There are so many different bottom bracket standards employed by frame manufacturers that buying the correct bottom bracket can be very difficult, and can even put people off replacing a bottom bracket themselves. There are three main measurements to consider when working out which bottom bracket you need; bottom bracket shell diameter, bottom bracket shell width and crank axle diameter. If you know these measurements, along with the fitment type of your frame (threaded, press fit, etc...) then hopefully with Westbrook Cycles' guide to bottom brackets you should be able to navigate the minefield of BB standards to find what you need.
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A design more common on older frames, the bottom bracket screws into threads in the frame and houses the bearings externally. Usually these bottom brackets are designed to take cranks with a 24mm diameter axle although some are available to fit a 30mm axle. A specific tool is required to correctly fit and remove threaded bottom brackets which fits into the notches on the external bearing housing. There are two types of thread - Italian and BSA/British, the latter being the more common version. These are not compatible with each other so be sure which thread type your frame has before buying a bottom bracket.
These are all bottom brackets that have cups that are pressed into the frame. They are referred to as PressFit or PF bottom brackets. The number in the name refers to the bottom bracket width - 86mm (Road), 92mm (Mountain Bike) and 121mm (Fatbike). There are two exceptions to this naming convention; Chris King use the term PF24 which refers to the crank axle diameter (24mm) instead of the bottom bracket width and Hope use the term PF41 which refers to the diameter of the frame's bottom bracket shell (41mm). These bottom brackets should be fitted using a bottom bracket press and the removal should be done using a pressfit removal tool.
BB30 bottom brackets rely on two sealed bearings being seated directly into the frame without any additional cups. They are often seated against circlips (c-clips) in the frame. They are intended to be used with cranks with a 30mm diameter axle although adapters are available to allow for cranks with a smaller diameter axle to be used.
These bottom brackets have cups that are pressed into the frame similarly to the BB86 / BB92 / BB121 / PF86 / PF92 / PF24 / PF41 standards, however they are intended to be used with cranks with a 30mm axle diameter and fit into a bottom bracket shell with a diameter of 46mm. Cranks with a smaller diameter can be fitted with adapters, or by using a bottom bracket specifically designed to convert the PF30 standard to be used with smaller crank axles. These bottom brackets should be fitted using a bottom bracket press and the removal should be done using a pressfit removal tool. Specialized bikes have their own version of this standard called OSBB.
The BBRight standard is another version of a pressfit bottom bracket, but features an asymmetrical design that has the non-drive side bearing pushed 11mm further out. These bottom brackets fit into 46mm diameter shells and as these bottom brackets are usually slightly wider than other variants you need to be sure that the crank axle is long enough when fitting aftermarket chainsets. Most BB386 EVO cranks will fit along with some 24mm axle diameter cranks and BBRight specific cranks. These bottom brackets should be fitted using a bottom bracket press and the removal should be done using a pressfit removal tool.
The BB386 EVO / BB392 EVO is very similar to the previous pressfit standards but is for use with frames with wider bottom bracket shells; either 86mm (BB386 EVO) or 92mm (BB392 EVO). The fitment is the same as the other pressfit options and this standard is intended for use with 30mm axle diameter cranks. 24mm axle diameter cranks can be used with either adapters or a specifically designed bottom bracket converter although you need to be sure that your crank axle is long enough to work with this version. Standard BB30 cranks will probably not have a long enough axle.
The T47, sometimes referred to as Thread Fit 30i, is a threaded bottom bracket that uses a 46mm diameter cup which is slightly larger that the conventional threaded bottom brackets. They are screwed into the frame in the same way a s a conventional threaded bottom bracket, although some manufacturers such as Chris King may require a specific tool for fitting and removing correctly. These bottom brackets come in variants to fit both 30mm and 24mm crank axle diameters.
DUB (or Durable, Unifying Bottom Bracket) is Sram's proprietary standard and comes in variations to fit all bottom bracket standards except Trek's BB90/BB95. All DUB bottom brackets are intended to exclusively fit DUB cranks as they have an axle diameter of 28.99mm. Fitment depends on which bottom bracket standard your frame uses.
In addition to the standards described above, there are some manufacturers that use their own proprietary standards. This can make buying components such as an aftermarket crankset very difficult. Here is a list of the single-manufacturer standards that you may encounter on your bike.
Trek's proprietary standards are BB90 (Road) and BB95 (Mountain Bike). This bottom bracket consists of two bearings that sit directly into the frame, similar to the BB30 standard, however they are positioned either 90mm or 95mm apart. A bearing puller is required for correct removal of bearings and a press is required for the correct installation of new bearings.
The "A" in BB30A stands for asymmetrical due to the BB30A standard being 5mm wider on the non-drive side of the bottom bracket shell than a standard BB30 frame. BB30A frames use BB30 bearings that press directly into the frame. Due to the BB30A using a wider spacing standard BB30 cranks will probably not fit, but most BB386 EVO cranks likely will. BB30-83 Ai is the mountain bike version of the same standard with an 83mm spacing between bearings.
This standard is the same idea of widening the bottom bracket shell by 5mm as the BB30A / BB30-83 Ai but uses a PF30 bottom bracket (bearings in cups pressed into the frame). The same crank compatibility will apply as with the BB30A / BB30-83 Ai standard. Again, in this case the PF30A is the road standard and the PF30-83 Ai is the mountain bike version.
Colnago's proprietary bottom bracket standard is a version of the T45 Thread Fit standard with a 82.5mm spacing. Usually Colnago's fitment method involves screwing cups into the threaded frame and then pressing a bottom bracket into these cups, however CeramicSpeed make a T45 bottom bracket that threads directly into the frame, simplifying the arrangement. The CeramicSpeed bottom bracket is compatible with a number of different cranksets, depending on which version you buy.