Keywords: bb standards,bb30,bb386 evo,bb90,bb92,crank fit,installation,installation,all you need to know
What you should know about Cranks & Bottom Brackets
Gerard Vroomen - 18-Aug-2016
There is probably no topic we get as many questions about as cranks & bottom brackets. Thanks to the bike industry brilliance of inventing a gazillion "standards", even Andy and I sometimes don't know every possible and impossible combination. So let's try to clear some of this up. The important points:
- Traditionally (not meaning 50 years ago but the oldest technology that still exists today for high-end products), BB shells are 68mm wide for road and 73mm for MTB and the crank axle is 24mm in diameter. The bearings are external, meaning they are embedded in cups which are threaded into the BB shell and hang outside of those 68/73mm.
- Then came along BB30, which uses a 30mm axle and bearings directly pressed into a 68mm wide BB shell (same width for road and MTB). IMPORTANT: A lot of people use "BB30" to talk about ANY 30mm axle set-up, but it really refers only to a 30mm axle in a 68mm wide BB shell. After an initial surge of popularity, It's now becoming less and less popular as most new crank axles are wider than 68mm so you may as well use a wider BB shell as well, instead of spacers on your crank.
- So then came along the wider BBs, like BBright (which is wider only on the non-drive side) to BB386EVO (road) and BB92 (MTB) which are wider on both sides. There are a ton more but we'll focus on the ones that are both common and common sense.
- For cranks, Shimano has always stuck to 24mm axles. SRAM produces 30mm and 24mm (called GXP) versions of their cranks, and most smaller brands focused on ultimate performance (THM for example but also RaceFace) only produce 30mm cranks.
- The ONE+ uses BB92, the U.P. uses BB386EVO. The reason for these choices are that they combine very good mechanical properties (meaning we can build a light, stiff, strong frame and use light, stiff, strong cranks) with a wide range of available cranks.
Let's talk about the ONE+ first:
- BB92 uses a 92mm (did you guess?) wide BB shell on the frame, with a shell diameter of 41mm. Why 41mm? Well, remember that traditional set-up with the 24mm axle and the 73mm wide shell and the outboard bearings? BB92 is basically that same setup, but the cups from those outboard bearings are integrated into the BB shell and form one wide BB shell. And the bearings (which still have a thin sleeve around them for ease of installation) press into that wider shell. So instead of having bearings that sit in cups that thread into a narrow BB shell, you have the bearings with a sleeve wrapped around them press directly into the BB shell. The end result is that the bearings are in the same spot as they would be with threaded cups outboard of a 73mm BB shell, and so all the dozens and dozens of cranks designed for that traditional system can be used in BB92. This means all 24mm high-end Shimano (Hollowtech II) and SRAM (GXP) cranks fit into BB92.
- One note on "all the cranks": the ONE+ is not only BB92, it is also BOOST. That has nothing to do with the axle and bearings, but it does have to do with the position of the chainrings. So while the axle of a five year-old XTR crank will work in a BB92 frame, the chainrings would be off. So you have to make sure if you use a Shimano crank that it's a BOOST crank. Or you can use a double crank as a single (and use the outer ring position with a flat ring) to get a similar offset).
- The great thing of BB92 is that inside that 41mm diameter, you can also fit a super light and narrow bearing with a 30mm internal diameter (as opposed to 24mm). This means you can put a crank with an oversized 30mm axle inside this compact 41mm BB shell. For example, a BB90 system will not allow this; its shell is almost as wide, but the diameter is 37mm, too small for any bearing with 30mm internal diameter.
- The one caveat for using a 30mm crank in a BB92 shell is that the crank axle has to be long enough. A dedicated BB30 crank (remember, for a 68mm BB shell width) would not fit, the axle is too short. But nowadays more and more BB30 cranks actually use a long axle and spacers to reduce it to 68mm. Remove the spacers and you have the perfect BB92 crank. Stiffer than BB30 (no spacers, wider bearing stance) and lighter (smaller bearings, no spacers). The THM M3 and Race Face Next SL are the two most popular cranks of this type.
Then the U.P.:
- BB386EVO uses a 86mm (no really!) wide BB, with a shell diameter of 46mm. 86mm equals that traditional 68mm BB shell width plus the width of two outboard bearing cups. The 46mm equals the outside diameter of a standard bearing for 30mm axles plus a thin sleeve for easy installation.
- Standard 24mm cranks from Shimano, SRAM and others fit inside BB386 EVO without a problem. The width matches the axle width and there are a host of bearing and cup options to connect crank and BB shell.
- For 30mm axles, most cranks also fit. Again a word of caution, a true BB30 crank would not fit because it is too narrow. If the axle is 68mm, you can't connect the two cranks through an 86mm BB shell. Pretty logical. BUT, more and more crank manufacturers make their 30mm axles long and use spacers to reduce for true BB30. Without those spacers, they are perfect for BB386 EVO as they are stiffer (no spacers, wider bearing stance) and lighter (no spacers) than a 68mm BB30 set-up.
- It's not always clear if a BB30 crank can be used for BB386 EVO. For the U.P. but far the biggest question mark for our customers is around the SRAM Force 1 crank. And yes, the new version of this BB30 crank is suitable for BB386 EVO with a few simple steps and renders a great set-up. To make it easier to understand, Andy made this small video to explain. NOTE: This does NOT work with a Rival 1 crank as it uses a shorter axle even thought Q factor of the crank is the same, so this is just for the Force 1 crank:
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