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Bottom Brackets
Simply put, the Bottom Bracket (often shortened to BB) is the term for the bearings and axle unit, which fits into the frame, upon which the cranks are mounted, and rotate on.
Bottom Bracket Types
Bottom brackets come in all sorts of varieties and sizes to suit the hundreds of possible frame and component combinations on the market. The type and/or size is often determined by the type of bike in question, but it’s important to know which type you requires before trying to buy one.

Useful things to know will be; the brand of chainset you are using e.g. Shimano, FSA, Race Face etc; the shell width of the frame, which is the metric measurement across the widest part of the bottom bracket shell (part of the bike in which the BB is housed); where applicable, the axle length, measured end to end; and finally for some road bike applications English or Italian thread.

It will also help to be able to identify the style of bottom bracket that you require according to the following guidelines. Regardless of the system used, most modern replacement bottom brackets have sealed cartridge bearings for a long service life, instead of the old style cup and cone system, which required regular servicing.

Cartridge Unit

The term cartridge bottom bracket simply refers to a one piece unit that contains the whole of the bearings and axle required for the job. They are sealed to protect the bearings from dirt and water ingress, and are usually unserviceable. When they are worn out the whole unit can be replaced relatively cheaply.

Square Taper

A bottom bracket with a tapered, square axle, usually as part of a sealed cartridge unit, which marries with a corresponding square tapered hole in the crank arm. On this type of bottom bracket, as the tightening bolt is done up, the crank arm is forced onto the square taper, until a snug fit is achieved, and the crank is firmly fixed in place. It is a very simple system that has been widely used throughout the industry for many years. This type of bottom bracket requires the use of a special tool to allow the crank arms to be removed from the taper.

Shimano Octalink

This type of bottom bracket, again, usually a sealed cartridge unit, has an oversized, hollow axle, making it lighter and stiffer than its ‘square taper’ predecessor. At the end of the axle is a spline, formed of eight cut outs (hence Octalink) These splines are designed to locate exactly with the matching spline within the crank arms, so they form a very well engineered fit to reduce the chances of movement which can cause material stress. The benefits of this system are there is no chance the crak arm can be subjected to wear, as with the old square taper style, and the connection from the axle to the crank itself has a large surface area, making it stiffer and stronger. Octalink exists in two variants; Octalink v.1 and Octalink v.2. Shimano Ultegra 6500 and Dura Ace 7700 cranksets use version 1 with more recent mountain bike designs tending to use the deeper grooved version 2.


Created in response to Shimano's Octalink, ISIS Drive stands for ‘International Splined Interface Standard’. It was developed by manufacturers, such as Race Face, FSA, Truvativ etc, to create a standardised system that all brands could follow, and hence make it possible to mix and match components. An ISIS bottom bracket axle has many more, smaller splines, but similar benefits to the Shimano Octalink. It still has an oversized hollow axle for strength and stiffness, but it is not compatible with the Shimano system at all.


Hollowtech II, X-Type or ‘External’ These terms all cover much the same product. It is the most recent development in bottom bracket technology where the axle is no longer part of the BB assembly, but is part of the crank instead. This is done to reduce weight whilst also improving performance. A Hollowtech II, X-Type or other extrenal bottom bracket comprises just the two bearings, one for each side and a usually a central sleeve. Another key difference is that the bearings are fitted externally to the frame (i.e. not held within the bottom bracket shell) to spread the load across the widest possible area, designed to give greater axle support and improve stiffness. However this does have the downside of making them more vulnerable to dirt and grit attacking them. Other benefits of this modern system are that it can be very lightweight and the cranks can be quickly and easily removed, without the need for special tools. This system is probably the most commonly used now for all mountain bike and road bike applications.

Three Piece

‘Three Piece’ refers mainly to BMX style bottom brackets, which still come with separate bearing cups, and crank arms bolt directly to the bottom bracket axle.


Eccentric bottom brackets are usually found on tandems, where they are used to adjust the chain tension. This is made possible by being able to offset the bottom bracket, within a larger insert, allowing the chainset to be moved slightly to the front or rear, increasing or decreasing the distance the chain has to travel, and the tension can be adjusted accordingly. For the same reason they may sometimes also be found on single speed and fixed gear bikes, especially those without horizontal dropouts.


Ashtabula bottom brackets are more commonly referred to as one-piece cranks. Ashtabula cranks and spindle are a single piece, in a roughly s-shaped design. The bearing cups are pressed into the frame, such that the crank, and the cones will hold the bearings in place. Once fitted they can be adjusted to eliminate play or remove tight spots. Ashtabula cranks are simple and easily maintained, however they are heavy, but offer a good cost effective option for entry level kids and BMX bikes.


The Thompson bottom bracket employs adjustable spindle cones and cups pressed directly into the frame, similar to those used by Ashtabula systems. However, unlike Ashtabula cranks, Thompson bottom brackets offer a two piece system, with the non-drive crank being removable. This allows for a smaller bottom bracket shell than that required for the lesser Ashtabula system.


Now almost entirely obsolete, Cottered bottom brackets use a flattened spindle against which a taped ‘cotter pin’ was tightened to secure the crank arm. Today, very few outlets would even carry spares, as bikes have long since come with more modern equivalents.

This Buying Guide gives our customers general advice on Bottom Brackets. It is a guide only and we always recommend visiting one of our stores or contacting one of the experts in our sales team on +44(0)1293 574 900 if in doubt about your needs.
This story was last updated on 16/01/2012