Bottom bracket
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The bottom bracket on a bicycle contains an axle to which the crankset is attached and the bearings that allow the axle and cranks to rotate. (The chainrings and pedals are attached to the cranks.) The bottom bracket fits inside the bottom bracket shell, which is part of the bicycle frame.
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[edit] Bottom bracket types
[edit] Three-piece
In typical modern bikes, the bottom bracket axle is separate from the cranks. This is known as a three-piece crank. The cranks attach to the axle via a common square taper, a cotter or via a variety of splined interfaces.
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. [1] 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.
Earlier three-piece cranks consist of an axle (known as the 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, installing new ball bearings, replacing the spindle and adjusting the cups. Most modern bicycles use what is called a "sealed" bottom bracket instead. Sealed bottom brackets are normally two pieces, a unit holding the axle 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 axle/bearing assembly on the non-drive side. Other designs have been three piece, the axle is separate, but the bearing cups incorporate "sealed" bearings, but these are increasingly rare. 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.
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.
[edit] Eccentric
An eccentric bottom bracket is a cylindrical plug that fits in an enlarged bottom bracket shell. The plug is machined to accept a typical bottom bracket. The bottom bracket is offset in the plug, so by rotating the plug, the location of the bottom bracket (and hence the chain tension) may be adjusted. The plug is then fixed in place by a pair of set screws.
Eccentric bottom brackets are usually found on tandems, where they are used to adjust the chain that connects the stoker's and captain's cranks. They may also be employed in bicycles designed without a derailleur but with vertical dropouts, such as single-speeds, fixed-gears, or bicycles with internally-geared hubs.
[edit] Ashtabula
With an Ashtabula crank and bottom bracket, the axle 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.
Ashtabula cranks are also known as "one-piece" cranks.
[edit] 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.
[edit] External bearing
Many current designs are now using an integrated bottom bracket with outboard bearings. This is an attempt to solve the problems caused by 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. A more elegant solution would be to standardise on the larger BMX shell for all bicycles.
Two approaches have been taken towards adoption of this design. In one, the more popular, one crankarm and the bottom bracket axle 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 1990's. This design is similar to the external Bottom Bracket designs that are curently 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 now closer to the frame. 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/thru axle design was developed by Sweet Parts, a micro manufacturer of high end cranks and stems. Their Sweet Wings cranks from the early 1990's incorporated the thru axle 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 axle itself. The bearings were essentially identical in functional concept to the current external bearings systems, but the right side screwed completely into the bottom bracket shell while the left side was external and had a 6805-RS sealed bearing, too.
Truvativ's approach is an evolution of their ISIS Drive standard 'Giga Pipe' bottom bracket. The axle 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 to prevent installation of older ISIS Drive crankarms which are no longer compatible because 'Q-factor' and chainline can't 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.
[edit] Other
Schlumpf makes a bottom bracket that incorporates a two-speed transmission.
[edit] 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 axle.
[edit] Cottered
One of the earliest standards of crank interface, Cottered cranks are now almost entirely obsolete, with only a few manufacturers producing spares. The axle is a slightly tapered cylinder, with a flat region across it. The crank has a similarly conical hole through it, with another hole for the cotter pin. The cotter pin resembled a cone with one side flattened, to meet with the flatspot on the axle. When tightened, this produced a simple and fairly effective interface. One problem with this design is that the crank cannot be easily removed. The cotter pin generally has to be hammered out. Since the cotter pin is made of soft steel, it is often destroyed in the course of removal.
[edit] Square Taper
Often referred to somewhat ambiguously as 'cotterless', since this was the design that succeeded cottered axles, 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.
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 abandoing the square taper interface in favor of an outboard bearing design called Ultra-Torque, which uses a splined interface between spindle halves.
[edit] Octalink
This system was designed by Shimano to address the problems with the Square Taper system, most notably the lack of standardisation. Also, the Octalink system provided a greater contact area between crank and bottom bracket, 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.
[edit] 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 cannot connect to an ISIS crankset 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 calls for a bigger axle in the same sized shell, so the bearings are smaller. Arguably, it was this shortcoming that lead to the development of external bearing designs.
[edit] Other Designs
BMX 3-Piece Bottom Brackets typically use a spindle either 19mm 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.
[edit] Threading and Sizes
| Bottom Bracket Thread Name | Nominal Thread Description | Cup Outside Diameter | Shell width | Shell Inside Diameter |
|---|---|---|---|---|
| ISO/English | 1.37" x 24 TPI | 34.6-34.9 mm
Left-hand thread drive side |
68mm (73mm Oversize) | 33.6-33.9 mm |
| Italian | 36 mm x 24 TPI | 35.6-35.9 mm
Right-hand thread both sides |
70mm | 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 |
| Whitworth
(obsolete, found on |
1-3/8" x 26 TPI | 34.6-34.9 mm
Left-hand thread drive side |
71/76 | 33.6-33.9 mm |
[edit] 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.
An old American term for bottom bracket is "hanger". This is usually used in connection with one-piece cranks.
[edit] External links
- Bottom Bracket Size Database by Sheldon Brown
- ISIS Drive description from King Cycle Group
