From: Derek C on
On May 22, 4:10 pm, Sniper <Sniper8...(a)yahoo.co.uk> wrote:

Looking at it another way, wearing a cyclehelmet increases the
distance your head has to fall to cause a skull fracture by a factor
of at least four (source TRL). Worth having I believe!

Derek C

> None of which would alter the basic premise of my general statement...
>
> Had the forces involved been sufficient to cause a skull fracture then
> those forces would undoubtedly have caused a complete failure of the
> cycle helmet.. this being a certainty given the structural failure
> requirements of cycle helmets.
>
> If as you state the helmet did not fail then it is also proven that the
> forces involved were insufficient to cause significant head trauma: by
> which I mean a fracture to the skull.
>
> Ergo: the helmet did not mediate the danger of a fractured skull but may
> have reduced the incidence of minor facial and dermatological injury.
>
> FYI compression fracture resistance of the human skull is 170 MPa
> (Cameron, et al.) or 24656.41 psi.
>
> A cycle helmet produced to the current standard (EN 1078:1997)(BS EN
> 1078:1997) is designed to mitigate impacts by decelerating the human
> brain within a deformable shell e.g. polystyrene foam.  In order to keep
> deceleration below 300g this representing the maximum deceleration that
> can be applied to the human head without causing brain injury the
> deformability of the foam insert cannot rise above 0.9MPa or 130.53 psi.
>
> If one allows that an impact had the potential to cause death by
> compression fracture the spent energy absorbed by the wearing of a
> helmet(assuming it did not fail)could be no more than 130.5 psi leaving
> some 24525.88 psi to be dissipated by the skull before a compression
> fracture is likely to occur.
>
> The question one might then legitimately ask is: is there any likelihood
> that an absorption potential of 130.5 psi will make any significant
> contribution to the saving of a life where the impact force required to
> cause a fracture so far exceeds the capability of the helmet structure
> to mitigate that force?
>
> I would suggest it is highly unlikely.
>
> One might also argue that a fractured skull in itself does not
> necessarily result in death so that even in an unprotected head the
> likelihood of brain injury is not a certainty provided that the
> deceleration applied does not rise above 300g.  Particularly if one
> notes the human skull will distribute forces below the fracture
> potential load over the surface area of the skull where as the cycle
> helmet will not as it must by design fail at 130.53 psi.
>
> Sniper.
>
> On 22/05/2010 12:39, Derek C wrote:
>
>
>
> > On 22 May, 12:06, Sniper<Sniper8...(a)yahoo.co.uk>  wrote:
> >> The best that can be said of cycle helmets is they may save some minor
> >> injury...
>
> >> The worst that can be said of cycle helmets is that they may cause some
> >> avoidable fatalities...
>
> >> The assertion that a cycle helmet may or will save your life is
> >> undoubtedly a fallacy based in poor understanding of basic principles of
> >> physics and engineering.
>
> >> A cycle helmet, when smashed to bits, is frequently described as 'having
> >> absorbed the impact' or having 'done its job' Whereas in truth it has
> >> simply failed... it failed to absorb any impact as the load applied
> >> exceeded its capability to transfer that load across its surface hence
> >> it failed.
>
> >> A motorcycle helmet exhibiting a similar result would not be passed as
> >> fit for sale... in a collision, even with a head on impact a motorcycle
> >> helmet is expected not to fail e.g. be reduced to a broken shell.
>
> >> Ergo: a cycle helmet in pieces has failed...
>
> >> Ergo: a cycle helmet that will not transfer loads without failure above
> >> the fracture frequency load currently in use is unlikely to save one's
> >> life in a cycle/motor vehicle collision: it may dissipate a bump to the
> >> head and save minor injury in a 'non direct impact' fall, e.g. where
> >> impact of the head is secondary: but, where the forces involved are
> >> sufficient to cause death in a non helmet wearing cyclist then the
> >> wearing of a cycle helmet will not alter that outcome until cycle
> >> helmets are sufficiently strong enough to withstand the forces applied
> >> to them during 'direct impact - fatal load' collisions... at which point
> >> a new series of fatality questions might possibly be presented.
>
> >> Going back to my law degree I leave you to your musings...
>
> >> Sniper:
>
> > The helmet in the example given had not split or failed in any way. It
> > was slightly dented at the front and some bits of foam had been gouged
> > out to the depth of a few millmetres, presumable as a result of
> > hitting a paved or tarmaced surface. Cycle helmets are essentially one
> > shot devices that should be thrown away after any significant impacts.
> > Actually they are two shot to absorb initial and secondary impacts.
>
> > Motor cycle helmets would be much too heavy and poorly ventilated for
> > cycling use, and also impair your hearing which is a good defence
> > mechanism for cyclists who can hear vehicles coming (except for
> > electric hybrids). Heavy helmets increase the risk of a neck injury in
> > high g decelerations, which is why racing drivers wear those helmet
> > restraint devices on their shoulders.
>
> > Derek C- Hide quoted text -
>
> - Show quoted text -

From: Tony Raven on
Derek C wrote:
>
> Looking at it another way, wearing a cyclehelmet increases the
> distance your head has to fall to cause a skull fracture by a factor
> of at least four (source TRL). Worth having I believe!
>

Another piece of junk science from that report given meaning by you far
beyond what is claimed.

What TRL claim, and all that they claim, is an un-helmeted head will
suffer a skull fracture in a fall from >15" (376mm) onto a flat surface
and that with a helmet it will be 5ft (1.5m) (actually the best case
from the 8 helmets they tested).

Now if you really believe, as they do, that hitting your head in a drop
from 15" is going to fracture your skull then feel free to wear a helmet.

--
Tony

" I would never die for my beliefs because I might be wrong."
Bertrand Russell
From: Ben C on
On 2010-05-23, Tony Raven <traven(a)gotadsl.co.uk> wrote:
[...]
> What TRL claim, and all that they claim, is an un-helmeted head will
> suffer a skull fracture in a fall from >15" (376mm) onto a flat surface
> and that with a helmet it will be 5ft (1.5m) (actually the best case
> from the 8 helmets they tested).
>
> Now if you really believe, as they do, that hitting your head in a drop
> from 15" is going to fracture your skull then feel free to wear a helmet.

What sort of drop are we talking? A scenario like you lift someone
upside down by their feet to a height of 376mm above a concrete floor
and then let go?

I reckon that would kill most people, although I haven't tried it.
From: Tony Raven on
Ben C wrote:
>
> What sort of drop are we talking? A scenario like you lift someone
> upside down by their feet to a height of 376mm above a concrete floor
> and then let go?
>
> I reckon that would kill most people, although I haven't tried it.

No because the tests assume there is no body attached to the head, just
the head and the helmet being dropped. So its equivalent to holding
your body horizontally 15" above the floor and letting it drop. If you
want to do your test of holding them by their feet and dropping them,
assuming a 4kg head mass and average 70kg body weight, you would drop
them from 21mm above the concrete floor to get the same effect

--
Tony

" I would never die for my beliefs because I might be wrong."
Bertrand Russell
From: Ben C on
On 2010-05-23, Tony Raven <traven(a)gotadsl.co.uk> wrote:
> Ben C wrote:
>>
>> What sort of drop are we talking? A scenario like you lift someone
>> upside down by their feet to a height of 376mm above a concrete floor
>> and then let go?
>>
>> I reckon that would kill most people, although I haven't tried it.
>
> No because the tests assume there is no body attached to the head, just
> the head and the helmet being dropped.

OK, although if your body's not attached to your head you're probably
going to suffer from a few other complications.

> So its equivalent to holding your body horizontally 15" above the
> floor and letting it drop.

Not exactly I don't think.

There are two main reasons why holding a whole person up by their feet
is different to just dropping their head on its own: (a) their centre of
mass will be much higher than 376mm (but only a bit higher in the case
of the severed head), and (b) the whole person weighs more than just the
head.

Dropping them from a horizontal orientation fixes (a) but not (b) (and
there are some other complications since the neck is not rigid).

> If you want to do your test of holding them by their feet and dropping
> them, assuming a 4kg head mass and average 70kg body weight, you would
> drop them from 21mm above the concrete floor to get the same effect

Sort of, but that only solves problem (b) and not (a). So it would be
equivalent to quite a bit less than 21mm.