I have a Timbuk2 backpack, and recently on international flights I have noticed that the nylon straps are stiff when I’m deplaning.

Within an hour or so they return to their normal pliable state.

Has anybody ever noticed this phenomenon? I can only assume it has something to do with the cabin pressure.

  • Congrats on the Timbuk2. Mine's lasted 13 years and counting. Only fix I've had to make was to reattach the little carrying handle at the top of the bag after 10 years, which was about 5 bucks. – user91988 Aug 26 '20 at 14:07
  • Fascinating! Worth a post to SE Physics :-)! – Russell McMahon Aug 29 '20 at 1:55

It is most probably because the straps are filled with some kind of foam rubber. The tiny pockets of air in foam rubber are not necessarily completely sealed, but they may take some time to ventilate and adapt to a surrounding change of pressure.

If you have a piece of foam rubber and rapidly decrease the surrounding air pressure, the foam will initially expand, since it takes some time for the trapped air to escape, allowing the internal pressure in the foam to adapt to the external pressure and the foam to go back to its original shape.

When you are landing, the air pressure in the cabin increases and the remaining lower pressure within the foam causes it to collapse and stiffen. Also this way around, it will take some time for the foam to 'suck up' air and align the internal and external pressure.

  • 2
    A fun experiment next time you're flying: Buy a bottle of water, drink it at altitude, and close the lid tightly. When you land, see how much it's crushed. (Might not be so much if you're landing in Denver, mind you.) – Ed Daniel Aug 26 '20 at 17:54

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