OK i've been thinking about this and I have a few questions:
Air springs are not linear; I think in terms of the equations that govern the actual spring rate as they move through their cycle they are exponential or something. Which is why they are progressive in terms of spring rate, i.e. the spring rate the air damper sees at the start of the stroke is lower than that it sees at the end of the stroke because of the way that air pressure increases faster than volume decreases.
So that means that the heavier you are, the more pronounced the effect of this progressiveness will be.
Which means that a heavier person, relative to a significantly lighter person, will feel that their air can will either be:
a)Too soft in the middle part of the stroke in order for the end stroke to feel about right
or
b) Too harsh at the end of the stroke in order for the middle part of the stroke to feel right.
or even worse
c)In order to get the bike sagging at the right amount (25%-30%) they have so much air in the can that anything from about the first third of the stroke onwards feels very harsh and over sprung.
I am heavy and feel as though this has been a problem in the past that a coil spring has not presented me with, but i am perfectly happy to be re-educated on the physics behind this thinking.
Obviously things will be effected considerably by the leverage curves of the bike and these can be matched to compensate for the way an air spring behaves.
Brant et al, how much of this is correct and if it is, how does running a coil mean that, assuming you have the correct spring rate, the shock 'blows through its travel'?