Think skull
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Wobblers
Euthermic
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- Minkowski Space
Adrian. I'm sure I've pointed out that you're a git before..
I have to admit, I was rather pleased to see the demise of That thread. And this, it was just a "smash and run" sarky comment. But no, you actually want me to sodding think. And do work. Git. It's not even as if you'll give me a sodding like either, is it? <insert "tongue-firmly-in-cheek" smiley here>
Wobblers
Euthermic
- Location
- Minkowski Space
I'm going to regret this, I'm sure, but [1]...
In that Other thread, @User9609 expressed some surprise at the difference between what a helmet is rated for and the energy needed to cause a skull fracture (500 Joules, on average). The skull can absorb ten times more impact energy than a EN1078 helmet. It shouldn't be that surprising when you think of it: bone is not expanded polystyrene - and skulls have had some 400 million years of evolution selecting for their qualities to absorb impact. Bone is a composite material. It is rigid thanks to crystals of calcium hydroxy-apatite, IIRC, all bound in a matrix of collagen fibres. Calcium hydroxy-apatite is brittle, the collagen serves in a crack stopping role to prevent those cracks from propagating. Thus much energy is needed to break all those little crystals and then yet more to shear the collagen fibres before you get a fracture. Expanded polystyrene doesn't have that complex microstructure so it can't be expected to be anything like as good in impact.
It's complicated, or, as a physicist would say "non-trivial". Firstly, it has to be pointed out that a cyclist's head has gravitational potential energy, which will get converted to kinetic energy should he or she fall off. Typically, that potential energy's about 60-90 J: more than the impact rating of a EN1078 helmet. Thus a helmet may not be adequate protection in even a stationary fall. Any kinetic energy will be added to that. But in most falls, the head won't bear the full force of the impact. But it is certainly possible to fall awkwardly - head first! It's rare, but there are the occasional fatality from a simple fall whilst walking. The best we can say is that the probability of enduring a severe head impact will rise with the speed. While a helmet may be expected to reduce that, given that the strength of the skull is so much greater than that of a helmet, any beneficial effect may be small. So small that confounding factors may eliminate it: one major confounding factor is the bulk of a helmet makes a head impact more likely, for instance. This may be why that whole population studies have failed to demonstrate any evidence of a beneficial effect with helmet use.
[1] And may Adrian endure having to fix punctures on the rear wheels of Bromptons forever whilst TEC-ing
In that Other thread, @User9609 expressed some surprise at the difference between what a helmet is rated for and the energy needed to cause a skull fracture (500 Joules, on average). The skull can absorb ten times more impact energy than a EN1078 helmet. It shouldn't be that surprising when you think of it: bone is not expanded polystyrene - and skulls have had some 400 million years of evolution selecting for their qualities to absorb impact. Bone is a composite material. It is rigid thanks to crystals of calcium hydroxy-apatite, IIRC, all bound in a matrix of collagen fibres. Calcium hydroxy-apatite is brittle, the collagen serves in a crack stopping role to prevent those cracks from propagating. Thus much energy is needed to break all those little crystals and then yet more to shear the collagen fibres before you get a fracture. Expanded polystyrene doesn't have that complex microstructure so it can't be expected to be anything like as good in impact.
It's complicated, or, as a physicist would say "non-trivial". Firstly, it has to be pointed out that a cyclist's head has gravitational potential energy, which will get converted to kinetic energy should he or she fall off. Typically, that potential energy's about 60-90 J: more than the impact rating of a EN1078 helmet. Thus a helmet may not be adequate protection in even a stationary fall. Any kinetic energy will be added to that. But in most falls, the head won't bear the full force of the impact. But it is certainly possible to fall awkwardly - head first! It's rare, but there are the occasional fatality from a simple fall whilst walking. The best we can say is that the probability of enduring a severe head impact will rise with the speed. While a helmet may be expected to reduce that, given that the strength of the skull is so much greater than that of a helmet, any beneficial effect may be small. So small that confounding factors may eliminate it: one major confounding factor is the bulk of a helmet makes a head impact more likely, for instance. This may be why that whole population studies have failed to demonstrate any evidence of a beneficial effect with helmet use.
[1] And may Adrian endure having to fix punctures on the rear wheels of Bromptons forever whilst TEC-ing
Wobblers
Euthermic
- Location
- Minkowski Space
<Snort!>
Wobblers
Euthermic
- Location
- Minkowski Space
But what if there isn't a ditch?
This is why the EN1078 needs to be revised to something that is not a laughing stock!
It is impossible to "test" a helmet for every single collision, accounting for angle and impact energy.
There is also controversy over design with the "Smoother rounder safer" campaign decrying the modern aero shape and "snag points"
All one has to go on is what the manufacturer's blurb claims (and doesn't tell you) and the test standards that it passes
These test differently, and a helmet that passes EN1078 will possibly fail when tested at a different angle, or energy by ANSI, or SNELL
FInally there is a consistent argument against modern design that is tackled by some not others.
The argument is that to make helmets lighter and more ventilated you remove material. This means that the layer of material absorbing the energy is thinner and less in volume, therefore less energy can be absorbed.
Finally with less material, the remaining part needs to be stiffer and denser to support the shape of the helmet or to insert stiff carbon cages or similar.
This again reduces the ability of the helmet to absorb impact
So in theory the original Bell helmets were a better bet in the protection stakes than those being sold 30 years later!
It is impossible to "test" a helmet for every single collision, accounting for angle and impact energy.
There is also controversy over design with the "Smoother rounder safer" campaign decrying the modern aero shape and "snag points"
All one has to go on is what the manufacturer's blurb claims (and doesn't tell you) and the test standards that it passes
These test differently, and a helmet that passes EN1078 will possibly fail when tested at a different angle, or energy by ANSI, or SNELL
FInally there is a consistent argument against modern design that is tackled by some not others.
The argument is that to make helmets lighter and more ventilated you remove material. This means that the layer of material absorbing the energy is thinner and less in volume, therefore less energy can be absorbed.
Finally with less material, the remaining part needs to be stiffer and denser to support the shape of the helmet or to insert stiff carbon cages or similar.
This again reduces the ability of the helmet to absorb impact
So in theory the original Bell helmets were a better bet in the protection stakes than those being sold 30 years later!
Profpointy
Legendary Member
[QUOTE 3023400, member: 9609"]Well actually I didn't, I think skulls and helmets are very different materials and have very different jobs and can't be compared that way - I will come to that later.
I was surprised at the high figure needed to fracture a skull (my steel toe capped boots are only rated at 250J) So I tried to understand what a 500J impact would be (the following took me a very long time to work out and I'm not confident of its accuracy or worth) What I come up with was a concrete building block (typical block in the building trade - weighs 3½ stone) dropped from 6 foot. On impact that would deliver about 500J of energy - far too much for any skull surely??
But really I don't understand any of that, the angle of the strike would also be a big factor. And how would the test be done, skull pressed against a solid surface then the block dropped, because if the person was for arguments sake swimming, the chance of damage would be different as the water under the head would take some of the energy.
So back to the helmet - a helmet is not only in addition to the skull, it creates a space for deceleration, The head coming to a stop in a 1/10 of a second is a lot better than it coming to a stop in 1/1000 of a second (which it might if the skull made a direct strike against a kerb stone). For instance hitting your unprotected head onto a grass surface could be similar to a helmeted head striking concrete. The grass and the polystyrene both offer a little give. The helmet has to be a good idea (even if I rarely chose to wear one)[/QUOTE]
Some good points there - I too was a bit sceptical of the 500J thing, 1kg from 10m height is going to be quite a wallop after all, though on the other hand fall head first from a couple of feet isn't far off the same energy (80kg x 10 x 3/4m). But as you say it's the impact (impulse or perhaps power even) which might be more to the point. Absorbing / dissipating 500J over half an hour is a different thing than over 1s or 10ms..
That said, your final line "helmet has to be a good idea" is very much begging the question (assuming what you are trying to prove) as Australian & Ontario experience post compulsion shows no improvement in safety overall - so assuming the must help some of the time, they seem to make things worse other times - at the very least make your head bigger so you're more likely to be hit in the first place. For the record, I am in the sceptic camp these days
I was surprised at the high figure needed to fracture a skull (my steel toe capped boots are only rated at 250J) So I tried to understand what a 500J impact would be (the following took me a very long time to work out and I'm not confident of its accuracy or worth) What I come up with was a concrete building block (typical block in the building trade - weighs 3½ stone) dropped from 6 foot. On impact that would deliver about 500J of energy - far too much for any skull surely??
But really I don't understand any of that, the angle of the strike would also be a big factor. And how would the test be done, skull pressed against a solid surface then the block dropped, because if the person was for arguments sake swimming, the chance of damage would be different as the water under the head would take some of the energy.
So back to the helmet - a helmet is not only in addition to the skull, it creates a space for deceleration, The head coming to a stop in a 1/10 of a second is a lot better than it coming to a stop in 1/1000 of a second (which it might if the skull made a direct strike against a kerb stone). For instance hitting your unprotected head onto a grass surface could be similar to a helmeted head striking concrete. The grass and the polystyrene both offer a little give. The helmet has to be a good idea (even if I rarely chose to wear one)[/QUOTE]
Some good points there - I too was a bit sceptical of the 500J thing, 1kg from 10m height is going to be quite a wallop after all, though on the other hand fall head first from a couple of feet isn't far off the same energy (80kg x 10 x 3/4m). But as you say it's the impact (impulse or perhaps power even) which might be more to the point. Absorbing / dissipating 500J over half an hour is a different thing than over 1s or 10ms..
That said, your final line "helmet has to be a good idea" is very much begging the question (assuming what you are trying to prove) as Australian & Ontario experience post compulsion shows no improvement in safety overall - so assuming the must help some of the time, they seem to make things worse other times - at the very least make your head bigger so you're more likely to be hit in the first place. For the record, I am in the sceptic camp these days
And, what?
Profpointy
Legendary Member
regarding bigger head - diameter of head - let's say 7" - add a helmet - let's say 10 - that's actually twice as bit in target area not 50% bigger in linear size. people forget this. This would be twice as many biffs even at a first approximation. could be much worse if "evolved to just miss head" is really true.
OP
OP
srw
It's a bit more complicated than that...
[QUOTE 3023400, member: 9609"]
I was surprised at the high figure needed to fracture a skull (my steel toe capped boots are only rated at 250J) So I tried to understand what a 500J impact would be (the following took me a very long time to work out and I'm not confident of its accuracy or worth) What I come up with was a concrete building block (typical block in the building trade - weighs 3½ stone) dropped from 6 foot. On impact that would deliver about 500J of energy - far too much for any skull surely??
[/QUOTE]
As @Profpointy has pointed out, if you were right then anyone who fell head-first off a low step would end up with a skull fracture.
I was surprised at the high figure needed to fracture a skull (my steel toe capped boots are only rated at 250J) So I tried to understand what a 500J impact would be (the following took me a very long time to work out and I'm not confident of its accuracy or worth) What I come up with was a concrete building block (typical block in the building trade - weighs 3½ stone) dropped from 6 foot. On impact that would deliver about 500J of energy - far too much for any skull surely??
[/QUOTE]
As @Profpointy has pointed out, if you were right then anyone who fell head-first off a low step would end up with a skull fracture.
They are a legal requirement unless it is "hot", you are in a built up area,
Profpointy
Legendary Member
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