Perhaps we can consider the bottom bracket to be the heart of the bicycle. The small, thicker walled tube, where the crankset attaches to the frame, provides the foundation for the drive of the bike. It has to be able to withstand the weight and power output of the rider, and serve as a reliable centre point for the bike as a whole. The bearing and component specifics have to be carefully considered as we work to design a bike that addresses these points. Historically, working to the limitations of common standards was a game of compromise, until about ten years ago. It was then that we saw the introduction of a new bottom bracket standard, intended to remedy some of the shortfalls of existing standards, and to move bicycle design forward. The T47 bottom bracket was born.
The quest for the perfect bottom bracket interface has been a long one, and with many chapters along the way, each being influenced by frame design and crank axle specification. Rarely did bicycle frame bottom bracket shell diameters exceed 36mm, meaning that finding the perfect ratio of axle diameter to bearing size was a bit of a toss-up. As far back as the latter half of the 20th century, American manufactures were building bikes with 2 inch/ 51mm diameter bottom bracket shells. These were for specific use with OPC (one piece crank) or Ashtabula crankset systems, utilising press-fit bearing cups. They were known for their durability and ease of maintenance, with their accommodation for large ball bearings and a lack of need for specialist tools for installation and removal. The configuration was also attractive to bike frame mass-production, as the necessity to tap threads into the bottom bracket shell was bypassed. However, the very basic design of the crank and bearing unit meant that efficiency was lesser than the three-piece cranks of the time, so the design never really found itself onto higher end bikes. Today we can still find examples of this interface being used, but it has been relegated to kids bikes and lower end machines.
Fast forward to the early 2000’s where the press-fit BB shell was considered again. A few different iterations of the press-fit system appeared, these being encouraged by bike frame manufacturers. As mentioned earlier, the standard allowed for simpler manufacturing techniques. This saw the development of the wider diameter PF46 standard (PF standing for ‘press-fit’). It allowed for a better ratio of axle diameter to bearing size, but still harboured some of issues encountered with existing press-fit systems, such as creaking, and in extreme instances, BB shell ovalisation. The latter was often a result of repeated bottom bracket replacement – a less than ideal side effect of routine maintenance. Naturally, the hunt for a better solution materialized and in 2015 the PF46 standard was re-examined. Chris King Components collaborated with Argonaut Cycles, and PF46’s overall shell diameter was adopted as the basis for T47 (ThreadFit 47mm). This time, a threaded interface was incorporated instead of a press-fit system. The threaded design utilised the same, well proven, configuration of thread direction as the BSA bottom bracket (Birmingham Small Arms – threaded BB standard of British origin, archaic but dominant in modern bicycle industry). Left hand threads on the driveside and right hand threads on the non-driveside serve to counteract pedaling forces – this prevents the unit from unscrewing as the pedals turn. The positive aspects of PF46 were maintained; a wider internal aperture of the BB shell meant that there was more potential for internal cable routing, without risk of fouling the bottom bracket sleeve. The generally larger outer diameter of the T47’s BB shell also meant an improvement in overall frame stiffness. At the downtube/ bottom bracket/ seat tube intersection, larger diameter tubing could be specified, making a more rigid structure. This is obviously a desirable outcome in terms of power transfer, so it is becoming increasingly more common to see T47 being used on race frames.
The threaded, wider diameter shell of the T47 makes for a versatile and practical interface. Typically, it is easier to service than its press-fit contemporaries, highlighting the user-friendly nature of the standard. Because of this simple, thread-based design, the user is not just limited to T47 bottom bracket units – available on the market are adapters for converting down to the BSA standard, meaning that crankset compatibility is as very broad horizon. If we wanted to, we could even fit a square taper bottom bracket to the same T47 shell. More conventionally, there are T47 bottom brackets available to work with all the popular sizes of crank axle – versions to work with Shimano 24mm, BB30, and SRAM Dub standards are commonplace.
The decision to specify bikes with T47 bottom brackets is becoming more widespread. The benefits from both the end user’s and the manufacturer’s point of view are widely accepted, therefore brands are producing more and more T47 orientated components, making it an easily accessible standard. In the Bombtrack line-up, T47 is in the majority, and Seido Components are advocates too – you will see this reflected in our bottom bracket product line-up. Sometimes brands are hesitant to embrace new standards, it can be a labourious task to commission new tooling and revisit bike frame designs, after all, the bottom bracket being the heart of the bike. However, ten years after it’s conception, T47 is proving itself to be more than just a passing trend, and it’s practically was simply impossible to ignore.
Perhaps we are finally seeing a shift in the evolution of the bicycle. After more than a century of the BSA being king, this simple component area has been pulled apart and dissected. Ease of manufacture and consumer usability have been achieved, as well as potentially simplifying the bottom bracket market. Will T47 replace the BSA standard? The current trend in the bike world seems to the production of ‘do-it-all’ type bicycles, so perhaps the T47 is a natural progression of this movement.
SEIDO Components
August 2025
By Peter Skelton