It seems almost all BMXs these days use the “mid” size BB, which uses a direct-fit 41.2mm bearing (19, 22 or 24mm ID). Because BMXers are actually able to agree to all use the same standards and everything is cross-compatible, I can’t find any dimensions or compatibility charts online like you would for the plethora of road/gravel/mountain BBs.
Does anyone know the ID and width specs? More to the point, is it the same ID as a BB86 (41mm press-fit)? Reason I ask is I’d like to know if it’s possible to use a 30mm spindle in a mid BB (something tells me that it is but the bearings will last about a week).
Good question. I had a dirt jumper with Mid BB. I had to get a BMX crankset with a wide spindle… It was a nightmare.
I’m pretty sure BB86 is 41mm nominally, meaning a 41.2mm MID bearing will be too big to pressfit.
Here is an interesting article on 30mm bearings in 41mm shells. The takeaway is: no
Looks like Wheels MFG makes bearings with custom outer races to fit 30mm spindles in BB86 shells:
All this wheel spinning to say, I couldn’t find a way to get a 30mm spindle into a mid BB. If you can figure out the bearing situation, you can easily use spacers or different 30mm spindles to get the proper OBW (outer bearing width) of the mid BB.
Someone correct me if I’m wrong here. If the mid shell is 68mm wide, and given that the OBW for (eg) Hope Evo cranks is 96.5mm, that gives you 14.25mm width per side to have an external cup with a bearing of a larger OD, for the same OBW. Plenty of room to make a sturdy-enough cup with a decent size bearing.
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Ah, I see where you are going. I think you are right:
Looks like there is enough room to make a press fit cup, even with a 73mm wide BB:
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