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Bottom bracket
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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 "bracket" refers to the tube fittings that are used to hold frame tubes together in lugged steel frames[1] which also form the shell that contains the spindle and bearings; the term is now 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 unit is stationary, as in a hub, or rotates, as in a bottom bracket.[2] American bicycle mechanic and author Sheldon Brown uses axle once and spindle four times in his bottom bracket glossary entry.[3] 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,[4] a lack of standardization, or rather the constant introduction of new standards[5] that disappear after relatively short periods, has been described as a complex topic to deal with[6] for those who want to buy bicycle components or maintain bicycles. Many bicycle brands have introduced their own dimensions for bottom bracket bearings,[5] and the different use of terminology by the various manufacturers has been described as confusing.[7]
An old American term for the bottom bracket is hanger. This is usually used in connection with Ashtabula cranks, alternatively termed one-piece cranks.
Bottom bracket types
[edit] | This section needs additional citations for verification. (October 2014) |
Three-piece
[edit]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)
[edit]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
[edit]
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.[8]
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.[8]
Cartridge bearing
[edit]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.[1]
In general use, the term 'three piece' refers to the former design,[which?] 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)
[edit]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.[9]
Thompson
[edit]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
[edit]Since around the late 2000s,[citation needed] 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[10] originally introduced by Cannondale.
Several implementations of external bearings have been brought to market.
X-type and Hollowtech II
[edit]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
[edit]
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
[edit]
In late 2006, Campagnolo introduced an outboard bearing design called Ultra-Torque, 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.[11]
Pressed bearing standards
[edit]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 the frame design, and can offer greater stiffness and reduced mass.
A disadvantage is that assembly and disassembly of press-fit bottom bracket bearings require expensive special equipment.[12][13][14] Some hobby-mechanics assemble and disassemble themselves using simple hand-tools, but there is a certain risk that the frame may become damaged. Another disadvantage is that many users report that they start to creak after a certain time. A third disadvantage is that there has become a wealth of competing and incompatible press-fit bottom bracket standards introduced by various bicycle manufacturers.
The current pressed-bearing standards[when?] (and the manufacturers who developed them) are:
- BB30 (Cannondale)
- PF30 (SRAM)
- BB90 and BB95 (Trek)
- BB86 and BB92 (Shimano)
- BB79 (Cervelo's BBRight)
- BB386EVO (FSA and BH Bicycles)
In the BB30 (Cannondale), BB90 and BB95 (Trek) systems, the bearings are pressed directly into the frame. For PF30 (SRAM), BB86 and BB92 (Shimano), BB79 (Cervelo), and BB386EVO (FSA) 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, or at least a spindle which can be threaded through after the bearings have been mounted. 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 third-party adapters can be used to fit a crankset made for one standard into another. For example, a Shimano (two-piece Hollowtech II 24 mm outer diameter spindle) road crankset can fit into a BB30 bottom bracket shell (42 mm inner diameter) using aftermarket adapters.
T47
[edit]T47 is a size for bottom-brackets in 47mm with threads. This will account for enough room for an oversized axle, while also having enough room for big ball bearings, this in comparison to the traditional BSA in 34.6mm. A bottom-bracket made for 46mm pressfit can be threaded up to a fit for a 47mm bottom-bracket and will have approximately a 46mm inside diameter. The T47 standard had a shell width of 68mm. The T47a has a width of 77mm. The T47i has a width of 86.5mm.
Other
[edit]Lightning Cycle Dynamics, Inc. offers a carbon crank bottom bracket assembly[15] with semi-axles that connect in the middle via a hirth-like joint to form the spindle.[16]
Schlumpf makes a bottom bracket that incorporates a two-speed epicyclic transmission.[17]
Interface between spindle and crankset
[edit]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 an "open" standard called "ISIS Drive" or simply "ISIS", for International Splined Interface Standard.[18]
Cottered
[edit]One of the earliest standards of crank interface, 'cottered cranks are now considered obsolete in developed countries, but are still in common use in developing nations.[citation needed] The spindle is a cylinder and has 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 pin press, a highly specialized type of clamping tool,[19] 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.
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 must therefore be replaced regularly. If this is not done the crank and the spindle wear and must be replaced. The rider will get a warning through a characteristic creak sound that aging pins cause the cranks to make.
Square taper
[edit]
Previously 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. It is still manufactured in great numbers for bicycles and maintains popularity for such applications as bicycles.[20]
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 Nihon Jitensha Shinkokai (NJS) keirin standards (Sugino 75). Some manufacturers make cranks and bottom brackets to both specifications.
Some square tapered bottom bracket axles are threaded at the ends and use nuts. Other square tapered bottom brackets are hollow, allowing for crank bolts to thread into the ends.
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 for improved reliability.
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.
Splined
[edit]Several different competing standards for splined interfaces exist. The tool drive for installing and removing these bottom brackets is often also a spline, instead of drives for regular hand tools.
Octalink
[edit]
The Octalink system uses a spindle with eight splines. The splines provide a contact area between crank and spindle for an interface. Octalink exists in the marketplace in two variants Octalink v1 and Octalink v2, and the two are not compatible with each other. The difference between the two can be seen by the depth of mounting grooves on the bottom bracket spindle. v1 spline grooves are 5 mm long, while v2 grooves are 9 mm long. Shimano 105, Ultegra 6500 and Dura Ace 7700 cranksets mate to v1 spindles, while later mountain bike designs use the deeper-grooved v2. The system is proprietary and protected by Shimano patents and license fees.
ISIS Drive
[edit]ISIS Drive, the International Splined Interface Standard, is a non-proprietary 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. ISIS Drive is open source and free to the public to be used as seen fit.
Other designs
[edit]BMX 3-Piece bottom brackets typically use a spindle either 19 mm or 19.05 mm (3/4″), 22 mm or 22.2 mm (7/8″), or 24 mm in diameter. The majority of newer or Asian parts are made to metric round numbers, and mixes of metric and non-metric (e.g. 19 mm and 19.05 mm) spindles and bearings may not be compatible, and can result in stuck parts.[citation needed] In some cases, the spindles are splined and the number of splines depends 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 PowerSpline 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 an 18-tooth versus a 20-tooth as per the Shimano design.
Bottom bracket shell sizes
[edit]Bottom brackets have several key size parameters: spindle length, shell width, and shell diameter.
Shell width and spindle length
[edit]There are a few standard shell widths (68, 70, 73, 83, or 100 mm [2.7, 2.8, 2.9, 3.3, or 3.9 in]). Road bikes usually use 68 mm (2.7 in); Italian road bikes use 70 mm (2.8 in); Early model mountain bikes use 73 mm (2.9 in). Later models (1995 and newer) use 68 mm (2.7 in) more commonly. Some downhill bikes even use an 83 mm (3.3 in) bottom bracket shell. Snow bikes use a 100 mm (3.9 in) shell.
Spindles come in a wider range of lengths (102–140 mm [4.0–5.5 in]), 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 the crank's shape, determines the Q factor or tread.
Shell diameters and threading
[edit]There are a few standard shell diameters (34.798–36 mm [1.3700–1.4173 in]) with associated thread pitches around 1 mm (24–28 TPI).
Most (except for Italian and 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 precession. These have become rare to encounter on newer bikes.[as of?] As of 2015, bikes with French bottom brackets were very rare to encounter, but there was still some aftermarket support for components.[21] According to an article by Parktool from 2019, the French threading standard is "considered obsolete".[22] As of 2017, bikes with Italian bottom brackets were also rare to encounter, but aftermarket Italian bottom brackets were nonetheless still well-supported by many component manufacturers, including top-end kits from Shimano and Campagnolo.[23]
With the development of external bearing designs, the standard shell diameter has become a considerable constraint,[according to whom?] 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 promoted new standards for larger diameter bottom bracket shells, the best known of which is BB30 promoted by Cannondale. The name BB30 refers to the 30 mm (1.2 in) diameter of the spindle, not of the shell.[24] Most of these larger diameter designs are using bearings that are pressed in. Since about 2015 there is now a T47 standard with a larger shell diameter and also threading. Some manufacturers (like Trek) are starting to implement this into their bicycles, while it is also possible to have threading cut into an existing 46 mm pressfit and have T47 cups fit in there.
| Bottom bracket thread name | Nominal thread description | Thread orientation | Cup outside diameter | Shell width | Shell inside diameter |
|---|---|---|---|---|---|
| ISO/English or BSC or BSA,[25] Euro (BMX) | M34.798×1.058 mm (1.37″-24 TPI) |
Left-hand thread on drive side | 34.6–34.9 mm (1.36–1.37 in) |
33.6–33.9 mm (1.32–1.33 in) | |
| Italian (uncommon, but available per 2015[23]) | M36×1.058 mm 1.42″-24 TPI |
Right-hand thread on both sides | 35.6–35.9 mm (1.40–1.41 in) |
70 mm (2.8 in) | 34.6–34.9 mm (1.36–1.37 in) |
| French (very uncommon, but available per 2015[21]) | M35×1 mm (1.38″-25.4 TPI) |
Right-hand thread on both sides | 34.6–34.9 mm (1.36–1.37 in) |
68 mm (2.7 in) | 33.6–33.9 mm (1.32–1.33 in) |
| Swiss (uncommon) | M35×1 mm (1.38″-25.4 TPI) |
Left-hand thread on drive side | 34.6–34.9 mm (1.36–1.37 in) |
68 mm (2.7 in) | 33.6–33.9 mm (1.32–1.33 in) |
| ISIS Overdrive[22] | M48×1.5 mm (1.89″-16.93 TPI) |
Left-hand thread on drive side | 68 mm (2.7 in) | 46.5 mm (1.83 in) | |
| Chater-Lea (uncommon/older vintage) | M36.83×0.977 mm (1.45-26 TPI) |
Left-hand thread on drive side | Oversized | 64 mm (2.5 in) | ~35.8 mm (1.41 in) |
| Whitworth (Raleigh 3-speeds + Raleigh road frames except Super Course) | M34.925×0.977 mm (1-3⁄8″-26 TPI) |
Left-hand thread on drive side | 34.6–34.9 mm (1.36–1.37 in) |
|
33.6–33.9 mm (1.32–1.33 in) |
| Ashtabula (one-piece crank, OPC) or American (BMX)[22] | 51 mm press fit, not threaded | Male threads on crank
|
No cups | 68 mm (2.7 in) | 51.3 mm (2.02 in)[citation needed] or 51.5 mm (2.03 in)[22] |
| Fauber (one-piece crank, OPC) | 45 mm press fit, not threaded[26] | Male threads on crank | 68 mm (2.7 in) | 45 mm (1.8 in) | |
| Raleigh | M34.925×0.977 mm (1-3⁄8″-26 TPI) |
Left-hand thread on drive side | 34.6–34.9 mm (1.36–1.37 in) |
33.6–33.9 mm (1.32–1.33 in) | |
| BB30 (Cannondale)[27] | Press fit, not threaded | Threads on crank spindle | No cups |
|
42 mm (1.7 in) |
| PressFit 30 (SRAM)[28] | Press fit, not threaded | Threads on crank spindle | 46 mm (1.8 in) composite cups |
|
46 mm (1.8 in) |
| BBright[29] (BB79) | Press fit, not threaded | Threads on crank spindle | 46 mm (1.8 in) composite cups |
79 mm (3.1 in) | 46 mm (1.8 in) |
| Shimano Press Fit (BB86)[30] | Press fit, not threaded | Threads on crank spindle | 41 mm (1.6 in) composite cups |
|
41 mm (1.6 in) |
| BB386EVO | Press fit, not threaded | Threads on crank spindle | 46 mm (1.8 in) composite cups |
86.5 mm (3.41 in) | 46 mm (1.8 in) |
| Spanish (BMX) | 37 mm press fit, not threaded (19 mm or 22 mm spindle) | Threads on crank spindle | No cups | 68 mm (2.7 in) | 37 mm (1.5 in)[22] |
| Mid (BMX) | 41 mm press fit, not threaded | Threads on crank spindle | No cups | 68 mm (2.7 in) | 41.2 mm (1.62 in)[22] |
| T47[31] | M47×1 mm (1.85″-25.4 TPI) |
Left-hand thread on drive side | 68 mm (2.7 in) | 46 mm (1.8 in) |
Bottom bracket height
[edit]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 but is not always achieved.
A lower bottom bracket creates a lower center of gravity and allows for a larger frame without creating an uncomfortable standover height.
Eccentric bottom brackets
[edit]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.
Compatibility issues
[edit]The bottom bracket shell and bottom bracket assembly as a whole have changed drastically as bike frames and part manufacturing continues to evolve. While the progression in technology has led to many new standards which is great for the consumer,[dubious – discuss] it has proven to play a challenging role in frame design and aftermarket parts, as well as servicing and changing of parts. Around 2001, Shimano was manufacturing proprietary bottom bracket components with patents. Bike frame manufacturers would then create bikes with the patented bottom brackets as a guiding light around the shell area. This caused aftermarket brands to struggle in creating a bottom bracket that would fit into bikes that came with OEM Shimano parts. Companies such as RaceFace, Chris King, and Truvativ (SRAM Corporation) sat down in 1998 and collaborated on a standard bottom bracket specification. In August 2001, the ISIS Drive Standard was published and made into open source for anyone to create products using the designated specifications. Frame manufacturers took to the idea and created bikes that accepted the standard, allowing for a broader market. This success would prove to be short-lived due to failures and need for the industry to continue progressing and creating better specifications. Now in 2019, Shimano creates non-proprietary as well as proprietary bottom bracket designs such as HollowTech II and BBR60. Many "non OEM" part manufacturers have created bottom bracket conversion kits, giving consumers the ability to install crank sets designed for one standard into another (Example BB/PF30 cranks into a 22/24mm spindle).[32][33]
References
[edit]- ^ "British Council Film Collection - How a Bicycle is made". British Council Film Collection. 1945. At 03:25. Retrieved 2016-03-16.
- ^ 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.
- ^ "Understanding bottom bracket issues. And why the standards are here to stay". Kogel Bearings. Retrieved 2023-06-03.
- ^ a b "The complete guide to bottom bracket standards". BikeRadar. 2019-03-14. Retrieved 2023-06-03.
- ^ "Bottom Bracket Standards and Terminology". Park Tool. Retrieved 2023-06-03.
- ^ Wilson, Mark (2022-11-23). "Bottom Bracket Basics: Cyclists' Guide to Standards". GearJunkie. Retrieved 2023-06-03.
- ^ a b "Royal Enfield Revelation 1965". Archived from the original on 2013-10-15. Retrieved 2013-08-26. Royal Enfield Revelation (retrieved 26 August 2013).
- ^ 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. Archived from the original on 2007-01-04. Retrieved 2006-12-14.
- ^ "Bottom brackets - get the insider info on your bike's beefiest bearing". road.cc. 2020-07-05. Retrieved 2023-06-04.
- ^ Palmer, Ryan (2014-07-27). "Bottom Bracket Racket: Chris King's answer to press-fit woes". BikeMag. Retrieved 2023-06-04.
- ^ "AngryAsian: I've had it with press-fit bottom brackets". BikeRadar. 2013-08-27. Retrieved 2023-06-04.
- ^ "The Lightest Integral Crankset and Bottom Bracket Assembly!". Archived from the original on 2003-12-07. Retrieved 2009-01-21.
- ^ "Lightning Carbon Cranks/BB Installation Procedure" (PDF). Archived from the original (PDF) on 2009-01-24. Retrieved 2009-01-21.
- ^ "Schlumpf Innovations: Mountain Drive". 2005. Archived from the original on 2007-10-30. Retrieved 2007-11-18.
- ^ "International Spline Interface Standard web site". Archived from the original on 2015-03-27. Retrieved 2015-06-04.
- ^ "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 the single-chainring application.
- ^ a b "Sheldon Brown's Threaded Bicycle Bicycle Bottom Bracket Crib Sheet". www.sheldonbrown.com. Retrieved 2023-05-31.
- ^ a b c d e f Bottom Bracket Standards and Terminology | Park Tool
- ^ a b "What is the difference between an Italian and BSA bottom bracket?". BikeRadar. 2017-07-20. Retrieved 2023-05-31.
- ^ "Home". bb30standard.com.
- ^ "AASQ #1: What is BSA? How do I get a BSA crank in BB30 bottom bracket?". BIKERUMOR. 2017. Retrieved 2019-08-30.
- ^ Fixed Gear Blog: Fauber Bottom Bracket Adapter
- ^ "BB30 Standard" (PDF). 2006. Retrieved 2015-06-04.
- ^ sram.com — frame fit specifications
- ^ BBright.net web site
- ^ "Press-Fit bottom bracket adapter" (PDF). 2007. Retrieved 2015-06-04.
- ^ "Bottom Bracket Standards and Terminology". PARKTOOL. 2019. Retrieved 2019-08-30.
- ^ Smith (March 23, 2001). "International Spline Interface Standard". www.isisdrive.com. Archived from the original on 2018-07-14. Retrieved 2019-02-14.
- ^ "The International Spline Interface Standard" (PDF). ISIS Drive. March 23, 2001. Archived from the original (PDF) on August 20, 2019. Retrieved February 13, 2019.
Standards
[edit]- 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
External links
[edit]
- Bottom Bracket Size Database by Sheldon Brown
- How to fit a bottom bracket by Ceramic Speed
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| Peripherals | ||
Bottom bracket
View on GrokipediaIntroduction
Definition and Function
The bottom bracket (BB) is the bearing assembly housed within the bicycle frame's bottom bracket shell that connects the crankset to the frame, enabling the smooth and independent rotation of the pedals and cranks relative to the rest of the bicycle.[1][2] Its primary function is to accommodate bearings and a spindle, or axle, that links the left and right crank arms, thereby transferring the rider's pedaling force from the cranks through the chain to the rear wheel while minimizing rotational friction.[8] In addition, it supports the vertical weight of the crankset and absorbs lateral forces generated during pedaling or cornering.[1] In bicycle design, the bottom bracket is essential because its dimensions and configuration directly impact pedaling efficiency by reducing energy loss to friction, determine the Q-factor—the lateral distance between the pedal mounting points on the cranks—and contribute to the bike's overall geometry, including chainline and stance width.[9][10] At its core, the bottom bracket achieves rotational freedom via low-friction bearings that allow the spindle to spin with minimal resistance, and modern sealed cartridge units are engineered to protect against contamination from dirt, water, and debris, which helps maintain performance and prolong durability.[5]Basic Components
The spindle, often referred to as the axle, serves as the central rotating shaft that connects the left and right crank arms, typically constructed from high-strength steel or chromium-molybdenum alloy for rigidity and resistance to torsional stress, with standard lengths ranging from 102 mm to 127 mm to accommodate various frame geometries and chainline requirements.[11][12] Bearings, which may include ball or cartridge configurations, are positioned around the spindle to support axial and radial loads while enabling low-friction rotation essential for efficient pedaling.[1] The cups or shells encase these bearings and interface with the bicycle frame's bottom bracket shell, featuring either fixed or adjustable designs that allow for precise installation and alignment.[1] Supporting elements enhance the assembly's performance and longevity; seals, typically made of rubber or synthetic materials, prevent contaminant ingress and retain grease lubricant within the bearing area.[1] A lockring or preload adjuster, usually threaded steel, secures the non-drive-side cup and applies tension to eliminate bearing play.[13] Flanges or interfaces at the spindle ends provide attachment points for the crank arms, ensuring secure power transmission.[14] Material choices prioritize durability and weight savings: spindles and cups often use hardened steel or aluminum alloys, with corrosion-resistant coatings such as nickel plating applied to withstand environmental exposure and extend service life.[12][15] In assembly, the cups are installed into the frame's bottom bracket shell—threaded counterclockwise on the drive side and clockwise on the non-drive side—followed by insertion of the spindle through the bearings, attachment of cranks, and final tightening of the lockring; manufacturer-recommended torque for cups typically ranges from 35 to 50 Nm to ensure stability without damaging threads.[13][16]Historical Evolution
Early Designs
The earliest bottom bracket designs emerged in the mid-19th century alongside the invention of the pedal-driven velocipede, often credited to French mechanics Pierre Michaux and Pierre Lallement. In 1861, Pierre Michaux and his son Ernest added pedals and a simple bottom bracket to the front wheel of a draisine, creating the boneshaker velocipede with a simple crank assembly housed in basic cups for rotation.[17] Lallement, working in Michaux's workshop before patenting his own version in 1866, similarly employed rudimentary lubricated bronze bearings in the front hub assembly to connect the cranks, marking the initial integration of powered propulsion in two-wheeled vehicles.[18] These designs prioritized simplicity and durability on iron-wheeled frames but suffered from high friction and maintenance challenges due to unsealed components exposed to dirt.[19] By the 1880s, the bottom bracket evolved significantly with the advent of the safety bicycle, which relocated the mechanism from the front wheel to the frame's lower bracket shell for improved stability and chain-driven rear-wheel propulsion. Early safety bicycles retained loose ball bearings within adjustable cups and cones threaded into the shell, allowing for preload adjustments to minimize play.[20] This configuration became foundational, tying bottom bracket development to broader frame standardization. Cottered cranks dominated these pre-1950s systems, featuring a steel spindle with tapered slots where wedge-shaped cotter pins secured the crank arms, often using 9 mm to 9.5 mm diameter pins depending on regional standards like French or British.[21] The loose ball bearings, typically 1/8-inch steel balls, were packed with grease and retained by cones, enabling easy roadside servicing but requiring frequent adjustments to prevent creaking or binding.[20] In the 1930s, the one-piece Ashtabula crankset introduced a more integrated approach for affordable American bicycles, combining the spindle and both crank arms into a single forged steel unit pressed directly into an unthreaded 51.3 mm shell without cups or cones.[22] This design, named after the Ashtabula Forge company in Ohio, gained popularity for BMX and low-cost utility bikes through the 1970s due to its robustness against impacts and simplicity in manufacturing, though it added weight and limited chainring options.[23] Loose bearings remained standard, pressed into the shell ends with basic retainers. The Thompson bottom bracket, developed in the 1960s, offered a pressed-in alternative for three-piece cotterless cranks, using adjustable cones on the spindle for preload and serviceability in touring applications.[24] Unlike the Ashtabula's fixed integration, it allowed crank removal via square taper interfaces while maintaining a non-threaded shell, appealing to cyclists valuing on-the-road adjustments without specialized tools.[25]Key Milestones and Modern Developments
The late 1970s marked a significant advancement in bottom bracket design with Shimano's introduction of the square taper spindle in 1978, which featured squared ends on the spindle for improved crank alignment and secure fixation, addressing inconsistencies in earlier cotterless systems. This design enhanced power transfer and durability, quickly becoming the industry standard by the early 1980s due to its simplicity and compatibility across various bicycles.[23] This was followed by Shimano's Octalink system in 1996, which introduced a splined interface for cotterless cranks, improving torque transmission before the shift to external bearings.[26] Building on this, the 1980s saw the rise of cartridge bearings, sealed units that replaced loose ball setups to reduce maintenance and contamination, influenced by Phil Wood & Co.'s pioneering work on sealed bearings in the late 1970s. These pre-assembled cartridges simplified installation and extended service life, dominating the market for both road and mountain bikes. The shift to external bearing configurations accelerated in the 1990s and 2000s, exemplified by Shimano's Hollowtech II system introduced in 2003, which relocated bearings outside the frame shell to accommodate larger 24 mm spindles for greater stiffness and reduced flex. Similarly, SRAM's GXP (Guttered X-Axis Profile) in the early 2000s adopted an external bearing approach with integrated spindle steps, further prioritizing rigidity and efficiency in high-performance applications.[27][26][28] The 2010s introduced press-fit standards like Cannondale's BB30 in 2000, which eliminated threads for lighter weight and easier manufacturing by pressing cups directly into the frame shell, enabling wider 42 mm bearing diameters. However, persistent creaking and installation challenges led to a backlash, favoring threaded alternatives such as T47, proposed in 2015 by Chris King and Argonaut Cycles, which combined a 47 mm threaded shell with support for 30 mm spindles to restore reliability without press-fit drawbacks. In 2023, Shimano addressed Hollowtech II issues through a voluntary recall and inspection program for pre-2019 bonded cranksets, implementing enhanced bonding processes and preload mechanisms in subsequent models to prevent separation and improve adjustment. By 2024-2025, professional cycling teams increasingly adopted threaded bottom brackets like BSA and T47 for their superior torque retention amid ongoing press-fit creaking complaints, prioritizing race reliability.[29][30][31] Market influences have further shaped developments, with e-bike proliferation demanding wider bottom bracket shells—often 73 mm or 83 mm—to accommodate integrated motors, batteries, and boosted geometry for stability under higher torques. Sustainability efforts have also gained traction, with manufacturers incorporating recyclable aluminum alloys and modular designs to reduce waste, aligning with industry pushes for longer-lasting components.[7][32]Bottom Bracket Types
Internal Bearing Types
Internal bearing types house the bearings within the bicycle frame's bottom bracket shell, providing a compact and protected setup for the spindle and cranks. The traditional three-piece design consists of a separate spindle, threaded cups, and loose ball bearings, known as the cup-and-cone system. In this configuration, the fixed cup threads into the drive-side of the frame shell, while the adjustable cone on the non-drive side allows for precise preload adjustment using a locknut to minimize play and ensure smooth rotation. This setup was the pre-1990s standard for most threaded bottom brackets, offering serviceability by allowing bearings to be cleaned, repacked, and replaced individually.[33][7] Cartridge bearing internals represent an evolution for easier maintenance, featuring pre-assembled sealed bearing units that drop into the frame shell and are retained by threaded cups. These units encapsulate the bearings in a sealed cartridge, protecting them from contaminants and eliminating the need for individual ball handling. The Shimano UN series exemplifies this type, introduced in the early 1990s as a reliable, low-maintenance replacement for loose-bearing systems, compatible with square-taper spindles and threaded BSA shells.[33][7][34] An earlier example of a unitized internal design is the Bayliss Wiley unit, a British innovation from the early 1930s that integrated the cone and bearings into a non-threaded enclosed assembly fitting directly into the frame shell. This rare design anticipated modern sealed concepts by combining components for simplified installation, though it saw limited adoption due to production challenges and axle durability issues in later variants.[35][36] These internal types offer advantages in compactness, fitting within standard 68mm or 73mm frame shells, and protection from environmental elements due to their in-frame positioning, which reduces exposure to mud and water. However, the constrained space limits bearing diameter and quantity, potentially leading to higher friction under high-torque pedaling compared to external bearing systems that allow larger outboard bearings for improved stiffness.[7][37]External Bearing Types
External bearing bottom brackets position the bearings outside the frame shell, enabling the use of larger angular contact bearings that enhance durability, reduce friction, and improve power transfer by increasing lateral stiffness compared to compact internal designs.[38] This outboard placement allows for wider bearing spacing, which minimizes spindle flex during pedaling and supports stiffer cranksets for high-performance applications.[7] While internal types prioritize a sealed, cartridge-style housing within the shell for simplicity, external systems offer superior bearing size and serviceability, though they require precise installation to avoid misalignment.[39] Shimano's Hollowtech II system, introduced in the early 2000s, employs threaded aluminum cups that house external sealed angular contact bearings, paired with a hollow 24mm steel spindle to reduce crankset weight while maintaining rigidity.[7] This design integrates the spindle directly into the drive-side crank arm, optimizing power transmission and allowing compatibility across Shimano's road and mountain bike groupsets.[38] Shimano's Dura-Ace (DA) and Ultegra (UT) series both utilize the Hollowtech II external bearing bottom brackets, such as the SM-BBR60 threaded model or SM-BB92-41B press-fit model depending on the generation. These bottom brackets share identical structure, bearing types, installation methods, and compatibility with 68mm and 73mm shell widths. The bottom brackets themselves show no significant differences and are fully interchangeable. Differences appear primarily at the groupset level: Dura-Ace is lighter, incorporates higher-grade materials (including more carbon fiber and titanium), commands a higher price, and is positioned for top-tier racing; Ultegra offers better value with slightly higher weight but near-equivalent performance.[38][7] Similarly, Shimano's X-Type variant extends this concept with reinforced external bearing cups for mountain bike durability. Shimano's BB-RS501 bottom bracket, compatible with Hollowtech II, features an enhanced seal construction for improved durability and service life in wet conditions, as introduced with the Tiagra series around 2020.[40] SRAM's Giga-X-Pipe (GXP), developed in the mid-2000s following SRAM's acquisition of Truvativ, adopts a comparable threaded external bearing setup but features an integrated spindle-crank interface with a tapered design—24mm on the drive side tapering to 22mm on the non-drive side—for balanced stiffness and weight savings.[41] This taper enhances crank arm attachment security without adding material, making it suitable for both road and MTB applications. Campagnolo's Ultra-Torque, launched in 2009 with the Record groupset, innovates further by unifying the spindle and cranks into a single asymmetric assembly, using external ceramic or steel bearings in dedicated cups and a 25mm spindle diameter to maximize torque transfer and reduce overall system weight.[42] Pressed bearing standards represent an evolution of external concepts by eliminating threads for direct frame integration. Cannondale's BB30, introduced in 2000 as a proprietary system and opened to other manufacturers in 2006, uses press-fit aluminum cups with 30mm aluminum spindles in a 42mm-diameter shell, achieving significant weight reductions—up to 100 grams lighter than threaded equivalents—through wider bearing positioning and simplified construction.[43] BB86 (86.5mm width for road) and BB92 (91.5mm for MTB), standardized in the late 2000s, extend this press-fit approach to narrower shells with 41mm inner diameters, supporting 24mm or 30mm spindles and further cutting weight via plastic or composite cups while maintaining outboard bearing benefits.[7] However, these press-fit designs have faced critiques in the 2020s for creaking due to tolerances between cups and carbon shells, often requiring adhesives or precise machining for reliable performance.[7]Specialized and Integrated Types
The one-piece Ashtabula bottom bracket, also known as the American standard, features a pressed-in cup system with a 51.3 mm unthreaded shell diameter and integrates the crank arms and spindle into a single forged unit.[22] This design originated in the 1930s with manufacturers like Ashtabula Forge producing components for American bicycles and became widespread on low-cost models, including Schwinn bikes and BMX frames, due to its simplicity and affordability.[1] While it uses loose ball bearings that require periodic servicing, the one-piece construction eliminates separate bearing cups, making it inexpensive but prone to flex under high loads compared to modern external bearing types.[22] The T47 standard, introduced in 2015 by Chris King Components in collaboration with frame builders like Argonaut Cycles, employs a threaded 47 mm diameter shell with M47x1.0 mm pitch to accommodate larger spindles such as 30 mm or 29 mm DUB.[44] It bridges the gap between press-fit and traditional threaded systems by using a 46 mm inner diameter shell that is machined with threads, offering improved stiffness and reduced creaking while supporting shell widths from 68 mm to 132 mm.[45] By the 2020s, T47 gained traction among custom steel and titanium frame manufacturers for its compatibility with oversized spindles and ease of installation in bespoke designs.[45] The Thompson bottom bracket, a pressed-in unthreaded design from the 1960s, uses an oversized shell with separate adjustable cone bearings to provide preload control and was commonly fitted to fat-tire bicycles for enhanced durability on rough terrain.[46] It supports spindle diameters ranging from 30 mm to 45 mm and features adjustable cones on both sides for precise bearing tension, distinguishing it from fixed one-piece units.[46] This configuration allowed for better accommodation of wider tires on European-style balloon tire bikes, though it requires regular adjustments to maintain smooth rotation.[46] Shimano's BB-UN series represents a cartridge-based internal bottom bracket with square taper spindles, available in lengths from 107 mm to 127 mm to fit 68 mm or 73 mm threaded shells, providing stable chainline performance through integrated stabilizers.[47] Emerging post-2020 e-bike designs incorporate wide-shell bottom brackets, often 100 mm to 150 mm in width, to integrate mid-drive motors like those from Bafang or Bosch directly into the frame for improved power transfer and compatibility with fat tire setups.[48] These unified systems prioritize motor alignment and rigidity, addressing the demands of electric propulsion in off-road applications.[7]Crank-Spindle Interfaces
Traditional Interfaces
Traditional bottom bracket interfaces primarily include cottered and square taper designs, which were the dominant methods for attaching cranks to the spindle before the widespread adoption of splined systems in the late 20th century. These interfaces emphasize mechanical simplicity and ease of adjustment, particularly in early bicycle designs. The cottered interface, prevalent in bicycles before the 1970s, secures the crank to the spindle using a wedge-shaped pin, known as a cotter, that passes through a hole in the spindle and expands to grip the crank arm.[21] This design allows for adjustability by filing the cotter flats to achieve proper alignment and tightness, making it suitable for low-cost production and field repairs.[21] However, cottered cranks are prone to loosening under repeated pedaling stress, which can lead to axle damage if not periodically re-tightened after initial rides of a few dozen miles.[21] By the 1970s, this interface had largely been phased out on higher-end bicycles in favor of more reliable alternatives, though it persisted on budget models.[49] In contrast, the square taper interface, first developed in the 1930s and which became the dominant standard in the 1970s under JIS (Japanese Industrial Standards) and ISO protocols, features a tapered square end on the spindle that mates with a matching hole in the crank arm, providing a self-centering friction fit without additional fasteners like cotters.[50][51] The taper angle is 2 degrees per side (4 degrees included), ensuring the crank pulls tightly onto the spindle when the fixing bolt is torqued, with JIS spindles having a slightly larger end dimension (12.7 mm) compared to ISO (12.3 mm), which affects compatibility if mismatched.[50] Spindle lengths typically range from 110 mm to 130 mm to optimize chainline for different frame geometries and drivetrain configurations, such as road or mountain bike setups.[50] Installation for both interfaces involves cleaning the mating surfaces and applying lubricant where appropriate, followed by torquing to specifications. For cottered cranks, cotters are inserted in opposite directions for 180-degree alignment, filed if necessary for fit, and secured with nuts, often requiring re-tightening after bedding in.[21] Square taper installation commonly uses a light grease or anti-seize on the spindle taper to prevent corrosion and galling—though some manufacturers like Campagnolo recommend dry assembly to avoid slippage—before sliding the crank onto the spindle and tightening the M8 bolt to 35-50 Nm, ensuring even preload without over-torquing that could deform the crank bore.[52][53] These methods are typically associated with internal-bearing bottom brackets, where the spindle is integral to the assembly.Splined and Advanced Interfaces
Splined interfaces represent an evolution in crank-spindle connections, replacing friction-based tapers with keyed engagements to enhance torque transmission in high-performance bicycles. These designs use multiple grooves or teeth on the spindle that mate with corresponding features on the crank arms, providing positive mechanical interlocking for more efficient power transfer, particularly under high loads in mountain biking and road racing. Unlike earlier square taper systems, which rely on wedging action, splines minimize rotational slippage and allow for larger, stiffer spindles while maintaining compatibility with various frame types.[23] Shimano introduced Octalink in 1996 as a proprietary 8-splined interface, initially for road applications and later adapted for mountain bikes, aiming to address the limitations of square tapers by increasing spindle diameter and contact area for improved stiffness.[54] The system features a hollow spindle with splines that engage the crank arms via bolts, supporting spindle lengths optimized for specific chainlines in off-road setups. Octalink gained popularity in the late 1990s for its robust construction but faced criticism for creaking issues due to non-interference fits in some implementations.[55] In response to Octalink's proprietary nature, the International Splined Interface Standard (ISIS Drive) emerged in the late 1990s as an open, non-patented alternative supported by multiple manufacturers including Truvativ and King Cycle Group. Featuring a 10-splined design, ISIS offered similar benefits of enhanced rigidity and power transfer while promoting wider aftermarket compatibility and avoiding Shimano's licensing restrictions.[56] The standard specified spindle dimensions and spline profiles to ensure interchangeability, leading to broader adoption in budget and custom builds during the early 2000s.[57] Other advanced interfaces include Shimano's Hollowtech II, introduced in 1995, which uses a 24 mm diameter hollow spindle integrated into the driveside crank arm, with the nondriveside crank secured by a two-bolt clamping mechanism for high stiffness and lightweight construction across road and mountain bike applications.[38] Similarly, SRAM's GXP (Guttered X-Axis Profile), launched in 2009, employs a stepped spindle (24 mm driveside, 22 mm nondriveside) with a similar clamping interface, optimizing bearing sizes and chainline in external bearing systems.[58] SRAM's DUB (Durable Unified Bottom Bracket) system, launched in 2018, incorporates a 28.99mm diameter splined spindle to balance bearing size and contamination resistance in modern cranksets. This design allows for larger bearings than traditional 30mm spindles while maintaining a compact profile, making it suitable for both mountain and road applications across threaded and press-fit frames.[59] DUB's splines provide secure crank attachment, contributing to its integration in SRAM's ecosystem of drivetrains. Campagnolo's Ultra-Torque, introduced in 2009, employs an integrated approach where each crank arm is fixed to a half-spindle, connected at the center via a Hirth joint—a high-precision spline-like coupling that transmits torque without traditional full-spindle rotation. This configuration uses external bearings pressed onto the spindle halves, reducing weight and increasing stiffness for road racing. The Hirth joint requires significant torque (around 50Nm) for assembly, ensuring a rigid connection but demanding exact machining tolerances.[60][61] In the 2020s, trends in splined interfaces have shifted toward hybrid designs combining spline engagement with tapered elements to optimize chainline versatility and frame compatibility, as seen in evolving standards like T47 threaded systems that accommodate larger spindles. These advancements aim to reduce creaking and improve durability in diverse riding conditions.[7] The primary advantage of splined interfaces lies in their positive engagement, which reduces slippage under torque compared to tapered predecessors, enabling more direct power delivery and structural integrity. However, they necessitate precise machining and alignment during manufacturing and installation to avoid play or noise, potentially increasing production costs and service complexity.[62][63]Frame Shell Specifications
Dimensions and Sizing
The dimensions and sizing of the bottom bracket shell and spindle are essential for ensuring compatibility with the bicycle frame, proper drivetrain alignment, and optimal pedaling efficiency. The shell width, measured across the frame's bottom bracket area, varies by bicycle type to accommodate different geometries and components. Standard widths include 68 mm for most road bicycles and 73 mm for mountain bicycles, while e-bikes and fat-tire models often require wider shells of 100 mm or more to integrate motors or handle increased torque.[64][65] These shell widths directly dictate the necessary spindle length, which typically ranges from 102 mm to 152 mm to span the shell while providing adequate clearance for chainrings and frame tubes.[66] Spindle length variations are primarily determined by the shell width and the crankset's offset, ensuring the chainline—the lateral position of the chainring relative to the frame centerline—remains centered for efficient power transfer and reduced wear. The chainline is determined by the spindle length and the crankset's offset design to ensure proper centering.[67] For instance, road setups often use shorter spindles around 110–113 mm for a chainline of 43–45 mm, while mountain bike configurations may employ longer ones up to 130 mm or more to optimize clearance for wider tires and off-road conditions.[11] The inner diameter of the bottom bracket shell also follows established standards that influence bearing fit and overall stiffness. Threaded systems like BSA typically specify a diameter of 34.8 mm, whereas press-fit designs such as BB30 use 42 mm to allow direct bearing installation into the frame.[68] These diameter choices impact the Q-factor, defined as the lateral distance between the pedal mounting points, which generally ranges from 140 mm to 170 mm across bicycle types to balance stability and biomechanics.[69] Narrower Q-factors suit road bikes for aerodynamic positioning, while wider ones accommodate the robustness needed in mountain biking. To determine precise sizing, cyclists and mechanics use vernier calipers to measure the shell width and inner diameter accurately, often to within 0.1 mm for press-fit compatibility.[70] Compatibility charts from manufacturers then guide spindle selection based on the measured dimensions and intended crankset, preventing issues like chain rub or misalignment.[11]Threading and Installation Methods
Bottom brackets are secured to the bicycle frame using either threaded or press-fit methods, each offering distinct approaches to attachment and maintenance. Threaded systems involve cups or adapters that screw into matching threads in the frame's bottom bracket shell, providing a reliable mechanical lock that resists loosening under pedaling forces. These have been the traditional standard since the early 20th century, with variations in thread pitch and handedness to accommodate different regional specifications.[1] The most common threaded configuration is the BSA/English standard, which uses a shell diameter of 1.37 inches (34.8 mm) with 24 threads per inch (TPI); the non-drive side (left) employs right-hand threading, while the drive side (right) uses left-hand threading to counteract rotation from pedaling torque. Italian threaded shells, less prevalent today, feature a larger 36 mm diameter with 24 TPI and right-hand threading on both sides, originally designed for Italian-manufactured frames but now rare due to compatibility challenges. Installation of threaded bottom brackets requires cleaning and lubricating the shell threads with anti-seize compound to prevent corrosion and ease future removal, followed by threading the cups using a dedicated bottom bracket tool—typically a splined or notched wrench that engages the cup's external notches. Torque is applied to 35-50 Nm, depending on the manufacturer, using a torque wrench to ensure secure fit without damaging the threads; for example, Shimano specifies up to 50 Nm for their threaded cups.[1][71][1][72][13] Press-fit bottom brackets, introduced in the 2010s with standards like BB86 for road bikes, rely on an interference fit where the bearing cups are pressed directly into an unthreaded shell, typically 41 mm inner diameter for BB86, eliminating the need for frame threading during manufacturing and allowing for lighter, more aerodynamically shaped shells.[1][65] This method requires specialized press tools, such as headset-style presses with adapters to evenly distribute force and avoid damaging the cups or frame, often achieving a secure hold through slight deformation of the aluminum shell around the cups. Advantages include simplified frame production and potential weight savings, but drawbacks emerged in the 2020s, including creaking noises from micro-movements at the interface, particularly under torque, which prompted the development of hybrid solutions like threaded inserts for press-fit shells to restore mechanical security. To mitigate insertion difficulties and reduce creaking, the shell and cups are lightly lubricated with a thin grease before pressing, ensuring even contact without excess buildup that could attract contaminants.[73][43][74][75] For larger-diameter shells, the T47 standard employs M47 x 1 mm threading, providing a 47 mm inner diameter compatible with press-fit frame dimensions after retrofitting threads, and installation mirrors BSA methods with right-hand threading on both sides but using larger tools to handle the increased size. This approach combines the benefits of threading with modern shell widths, typically torqued to 40-50 Nm after applying thread lubricant. Specific wrenches, such as 16-notch or hex-socket bottom bracket tools from manufacturers like Park Tool or Shimano, are essential for all threaded installations to prevent rounding of the cups, while press-fit operations demand precision tools like the Shimano TL-BB12 to achieve uniform pressure up to several hundred pounds without misalignment.[76][77][78][79]Specialized Configurations
Height and Geometry
Bottom bracket height refers to the vertical distance from the ground to the center of the bottom bracket spindle, a key metric in bicycle frame design that determines pedaling clearance and overall stability.[80][81] For road bicycles, this height typically ranges from 265 to 285 mm, balancing efficient pedaling with sufficient ground clearance for smooth surfaces.[82] In contrast, mountain bikes often feature heights of 300 to 340 mm to accommodate rough terrain and larger wheels, enhancing obstacle clearance while maintaining control.[80] This measurement directly influences the rider's center of gravity: a lower height improves stability during cornering by reducing the risk of tipping, whereas a higher height provides better pedaling efficiency over uneven ground by minimizing pedal strikes.[83][84] Bottom bracket drop, measured as the distance below the line connecting the front and rear wheel axles (typically 11 to 75 mm), contrasts with rise configurations where the bottom bracket sits above this line.[85] A greater drop—common in road bikes at 68 to 74 mm—lowers the bottom bracket for enhanced cornering stability and a more planted feel, as the center of gravity shifts downward.[86] Conversely, a smaller drop or rise, often seen in mountain bikes (around 30 to 60 mm drop), elevates the bottom bracket to prioritize clearance over obstacles like roots and rocks, reducing the likelihood of pedal-ground contact during technical descents.[87] These choices reflect trade-offs in handling: lower setups favor predictability in turns but risk strikes on rough paths, while higher ones promote agility over barriers at the expense of lateral stability.[83] The bottom bracket height profoundly impacts overall bicycle geometry, including trail (the front wheel's contact patch offset from the steering axis), stack (vertical distance from the bottom bracket to the head tube top), and reach (horizontal distance from the bottom bracket to the head tube centerline).[88] A lower height decreases stack and can subtly increase trail by altering the frame's effective angles, promoting more stable steering on flat terrain.[89] Conversely, higher heights raise stack, potentially reducing trail for quicker handling in technical sections, while affecting reach by shifting the rider's forward position relative to the front wheel.[90] The effective bottom bracket height can be calculated as the wheel axle height minus the bottom bracket drop, or more precisely:Eccentric and Adjustable Variants
Eccentric bottom brackets feature an oval-shaped shell that rotates within the frame's bottom bracket to adjust chain tension by effectively lengthening or shortening the chainstay by approximately 5-10 mm.[94] This mechanism allows precise control without relying on rear dropout adjustments, making it particularly suitable for single-speed and derailleur-less bicycles where fixed chain lengths are common. Introduced in the 1990s by manufacturers like Bushnell, these units addressed chain slack issues in frames with horizontal dropouts, providing a more integrated solution than external tensioners.[95] Adjustable bottom bracket variants extend this functionality through mechanisms that permit lateral or axial movement of the bottom bracket assembly itself. In some designs, sliding dropout systems incorporate bottom bracket movement to fine-tune alignment and tension, though such configurations remain uncommon outside custom builds. Rare vintage units, such as those produced by Bayliss Wiley in the mid-20th century, allowed for bearing preload adjustments via threaded components, enabling mechanics to optimize play in cottered cranksets before modern sealed-cartridge standards dominated. These variants find primary application in frames with horizontal dropouts, where they simplify chain tensioning for single-speed setups by eliminating the need for constant wheel repositioning. However, installation complexity arises from the need for precise rotation and securing of the eccentric shell, often requiring specialized tools, while misalignment risks can affect pedaling efficiency if not calibrated correctly.[96] In modern contexts, eccentric bottom brackets integrate with e-bike mid-drive motors to accommodate chainline shifts during installation, ensuring compatibility with drivetrain components.[97] Since the 2020s, custom frames have increasingly adopted eccentric T47-threaded designs, combining threaded durability with adjustable features for high-end single-speed and adventure bicycles.[98]Compatibility and Standards
Interchangeability Issues
One common interchangeability issue arises from mismatches between bottom bracket spindle length and frame shell width, which can lead to chainline errors that affect drivetrain performance and shifting efficiency. For instance, installing a bottom bracket designed for a 73 mm shell width into a 68 mm shell without appropriate spacers shifts the chainline outward by approximately 2.5 mm on each side, potentially causing chain rub on the frame or suboptimal gear alignment.[67][11] This misalignment is particularly problematic in multi-speed setups, where even small deviations (e.g., 2-3 mm) can exacerbate cross-chaining issues or increase wear on cassette and chainring teeth.[99] Press-fit bottom brackets have been prone to creaking due to manufacturing tolerances in the interface between the bearing cups and carbon or aluminum frame shells, an issue that became widespread in the 2010s and persisted into the 2020s as more frames adopted these designs. Poor fits often result from variations as small as 0.1-0.2 mm in shell diameter, leading to movement under load and audible noise during pedaling.[7][100] This "creaking epidemic" affected systems like BB30 and PF30, prompting riders to apply retaining compounds or switch to threaded alternatives for resolution.[101] Brand-specific differences further complicate compatibility, particularly in spindle interfaces between major manufacturers like Shimano and SRAM. Shimano's Hollowtech II cranks use a uniform 24 mm diameter spindle, while SRAM's DUB system employs a 28.99 mm diameter, requiring dedicated bottom brackets or adapters to avoid loose fits or binding.[102] For older square taper systems, Shimano adheres to the JIS standard with a slightly larger square profile (12.73 mm at the base) compared to the ISO standard used by SRAM and other brands (12.6 mm), which can cause incomplete seating and slippage if mixed.[50] Additionally, converting from BB30 press-fit shells to threaded BSA requires specific adapters, such as those from SRAM or FSA, to thread into the 42 mm BB30 bore and accommodate 68 mm shells.[103][104] Diagnosing these issues often involves tools like chainline gauges to measure the distance from the frame centerline to the chainring midpoint, ensuring it falls within 43.5-50 mm for most road and mountain setups. Common fixes include adding spacers—typically 1-5 mm total, distributed as 0.5-2.5 mm per side—to adjust spindle positioning and correct chainline without altering the bottom bracket type.[67][105] Historically, transitions between splined interfaces like Shimano's Octalink and the ISIS standard (developed by Truvativ and Race Face) highlighted interchangeability challenges, as their differing spline counts (8 for Octalink V1, 10 for ISIS) prevented direct swaps and often necessitated replacing the entire crankset.[106] For example, upgrading from an Octalink-equipped bike to ISIS required a compatible bottom bracket and new arms, as adapters were ineffective due to the incompatible drive-side interfaces.[107]Current Standards and Trends
As of 2025, the most established bottom bracket standards in bicycle manufacturing include the BSA threaded system, which features a 68mm shell width for road bikes and 73mm for mountain bikes, providing a reliable threaded interface for external bearing cups.[7] Press-fit standards such as BB30 and PF30, with 42mm inner diameter shells and widths typically around 68mm or 73mm, remain prevalent for their compatibility with larger spindle diameters and stiffer frame designs.[108] SRAM's DUB (Durable Unifying Bottom bracket) standard, introduced in the late 2010s, unifies a 28.99mm spindle diameter across various shell types, including both threaded BSA and press-fit options like BB86 or PF30, to simplify crankset compatibility.[7] The T47 threaded standard, with a 47mm inner diameter and 1mm pitch threading, has seen rising adoption since 2015, particularly among custom and high-end frames for its balance of press-fit width benefits and threaded durability.[109] By 2025, T47 has gained traction in high-end and custom frames for its threaded reliability with press-fit widths, complementing the ongoing dominance of DUB for spindle unification.[77] In professional cycling, WorldTour teams show a mix of preferences, with threaded standards like BSA or T47 used for enhanced serviceability, often paired with Shimano Hollowtech II or SRAM DUB systems. Examples include UAE Team Emirates using BSA with Colnago frames and Bikone components, while Cofidis employs T47 with LOOK frames and Campagnolo components; many teams favor press-fit variants such as BB86 or BBright for aerodynamic and stiffness advantages in frames from sponsors like Specialized and Trek.[110][111][112] Press-fit systems continue to face scrutiny for potential creaking and installation challenges due to frame tolerance variations, prompting a cautious shift toward threaded options in reliability-focused applications, though no widespread decline in press-fit adoption has occurred.[73] For e-bikes, wider shell standards exceeding 100mm, such as those compatible with fat bike frames, are increasingly standardized to accommodate mid-drive motors and larger batteries while maintaining pedaling efficiency.[113] Standardization efforts are supported by organizations like ISO and JIS, which define spindle taper specifications—ISO for European-style square tapers (longer spindles tapering to about 12.5mm) and JIS for Japanese-style (shorter, tapering to around 12.7mm)—ensuring crankset interchangeability in traditional systems.[50] The Union Cycliste Internationale (UCI) regulates frame geometry indirectly through equipment rules, such as requiring the saddle nose to be at least 5cm behind the bottom bracket centerline in time trials, which influences bottom bracket positioning for aerodynamic compliance but imposes no explicit height limits on the component itself.[114] Looking ahead, trends point to modular adapters that enable cross-compatibility between standards, such as bolt-on mounts for integrating gearboxes or alternative drivetrains, enhancing universality in evolving frame designs.[115] Market reports from 2024 highlight a push toward sustainable materials in components, including recycled aluminum and lightweight alloys for bottom brackets, driven by environmental demands in the bicycle bottom bracket market, valued at approximately USD 1.2 billion in 2024.[116]Maintenance
Modern sealed cartridge bottom brackets, such as Shimano's Hollowtech II external bearing units (e.g., BB-MT801 in Deore XT series), are designed for low maintenance with sealed bearings protected against contaminants. Official Shimano guidelines emphasize preventive care to maximize longevity:- Clean the crank arms and bottom bracket externally using a neutral detergent; avoid high-pressure water jets, as they can force water into the bearing area, leading to noise or premature failure.
- If squeaking occurs from the bottom bracket axle or left crank arm interface, apply grease to the fastening section and tighten to the specified torque (consult model-specific manual).
- Regularly inspect for looseness, roughness, grinding, or play in the bearings; if detected and not resolved by tightening, replace the entire bottom bracket unit, as internal servicing is not prescribed.
- Keep the drivetrain clean, as dirt accelerates wear on related components.
- Every 3–12 months (depending on riding environment—more frequent for MTB in harsh conditions), remove cranks, clean spindle and interfaces with degreaser, and apply waterproof grease to contact points to prevent creaking and moisture ingress.
- For some models, carefully pry off dust seals (if removable), flush bearings with solvent, dry, repack with high-quality waterproof grease, and reseal—though this is unofficial and risks seal damage if mishandled.
- Torque specifications and tools (e.g., TL-FC series for Shimano) should be followed precisely during reinstallation.