Thanks! There's actually nothing stretchy. I used rope throughout rather than shock cording. This way the rope is in tension to support the weight of the load - which is why I don't need a webbing / camlock type strap.
The only movement of the rack is rotation up and down around the seatpost pinch bolt to accomodate different size loads.
Think of the aluminum like the hoop in one of those mesh laundry bags. It gives the rope a structure to pull against and provides an anchoring point at the seatpost clamp. Most everything else is in the rope itself.
I did try a webbing strap at first - that's why I have two larger holes near the rear. It was functional, but I figured: 1) a 1/2" webbing strap is WAY overkill for the typical load in a seatbag. The rope is 500lb test (assumes tension, doesn't apply when you go around things), and I have 6 vertical members taking up the load and 2) I always have rope with me. Easy fix if it does break.
Otherwise, I took it for an hour long shakedown ride loaded with mostly heavy packaged foods from the pantry. Rode some 6" deep pock-marked footprint junk snow on 2.3" tires, a few curb hops, and surface transitions with no movement at all. I actually forgot it was back there! Still rock solid by the time I got back
In terms of longevity, yes, the porcelain rocket with a tubular CrMo stay will last forever as steel has more forgiving cyclical loading properties....but I doubt that I'm stressing it to that point of causing any stress or plastic deformation. Another, more robust option would be tube steel or tube aluminum. Again, these felt like overkill to me.
and if I am fatiguing it to the point of failure, I still have 40" of aluminum left!
