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Bottom Bracket

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.

There are currently a lot of different bottom bracket formats and, of course, there are compatibility issues between all of them. It's always wise to know what types of bottom brackets your frame is made to accept and to know the type of bottom bracket your crankset is made to interface with.

The key features needed to be considered when picking between bottom brackets are compatibility, stiffness, durability and resistance (less is obviously better).
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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 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", which stands 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.

Most bicycles use what is called a "cartridge" bottom bracket instead. Sealed cartridge 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 "cartridge" bearings, however, 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. Cartridge bottom brackets generally have seals to prevent the ingression of water and dirt.

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 (One Piece)
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.

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 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.

Interface Types

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 resembles a cone with one side flattened, to meet with the flat spot on the axle. When tightened, this produces 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.

Square Taper
Often referred to as 'cotterless', since this was the design that was introduced after cottered axles, square taper is currently the most popular design by far. This interface consists of a spindle with square tapered ends that 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 it 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.

This system was designed by Shimano. The Octalink system provided a greater contact area between the crank and bottom bracket, so it has 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 license 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 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.

Height of the Bottom Bracket
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.
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