Fatbike frames typically come with 100mm or 120mm BSA BBs. Does the 120mm BB help anything, though? For example, if I have a 100mm bottom bracket and a 5" tire on a 100mm rim, and I switch to the largest cog of an Eagle 1x12 setup, will the chain start scratching the tire? Or will there be adequate clearance?
The math is easy to do.
According to Sheldon Brown, fatbike chainline is 66 or 76mm for dropout spacings of 170 and 190mm respectively.
Assuming a 100mm BB is used with a 170mm dropout spacing and 66mm chainline: a 5" tire is approximately 127mm wide so the tire would extend 63mm off the centerline of the bike.
That's only 3 mm clearance if the chain is running parallel to the centerline of the frame. With the chain on the largest ring it's conceivable that the chain would hit the tire.