CyclistThe bottom line: Everything you need to know about bottom bracket standardsBicycles may seem like simple machines, but their design is changing constantly. New tube profiles, frame materials and levels of integration appear at a staggering pace and, while thats all well and good, it is often the less glamorous elements of innovation that can have the biggest impact on the bikes that we ride. While most of us will have a cursory knowledge of a bottom bracket and how it functions, its easy to disregard as a realm for nerds and mechanics, so we thought it was time to demystify the humble BB and explain what all the fuss is about.The primary function of a bottom bracket is to facilitate the rotation of the cranks, which transfers power from your pedals to your drivetrain. To that end, bearings are needed to allow the cranks to move freely. While that is the basic principle, the factors affecting bottom brackets have become increasingly complicated over the years in the quest for peak performance and efficiency. Nowadays, its not only the bearings but also the shape, structure and size of the bottom bracket area known as the bottom bracket junction that are subject to endless tweaks and refinements. Related Posts Tech tussle: Is it time to ditch the press-fit bottom bracket? How to clean your bike chain and drivetrain in 5 minutes Watch: How to fit a press-fit bottom bracket Are ceramic bearings worth it? Factors at playIf the key purpose of the bottom bracket is simply to allow the cranks to rotate, the sheer number of different BB standards these days may seem bewildering. This isnt out of some perverse masochism on the part of bike manufacturers, however. Rather, it signifies the large number of elements at play and differing ideas about how best to reconcile them.While racy frames have become ever stiffer in the quest for ultimate performance metrics, these developments have focussed heavily on the bottom bracket junction as the area where riders drive power through their frames, and thus the area where the most power could potentially be lost. Lizzie CrabbThis has seen many frames develop bigger or burlier bottom bracket junctions to counteract torsion. But in recent years, the trend towards both bigger tyre clearances and bigger chainrings has seen the bottom bracket junction needing to strike a balance to allow space for both while maintaining its own integrity.Throw welding sites on metal frames, bike assembly, repairability and bearing longevity into the mix and the complexity of the bottom bracket area becomes more understandable. As such, when it comes to the different standards, its probably best to start at the beginningBSAOne of the oldest bottom bracket types still in use today is the BSA standard sometimes known as BSC for British Standard Cycling. BSA actually stands for Birmingham Small Arms, a name that Peaky Blinders fans will be familiar with, referring to the Midlands-based arms manufacturer that created the thread standard used for the BB cups. Like the fictitious Shelby family, the BSA bottom bracket is widely renowned and, despite dating back in various forms to the early 1900s, endures to this day and has experienced a resurgence in recent years.BSA bottom brackets are threaded, with both cups tightening in the direction of the rear of the bike. While they lost favour throughout the 2000s as other standards became popular, big brands such as Specialized have recently reverted to the BSA standard.Previously we made our own S-Works cranks and found the best bottom bracket for that was BB30, says Specializeds road and gravel leader, Stewart Thompson. But when groupset manufacturers started saying you had to use their chainrings, thats when we migrated towards BSA. We figured out that we could actually make frames lighter with BSA. It also allowed us to use the lightest and best bottom bracket for Shimano cranks at the time, which we saw as the default groupset manufacturer.Whereas previously BSAs required a threaded metal sleeve in the frame, Specialized developed a method that allowed the metal threads to be co-moulded during the fabrication process. The shell needed for BB30 was larger and required more metal, so switching to BSA, with its smaller bearing diameter, allowed a slightly lighter setup. But what is BB30 anyway, we hear you ask? Sit tight, well come back to it.BB86BB86 also known as PF41 is the Shimano standard for press-fit bottom brackets, introduced in the early 2000s and, as the name alludes to, generally 86.5mm wide. Whereas BSA bottom bracket cups thread into the frame with the bearings sitting outside it, press-fit BBs have smooth external cups that are pressed directly into the frame, with the bearings sitting inside the cups.BB86 works really well for Shimano because its designed with its system in mind, says WheelsMFG senior sales manager Dan DePaemelaere. The larger shell creates the potential for more frame stiffness as there is a larger surface area, and the system is traditionally lighter than threaded alloy cups.With press-fit bottom brackets, the margin for size discrepancy between cup and frame bore known as the tolerance is very low because the parts need to fit together almost perfectly to prevent creaking.The plastic cups are designed to absorb some of the tolerance issues, says DePaemelaere. Plastic is more forgiving than metal or carbon.Dreamed up by Shimano for its 24mm axles, it is unsurprising that conversion to other crank designs posed a bit of an issue, particularly when it came to SRAMs stouter spindles.Matthew Loveridge / CyclistTrying to adapt a shorter 29mm spindle to a 41mm deep, 86.5mm wide shell is not an easy thing to do and, while there are solutions out there, theyre not ideal, says DePaemelaere. This cup/spindle combination necessitates slimmer bearings, thus smaller ball bearings. Being smaller, the contact area between the spindle and inner bearing race is more concentrated and, as this oversized spindle is made of soft aluminium, spindle wear becomes an issue.While brands such as Giant, Canyon and Scott still champion BB86, many others are moving away from the standard. An evolution of BB86 was introduced by component company FSA in the form of BB386EVO, which uses the same 86.5mm shell but also accommodates a 30mm spindle. Given that BB86 was largely supported by those with a brand allegiance to Shimano, this new standard, focussed more on a 30mm spindle, remains limited in usage.BB30Herald of innovation and all things novel, Cannondale introduced the then-wacky concept of BB30 in the early 2000s. In BB30s case, bottom bracket cups were eschewed in lieu of bearings that pressed directly into the frame, and which were designed to take a 30mm spindle.Cannondale was the main disruptor of BSA, says CeramicSpeeds head of product management, Paul Sollenberger. There was a high focus on tolerance, where the bearings had to fit perfectly into the frame, and you could run that oversized aluminium [crank] spindle.The larger diameter allowed the spindle to be made of a lighter material while maintaining the same stiffness, and improved weight was an easy metric to show as an advantage. But, while tolerances were somewhat of an issue for BB86, they were taken to a whole new level for BB30.Whereas bearing manufacturers like ourselves can measure down to 0.001mm, frame manufacturers may measure tolerances to 0.1mm to keep the manufacturing process economical, says Sollenberger. As a result, the BB30 standard became notorious for migration issues, where even small amounts of bearing movement resulted in excessive creaking noises. Even its innovator, Cannondale, has since transitioned back to other standards.PF30Sram tried to rescue the BB30 concept towards the end of the decade with the PF30 derivation. In essence it combined BB30 bearings with the press-fit concept of BB86, placing big bearings inside a press-fit shell. It allowed those who wanted to use the chunkier SRAM cranks to do so, while keeping the claimed benefits of a press-fit system. As cups were used, there was less focus on the exacting tolerances required of BB30, while still allowing the same level of stiffness and crank compatibility.In an assembly line of bikes, pressing in [as opposed to screwing in] a bottom bracket is very simple, and so is quicker and cheaper to do, says Sollenberger. But as with BB30, tolerances were still an issue, and creaking continued.T47Matthew Loveridge / CyclistThere have been many other bottom bracket standards along the way, such as Treks BB90 (a wider version of BB30) and Cervlos BBright (an asymmetrical version of PF30), each with its own strengths and pitfalls, but one of the latest developments that seems here to stay is T47.The T stands for threaded, while the 47 denotes the specific thread pitch meaning the gap between each thread spiral. The standard was dreamed up by a working group including Chris King, White Industries and Argonaut Cycles in a bid to create a one size fits all solution to bottom bracket needs.We wanted to keep the benefits of a bigger bottom bracket shell, says Chris King design manager Jay Sycip. In the case of carbon, this allowed for a stiffer junction, and for metal frames it allowed more area to attach tubing. It also allows more space for routing cables and hydraulic hoses. We saw the benefit of the large PF30 shell but really wanted to have threads because it reduced the margin of error when it came to tolerances.The larger shell meant that T47 could accommodate both SRAM and Shimano crank spindles, unlike the original BSA threaded bottom brackets, making it more versatile. As such, big brands such as Trek have made the switch to T47 as their preferred standard.What does the future holdTapestry / CyclistGoing forward, it really seems like BB86 is the way for those wanting a press-fit solution. While BB386EVO should have been well adopted, its similar proportions to T47 made for an easy transition for brands to adopt and promote a threaded BB interface. In that regard, T47 is leading the way for those after a threaded BB, says CeramicSpeeds Sollenberger. T47 is the biggest disruptor weve seen since Cannondales BB30, and Cannondale was making its own frames and cranks, so I dont think well see anything like that again for a while.CeramicSpeed isnt alone in these predictions, either: Weve had some small frame manufacturers wanting to use BB86 on their new models, but by and large were seeing a huge growth in T47, says WheelsMFGs DePaemelaere. It hits all those keys areas, with crank adaptability, the large junction area and the easier tolerances. It kind of has it all.Its unlikely that press-fit, in whatever form, is going anywhere anytime soon. It has been around too long to simply vanish, and many frame manufacturers will still rely on it for its more economical build process as well as its performance advantages as its lighter than T47 while still offering a wide tyre clearance. Its clear, however, that T47 has made its mark, though recent developments show concerning signs that the variants of the standard will dilute and complicate its premise in a similar way to standards that have come before it.Will someone else throw a new standard into the ring in coming years to shake things up? We think we speak for most when we say we dearly hope not. Lets all settle on one or two standards and be done with it. With SRAM looking to be well on its way to standardising derailleur hangers with its UDH design, bicycles could well go back to being the simple machines they once were. Related Posts Tech tussle: Is it time to ditch the press-fit bottom bracket? How to clean your bike chain and drivetrain in 5 minutes Watch: How to fit a press-fit bottom bracket Are ceramic bearings worth it? BB cheat sheetThe good and bad of modern standardsBSAThreaded BB, 68mm shell width for roadPros: Less tight tolerances needed, less creaking, longevity means replacements are easier to findCons: Narrower junction means frame is potentially less stiff, traditionally heavier than alternativesBB86 Press-fit BB, 86.5mm shell width, designed by Shimano and best suited to Shimano cranksPros: Economical frame manufacture process, wider so frame is generally stiffer, easy compatibility with Shimano cranksCons: Poor compatibility with other cranks, requires tight tolerances, trickier to servicePF30Press-fit BB, 68mm shell width for road, using BB30 bearings and best suited to Sram cranksPros: Economical frame manufacture process, better suited to wider range of cranks than BB86Cons: Narrow junction, bias towards 30mm cranks, BB30 creaking issues still presentT47 Threaded BB; 68mm wide for outboard/external bearings; 86.5mm for inboard/internal bearings; or 86.5mm asymmetric outboard bearing driveside, inboard non-drivesidePros: Tight thread pitch reduces chance of creaking, multiple width options, multiple spindle compatibility optionsCons: Potentially heavier than non-threaded optionsGreatest hitsFavourite BB designs from the people that make themDan DePaemelaere, WheelsMFGs senior sales manager: I would have to say T47 you can adapt it to any crank spindle you want and its got threads, so thats a win in my books.Paul Sollenberger, CeramicSpeeds head of product management: I love BSA because its been around for so long and you know it isnt going anywhere. I like the classics and BSA is pretty hard to beat.Jay Sycip, Chris Kings design manager: On my newer bikes, I chose T47. The junction is bigger and stiffer, and theres more space for cable routing and hoses.Tapestry / CyclistInnovation never stopsT47 has not long arrived, but some brands are already moving it forwardNot content with conventional T47, US brand Bridge Bike Works has developed an Integrally Threaded Carbon Bottom Bracket for its frames.In this case, the BB threads are moulded directly into the carbon frame and coated with a Cerakote ceramic finish. Bridge Bike Works claims its bottom bracket is less prone to creaking, ensures the best alignment, holds the bearings securely and is better at keeping road grime out.It adopts the same dimensions as the T47 inboard standard but is said to save up to 100g in comparison to the metal sleeves normal T47 necessitates. Its a complex and comparatively costly process so its unlikely to become an industry standard any time soon, but we applaud the cutting-edge approach.Elsewhere Felt and Factor have also played around with the T47 concept, mixing and matching the inboard and outboard cups to create T47A, where the driveside cup uses the outboard version and the non-driveside uses the inboard version of the standard. The forces a frame copes with are asymmetrical, so bottom bracket designs should be as well in order to best support frame design, says Factors head of engineering, Graham Shrive. This article originally appeared in issue 151 of Cyclist magazine. Click here to subscribe Related Posts Tech tussle: Is it time to ditch the press-fit bottom bracket? How to clean your bike chain and drivetrain in 5 minutes Watch: How to fit a press-fit bottom bracket Are ceramic bearings worth it? The post The bottom line: Everything you need to know about bottom bracket standards appeared first on Cyclist.