Bottom bracket “standards”
Bottom Bracket ‘standards’: which BB is which and does it actually matter..?
The current state of the road bike bottom bracket maker’s art is one of complex interaction and plenty of choice. Essentially, there are two axle diameters: 24mm (usually steel), 30mm (almost always aluminium); three bottom bracket shell internal diameters: threaded (two of these if we are picky), 42mm, 46mm; and four main “widths”: 68mm, 79mm, 86.5mm, 90mm…
At a push, many of these components can be made to work with others by using spacers and sleeves; a 24mm axle can be sleeved up to match a 30mm ID bearing and excess axle width can be taken up with spacers.
One of many: Look’s Zed2 crank and BB65
Obviously, the opposite is not the case… As a rule, the best combination will be the one assembled as proposed by the manufacturer; spacers and sleeves add weight and, moulded in hard polymer, introduce a degree of compressibility absent where axle sits directly in bearing. In other words, expect optimal performance from matched components, correctly installed.
“Essentially, there are two
two axle diameters…
three bottom bracket shell
and four main shell widths”
In an attempt to shed some light on the state of this essential element of any bicycle there follows a summary of current road frame bottom bracket shell formats…
The original “standard”, which may have either BSC or Italian threads, is not only the norm with custom and entry-level frames but still a popular option with top-flight frame manufacturers working in carbon-fibre such as Pinarello thanks to its compatibility with major groupsets. It has the advantage of being compatible with any bottom bracket assembly that uses threaded cups, cartridges or housings including conventional square-taper and Octalink or ISIS Drive cartridge as well as more modern systems with outboard bearings.
The traditional threaded bottom bracket seen here in a brushed titanium frame
In practice, such wide compatibility is of lesser importance when choosing a high-performance cycle equipped with outboard bearings. In any case, an insert to provide the threads is needed in a “threaded” carbon-fibre frame, adding weight in comparison to a carbon frame with housings net-moulded in the composite structure for press-fit bearings. Set against this is the ease with which threaded housings can be installed and their ready availability in the event of untimely failure. A shell with faces cut parallel is required for correct bearing alignment and long service life.
With outboard bearings axle diameter is almost always 24mm. There is little perceptible performance differential between a 24mm steel axle and 30mm aluminium axle so this may be a minor consideration when buying. As may be the fact that the use of outboard bearings limits bracket shell width to 68mm and therefore area over which frame spars can mate with the bracket housing structure.
For: adaptable; compatible with any bottom bracket assembly that uses threaded cups, cartridges or housings; parts readily available and easy to install
Against: axle diameter limited to 24mm
The original alternative bottom bracket “standard”, BB30 was dreamt up by Cannondale back in 2000 as an improvement on the firm’s Coda crankset and still retains many of its design features. These include a massively oversized 30mm diameter axle turning in large, durable and smooth-rolling 6806 cartridge bearings. But where the Coda design placed the bearings outside the bracket shell, BB30 places them inside a bigger shell with 41.960mm internal diameter.
FSA BB30 left hand crank showing interface with bottom bracket axle
During development, Cannondale opted to retain the existing 68mm shell width, which keeps the axle short and stiff and maximises ankle clearance for a given “tread” or Q factor. As with the threaded shell standard, it also limits the area over which the various frame spars can mate with the bracket housing structure.
The bracket shell must be carefully machined to ensure accurate bearing alignment, without which they will fail prematurely. Ideally this is done in one pass to guarantee concentricity, leaving it to the internal circlips to determine axial bearing placement. Bearings must be pressed into place and are likely to be damaged by removal. They are widely available from cycle outlets and readily available at low cost from bearing suppliers who need only the code – 68062RS, the 2RS suffix meaning that both sides of the bearing have a rubber seal.
A BB30 bottom bracket is compatible with BB30 cranksets, with any crankset using a 30mm axle e.g. BB386Evo provided it is correctly shimmed and, using adaptors, with cranksets with a 24mm axle.
For: compatible with the stiff and light BB30 axle; large, durable bearing size; light, stiff complete assembly
Against: limited crankset availability and market compatibility unless adaptors used; shell needs accurate machining; bearings press-fitted
BB30 Pressfit conforms to the BB30 axle standard (axle OD 30mm, bearings 6806, BB shell width 68mm) but requires less precision on the part of the frame manufacturer as it uses plastic inserts to house the 42mm OD bearings pressed into a 46mm ID bracket shell. The compressibility of the plastic allows it to deform to accommodate imperfections left by inaccurate machining of an aluminium bracket shell. Pressfit BB30 is also preferred for use in carbon-fibre frames where the larger surface area and softer interposed material improves the interface between the bearings and frame. Containing inserts and requiring a significantly larger diameter bracket shell, it is likely to be heavier than standard BB30.
Cannondale press fit BB30 bottom bracket and chainset installed
For: Requires less precision in frame manufacture than BB30, compatible with BB30 axle and cranksets
Against: as with BB30; inserts add weight and complexity
The most recent arrival in the field, it is based on the BB30 PressFit concept but uses a wider shell. Shell width 86.5mm; shell ID 46mm; bearing size 6806.
For: accepts wide range of cranksets when using BB386Evo adaptors; large shell allows junction with round tube up to 83mm diameter
Against: best when combined with dedicated BB386Evo crankset; some non-dedictated applications will perform sub-optimally thanks to requirement for spacers
Variants on a theme first introduced by Scott working with Shimano; instead of sitting in a threaded aluminium housing, the 6805 size bearings used in regular outboard bearing systems are pressed into the frame. The number indicates the effective bottom bracket shell width; a standard threaded shell is 68mm wide; outboard bearing housings add 11mm each, so any crankset designed to fit threaded outboard bearings works with 6805 bearings having their outer faces spaced roughly 90mm apart. For BB86 versions, the bearings are retained in a hard plastic housing that is then pressed into sockets net-moulded in the frame; the Shimano SM-BB91 Road Press Fit Bottom Bracket fits frames with a bottom bracket shell width of 86.5mm, hence the BB86 code, the flanges adding the required width. Trek’s “proprietary” BB90 standard has the bearings pressed directly into net-moulded sockets in the 90mm-wide structure of the carbon-fibre bracket moulding.
Trek BB90 right-hand bulge
The advantage over a threaded shell – greater with the Trek version – is that the bearings sit directly in the frame instead of in a heavy metal housing, which also obviates the need for a heavy threaded insert. The Trek design, with a wider moulding and no intermediate plastic sleeve, should, in theory, be stiffer. One possible disadvantage is that installation is best done with some sort of press – a headset press can be used – to ensure the bearings are properly seated and aligned. If they aren’t, the bearings will fail very quickly. In any case, the layout is designed to work with cranksets with a 24mm axle and won’t accept a 30mm axle.
For: light, simple assembly; bearings widely spaced and directly supported by frame moulding, bearings readily available from general suppliers
Against: bearings are smaller with less load capacity than BB30 and must be accurately installed for maximum longevity
A format initiated by Cervelo with the intention of improving on BB30 by keeping its 30mm OD axle and 6806 bearing but widening the bottom bracket shell to provide a greater area for the frame spars to support the bottom bracket shell.
The bracket shell is wider by 11mm – the width of an outboard bearing – on the non-drive side so that the bearing sits directly against the crank. The right-hand bearing is in the same position relative to frame centre line as with BB30, 11mm closer to the frame centre line. Shell width is therefore 79mm, precluding the fitment of a BB30-specific crankset.
To take full advantage of the system by minimising axle length, the crankset would need to be asymmetric, with the drive-side crank orientated as per standard BB30 and the left-hand crank as per a system with cranks 90mm apart.
In fact, even Rotor’s 3D+ crankset, which is claimed to be BBRight™-specific, is symmetrical and can be made to fit BB30 using spacers on both sides. Used with BBRight™, it requires the fitment of an 11mm spacer on the drive side, leaving that length effectively unsupported on the axle’s right hand end as it overhangs the bearing. The BBRight specification claims that the design is stiffer than housing the drive-side bearing in a bulge or protrusion built out from the shell but necessarily kept small to clear the inner chainring; whether a spacer taking up the excess axle overhang is a better solution is debatable.
The obvious direct comparison is with BB30; BBRight™ features a wider bracket shell that offers the potential for a stiffer frame but also has a 16percent longer axle which, assuming equal axle wall thickness, is heavier and will twist more by the same amount under the same pedal force. It also overhangs the right-hand bearing.
To date there are few cranksets with a ‘long’ 30mm axle suited to BBRight™. The closest is the Rotor 3D+; an FSA BB386 Evo crankset should fit with adaptors. Adaptors are also available to allow the fitment of cranksets with 24mm axle and nominal 90mm bearing face spacing.
Cervelo’s BBRight viewed from underneath
For: stiff bottom bracket shell, 30mm axle, large bearings
Against: market paucity of cranksets with dedicated 79mm axle; paucity of cranksets with 90mm length 30mm OD axle; 90mm axle unsupported on drive side, needs 11mm spacer
So you have a bottom bracket. What fits? Here’s a quick guide to the common road crankset, bottom bracket bearing and axle formats…
Square taper / Octalink / ISIS Drive®
Suitable for fitment to threaded bottom bracket shell, these are now obsolete, with bearings usually housed in a cartridge that sits inside the bottom bracket shell, limiting axle and bearing dimensions.
For: compatible with threaded bottom bracket shells
Designed to fit bearings in threaded cups outside the shell, which take a 24mm OD axle, this format is favoured by major component manufacturers e.g. Shimano Hollowtech II, SRAM/Truvativ GXP, Campagnolo Ultra-Torque. A typical design has the axle embedded in the right-hand crank/spider.
For: readily available; widely compatible, cranks with 24mm axle adaptable to BB30 etc. using sleeves/spacers; easily installed
Against: axle diameter limited to 24mm
Green-anodised aluminium Cannondale BB30 axle – too pretty to hide away!
The cranks interface with a 30mm axle via an eight-lobe taper; some systems e.g. FSA have the axle permanently embedded in the right-hand crank. A BB30 axle is usually machined from aluminium; steel can be used to add weight where the cycle needs to be ballasted to meet the UCI weight limit as has been done with Liquigas star Peter Sagan’s Cannondale SuperSix EVO. BB30-specific cranksets are only compatible with BB30 and BB30 Pressfit bottom bracket assemblies.
For: stiff and light complete assembly – Hollowgram Si is claimed to be the best performing transmission by Cannondale
Against: lack of compatibility
FSA’s BB386Evo crankset is based on BB30 and, conceived to allow one bottom bracket axle size to fit most of the current bottom bracket shell standards, may require specific bearing and spacer
types. Featuring a 30mm diameter axle, the crankset will fit its own BB386 shell and, with spacers, fit standard and press-fit BB30-type shells. With FSA’s outboard bearings it will also fit conventional 68mm wide threaded shells.
For: Wide compatibility, 30mm axle
Wilier’s BB386EVO as fitted to the superb Zero.7
French manufacturer LOOK’s proprietary crankset is unique although similar in layout to the one-piece Ashtabula type seen in some BMX machines. The composite cranks and axle are moulded in one piece and rotate in 64mm ID bearings chosen to permit the assembly to be passed through the bracket shell during installation. The system is exceptionally stiff and light but is incompatible with any frame not made to take it, which means most of them.
And which is best?
Don’t worry; whichever type of bottom bracket housing sits in your frame, it will do its job, which is to locate and support the bearings for the axle. The next
Look’s fabulous 695 uses their BB65 ‘standard’
question is, how heavy does it have to be to do so or, more precisely, how much support does it offer the bearings per gramme? On this basis, arguably the most effective design is Trek’s BB90, which places its bearings as far apart as possible and does so without recourse to metal inserts, threaded or not. The construction of BB86 must be slightly heavier and offer marginally less lateral support, but a significant difference is surely impossible to discern from the saddle.
Install the crankset, and the balance of the overall package should shift in favour of a system with 30mm axle, which is torsionally stiffer than a 24mm steel axle of the same weight. Here, BB30 offers the shortest – and therefore stiffest – axle but supports it in bearings set close together in a narrow shell. Provided a genuine BB30 crankset is fitted, with no axle overhang, the narrow shell is of less than marginal consequence. Fit a BB30 shell and bearings with a crankset that requires spacers and both weight and distortion under load will increase. Ideally, use the axle for which the bearing and shell system was designed; using spacers or sleeves will inevitably add weight and reduce overall rigidity, as will opting for a smaller-diameter axle.
And if you are on holiday and faced with a failed bearing? You are most likely to be able to find and fit replacement bearings in a hurry if you ride a frame with a threaded shell.
BB90 bearing in frame housing
Words by Richard Hallett
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