Bottom bracket
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Image:BB-UN73.jpg Image:Road bicycle 68mm bottom bracket shell.jpg 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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Bottom bracket types
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 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 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) with 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" one-piece cartridge bottom bracket instead, with the spindle and bearings sold as a single unit. This 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 'one piece' bottom brackets being seen as the 'standard'. The former designs, with separate bearings are still used very often on BMX bikes, due to the means of installation.
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.
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.
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 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" (36mm 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 HollowtechII, RaceFace's X-type, FSA's MegaExo. The terms 'X-Type' and 'Hollowtech II' are both used, incorrectly, to refer to any design of this type.
Truvativ's approach is a modified ISIS bottom bracket. The axle is made longer, and the bearings sit outside the bottom bracket shell. They refer to this design as 'Howitzer', and Bontrager also make compatable cranks.
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 axle.
Cottered
Image:Cottered-axle.jpg One of the earliest standards of crank interface, Cottered cranks are now almost entirely defunct, with only a few manufacturers producing spares. The axle is a slightly tapered cylinder, with a flat region accross 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 the lack of easy means with which to get the crank off. One would have to hammer out the cotter pin generally, often resulting in a damaged pin that then had to be replaced.
Square Taper
Image:Shimano-UN25-68x110-ST.jpg Often referred to somewhat ambiguously as 'cotterless', since this was the design that succeeded cottered axles, square taper is by far the most popular design currently. This interface is based on a square tapered rod (the axle) and a square tapered hole (in the crank). Tightening the two together creates a reliatively efficient and simple interface. The most common square taper bottom bracket type is known as the JIS (Japanese Industrial Standard), used by Shimano and most other asian manufacturers. In recent years Shimano has migrated much of their product line away from square taper to Octalink and external bearing types. Crank arms designed for the JIS square taper standard are not compatible with Campagnolo bottom brackets. Campagnolo bottom brackets use two different degrees of taper. Mirage, Xenon, Veloce and Centaur bottom brackets are compatible with all of Campagnolo's budget and mid-range crank offerings. Chorus and Record bottom brackets are only compatible with Chorus and Record series cranks.
Octalink
Image:Shimano-octalink.jpg 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 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. Ultegra 6500 and Dura Ace 7700 cranksets mate to version 1 bottom brackets, with more recent mountain bike designs manufactured the deeper-groved version two. The Octalink illustration photograph to the right is of a version one bottom bracket. 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.
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 properly 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.
Threading and Sizes
| Bottom Bracket Thread Name | Nominal Thread Description | Cup Outside Diameter | Shell Inside Diameter |
|---|---|---|---|
| ISO/English | 1.37" x 24 TPI | 34.6-34.9 mm
Left-hand thread drive side | 33.6-33.9 mm |
| Italian | 36mm x 24 TPI | 35.6-35.9 mm
Right-hand thread both sides | 34.6-34.9 mm |
| French
(obsolete) | 35mm x 1 mm | 34.6-34.9 mm
Right-hand thread both sides | 33.6-33.9 mm |
| Swiss (very rare) | 35mm x 1 mm | 34.6-34.9 mm
Left-hand thread drive side | 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 | 33.6-33.9 mm |
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.
External links
- Bottom Bracket Size Database by Sheldon Brown
- ISIS Drive Group
- ISIS Drive description from King Cycle Groupde:Innenlager