bottom bracket

 

The bottom bracket on a bicycle contains a spindle to which the crankset is attached and the bearings that allow the spindle and cranks to rotate. (The chainrings and pedals are attached 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 controversy as to whether "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."^[1] 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

Three-piece

In typical modern bikes, the bottom bracket spindle is separate from the cranks. This is known as a three-piece crank. 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

Most 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 in to 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; Specialites TA's "Axix" range is one example. 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 usage, the term 'Three piece' refers to the former design, with sealed bottom brackets being seen as the 'standard'. Designs utilizing separate bearings are still very often found on BMX bikes, due to the means of installation.

Ashtabula (One-Piece)

With an 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.

Ashtabula cranks are easily maintained and reliable, but heavy. They are found on BMX bikes as well as older low-end road and mountain bikes. They fit only frames with American sized (also known as "Pro size") bottom brackets.

Thompson

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. This having been said, the Thompson bottom bracket is 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 Italian frames) 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.

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 designed to be cross compatible with those from other manufacturers. With this new standard has 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 which are made of two halves glued together. Cannondale moved on and developed the SI cranks and bottom brackets. 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 was between both worlds. The right side was internal while the left side was external (and had the 6805-RS sealed bearing, too).

Giga-X-Pipe

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 drive side crankarm. The non-drive side spline interface looks similar, but is in fact different from prevent installation of older ISIS Drive crankarms which are no longer compatible because Q-factor and chainline can not be maintained using these older crankarms 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 drive side crankarm. 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 crankarms on the external bearing BB, as doing so would cause incorrect chainline and Q-factors.

Ultra-Torque

In late 2006, Campagnolo introduced an outboard bearing design called Ultra-Torque^TM, 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.^[2]

Other

Schlumpf makes a bottom bracket that incorporates a two-speed 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.^[3] 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.

Cottered

One of the earliest standards of crank interface, Cottered cranks are now almost entirely obsolete, with only a few manufacturers producing spares. The spindle is a cylinder, with a flat region across it (a land). The crank has a similar hole through it, with a transverse hole for the cotter pin. The cotter pin is cylindrical with one side flattened at an angle form a wedge, this meets spindle land. When tightened, this produced a simple and slightly effective interface.

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 needs to be replaced regularly. If this is not done the crank and the spindle wear and have to 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 is currently the most popular design by far. This interface consists of a spindle with square tapered ends which fit into square tapered holes in each crank. Tightening the two together creates a relatively efficient and simple interface.

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 for the thickness of 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. 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.

In an effort to make bottom brackets lighter, early attempts at using titanium for the spindles were not entirely successful. Several makers have built bottom brackets with alloy cups and titanium spindles but their durability were 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 titanium that is less strong than titanium available today, current titanium bottom brackets are equally reliable and lighter but more expensive than steel bottom brackets.

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 dimeter 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 splined attachement called Octalink and to external bearing types. In late 2006, Campagnolo announced that it was abandoning the square taper interface 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 being a notable exception) recommend that square-taper cranks be fitted to the bottom bracket "dry", with no grease or other lubricant. 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. The argument against greasing is that the crank may slide too far onto the spindle, reducing the designers' intended chainline and potentially cracking the crank.

Octalink

This system was designed by Shimano. The Octalink system provided a greater contact area between crank and spindle, so it had 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. 105, Ultegra 6500 and Dura Ace 7700 cranksets mate to version one bottom brackets, while more recent mountain bike designs use the deeper-grooved version two. The system is proprietary and protected by Shimano patents and licence fees, thus relatively few companies aside from Shimano produce Octalink cranksets. Many competitors have adopted the square taper and ISIS designs as an alternative. In use, Octalink has been shown to loosen because it is not a taper-fit but merely a tight spline fit. Reverse torque loads can cause the crank bolt to undo, and the crank can be irreparably damaged if this is not checked.

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 lead to the development of external bearing designs. ISIS appears to be more durable in a road bike application than for mountain bikes.

Other Designs

BMX 3-Piece Bottom Brackets typically use a spindle either 19(.05)mm (3/4") or 22mm 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.

Sizes

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 or 73 mm). Road bikes usually use 68 mm; Italian road bikes use 70 mm; mountain bikes use 73 mm.

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 crank arm 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.

Bottom Bracket Thread Name	Nominal Thread Description	Cup Outside Diameter	Shell width	Shell Inside Diameter
ISO/English	1.37 in x 24 TPI	34.6-34.9 mm Left-hand thread drive side	68 mm (73 mm Oversize)	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 (obsolete)	35 mm x 1 mm	34.6-34.9 mm Right-hand thread both sides	68	33.6-33.9 mm
Swiss (very rare)	35 mm x 1 mm	34.6-34.9 mm Left-hand thread drive side	68	33.6-33.9 mm
Chater-Lea (very rare)	1.450 in x 26 TPI	oversized Left-hand thread drive side
Whitworth (obsolete, found on older English 3 speeds)	1-3/8 in x 26 TPI	34.6-34.9 mm Left-hand thread drive side	71/76	33.6-33.9 mm
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)

Updating older bottom brackets

Phil Wood & Company makes retaining rings for nearly all historic 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 which allows both cups to be removed can also be mounted with the Phil Wood rings and tools. In particular, the Shimano UN-72, Campagnolo Chorus/Record 2006 (all obsolete now), and the Tange square-taper bottom brackets can have both cups removed, although the Tange diameter is 2mm larger than necessary, and so some filing of the mounting rings would be necessary.

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 in which 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 respray anyway.

Bottom bracket height

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.

For touring bicycles, a lower bottom bracket creates a lower center of gravity and allows for a larger frame without creating an uncomfortable standover height.

Bottom bracket eccentric

An eccentric is a cylindrical plug 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 is desired, up and down is unavoidable and usually inconsequential). Once properly adjusted the plug is then fixed in place by a pair of set screws.

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.

References

Standards

• 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)

External links

• Bottom Bracket Size Database by Sheldon Brown
• How to fit a bottom bracket by Park Tools

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