Bottom bracket is simply what mates your frame and crankset together. Other then that, it’s far from being simple. There are millions of frame shells sizes that accepts bottom brackets, and also millions of bottom bracket types.
You may come across many, many categorisation of bottom brackets. Like, press fit, outboard bearing, square taper, etc. Nomenclature is vast; yet it’s not really hard to determine what type of bottom bracket you have, or what you will need for replacement.
I’ll follow a simpler categorization.
Since a bottom bracket is fitted to your frame shell, it can be fitted in 3 ways: Either pressed (no threads on your frame shell) or, threaded inside, or threaded outside. What that means? Modern bottom brackets like Shimano’s Hollowtech II, Sram’s GXP or DUB, have 2 exterior cups that is bolted outside your frame shell. While older standards like square taper, Truvativ ISIS, former Hollowtech ones fits inside the frame shell. They sit (almost) flush with the shell, just the spindles protruding outside.
Things get a bit of complicated here, as the shell sizes are not just one size. Also, Hollowtech II and others have press-fit versions, too!
To get a new bottom bracket, you need to know at least 2 parameters: frame shell type, and bottom bracket type. But that’s not all. For example, BSA threaded standard is the most common type. So does Hollowtech II these days. Is this data enough? No! Generally, road bikes with BSA standard frame shells have shell width of 68mm, whereas MTB’s have 73mm. The newer, mostly downhill MTB’s have 83mm frame shells. These have longer crank spindles (axles). And bottom brackets are different, too. Normally, same 68/73mm Hollowtech II bottom brackets will fit both 68mm or 73mm frames. It’s easy: If your frame has 73mm shell, you just bolt on the cups. (or, press them). If its 68mm, you add 3 spacers that mostly comes with the bottom brackets. Generally, 2 on drive side, depending on chainline.
Longer spindle cranksets like newer Shimano Saint’s, or Zee’s, are meant to use with 83mm frame shells thus won’t fit to 68/73mm frames.
And, it wasn’t all rosy 30 years ago, too. Italian frames have different threading, for example. And for square taper, spindle size matters.
Frame Shell standards
Also known as ISO or English. This is the most common frame shell type. Shell internal diameter is approximately 34mm, and shell width can be 68,73,83mm for downhill or 100mm for fat bikes.
Drive side is left handed, while left side is right handed. Most people don’t know all pedals and most bottom brackets are reverse threaded on drive side, as otherwise, they can get loose. This is why we see so many damaged threads on cranks, and frame shells!
Italian and French
These bottom brackets come with “vintage bikes” and quite rare, especially the French. Their inner diameter is almost identical to BSA; though threads are a bit different. Italian frame shells have 70mm width, while the French shells have 68mm width. I’ve seen modern bottom brackets made for Italian shells, but not French.
They are both right threaded.
BB30, mostly Cannondale
Meant to be used on road bikes, these are all press-fit. Internal diameter 42mm; with 68mm or 73mm shell width. Note that, since made by Cannondale, it’s open standard, so you can come across in some other bikes..maybe.
Used by Campagnolo, FSA, Rotor, SRAM. BBRight is the company who licences the standard. Looks like a derivative of PressFit 30 by SRam.
79mm shell width, 46mm diameter.
Yet another standard by Sram. 46mm diameter, but ISO-ish 68/73mm shell width spec. For both road and MTB.
A press-fit shell type by Shimano. 41mm diameter, 86.5mm/89.5mm/92.5mm shell sizes.
A promising new kid on the block by the headset king, Chris King. Shell inner diameter is 46mm, shell width is either 68mm or 73mm. Being an umbrella company for many boutique brands, they offer not just the bare minimum bottom brackets, but many optional accessories and options, including “converters” for smaller frame brackets like Sram DUB.
I do not think they will get popular, though: Chris King parts are very expensive, and they’ re small compared to Sram, Shimano, even Race Face. And the companies that are implementing their own standards already did that; like Trek, Cannondale, even Look.
If I ever build my own frame, this is the one I’ll go for, most probably.
Oldest surviving bottom bracket type. Not sure, but I haven’t seen any square taper brackets made other then BSA and Italian
This is also the most common interface. If your bike is less then 50 years old, you probably have this.
For this type of bottom bracket, you have to pick the one with correct width (frame shell width), correct spindle width, and correct frame shell type. Spindle sizes start from 103mm, and goes far up to 127mm for MTB bikes.
These days, even the cheapest ones use sealed cartridge bearings.
When Hollowtech II came out, followed by other external bearing type bottom brackets, its tought that square taper will go extinct. We don’t see high quality stuff from big guns like Shimano, Sram or Campagnolo these days in square taper form, but some uber-high quality, boutique manufacturers like Rene Herse makes lightweight and high quality cranksets for square taper; if the vintage look fits your taste. Not to mention, they are more expensive then a entry level bike.
One major problem with square taper bottom brackets is the bolt that connects cranks to the bottom brackets. It has to be properly tightened. If not, they’ll get loose, causing stripped holes in cranks. I highly suggest using copper grease. Copper grease stops corrosion, and ensures a better fit. You should have copper grease in your toolbox for many applications, such as greasing seat posts. They provide far superior corrosion resistance and since copper particles fills micro-gaps, far better retention.
Octalink by Shimano
A proprietary Shimano bottom bracket standard, characterised by bigger, stiffer spindle. It also addresses the retention issues with square taper. Octalink splines have 8 splines; providing much better grip. Much like the difference between a Phillips and Torx screws, actually. Second version has deeper splines.
It was a short lived standard. Previously, Shimano used this bottom brackets for high end cranksets like XTR, 105, Ultegra 6500 and Dura Ace 7700. Then, Hollowtech II replaced it.
International Splined Interface Standard, brainchild of Race Face, Germany’s Truvativ, now a part of Sram, and Chris King. This is an open standard. I think these guys thought the whole world would flock to Octalink, so they decided to build a better, but similar bottom bracket. Better? Yeah. Bigger spindle and of course, more splines. This thing has 10 splines, compared to Shimano. But they forgot to think about bearings. Spindle diameter is about 22. For the BSA shell, which is barely 34mm, this doesn’t leave much space to put in some bearings, right? And don’t forget; you have to use collars to constrain the thing in the frame shell..Maybe 3mm roller bearings ?! Some Chris King versions use pathetically sized 22x31x7mm cartridge bearings. Funny thing is, this “open source” design uses proprietary bearings.
It’s a shame that many nice Truvativ cranksets used this interface in the past.
Original Hollowtech is …Octalink v1 and v2. It’s funny that bottom brackets we know as “Hollowtech II” is not even the bottom bracket standard; Hollowtech is the name of the patented production technique Shimano uses to make “hollow arm and body” cranksets. Weird, but true. I don’t know how this happened; maybe it got so popular that Shimano keep using the “wrong” nomenclature, who knows? (older cranksets used have huge “Hollowtech” logos in them; probably this made people to think its the new bottom bracket standard)
Hollowtech v1 is meant for road bikes, which had shallower splines, while the v2 version of Hollowtech (I) have deeper grooves and used in MTB cranksets.
The most popular, outboard bearing bottom bracket type. I’m not a fan of it. Yet, I think, there is no better alternative, too!
Hollowtech II bottom brackets comes in 2 forms; either threaded or pressfit; different versions for road and MTB.
Although a proprietary design, there are many brands available. Threaded, original Shimano bottom brackets are available for BSA and Italian frame shells; and all the range except Saint and Zee is for 68mm and 73mm shells. (One size fits all). Larger versions for 83mm shells are only available in Saint and Zee series, meant for downhill bikes mostly.
Chris King manufactures a rather crowded range of Hollowtech II compatible bottom brackets for its own design, T47, as well as BSA and ITA.
For pressfit, available sizes are 86.5mm (road), 89.5/92 (MTB), 104.5/107 (MTB).
Shimano, likes SRAM, uses spacers to fit cups, if the specified width is smaller. For example, MTB cranksets (non-boost) are specified as 68/73mm. That means, if your shell width is 68mm, you use spacers. If 73mm, you fit cups directly. That way, they only have to produce one spindle size crankset for 2 types of frame shell width (68mm, 73mm).
This is almost the same as Shimano Hollowtech II. Main difference is, spindle is 24mm in drive, 22mm in non-drive side.
They have threaded and pressfit versions as well, and like Shimano counterparts, are either produced for BSA, ITA (Italian) frameshells, as well as 41mm pressfit versions.
SRAM introduced DUB in 2018, maybe for the reason that GXP has a bad reputation. DUB spindle diameter is 29mm; but the spindle is aluminium – another bold step from SRAM; more on that later!
DUB uses tiny cassette bearings, with miniscule bearings like many others, and they are non-standard, of course! They use a modified 6806 bearing. Off-the-shelf 6806 is 30x47x7mm in size, while SRAM “invention” is 29x47x7mm; because using standard, high quality bearings at a reasonable price will cause crankset to burn, your bike frame to collapse, and kill you in agony.
I have seen Hollowtech II spindles, which are super quality alloy steel, cut where bearings sit, because of sand-water ingress and corrosion. (They tend to turn inside the plastic collar when it erodes, and cuts the spindle like an angle grinder!) I’m very curious to see what will happen to DUB spindles that are aluminium, having Brinell hardness at least half to 1/3 of steel alloy, and modulus of elasticity 1/3 of steel…
BB30 is not a bottom bracket, actually! If you buy a BB30 bottom bracket, what you get is, 2 bearings, (maybe) 2 snap rings, and (maybe) extra seals. You put the bearings onto your crankset, and insert crank into frame shell, screw and go.
Uses standard 6806, unlike SRAM’s DUB. Frame shell width is 68 or 73mm.
For both road and MTB.
Cranksets available for BB30 are Cannondale (its owner; but royalty-free) some low to mid end SRAM road cranks, and Rotor.
BB30 looks great on paper, with one downside: frame shells should be perfectly finished, otherwise its known to creak. That gives BB30 a bad name, which is a shame. Even some 5 figure bikes have badly aligned and machined frames, which is attributed to BB30.
BB30 is a dying breed. I wouldn’t invest on a BB30 crankset; but would definitely buy a BB30 frame, as there are adapters available to use other bottom brackets like Hollowtech, GXP, etc thanks to its bigger diameter.
This is almost the same in every other aspect, except it uses heftier 46mm frame shell diameter.
Other bottom brackets
There are over 20 types; which are very exotic, like the Look’s proprietary system; or FACT cranksets used on some Specialized bikes. It’s not worthwhile to cover all those. For the %99 of the audience, we have Square Taper, Hollowtech II, SRAM GXP, maybe DUB, and a little bit of dying BB30.
Keep your bottom brackets dry, and happy riding!