Your bottom bracket is that hollow bit at the bottom of the frame into which the bike’s cranks fit, to allow you to pedal. There’ll be an axle in there, which turns in a pair of bearings: one on the right-hand, drive side of the bike, the other on the left-hand non-drive side.
Once upon a time, all bikes were made of steel. It’s a material which lends itself to reaming and threading, so all bottom brackets had threads into which the chainset bearings screwed. And the bottom bracket axle was a solid steel rod. There were cups in the bearings into which loose ball bearings fitted, with the axles having a curved surface which pushed against the ball bearings.
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The bearing cups threaded inside the bike’s bottom bracket shell. There were just two main standards: BSA threaded, where the shell was 68mm side to side and 33mm in diameter; or Italian threaded, where the shell width was 70mm and diameter 34mm. Lovely and simple.
You’ll still find inboard bearings on many budget bikes, often with a solid square taper axle. Or if you’ve got a really old bike, the cranks may be connected to the axle with a bolted-on transverse cotter pin. And track bikes usually have inboard bearings, with an Octalink connection to the cranks.
But there were a few issues with this simplicity: first, all that steel is heavy; second, the bearings are quite close together, so there’s a lot of free axle either side of the bottom bracket shell, resulting in a loss of rigidity and pedalling efficiency; third, carbon fibre came along and it really doesn’t like having threads cut in it.
So the explosion of bottom bracket standards is really just trying to address these three problems.
There are two basic solutions to improving bottom bracket lightness, stiffness and compatibility with carbon frames: use the same 68 width shell but place the bearings further apart; or make the shell larger altogether.
There’s a general trend to longer axles with larger diameters. Get the bearings further apart and you increase stiffness and power transfer, and can make the frame more substantial, while larger diameter axles can be made with hollow axles with thinner walls, which are stiffer and lighter.
But also coming into play is Q-Factor. This is the distance between the outside edges of the two crank arms. There’s a sweet spot for this, where pedalling is most efficient. If the bottom bracket is too wide, it’s like trying to pedal a horse, but if it’s too narrow, although you might enjoy aero gains, power delivery can drop off.
External threaded bearings
Shimano was an early advocate of moving the bearings outside the bottom bracket shell. It’s a widely used solution that increases the distance between the bearings for extra rigidity, while continuing to use the threads present in steel and alloy bottom bracket shells.
It also means that the axle diameter can be increased to 24mm, which increases axle stiffness. The axle is hollow, which reduces the weight too. The bearings are usually housed in sealed races, which makes them easy to remove or replace. And there’s a really solid interface between the bearings and the frame, which should eliminate creaking (see later).
Press fit bearings
BB30, PF30 and BB30a Pressfit
With the rise of carbon fibre as a frame material, threaded bottom brackets presented a problem. Since you can’t easily cut a thread into carbon, manufacturers needed to insert a threaded metal piece into the bottom bracket shell to screw in the bearings, which added weight.
So Cannondale promoted its BB30 system as an open solution which could be adopted by other manufacturers. Rather than screwing in, the bearings are pushed into the carbon bottom bracket shell, using a threaded press, like that used for headset bearings, and a special adapter to fit the bearings and make sure that they end up parallel. There’s a circlip inside the shell on either side to ensure that the bearings stay in the right position.
The axle diameter is increased from 24mm to 30mm too, which means that it can be made lighter without losing rigidity, although the shell width is still 68mm. Having the bearings inboard in the shell means that there’s extra room for wider frame tubes, allowing bike designers to add frame rigidity for more efficient pedalling.
Pressfit bearings are widely used for modern carbon frames. But the BB30 standard requires close tolerances in the frame to ensure that the bearings fit without play. Early BB30 frames were notorious for creaky bearings, although Loctite and more accurate frame building have reduced the problem significantly.
Another way of making BB30 bearings more robust, which is widely used, is the PF30 bearing. This has the same dimensions as BB30, but instead of using circlips to retain the bearings, these are housed in a plastic sleeve. This pushes through the bottom bracket shell, keeping the bearings aligned and reducing the risk of squeaking. It also means that frame tolerances don’t have to be so exact.
But the plastic sleeve can be prone to wear, so PF30 bottom brackets may need replacing more frequently.
Although the BB30 and PF30 standards have been widely adopted, there’s still a little distance between the crank arm and the bearing on the non-drive side. So Cannondale is increasingly using BB30a bearings on its more recent bikes. This is essentially the same as BB30, but just moves the left hand bearing outwards by another 5mm, adding a bit of extra crank stability.
Campagnolo and SRAM offer BB30 cranksets, as do FSA and other groupset makers. But Shimano has never joined the BB30 party, preferring to promote its own press-in bearing standard. Which is why you’ll often see bikes sold with FSA chainsets in BB30 bearings, but otherwise using Shimano mechanicals.
Shimano’s preferred press-in bearing solution is BB86. This places the bearings 86mm apart – the same distance as its screw-in external bearings. But the BB86 bearings are totally enclosed within the bottom bracket shell, so there’s even more bottom bracket real estate, allowing designers to beef up the bike’s down tube and chainstays.
The axle diameter remains 24mm though, so its walls need to be thicker than a BB30 axle to retain rigidity and BB86 systems tend to be slightly heavier than their BB30 equivalents.
Yet more bottom bracket standards
So all of the main standards available have their downsides, as well as their advantages. And bike manufacturers have been quick to tweak them, which has led to yet more “standards”, which we’ll go through.
Cervélo’s bikes use the BBright bottom bracket system,. It has a 30mm diameter axle with pressfit bearings like BB30, but pushes the non-driveside bearing out by 11mm, for a bottom bracket width of 71mm. This makes the bottom bracket shell asymmetric, displacing its centreline relative to that of the frame. Cervélo says that this allows it to increase left side chainstay width for – you guessed it – yet more frame rigidity.
Is Trek’s favourite standard for its road bikes. Trek has bearing seats moulded directly into the bottom bracket shell, so there’s no need for spacers, circlips or sleeves. The bearings just press directly into the frame.
As the name suggests, the bottom bracket shell is 90mm wide. Despite this, a standard 24mm spindle will still fit into the shell, so it’s compatible with Shimano chainsets.
Yet another maker-specific standard, OSBB (which stands for oversize bottom bracket) is used on some Specialized machines. It’s very similar to BB30 and will fit a BB30 chainset. But the original all-carbon OSBB shell has a plastic sleeve insert and is 61mm wide by 46mm in diameter, as against BB30’s 68mm x 42mm.
Confusingly, Specialized also uses the OSBB name for its BB30 standard (68x42mm) bottom bracket shells, where there’s an alloy insert in the frame.
Compatible with BB86, Colnago uses this system on some of its high end frames. It consists of an alloy sleeve through the frameset, into which alloy cups are threaded. The replaceable bearings are then pushed into the cups.
Colnago says that its design improves rigidity, reliability and ease of maintenance. It says that the frame life is extended too – a nice extra in a premium machine.
Look’s high end bikes use its Zed2 or Zed3 single piece chainset, which has a 50mm axle diameter. It’s threaded through the bottom bracket shell, before the bearings are pushed into the frame, using a proprietary tool.
This means a bottom bracket shell which has a higher diameter than others – and a unique bearing size and bottom bracket shell width: 90x65mm.
Cannondale’s SuperX cyclocross bike uses yet another variant of BB30. To add clearance for extra-wide tyres or extra-muddy cyclocross courses, this goes one up on the brand’s BB30a, by increasing the bottom bracket shell width to 83mm and, like Cervélo’s BBright system, positioning the bearings off-centre from the frame.
Designed to unite multiple bottom bracket standards, BB386Evo is FSA’s solution to improve frameset compatibility. FSA makes a wide variety of adapters to deal with this, but the design is based on a 30mm spindle. It will cope with BB30 and BB86, as well as threaded BSA shells.
The bottom bracket shell width is 87mm, while the 46mm shell diameter means that it will accept PF30 bearings. Although the bearings are placed further apart than in a BB30 set-up, the downside is that the axle is longer than a BB30 axle and so is a bit less stiff. And you need either a 24mm diameter or BB386Evo chainset (which FSA would be pleased to sell you).
Praxis sells chainsets with its own M30 standard. These have a tapered spindle with a 30mm drive side bearing and a 28mm left side bearing. Praxis sells a range of bottom bracket bearings to fit its cranks, compatible with the main bottom bracket standards, including threaded and press fit options.
Finally, there’s the Threadfit T47 standard (no relation to Colnago’s ThreadFit system) used by Argonaut Cycles. It’s been developed for the brand by Chris King and uses an oversize, threaded alloy sleeve to house the bearings. Among its advantages touted by Argonaut are a reliable threaded interface and compatibility with all known crank standards.
But having a threaded alloy sleeve inside your carbon bottom bracket shell sounds a bit regressive – avoiding this was the whole reason for all those press-in standards in the first place.
There’s a whole industry which has sprung up to increase compatibility between the different standards, as well as selling replacement bottom bracket bearings when yours gives up. So, for example, you can get adapters which let you run a 24mm crank spindle in a BB30 frame, and which take care of the extra axle length too.
They’re often designed with threaded sleeves which connect one bearing to the other. When the bearings are tightened, these expand and press against the sides of the bearing shell. If you’re suffering from a creaky bottom bracket, it can be an effective way of remedying it.
And we haven’t talked about MTB and Fatbike bottom brackets – another whole world of pain.