Originally Posted by
revelo
That would be 15kg on rugged roads and even that is dubious. What is needed in a rack is stiffness, not static strength. A rack which flexes is much more likely to break than one which is stiff. Or the attachment bolts will break from all the flexing. A thin titanium tube can be half the weight but the same strength as a thicker steel tube, but the titanium tube will not have the same stiffness as the thicker steel tube.
I'm not sure where you get this notion that the stated weight limit of the rack is only for loads on an even surface, let alone that you have to halve it for dirt. I've used a bunch of racks and found their ratings to be very conservative. I've ridden on racks (all 180 pounds of me) that were rated for 40 pounds.
I've only broken one rack, and that was rated for fifteen pounds. I had it loaded up with maybe 40 pounds of groceries, and was taking a single-track shortcut back from the store. It wasn't the first time I had done that either.
I think you're being overly conservative with your estimations; I would have no trouble trusting an Airy with 30 kg on rough roads. I would feel pretty confident in it's strength even if it were overloaded.
As far as rack stiffness and strength, stiffness is usually in proportion to a tubes outer diameter, rather than wall thickness. That was why Blackburn made the move to tubular aluminium racks (and why so many people followed them.