Helmet damage

First up, I'm not interested in a debate on the merits/uselessness of bike helmets. Wear one, don't wear one, whatever makes you happy.

I am interested in their design. I once got into a discussion regarding one of my old lids: I faceplanted at fairly high speed on a road, and the helmet (nearly new MET) had a clean split, right through, at the point of impact. I considered this to be a good thing - it had absorbed the impact by breaking, and while my face was a complete mess for weeks, I only had minor concussion. However, I was also told by a few people that it should never have actually cracked all the way through, and that a decent helmet should always remain completely intact during an impact between the head and a stationary object.

Any informed opinions? Or half-baked ones?

I spoke to an a&e doctor about this many years ago. He said someone was brought in where his helmet had split and he was confident that had the energy not been absorbed by the splitting the his skull was next in line and he surely would have karked it. That said its never been proved that a helmet has saved someone's life as its too speculative but the simple physics of the event suggests that if the helmet isn't there to absorb some if not all of the impact then your hair isn't going to offer much resistance. I think a helmet splitting is about right, it would depend on lots of factors such as construction, what you hit, how hard you hit, what angle you hit it at ect. Saying it should remain intact isn't taking these things into account imo, this is my half baked opinion.

The helmet I have is quite old. When I actually wear the thing I would like to think its going to actually do something. So, I'm quite interested in how age and UV is likely to degrade the helmet.

My opinion is it should destroy itself. Basically, it's the same as a car's crumple zone.

they say to replace a helmet every 2-3 years, or after an impact or drop over 4 feet.

Nothing useful to add other than to note that the only helmet I have ever split completely in two was an MET. Design fault, or feature I wonder?

I've split a Giro, and a Bell into 3. I've also seen another Bell in 3, and some others that I forget the brand in more than 1 piece.

I understand they are supposed to split, as said by others.

The way I see it, if it split/fractured/fell apart/exploded it absorbed energy. Less splits/deformation means that it has (probably) absorbed less energy.

If it's still on your head after the incident, It succeeded. It's sole purpose is to absorb a single blunt force, after which it's junk. I may be concerned if it fell apart and off your head during the incident?

Well, all the above echo my own lines of reasoning. I can only assume that the people who disagreed with me at the time, had never actually had a head impact while riding, and thought that cycle helmets were basically scaled-down motorbike helmets :?

Anyways, have an appropriate pic:

Motorbike helmets crack and break on impact too. They too need to be binned if "used" or dropped from 4'+

True, but I guess the damage is less visible on a motorbike lid? Hence the assumption by some that the cycle helmet has 'failed' in some way.

At the time (about 17 years ago) I remember my Mum mentioned the episode to a local copper, and my broken helmet ended up having pride of place in a local police mobile display on bike safety