Originally Posted by wphamilton

Understanding one basic, core concept of how bike helmets protect us bypasses several mistakes that we hear repeated often. Fortunately it's very simple to grasp and I'll only address this one concept.


The helmet reduces the forces felt by our skull by stretching out the time of the impact. It takes a certain amount of time to crush the foam, depending on how fast the helmet is going, and how massive your head is of course. Simple as that, and that's all of it.


It does not "spread out the force" very much. It does not "dissipate energy". Energy is absorbed by crushing. And it's basically linear in the crush range - ie, twice as thick gives you twice as much protection. No amount of shaping and mysterious designs change that.


Check it yourself: hit a styrofoam sheet with a hammer and look at the damage. There will be a dent where the hammer hit, and maybe just a little slope in the surface closely surrounding the round hole. You can see that there is little if any "spreading out" of the force, because there would be visible damage if there were. Now crack it in half at an undamaged section, push the edges together and hit it with a hammer again right on the crack. You'll see the same damage as before, some on each half. As long as the crack is held together, it does not harm the impact protection. Surface (UV) damage of a mm thick or less is insignificant, because the only affect is reducing the crush by a mm.


That's all, valid corrections are welcome.