I understand how you work out the maximum capacity of gearing to compare to the maximum capacity of a rear derailleur, (largest crank - largest cog) + (smallest crank - smallest cog). What I don't understand is when they tell you the maximum difference between largest and smallest cranks. Why does it need to know ? It's on the back of the bike !! Or is it to do with amount of fore and aft movement it has to take up the slack in the chain ? This, BTW, is a purely theoretical question at the mo, I have no intention of changing anything just yet, although when needed I would like to go from 48/38/28 crank to 52/42/32, but it appears my only option would be 52/40/30 or similar, 2 outside the range of my rear derailleur (20T).