Derailleur capacities

[Elmer Fudd]

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).

 

[TheDoctor]

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).

Think you've hit the nail on the head there. The geometry of the parallelogram bit lets it track closer ratio (ie road) cassettes or wider ratio (ie MTB) cassettes. The cage length lets it wrap more or less chain - so a long cage one wraps enough chain to cope with triples.

(and it'll probably cope OK with being a bit outside its nominal spec anyway)