Another thing that seems to be certain is that virtually every helmet wearing cyclist who crashes had their life saved by their helmet and yet virtually every cyclist who crashed without a helmet survived it quite happily.
Helmets why doesn't everyone wear them?
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Another thing that seems to be certain is that virtually every helmet wearing cyclist who crashes had their life saved by their helmet and yet virtually every cyclist who crashed without a helmet survived it quite happily.
Maybe it would have compressed as designed at lower speeds, but the force was too much for the material to do that in this case, so it fractured. I'm not an expert in materials science, so have no idea how polystyrene should behave under different stresses.
My point was that, as the impact was far outside the design specifications, it's not really correct to say the helmet failed - it was simply subjected to forces outside of its parameters.
If you fired an RPG at a kevlar vest, thereby destroying it, you wouldn't really say the vest had failed, as it was never designed for that.
mcshroom
Bionic Subsonic
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I thini you miss the point benb. The helmet wearer believes the helmet worked, when it has in fact entered a failure mode that provides less marginal protection than it would do if it hadn't cracked.
Of course the material failed, and it would be surprising if it had not done so considering the extra load it has been subjected to, but the point is that despite it being obviously cracked the poster still believes it was effective.
Of course the material failed, and it would be surprising if it had not done so considering the extra load it has been subjected to, but the point is that despite it being obviously cracked the poster still believes it was effective.
To be honest, any impact there is outside its design specification. Its designed and self certified by the manufacturer to protect against an impact to the top of the helmet, not the side. AFAIK helmets are never tested for side impacts and what their performance is under those circumstances is anyone's guess. What you do see on a lot of those side impacts with an unsupported edge is the side of the helmet breaking off. Exactly the same happened with the previous photo posted here of a helmet after an accident. I might see if I can persuade a test house to test a helmet in a side impact and see what the results are.
And on the semantics point I think most people think the Twin Towers in New York suffered a structural failure even though they were never designed to have a 737 flown into them.
A bicycle helmet can only take a low load. At best it can save the rider from road rash, or turn a major injury into a less serious injury or leave the cyclist seriously injured rather than dead. Or have I misunderstood the physics?
What is more interesting (except to the victim) is to look at serious head injuries to cyclists WEARING helmets. We must also remember the more important brain defender - THE SKULL. Just like the helmet it has a design limit before (catastrophic?) failure. If the skull is still intact then is the load taken by the helmet of much relevance?.
If the skull is deformed (And I've fractured mine twice) brain injury may still not occur (lucky me!). So we are getting into the area of wearing a helmet, having a fractured skull and some brain damage before we can really examine whether the helmet has significantly changed the extent of this damage. I'm not a structures man but it would seem quite possible to do some forensic modelling to estimate the load alleviated and the consequences. I've not seen any work done on this. Possibly because it is beyond the remit of mere brain surgeons and statisticians?
What is more interesting (except to the victim) is to look at serious head injuries to cyclists WEARING helmets. We must also remember the more important brain defender - THE SKULL. Just like the helmet it has a design limit before (catastrophic?) failure. If the skull is still intact then is the load taken by the helmet of much relevance?.
If the skull is deformed (And I've fractured mine twice) brain injury may still not occur (lucky me!). So we are getting into the area of wearing a helmet, having a fractured skull and some brain damage before we can really examine whether the helmet has significantly changed the extent of this damage. I'm not a structures man but it would seem quite possible to do some forensic modelling to estimate the load alleviated and the consequences. I've not seen any work done on this. Possibly because it is beyond the remit of mere brain surgeons and statisticians?
Yes, Ade but it is possible to start simplistically and ignoring the effect of brain damage on function - it should be possibly to measure the crushing and or/loss of brain tissue and estimate how much more, if any, would have occurred if the helmet had not been worn. You could then extrapolate that to the number of incidents sustaining these levels of damage to see whether the net effect is significant.
If it isn't there is not much use going into the consequences of damage since the amount of change may not be meaningful. Of course if the analysis came out the other way then the more difficult job of measuring the effect of damage on function is needed. However, it may be sufficient to change the minds and helmet wearing practices of those who currently assume they are not much use protecting the head in a crash anyway.
Or maybe not for some here.
If it isn't there is not much use going into the consequences of damage since the amount of change may not be meaningful. Of course if the analysis came out the other way then the more difficult job of measuring the effect of damage on function is needed. However, it may be sufficient to change the minds and helmet wearing practices of those who currently assume they are not much use protecting the head in a crash anyway.
Or maybe not for some here.
Not on the side of the helmet it will fracture as its unsupported.
Easiest way I can think to describe it take 2 identical sheets of polystyrene 1 inch thick. Take the first one and hold it by the edges, now get someone to start pushing in the middle of the sheet with 2 fingers. It won't take much to snap the sheet in 2 as it is not able to absorb much energy. Now take the 2nd sheet and hold it firmly against a wall and get someone to punch it. It won't break as it is fully supported, and it will deform, It has absorbed far more energy than the first sheet without fracturing. Polystyrene is good at absorbing energy when it is fully supported and it is a blunt impact, the only time you'll get this with a helmet is if you land on top of your head. If you land on the side of it chances are the helmet will just fracture.
classic33
Leg End Member
I had my injuries made worse due to the fact that one was being worn.
Yes, I do understand that completely. Sorry if I was unclear as to what exactly I was saying!
mcshroom
Bionic Subsonic
- Location
- Egremont, Cumbria
In which case I appologise for my lack of comprehension
No, I apologise for giving the impression I didn't understand! (this could go on some time)
Right on Ade. A new argument against helmets! It denies giving pleasure to the wind. And a mean wind is no friend of the cyclist. I shall remind you of that next time I see you wearing one
When I have wind, it is not a pleasant experience...........olfactorily speaking
The interesting question is whether cracking is more common now than 30 years ago.
The original helmets were large polystyrene foam with very few vents, even in the early 70's when Bell introduced the first hard shell, the foam later was about an inch thick. They all passed Snell B95, a standard that not a single helmet passes today!
As the helmet has "improved" the amount of absorbent material has been reduced for ventilation an styling.
The remaining material is thinner, and needs to be denser and harder thus reducing even further the amount of material absorbingthe energy.
Cycle helmets are unequivocally less safe than 30 years ago, and one wonders if the cracking is because a dense hard material cannot absorb energy by compressing, and hence cracks.
The original helmets were large polystyrene foam with very few vents, even in the early 70's when Bell introduced the first hard shell, the foam later was about an inch thick. They all passed Snell B95, a standard that not a single helmet passes today!
As the helmet has "improved" the amount of absorbent material has been reduced for ventilation an styling.
The remaining material is thinner, and needs to be denser and harder thus reducing even further the amount of material absorbingthe energy.
Cycle helmets are unequivocally less safe than 30 years ago, and one wonders if the cracking is because a dense hard material cannot absorb energy by compressing, and hence cracks.
Philip Osler
Active Member
one of the guys I cycle with now and again dropped his week old hat on some stones, that did a supprising amount of damage!
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