Even that stupid potato-sack pack could be made reasonably comfortable if we could put a large percentage of its weight on the hips. The bones of the pelvis can bear a far heavier load that our relatively wimpy shoulders. Obvious solution: Add a hipbelt. But how to get the weight onto the belt? The standard approach is to use one or more aluminum stays, curved to fit the bend of the spine. The stays extend from the top of the pack down to the hipbelt, typically two stays in a V-shape that meets near the center, i.e., the small of the back. The stays transfer weight effectively, yet the pack remains flexible (which is, after all, the primary advantage of an internal over the old-fashioned, rigidly framed external).
Many packs add a framesheet across the back. The framesheet, usually made of high-density polyethylene (HDPE, a flexible but strong plastic) stiffens the back of the pack for even better weight transfer, but at some sacrifice in flexibility. It also prevents sharp, bulky items from poking your back.
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