Upgrading mechanical disk brakes I'm considering upgrading from Avid BB5 to BB7 mechanical disk brakes on my fat-bike and upgrading to Avid Speed Dial 7 levers. While I'm at it I can easily move up a size or two from the existing 160mm rotors. The mounts are 51mm IS front and rear and there are no clearance issues apparent with the front fork -- it looks to be parallel well past the point that even 203mm rotors would sweep past.

Avid's come with adapters and am I correct in assuming that the adapters included for a 180mm rotor would correctly space the calipers out from the 51mm IS mounts?

Going from 160mm to 180mm would effectively increase the radius 10mm; going from 160 to 203mm would increase radius about 20mm. Is there any issue with rigidity or or alignment going with much larger adapters?

Thanks!
As the comment mentioned, if there is an issue it will be with the fork.  The rotors are designed to withstand the additional braking loads.  However, for what it's worth, I've always felt that larger rotors were easier to bend in a crash.  There's more area to strike on a rock or something, and you get better leverage on the rotor.  But there are plenty of people who think that's just superstition.

I think you might find that you don't need the larger rotors, though.  I switched from BB5s to BB7s (and then to SLX hydros), and felt that the BB7s stopped harder than the BB5s, even at the same rotor size.  And I was using crappy levers, too.