Howard wrote:
> On May 15, 9:57 am, Marc Brett <[email protected]> wrote:
>> For all you doubting-Thomases who think helmets don't work in
>> real-world
>> collisions with vehicles.
>>
>> Bike helmet crushed, but head
>> finehttp://www.madison.com/tct/news/index.php?ntid=133934
>>
>> A white paneled delivery truck ran over a UW-Madison graduate
>> student's
>> head on Division Street Friday afternoon and, except for a
>> concussion,
>> he wasn't hurt.
>
> Hmmm, Just goes to show that human skulls, like eggshells, can be
> surprisingly strong. Can't say the same for the polystyrene hat he was
> weaing though, it looks like it just fell to bits. This is not
> surprising given all the holes in modern cycle helmets and the fact
> that they are only designed to absorb around 100 Joules of energy in
> an 'ideal world' crash situation. (And this is for 'Snell' certified
> helmets, a standard which is so 'high' that almost no helmets
> currently available meet it).
The standard for the flat anvil test part of B95-A is actually 110J.
The helmet isn't required to absorb this amount of energy during
certification testing. The test is described as "impact management" and the
idea is that the helmet remains in place and intact during a 110J impact and
keeps the peak linear acceleration of the headform below a value of 300g. A
very strong helmet shell may absorb plenty of energy but still impart
unacceptably high linear accelerations, a decent amount of controlled
crushing is required to keep the forces under control. While the test
standards are minimum requirements, it is most unlikely that a primary
design target of any helmet is that it fails catastrophically after
absorbing 111J.
>
> In a classic 'over the handlebars' crash where a 75 kg cyclist is
> doing only 15.6 Mph (7 m/sec) when a car pulls out and is hit by the
> cyclist who summersault forward and lands head first the kinetic
> energy carried by the airborne cyclist will be 1837 Joules, almost 17
> times the load imposed on the helmet when certified to Snell
> standards. If the helmet is the first point of impact it will simply
> bottom out (or, as often happens with a modern helmet, break up) and
> even if the helmet were to absorb all the energy it was designed to
> 94% of the energy developed in the crash must be dissipated by other
> means, which potentially includes dissipation via the riders skull,
> brain and body.
For the head of our unfortunate hypothetical cyclist to attract the whole
1837J he would have to possess a totally rigid body and impact a similarly
rigid obstacle with his centre of mass directly behind the impact point as
defined by his direction of travel - in zero gravity. In reality the body
of the cyclist behaves as a loosely connected mass/spring/damper system
which largely decouples (not in a literal sense, of course) the head from
the mass of the body. If we consider the head of the U.H.C. having to deal
with just its own kinetic energy we see that a typical 5kg effective head
mass travelling at 7m/s has a touch over 122J of the stuff - rather closer
to the Snell standard. Another thing to consider is that the 7m/s accident
velocity may not translate directly into the speed of impact with the rigid
obstacle, and if the impact velocity has a component tangential to the
obstacle's surface then the energy requirements are again reduced.
An aside: Formula 1 racing cars regularly have spectacular high speed
accidents which are rendered survivable by well designed energy management
structures to their front, side and rear. The amount of damage suffered by
the front nosecone in a ~160mph encounter with the tyre wall can be roughly
the same as that measured in approval testing, i.e. the amounts of energy
absorbed are in the same ballpark which shows that the test is a realistic
representation of what the structure might be called on to do in a real
crash. The test is carried out against a rigid wall at 35mph which suggests
that about 5% of the energy in the crash goes into the first point of
contact. Taking some liberties with the U.H.C. example and reading the
proportions across we see that his head whould have to absorb about 88J in
the impact. OK, I know that this is not scientific and proves nothing, but
it does illustrate the point I'm trying to make that the claims of enormous
energies required to be absorbed by the head, based on the K.E. of the whole
body, are rather misleading.
Given the fact that the skull is supposed to absorb 1000J before fracturing
it seems to be a wonder that skull fractures happen at all based on energy
considerations. Well, the clue is that no biomechanical injury predictors
for bone fracture use energy absorption as their measurement. The much
maligned, flawed (but still useful) H.I.C. uses head acceleration (force)
and time to predict the likelihood of skull fracture and other injury
predictions are made from forces, moments and deflections of bones and
joints.
It is obvious that much larger forces than even these
> may be generated should someone be struck by a large, unyielding motor
> vehicle travelling at speed, and this is without considering other
> such all important factors such as the creation of rotational forces
> within the brain in a violent road crash which may kill even when '
> direct impact' injuries such as skull fractures do not occur.
Of course, any safety system will reach the limit of its usefulness if too
much is asked of it. Plenty of people die in cars with seatbelts and
airbags.
>
> Do helmets genuinely 'save lives'? I have serious doubts myself...
Maybe, at the margins, although I suspect that anyone who emerges from their
accident relatively unscathed was not "saved" by their helmet, any more than
they were "saved" by their track mitts.
I think that we get too hung up on the supposed life saving properties of
helmets. Look again at the picture of Mr. Lipscomb and his helmet. The
helmet does appear to be badly cracked (although it's not really possible to
see the full extent of the damage from this one picture) but his head
appears to be unscathed. It the truck had run over his unprotected head,
and his skull not been damaged, I would expect to see rather more evidence
of the truck's passing on Mr. Lipscomb's person.
--
Nigel