Anyone buying a new mountain bike or looking to build a frame these days will, sooner or later, come across an issue that regularly leaves even experienced mechanics scratching their heads: the bottom bracket standard. The multitude of terms can seem like an impenetrable jungle at first. Yet every bottom bracket fulfils the same function: it connects the crankset to the frame and enables the crankshaft to rotate with as little friction as possible whilst ensuring durability. In this overview, we explain why there are still so many standards, what advantages and disadvantages they offer, and what mountain bikers should look out for.
Why are there different bottom bracket standards in the first place?
In the 1990s, the world was still a simple place. Almost all mountain bikes used threaded BSA bottom brackets with square-tapered axles or, later, ISIS or Octalink axles. With the advent of aluminium and carbon frames, manufacturers sought ways to achieve larger bottom bracket diameters, wider frame areas and stiffer cranksets. The result was a host of new standards that promised greater stiffness combined with lower weight. At the same time, larger bottom brackets were intended to increase durability and simplify the manufacture of carbon frames. However, not all of these designs proved to be successful in the long term.
Which standard is the best choice today?
There is no one-size-fits-all answer to this question, yet a number of clear favourites have emerged in recent years. For most mountain bikers, BSA remains the safest choice. The classic threaded standard impresses with its high reliability, straightforward maintenance and the wide availability of spare parts. Furthermore, BSA is considered to be comparatively less sensitive to manufacturing tolerances and is therefore significantly less prone to clicking and creaking noises than many press-fit solutions.
Anyone operating in the high-end segment and building a modern carbon or aluminium frame should take a look at T47. This relatively new standard combines the advantages of large bearing diameters with the practicality of a threaded system. This makes it possible to create stiff frame designs and large-diameter bottom bracket axles without having to put up with the typical drawbacks of traditional press-fit systems. Many industry experts therefore regard T47 as one of the most technically mature solutions on the market.
The PF92 has also continued to hold its own in the mountain bike sector. The standard is particularly widespread on trail, enduro and cross-country bikes, and enables high frame stiffness whilst keeping the weight low. However, precisely manufactured frames and high-quality bearings are essential for trouble-free operation. Provided these factors are in place, PF92 usually performs just as reliably in everyday use as a bolted system. The choice of the appropriate standard therefore ultimately depends on the frame in question – as the bottom bracket shell is specified by the frame manufacturer and cannot be chosen freely.
Which bottom bracket is compatible with which crankset?
The topic of bottom brackets becomes particularly confusing when crank and bottom bracket standards are mixed up. In fact, two factors determine compatibility: firstly, the frame’s bottom bracket shell, and secondly, the diameter of the crank axle. Modern cranksets mainly use axle diameters of 24 millimetres, 28.99 millimetres (SRAM DUB) or 30 millimetres. For each of these axle standards, there are suitable bearing solutions for almost all common bottom bracket shells.
A Shimano crankset with a 24-millimetre axle, for example, can be fitted to a BSA frame as well as a PF92, PF30 or T47 frame – provided the correct bottom bracket is used. The same applies to SRAM DUB cranksets, which are available for almost all current bottom bracket standards thanks to specialised bearings. Cranksets with 30-millimetre axles, on the other hand, require larger bearings and cannot be optimally combined with every frame design. Particularly with narrow bottom brackets, the bearings reach their design limits due to the restricted installation space.
An overview of bottom bracket standards
BSA – The classic lives on
BSA (British Standard Cycle) remains the most widely used bottom bracket standard in the mountain bike sector to this day. Many manufacturers have returned to BSA after experimenting with press-fit systems. The reason: its reliability continues to impress.
Technical Specifications
- Case diameter: 34.8 mm
- Threads in the frame
- Case width: usually 68 or 73 mm
- Bearings are screwed in
Advantages
- Very easy to maintain
- Hardly any clicking noises
- Easy installation
- High compatibility
Disadvantages
- Slightly heavier
- Limited housing diameter
Pressfit – The revolution with side effects
In the early 2010s, Pressfit was regarded as the future of bicycle manufacturing. Instead of using threads, the bearing cups are pressed directly into the frame. The idea behind this is to allow for larger tube diameters, reduce material usage and simplify carbon fibre construction. In practice, however, it became apparent that even minimal manufacturing tolerances could lead to play or noise. As a result, Pressfit came under increasing criticism.
Advantages
- Lower weight
- Larger frame cross-sections are possible
- Greater design freedom
Disadvantages
- Wider manufacturing tolerances are required
- Prone to clicking noises
- More labour-intensive assembly and disassembly
BB30 – The Pioneer
BB30 was originally developed by Cannondale and was one of the first press-fit systems. These days, BB30 is mainly found on older mountain bikes.
Technical Specifications
- Inner case diameter: 42 mm
- Axle diameter: 30 mm
- The bearings are fitted directly into the frame
Advantages
- Lightweight
- Large, rigid crankshafts are possible
Disadvantages
- High demands on frame precision
- Changing bearings is relatively labour-intensive
- Sensitive to noise
PF30 – Further Development
PF30 was designed to address the shortcomings of BB30 and was particularly widespread in the cross-country and road cycling sectors; however, it now plays a lesser role in the MTB segment.
Technical Specifications
- Case diameter: 46 mm
- Axle diameter: 30 mm
- Bearings are housed in plastic or aluminium shells
Advantages
- Larger contact area than BB30
- Easier to fit than BB30
- Better options for compensating for tolerances than BB30
Disadvantages
- It can also crack
- Not fully compatible with other standards
BB92 / PF92 – The MTB standard of the 2010s
In the past, many mountain bike manufacturers relied on BB92 or PF92. PF92 was the dominant standard for many years, particularly for trail, enduro and cross-country bikes.
Technical Specifications
- Inner diameter: 41 mm
- Case width: 89.5 or 92 mm
- Press-fit system
Advantages
- Wide bottom bracket range
- High frame rigidity
- Integrates well with carbon frames
Disadvantages
- Pressfit-related noise issues may occur
- Special tools required
T47 – The modern solution
T47 is now regarded as one of the most technically interesting standards. The system combines the advantages of large bearing diameters with the benefits of a threaded system. T47 is becoming increasingly important, particularly in the high-end segment.
Technical Specifications
- Case diameter: 47 mm
- Threads in the frame
- Variants for different housing widths
Advantages
- Large bearing diameters are possible
- Very high rigidity
- Hardly any clicking noises
- Easy to maintain
Disadvantages
- Higher production costs
- Not yet widely available
The role of SRAM DUB
DUB is often mistakenly referred to as a bottom bracket standard. In fact, it is an axle standard developed by SRAM. The axle diameter is 28.99 millimetres and is designed to combine the advantages of 24- and 30-mm systems. DUB cranksets are available for numerous bottom bracket standards – including BSA, PF92, PF30 and T47. DUB therefore refers to the crank axle, not the frame’s bottom bracket shell.
Conclusion
Even professionals don’t always find it easy to make sense of the complex system of bottom bracket and crank axle standards. As I frequently swap components, I use only the threaded T47 or BSA standards on my personal bikes. Press-fit bearings may be lighter, but in my view the bottom bracket isn’t the best place to save weight. - Jan Timmermann, BIKE editor
Which bottom bracket standard do you think is best for a mountain bike? Let us know what you think in the comments!
Jan Timmermann
Editor
