Originally Posted by
dougmc
Yes, it took some energy to compress it to the point that it would split, and then some more to actually split, and that's energy that didn't go into splitting your head. The question is if this is a significant amount of energy, and I don't recall ever seeing anybody actually measuring this.
Yes, if the helmet split, it failed. But the assertion that it provided no protection up until that point is unsupported as far as I know. Common sense (or simply my own educated guess, take your pick) tells me that it probably provided a little protection up until that point -- but not very much -- but I have no evidence of this either way. Do you?
I do. I've read papers and articles by professional helmet engineers. The chances are that if a helmet fails that it provided about zero meaningful protection.
Why?
Helmets absorb energy when the foam liner compresses. But to compress properly the liner must have a shell to brace against - think of it as being like a fluid. If it is confined, then compressing it takes a lot of energy. If it isn't, the liquid flow freely at hardly any energy cost.
When helmets split they do so because the shell has failed. This takes place BEFORE compression. So meaningful energy absorption hasn't taken place - the helmet might have absorbed something 1/10 to 1/100 the energy it is designed to. To put this in perspective, that's take the 1/10 figure and call that amount of energy absorbed a "splat" and consider a 24mph crash:
- Helmet absorbs 1 splat
- Would have absorbed 12 splats in 12mph (design limit) hit
- Actual crash energy 48 splats (kinetic energy is a square law)
Important note: impact speed is the component of speed at right angles to the impact surface. If you hit a road and you were cycling at 30mph, then hopefully your impact speed will be vertical only. In theory. (In practice, some helmet shells may shatter from the "belt sander" effect of the 30mph collision - this isn't part of the helmet cert test, and in the real world helmets have an extremely high failure rate.)