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Comment by the_af | Hacker News Reader

> my helmet was cracked by this fall (hit the side of my head on the cement)

I'm not sure whether helmets really work, and just to be on the safe side I almost always wear one, but I seem to remember one European biking site -- possibly mentioned here in HN... I forget -- where the author argued that helmets are designed to crack so that it seems they work. But, he argued, whether they crack or not bears no relation to protecting from head injuries; the crack simply leaves you satisfied that the helmet "saved" your life.

The article seemed convincing. Sadly I cannot remember where I read it.

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They are definitively designed to crack as that's a core way for it to absorb the energy of the hit. Similar to how a car crumbles in an accident. Otherwise it's just transferring the kinetic energy straight to your head.

This is also why you should replace helmets after a fall and every few years since the material they are made of degrades with age.

I don't remember if the following link was the site, but in any case it makes similar points (disclaimer: as far as I can tell, this seems to be a "helmet skepticism" advocacy site, so take it with a grain of salt):

http://www.cyclehelmets.org/1209.html

> "It's not a simple matter to draw conclusions about the benefit a broken or deformed helmet might have provided in a crash. However, the fact that serious injury to unhelmeted cyclists is as rare as helmet damage is common, suggests that most of the claims of benefit from damaged helmets are likely to be exaggerated."

> "The next time you see a broken helmet, suspend belief and do the most basic check – disregard the breakages and look to see if what's left of the styrofoam has compressed. If it hasn't, you can be reasonably sure that it hasn't saved anyone's life."

The point seems to be that anecdotes of the type "my helmet worked and saved my life, see? It's cracked" are not conclusive.

If a helmet cracks, it's absorbed energy, cracking is simply how it's designed to dissipate that energy. Whether it cracks or not, it should be replaced when it absorbs energy from a fall.

No. The helmet, in the ideal conditions for which it is designed, should /crush/. Cracking is a non-ideal failure mode as it absorbs much less energy.

hmmm ... interesting point

Looking into the documentation around bicycle helmet design, it appears that the outer surface is actually less for energy absorption, and more for preventing any "catching" on the road surface which would add torque to your head in a crash.

Cracking, in this case, is the expected failure model, since the material has to be hard to provide a sliding surface.

Helmets should not be re-used after a crash.

http://www.bhsi.org/ideal.htm point 5 - outer shell

Well, when we talk about a helmet cracking as opposed to specifying "the outer shell of the helmet" I think we're talking about the whole polystyrene core.

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