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helmets and rotational acceleration



 
 
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  #11  
Old August 11th 08, 06:00 PM posted to uk.rec.cycling
Phil Armstrong
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Posts: 74
Default helmets and rotational acceleration

Just zis Guy, you know? wrote:
On Mon, 11 Aug 2008 15:01:34 +0100, Phil Armstrong
said in
:

Having scanned the two papers (which are related: the physical testing
generates paramenter values for the finite element analysis) the main
conclusion appears to be that the coefficient of friction between the
headform (with anatomically correct scalp + hair) and the helmet is
too low to lead to rotational forces in a 'typical impact' which are
large enough to cause brain damage.


I think this is a bit of policy-based evidence making.

Dr Mills is a non-medical doctor with no relevant qualifications who
has given important (to one side) testimony in over 100 cases of
contributory negligence. So far as he is concerned, it appears that
helmets are always effective as the only factor of importance is the
1.5m a cyclist falls vertically when he's hit. That he might be
propelled forwards at the same time by a 50 mph car is totally
irrelevant.


Yes, the "typical impact" in the paper appears to be about the same as
the one used to test that a given helmet reaches the relevant British
Standard.

Phil

--
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  #12  
Old August 11th 08, 09:16 PM posted to uk.rec.cycling
Slark[_2_]
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Posts: 61
Default helmets and rotational acceleration

Just zis Guy, you know? wrote:
On Mon, 11 Aug 2008 15:01:34 +0100, Phil Armstrong
said in
:

Having scanned the two papers (which are related: the physical testing
generates paramenter values for the finite element analysis) the main
conclusion appears to be that the coefficient of friction between the
headform (with anatomically correct scalp + hair) and the helmet is
too low to lead to rotational forces in a 'typical impact' which are
large enough to cause brain damage.


I think this is a bit of policy-based evidence making.

Dr Mills is a non-medical doctor with no relevant qualifications who
has given important (to one side) testimony in over 100 cases of
contributory negligence. So far as he is concerned, it appears that
helmets are always effective as the only factor of importance is the
1.5m a cyclist falls vertically when he's hit. That he might be
propelled forwards at the same time by a 50 mph car is totally
irrelevant.

Brian Walker and John Franklin, plus a few solicitors, are currently
largely warding off such nonsense but it could be more difficult if
he's now coming up with 'proof' that helmets guard against oblique
impacts. Note that the 'proof' in this case is largely dependent on
the assumption that he is right.

Guy



Please see http://www.cyclecraft.co.uk/digest/liverpool.html for an
example of Dr Mills other contributions. Assuming it is the same 'Dr
Mills' then this would seem to point to a more balanced perspective.

Graham
  #13  
Old August 11th 08, 09:53 PM posted to uk.rec.cycling
Ian Smith
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Posts: 3,622
Default helmets and rotational acceleration

On Mon, 11 Aug 2008, Martin wrote:

To me the second one seems to say, that linear impacts forces were
not affected by the helmet rotating. They also investigated
rotational forces, but do not have the results in the abstract.

The first one says that extra rotational forces were negligible
when the rider hits the ground at 4m/s (about 9mph).


So, for a collision where you probably won't have life-changing injury
when bare-headed, and for which you probably won't have life-changing
injury in a pure normal impact, you're almost as likely not to have
life-changing injury if it's an oblique impact?

Maybe that's unfair, I'd still like to read the papers.

regards, Ian Smith
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  #14  
Old August 11th 08, 10:18 PM posted to uk.rec.cycling
Nigel Randell[_2_]
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Posts: 54
Default helmets and rotational acceleration

Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith
wrote:

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.


Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury
from wearing a helmet is slight or negligible.
========


I would say you shouldn't conclude anything from the abstracts of two
technical papers. Which is why I'd be keen to hear the views of
anyone that does have access to the content. If it was someone I
trust to be able to read straight, it might even influence my opinion.

regards, Ian SMith


I will ask whether any of my former colleagues have access to the full
papers

--

Nigel


  #15  
Old August 11th 08, 10:29 PM posted to uk.rec.cycling
Nigel Randell[_2_]
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Posts: 54
Default helmets and rotational acceleration

wrote:
On Aug 11, 1:41 pm, Ian Smith wrote:
The peak rotational accelerations, the order of 5 krad s-2 when the
tangential velocity component was 4 m s-1, were only slightly greater
than in comparable direct impact tests.


Am I reading that correctly? 5000 rad per second per second in an 8mph
crash?

Surely we must be talking microseconds - which I'd have thought is as
likely to be an artifact of the modelling as a real acceleration. In
particular, I don't see how it's possible for those sorts of
accelerations to be transferred to the head from the helmet - or even
from the scalp to the head.


AIUI this was from the test result.

Absolute peak values for acceleration (in test or analysis) need to be used
very carefully. The peaks will be influenced by the mounting of the
accelerometer, data sampling rate and, most importantly, the filtering of
the signal. Often a test trace will be so noisy as to obscure the actual
result but careful application of filtering can reveal the true picture
without losing too much detail.

You are right about the effect of the high peak being dependant on its
duration. The usual way of quoting the maximum acceleration in biomechanics
is to use the 3ms clip, that is the acceleration level exceeded for more
than 3ms. This tends to discard the very short duration, non-damaging high
peak values.


Oblique impact tests were
possible on the front lower edge of the helmet, a site commonly
struck in crashes, without the headform striking the 'road'. Data
characterizing the frictional response at the road/shell and
helmet/head interfaces, were generated for interpretation via FEA
modelling.

And here we seem to be talking about crashes where the head would miss
the ground other than for the helmet.

It certainly would be interesting to see the papers - although I
suspect I'm not qualified to critique them as FEA is something I've
never done.


I would really like to do something like this in FEA, but unfortunately I
have a day job.

--

Nigel


  #16  
Old August 12th 08, 08:38 AM posted to uk.rec.cycling
Peter Clinch
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Posts: 4,852
Default helmets and rotational acceleration

Ian Smith wrote:

Maybe that's unfair, I'd still like to read the papers.


I did send you copies of the text, assuming your stated email was
valid... if it isn't, mine is, let me know where to send.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/
  #17  
Old August 12th 08, 09:48 AM posted to uk.rec.cycling
Roger Merriman
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Posts: 2,108
Default helmets and rotational acceleration

Peter Clinch wrote:

Ian Smith wrote:
On Mon, 11 Aug, Tom Crispin wrote:
On 11 Aug 2008 12:41:47 GMT, Ian Smith wrote:


Can someone confirm my interpretation of this:
========
From these tests, any additional danger of rotational head injury from
wearing a helmet is slight or negligible.
========


I would say you shouldn't conclude anything from the abstracts of two
technical papers. Which is why I'd be keen to hear the views of
anyone that does have access to the content. If it was someone I
trust to be able to read straight, it might even influence my opinion.


I would concur with Ian's analysis.

But, for the sake of argument, let's assume your take is a fair one.
The possibility of rotational injury aggravation by helmets has
primarily been a conjectured mechanism for how they might make things
worse. Even if you demonstrate that as a non-issue, it will not affect
in any way, shape or form the whole population studies that demonstrate
no clear advantage in terms of serious injuries to wearing a cycle helmet.

Pete.


helmets don't seem to attaully do much, niether harm or protect. though
in my experance getting sweaty is something they do rather well.

roger
--
www.rogermerriman.com
  #18  
Old August 12th 08, 10:01 AM posted to uk.rec.cycling
David Martin
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Posts: 1,059
Default helmets and rotational acceleration

My reading of this is that the authors deliberately used tests below
the minimum required by EN whatever. They showed that even at these
low levels the rotational acceleration is sufficient to get to 50% of
that required for causing brain injury. They claim that because the
tests do not exceed the limits (due to being essentially at almost
stationary fall off speeds where DAI is unlikely to occur anyway) that
the criticisms of Curnow etc are invalid.

There is a lack of relating this work to the real world in terms of
crash speeds and likely outcomes.

...d


  #19  
Old August 12th 08, 10:36 AM posted to uk.rec.cycling
Just zis Guy, you know?
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Posts: 1,612
Default helmets and rotational acceleration

On Mon, 11 Aug 2008 21:16:52 +0100, Slark
said in :

Please see http://www.cyclecraft.co.uk/digest/liverpool.html for an
example of Dr Mills other contributions. Assuming it is the same 'Dr
Mills' then this would seem to point to a more balanced perspective.


Not sure, but he does appear to amend his evidence according to
which side employs him.

Guy
--
May contain traces of irony. Contents liable to settle after posting.
http://www.chapmancentral.co.uk

85% of helmet statistics are made up, 69% of them at CHS, Puget Sound
  #20  
Old August 12th 08, 01:15 PM posted to uk.rec.cycling
Rob Morley
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Posts: 7,173
Default helmets and rotational acceleration

On 11 Aug 2008 20:53:46 GMT
Ian Smith wrote:

So, for a collision where you probably won't have life-changing
injury when bare-headed, and for which you probably won't have
life-changing injury in a pure normal impact, you're almost as likely
not to have life-changing injury if it's an oblique impact?

That's my interpretation too - move along, nothing to see here ...

 




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