
|Italian||36 mm x 24 TPI||35.6-35.9 mm
Right-hand thread both sides
|70 mm||34.6-34.9 mm|
|French||35 mm x 1 mm||34.6-34.9 mm
Right-hand thread both sides
|Swiss (very rare)||35 mm x 1 mm||34.6-34.9 mm
Left-hand thread drive side
|Chater-Lea (very rare)||1.450 in x 26 TPI||oversized
Left-hand thread drive side
(Raleigh 3-speeds + Raleigh road frames below Super Course)
|1-3/8 in x 26 TPI||34.6-34.9 mm
Left-hand thread drive side
|O.P.C. Ashtabula||Male threads on crank 24 tpi (most)
28 tpi (Schwinn, Mongoose)
|68 mm (2.68 in) wide||51.3 mm (2.02 in) (approximate)|
|O.P.C. Fauber||Male threads on crank||68 mm (2.68 in) wide||45 mm (1.77 in)|
|Raleigh||1 3/8 in x 26 tpi||34.6-34.9 mm
Left-hand thread drive side
|BB30(Cannondale)||Press fit, not threaded||No cups||68mm (road) or 73mm (MTB)||42mm|
|PressFit 30 (SRAM)||Press fit, not threaded||46mm composite cups||68mm (road) or 73mm (MTB)||46mm|
|BBright||Press fit, not threaded||46mm composite cups||79mm||46mm|
|Shimano Press Fit ||Press fit, not threaded||41mm composite cups||86.5mm (road) 89.5mm or 92mm (MTB)||41mm|
|BB386EVO||Press fit, not threaded||46mm composite cups||86.5mm||46mm|
|Spanish (BMX)||Press fit, not threaded||No Cups||68mm||37mm|
Phil Wood & Company makes retaining rings for all historic threaded bottom bracket sizes except for Ashtabula. Therefore, Phil Wood cartridges can be fitted to just about any type of bottom bracket shell. Any bottom bracket cartridge that allows removal of both cups can also be mounted with the Phil Wood rings and tools. In particular, the Shimano UN-71 and UN-72, Campagnolo Chorus/Record 2006 (all obsolete now), and the Tange/IRD square-taper bottom brackets can have both cups removed, although the Tange diameter is 2 mm larger than necessary, and so some filing of the mounting rings would be necessary. This solution is very popular for bikes with Swiss, Raleigh, or Chater-Lea bottom bracket shells.
In addition, most bottom bracket shells of the 33.6-33.9 mm size can be 'tapped out' to the larger Italian 34.6-34.9 size in situations where the threads are irreparably damaged. Before this is done, glues such as JB Weld or Phil Wood Red or Green Retaining Compound should be tried. If all else fails, there are several types of replacement bottom brackets that press-fit or self-tighten into the BB shell, for cases where threads are destroyed. These bottom brackets require further facing or machining of the bottom bracket shell, and it is worth comparing the cost of having a whole new bottom bracket shell brazed in (for a steel frame), especially if the frame is in need of a re-paint anyway.
Another alternative for damaged or uncommon bottom bracket updates is threadless bottom brackets that work by expanding against the existing bottom bracket threads regardless of what they are. This can offer a relatively cheap option. Inexpensive French bottom brackets are now back in production as well.
Bottom bracket height 
The bottom bracket height is the vertical distance from the center of the bottom bracket to the ground, and it expresses clearance of the frame over obstacles. 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 bicycle, the bottom bracket should be high enough to prevent the pedals from coming in contact with the ground while cornering.
A lower bottom bracket creates a lower center of gravity and allows for a larger frame without creating an uncomfortable standover height.
An eccentric is a cylindrical plug or bearing cup that fits into an enlarged bottom bracket shell. The plug is machined to accept a typical bottom bracket, but offset from the center of the plug, so that by rotating the plug, the location of the bottom bracket (and hence the chain tension) may be adjusted (fore and aft to tension the chain, the upper or lower eccentric position for a given chain length can be chosen to fine tune bottom bracket height). Once properly adjusted the plug is then fixed in place by a pair of set screws, a clamping bottom bracket shell, an expanding wedge in the plug, or the plug may be manufactured in left and right halves that clamp against the faces of the bottom bracket shell with screws that connect the two halves.
Eccentric bottom bracket shell inner diameters vary between manufacturers from 42mm-55mm (nominally).
Eccentrics are used in applications that require precise chain tension adjustment such as the timing chain of tandem bicycles, the chain that connects the stoker's and captain's cranks. They may also be employed on bicycles that do not have an adjustable rear wheel position, due to vertical dropouts or a rear disc brake, and that do not have an external rear derailleur such as single-speeds or bikes with an internal-geared hub.
- Brown, Sheldon. "Sheldon Brown's Bicycle Glossary:Axle". Sheldon Brown. Retrieved 2009-01-14.
- Brown, Sheldon. "Sheldon Brown's Bicycle Glossary:Bottom bracket". Sheldon Brown. Retrieved 2009-01-14.
- Brown, Sheldon. "Bicycle Glossary: Retainer". Retrieved 2010-03-26. "The advantages of retainers are ease of assembly, and economy because the manufacturer can get by with fewer balls, if they are spaced apart by a retainer."
- "BB30 Standard". 2007. Retrieved 2008-03-19.
- "Campagnolo Ultra-Torque Crankset". 2006. Retrieved 2006-12-14.
- "The Lightest Integral Crankset and Bottom Bracket Assembly!". Retrieved 2009-01-21.
- Cranks Installation Sheet "Lightning Carbon Cranks/BB Installation Procedure" (PDF). Retrieved 2009-01-21.
- "Schlumpf Innovations: Mountain Drive". 2005. Archived from the original on 2007-10-30. Retrieved 2007-11-18.
- "Chris King ISIS Standard". Retrieved 2007-03-11.[dead link]
- "Cottered Cranks by Sheldon "No Clunk" Brown". Retrieved 2008-02-27.
- Nick Legan (Aug. 31, 2011). "Eurobike Day One: The big brands can wait". VeloNews. Retrieved 2012-07-21. "Aluminum cranks for square taper bottom brackets seem to be holding strong as well. Most of these cranks are made for single-chainring application."
- "Installing Cranks", Jobst Brandt, on Sheldon Brown's website
- sram.com — frame fit specifications
- BBright.net web site
- "Bottom Brackets". Phil Wood & Co. Retrieved 2010-04-05.[dead link]
- "Grand Cru threadless bottom bracket". Velo Orange. Retrieved 2011-08-08.
- "Grand Cru Bottom Bracket, French Thread". Velo Orange. Retrieved 2012-06-19.
- ISO 6695: Cycles – Pedal axle and crank assembly with square end fittings – Assembly dimensions. International Organization for Standardization, Geneva, 1991. (also: British Standard BS 6102-14)
- ISO 6696: Cycles – Screw threads used in bottom bracket assemblies. 1989. (also: British Standard BS 6102-9)
- ISIS drive
|The Wikibook Bicycles has a page on the topic of: Bottom bracket overhaul|