There is some disagreement 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. 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.
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 usually found on low end bikes, due to the low cost.
Ashtabula cranks are easily maintained and reliable, but heavy. They are found on BMX bikes as well as low-end road and mountain bikes. They fit only frames with American sized (also known as "Pro size") bottom brackets.
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, or the BB30 standard originally introduced by Cannondale.
Several implementations of external bearings have been brought to market.
As well as the different means to fit the bottom bracket into the frame, there are a number of ways of connecting the crank arms to the bottom bracket spindle. Shimano introduced a proprietary splined interface named "Octalink". Several other manufacturers (King Cycle Group, Truvativ, and Race Face) created a competing open standard called "ISIS Drive" or simply "ISIS", for International Splined Interface Standard. The goal of ISIS was to increase interoperability of bottom brackets and cranksets. Previously, it was more difficult to match two components (from different manufacturers) to fit. Also, ISIS was designed to be stronger than the traditional square taper interface.
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.
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, 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. Some manufacturers make cranks and bottom brackets to both specifications. The overall length of the spindle has no bearing on crank compatibility but does affect frame clearance, chainline, and Q factor.
Lower-quality square tapered bottom brackets are threaded at the ends and use nuts. Higher-quality square tapered bottom brackets are hollow and crank bolts thread into the ends. This allows alloy crank bolts to be used, but only after steel crank bolts are first installed and removed, to securely tighten the crankarm onto the spindle.
Titanium has been used in an effort to make bottom brackets lighter, but early attempts were not entirely successful. Several manufacturers have built bottom brackets with titanium spindles and alloy cups but their durability was lower than that of steel. Early Campagnolo Super Record titanium spindles (which were hollow) were replaced by a later version that used solid, nutted spindles in the lower-quality pattern for improved reliability. However unlike these early titanium spindles which used a grade of titanium less strong than that available today, current titanium bottom brackets are as reliable as steel designs whilst being lighter and less stiff across the axle, but are more expensive.
Another method of reducing weight is to increase the diameter of a hollow spindle. But because the diameter of the bottom bracket shell of the frame is fixed, a larger diameter spindle would need to have the bearings and crank interface moved outside the bottom bracket shell, then a hollow steel bottom bracket could be built at a lower cost and with a reduced weight.
In recent years Shimano has migrated much of their product line away from square taper to a spline attachment called Octalink and to external bearing types. In late 2006, Campagnolo announced that it was abandoning the square taper interface in favor of an outboard bearing design called Ultra-Torque, which uses a splined interface between spindle halves.
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.
A very logical and convincing explanation of why greasing the crank spindle is necessary, written by noted cycling engineer Jobst Brandt, can be found at http://sheldonbrown.com/brandt/installing-cranks.html
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.
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.
Bottom brackets have several key size parameters: spindle length, shell width, and shell diameter.
Spindles come in a wider range of lengths (102 - 140 mm), and are sized to match not only the shell width, but also the type of crankset it will support (longer for triple, shorter for single, etc.). Spindle length, along with cranks shape, determines the Q factor or tread.
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)|
|Raleigh||1 3/8 in x 26 tpi||34.6-34.9 mm Left-hand thread drive side||33.6-33.9 mm|
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-71 and UN-72, Campagnolo Chorus/Record 2006 (all obsolete now), and the Tange/IRD square-taper bottom brackets can have both cups removed, although the Tange diameter is 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.
The bottom bracket height is the vertical distance from the center of the bottom bracket to the ground, and it expresses clearance of the frame over obstacles. The height of the bottom bracket is of concern when designing the frame. The height of the bottom bracket is the baseline for the rider's height while riding. Combined with the length of the cranks, it determines the bicycle's ground clearance.
A higher bottom bracket is useful for mountain bikes. In a fixed-gear bicycle, the bottom bracket should be high enough to prevent the pedals from coming in contact with the ground while cornering.
A lower bottom bracket creates a lower center of gravity and allows for a larger frame without creating an uncomfortable standover height.
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.