The bottom bracket or ‘BB’ is the engine room of your bike’s chainset and one of the two most important bearings on your machine (the other being the headset).
Your BB is located at the junction of your frame’s seat- and down-tubes (in a hollow tube referred to as the ‘BB shell’) and consists of two bearings, one at either side, through which your pedal axle is fitted, enabling the pedals and chainset to spin freely (many cranksets have an integrated pedal axle, three-piece BMX cranksets have a separate one). The BB assembly may also consist of spacers and lockrings, etc. depending on type and design.
A good-quality BB, correctly installed, should serve you with years of trouble-free riding, but bearings can wear out or the BB can become loose. This can cause annoying noises or lead to excess movement in the BB (‘play’) which can damage your frame, so it pays to ensure that your BB is secure and in good condition.
When replacing or upgrading your BB there are a number of things to take into account, with models varying according to bike type (BMX BBs are different from those used on road and MTB bikes), BB shell size (they can be different lengths so you need a BB to fit) and sometimes, frame design (many modern road and MTB bikes use ‘press-fit’ designs where the shell is moulded to enable the bearings fit directly in).
Find out more about the differences between BB standards and to help you choose which one is right for you.
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MTB and road bottom brackets are functionally the same as they ever were, consisting of two sets of bearings which allow the pedals and crankset to rotate freely.
However in terms of design they have evolved considerably in recent years, with a trend towards weight-shedding minimalism which means that many ‘bottom brackets’ are now little more than two bearings. A further trend has been towards integration of components, which has seen the pedal axle evolve to become an integral part of the crankset, and the BB’s bearing cups incorporated into the frame itself in some instances.
Which bottom bracket is right for you?
To some extent, the type of bottom bracket you choose will be determined by your frame type, as some frames – particularly carbon models above a certain price point – are made to use one type of BB, and one type only.
With some BB types (anyone remember cottered cranks?) having died out completely, there are three main designs remaining – the cartridge-type BB (itself a declining standard); the external or outboard BB and the press-fit BB (with this latter type becoming more and more commonplace).
Inexpensive, easy-to-fit and well-sealed cartridge BBs are still found on lots of MTB and road frames at the more budget or utilitarian end of the market, with external BBs at one point having largely replaced them on higher-end models. However the increased use of carbon fibre as a frame material has also led to a growth in press-fit BBs being specced as standard on mid- to high-end carbon bikes, as well as some aluminium models. It’s one area we are seeing a lot of divergence in at the moment, with different manufacturers using different standards, for different claimed benefits.
Whichever type you choose – or are obliged to stick with – make sure you choose the correct width to fit your BB shell. These are generally 68 or 73mm wide on MTBs, or 68/70mm on road bikes although again this is not such a consideration with press-fit BBs (of which, more below).
Common types of road/MTB bottom bracket:
• Cartridge BB: This older standard features a small diameter axle and internal bearings sealed in a metal cylinder which screws into the BB shell as a single unit. Whereas the predecessors of this design required painstaking bearing adjustment using two spanners, cartridge BBs became popular as a ‘fit and forget’ solution. Once they wore out, you simply replaced them, rather than fiddling around with bearings.
Again, this standard is in decline but can still be found on older and entry-level MTB and road bikes. When replacing, check that your axle type matches your cranks. Pedal axles for cartridge BBs may be square-taper or splined at the interface with the cranks, while some chainsets use integrated axles that are not designed for use with cartridge BBs. Most cartridge BBs will have their model no. and specifications printed on an external sticker, making them straightforward to replace. While cartridge BBs are heavier than their external or press-fit cousins and not as stiff, lots of people still like them – they are super-easy to fit at home, relatively inexpensive and have a good reputation for keeping out water and lasting for years.
• External BB or ‘conventional threaded’: This is where cartridge bearings sit in aluminium cups on the exterior of the BB shell (‘outboard’ bearings). The principle behind this is that it allows for the use of a larger-diameter axle (typically 24mm as opposed to the 17mm used in cartridge BB axles) for increased stiffness in the BB area and therefore improved power transfer.
The external BB setup means greater stiffness for less weight, but it can have drawbacks. Bearings can die quickly if they are not perfectly aligned, and it is often advised to have your BB shell professionally faced (where the opposite sides are ground down to ensure they are perfectly parallel) for maximum longevity. Ceramic bearings are also regarded as longer-lasting and smoother-running than standard steel ones.
• Press-fit: Press-fit bottom brackets are found on an increasing number of mid- to high-end carbon fibre MTB and road bikes, and some aluminium models. With these, the bearings are pressed directly into the frame, either in aluminium cups or into races that are moulded into the structure of the frame. The main advantage of the press-fit generation of BB standards is that is allows for a wider BB shells without affecting axle width, thus enabling bigger tube sizes to be used and increasing frame stiffness in the BB area. This, in turn, improves the power transfer between rider and frame by reducing flex at the BB. However there are many different sizes and iterations according to manufacturer – BB30, PF30, BB90 ,PF86/92 to name but a few – and not all are interchangeable.
Check which standard your bike is running when replacing a chainset or BB, and if in doubt have a professional mechanic do the replacing (many critics of the press-fit standard also point to it being difficult for home mechanics to tinker with). Also, if you’re riding a bike with a press-fit BB and keep wondering what that creaking sound is… well, there’s your answer.
BMX bottom brackets need to be tough and sturdy in order to deal with hard landings and the fact that most BMX riding involves standing on the pedals and pushing hard, which puts a huge amount of stress on the BB.
Basic BMX BBs will use loose ball-bearings with more expensive units having cartridge bearings which can be easily replaced when worn. In terms of quality, the better the bearings, the better the BB – top-end bearings will spin smoother for longer and be able to deal with more abuse.
There are two standard sizes for BMX bottom brackets – 19mm or 22mm according to axle length/shell width (check your frame before you buy) – and four main types: American, Mid, Euro and Spanish.
• American – These use bearings that sit inside cups that are pressed into the BB shell. They are more common on older and entry-level BMX bikes, especially those using one-piece cranks.
• Mid – Mid-type BMX BBs are similar to American in that the bearings are pressed into the frame, but in this case directly, without the need for extra cups. These can be lighter than American BBs but your frame needs to be machined to enable the bearings be fitted.
• Spanish – Spanish bottom brackets use integrated bearings but of a smaller diameter to mid-type BBs, meaning a lower overall weight and smaller BB shell size
• Euro – Generally only found on BMX race frames, Euro BBs use small-diameter bearings in a cup that sits externally to the shell (‘outboard’ bearings).
Keep your bottom bracket turning smoothly through all conditions with our selection of parts including conventional and ceramic bearings, replacement centre tubes, conversion kits, bearing caps, spacers and more.