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| The whole idea is for the pack to transfer the weight of the load to your hip belt, right? A pack that does so will keep the weight off your shoulders, and is much more comfortable to carry, especially with heavy loads. Most modern, commercial packs accomplish weight transfer by incorporating a rigid, two-dimensional frame into the back of the pack. Some packs use a sheet of plastic, some use metal stays, others use combinations of sheets and stays, and at least one uses a fiberglass hoop. Most of these systems work as they're intended, and if I regularly carried loads in excess of 40 pounds I would go with one of these so-called "load monsters." In fact, in my painful backpacking past, I did. |
| However, for loads up to 40 pounds, two years of testing indicate that a rigid internal frame is overkill, and just adds unnecessary weight and complexity to a pack. Perfectly adequate weight transfer can be accomplished by using a ground pad (which you're carrying anyway) as the pack's "frame." The secret is to take advantage of the pad's strength and rigidity when it is configured as a "constrained tube." |
| Have you ever noticed how difficult it is to compress a tube longitudinally ? If you can keep the tube from buckling, even otherwise flimsy material can support a lot of weight in this configuration. That's why even a thin aluminum can with liquid sealed inside it can support the weight of a man standing on one end. The pressure created by the contents resisting a reduction in its volume helps the can maintain its cylindrical shape, and therefore its compression strength. The can is constrained by its contents. |
| The Mithril’s "tubular frame" is constrained both by its contents and the walls of the pack. These opposing forces help maintain the tube’s cylindrical shape (keep it from buckling), and allow you to take advantage of the compression strength of the two or more layers of foam material that surround the load on its longitudinal axis. It’s certainly not as stiff as a titanium or aluminum stay, but it is surprisingly non-compressible. I normally don't carry much weight this way, but I've found the configuration to be sufficiently non-compressible to maintain its weight-transferring abilities with loads up to 45 pounds. |
| The pack is designed to be worn with the hip belt at the level of the small of your back, and with the hip belt and shoulder-level load control straps snugly tightened. This molds the tube to your back without sacrificing any of the its weight-transferring properties. Also, worn this way the Mithril conforms your back perfectly and is extremely stable. |
| Need more convincing? Do this experiment. Roll any ground pad (except a Z-RestTM) into a tube with a twelve to fourteen inch inside diameter. Wind string or masking tape around the outside to maintain the tubular shape. Then stuff your sleeping bag into the center of the tube, and load as much weight as you can pick up with both hands on top of the bag. Now, carefully pick up the packed tube. If you've secured it well and packed it tightly, the whole thing will come off the floor as a unit. If you're strong enough you'll be able to hold it upright by balancing it on your palms. Then carry it across the room and set it upright on a table. |
| What you've just demonstrated by picking the tube up as a unit, holding it on your palms and carrying it across the room is perfect weight transfer. The weight of the load is being transferred by the walls of the tube to your palms or the table top. |
| You might also want to give the "tubular frame" a try with a frameless pack. Just make sure that the tube lines the pack walls snugly, that the contents occupy as much volume as possible, and that you firmly enclose the contents longitudinally (with your top closure) without deforming the tube. Also, don't deform the tube laterally with side compression straps. |
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