Like all spanner grouches I have various axes to grind when it comes to press fit bottom brackets, but I’m not going into that now. This is about nomenclature, or the lack thereof.
A great many mainstream road bikes and mtbs (cf. Scott, Giant etc.) are specced with Shimano pressfits, variously described as “Shimano Integrated”, BB86, BB92, BB71, BB91 and countless other helpfully similar alphanumeric combinations. They’re all talking about things that look like this:
Looks pretty innocent, amirite? (Photo from the Haribo hounds at Wiggle)
Park Tool has a pretty good article on dealing with these bad boys, but it skips rather quickly over the naming confusion. Shimano further muddies the waters by using the codes SM-BB71 and SM-BB91 which are product hierarchy designations (like groupset levels), and not the names of different standards.
Essentially, Shimano pressfits fit Shimano Hollowtech II cranks (which have a 24mm spindle) into bottom bracket shells of 41mm diameter with a width of 86.5mm (road) or 89.5/92mm (mtb). The 86.5 and 92mm dimension give rise to the BB86/92 name. The cups themselves appear to be identical between road and mtb, the only difference being the length of the plastic tube doohickey in the middle.
You’ll sometimes hear people call the Shimano pressfits “BB90″, which is extremely misleading. BB90 (or BB95 for mtbs) is a Trek proprietary standard that is completely different. Although it also accepts 24mm spindle cranks, it uses ‘slip-fit’ bearings that sit in recesses moulded into the bottom bracket, a system which probably seemed terribly elegant when the engineers drew it in CAD but which is a pain in the arse in practice.
By the way, Shimano isn’t the only company that makes bottom brackets to fit 41×86.5/89.5/92mm shells, but if you have a Shimano cranks then you’ll keep things simple if you stick with their product. SRAM have their own version to match their cranks, and there are expensive third party options too if you like that sort of thing:
If you’re too lazy to read all that, here’s the short version:
- BB86 and BB92 are both the ‘Shimano standard’ using the same cups but with different shell widths, to accept Hollowtech II cranks.
- BB71 and BB91 are two levels of the same Shimano standard, and are not bottom bracket standards themselves.
- BB90/95 is a different, Trek specific standard that you only need to know about if you have a Trek.
- If you don’t have Shimano cranks, you probably didn’t need to read this.