I am delighted to report that the Tamarisk is finished. If by finished I mean that the prototype I completed a month ago and have been testing exhaustively since requires almost no changes. The patterns can now be set in stone, and the road towards production begin. This may not be a short road: I’m still trying to nail down a foam supplier who will provide relatively small wholesale quantities of the exact right thickness and density; I’ve all but resigned myself to sourcing the best ladderlocks and quick release buckles from different sources; I’m using this prototype to see if this stuff might be a substitute for 500D Cordura on the pockets and suspension components. All that and everything else might yet take months, but having the shape, features, and especially suspension where I want it to be is deeply satisfying.
The first goal for this pack, when I started working on it almost 3 years ago, was to have a ~50 liter package that would carry 50 pounds but be optimized for 20-30. More specifically, I wanted a hipbelt and harness wouldn’t feel clumsy with a daypack type load, and would also be substantive enough that the structure of the suspension (a single stay in this) would be the limiting factor in load carriage. To make sure that this can be checked off as mission accomplished, I’ve spent the last week and half using my workday workout time (6-7 am) to load the pack with ever increasing weight for the same 4 mile loop. This isn’t enormously exciting, but does allow for an extended and exacting focus on just how the various elements in the pack respond to another 5 pound increase. The last three mornings have seen this number creep above 50, this morning, in the form of a painters drop cloth in the bottom, and 26 liters of water on top. This is a lot, enough to get me sweating even on the flats, at -10 F. My 4 inch wide, sub 8 ounce hipbelt has been holding firm around my hips, the single stay just beginning to bounce vertically in the way I’ve to recognize as how you want to see a suspension system using aluminum start to reach its limit.
The definitive beginning to defining load carriage in a backpack remains Ryan Jordan’s 2003 article on torso collapse in packs, the thesis being that when a correctly sized pack looses a certain amount of its torso length (10% being a useful threshold) to load induced collapse, the load limit of that pack has been reached. The other dimension of that puzzle, one which took me the better part of a decade to fully understand, is that the ability of the hipbelt to resist slipping and appropriately contour to the user must at least keep pace with the suspension. A hanging belt with the right mix of flexible yet supportive structure is the abbreviated answer here, and leaves one with the fairly simple design challenge of optimizing vertical structure for the weight to be carried. In this case, a single 3mm by 13mm 7075 stay.
It is the simplest suspension system I could design, because it minimizes things like the number of fabric panels and yards of thread, as well as because there are as few performance elements in action as possible. The theoretical and practical limits of that single stay are in the Tamarisk identical, which is why I’m content that I did what I wanted.