Coleman had the 1st "composite" pack frame and it was very nice. It was made of fiberglass reinforced resin (Delrin methinks) ans flexed with the wearer.

The military did use some frames like this for a while for special ops troops's ALICE packs. Much better than the original ALICE frames. Then the military went to internal frame packs with aluminum stays.

Dana Designs and other pack makers have used composite carbon fiber frame rods as well, mainly in combination with aluminum maiin frames.

Eric
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"There are no comfortable backpacks. Some are just less uncomfortable than others."