helmets and rotational acceleration

Two papers available online from today, which appear to address one
of the significant uncertainties regarding helmet performance.
They also render some previous comments ('no testing ever looks at
oblique impacts') untrue, which might require minor revisions of some
web sites (Guy?).

I'd need to pay for them (so have only read the abstracts), but
others with free-er access to these sorts of things might be
interested:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

Abstract

The performance of bicycle helmets was investigated in oblique impacts
with a simulated road surface. The linear and rotational accelerations
of a headform, fitted with a compliant scalp and a wig, were measured.
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. 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.



Finite-element analysis of bicycle helmet oblique impacts

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1087-1101

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006

Abstract

Finite-element analysis (FEA) was performed for bicycle helmets making
oblique impacts with a road surface, to evaluate the linear and
rotational accelerations of the headform. Helmet rotation on the head
was considered, modelling the helmet and retention strap interactions
with the headform. The effects of frictional parameters on the
response were explored, and parameters selected to reproduce
experimental results. Predictions were made for two helmets, for a
range of impact locations and tangential velocities. The design method
for the peak headform linear acceleration was confirmed; it was hardly
affected by the tangential component of the impact velocity. The peak
headform rotational acceleration was investigated as a function of the
helmet geometry, impact sites and velocities and the contributing
mechanisms established.


regards, Ian SMith

--
|\ /| no .sig
|o o|
|/ \|

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.
They also render some previous comments ('no testing ever looks at
oblique impacts') untrue, which might require minor revisions of some
web sites (Guy?).

I'd need to pay for them (so have only read the abstracts), but
others with free-er access to these sorts of things might be
interested:

Oblique impact testing of bicycle helmets

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1075-1086

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.005

Abstract

The performance of bicycle helmets was investigated in oblique impacts
with a simulated road surface. The linear and rotational accelerations
of a headform, fitted with a compliant scalp and a wig, were measured.
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. 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.



Finite-element analysis of bicycle helmet oblique impacts

International Journal of Impact Engineering, Volume 35, Issue 9,
September 2008, Pages 1087-1101

N.J. Mills, A. Gilchrist

http://dx.doi.org/10.1016/j.ijimpeng.2007.05.006

Abstract

Finite-element analysis (FEA) was performed for bicycle helmets making
oblique impacts with a road surface, to evaluate the linear and
rotational accelerations of the headform. Helmet rotation on the head
was considered, modelling the helmet and retention strap interactions
with the headform. The effects of frictional parameters on the
response were explored, and parameters selected to reproduce
experimental results. Predictions were made for two helmets, for a
range of impact locations and tangential velocities. The design method
for the peak headform linear acceleration was confirmed; it was hardly
affected by the tangential component of the impact velocity. The peak
headform rotational acceleration was investigated as a function of the
helmet geometry, impact sites and velocities and the contributing
mechanisms established.

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.
========

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
--
|\ /| no .sig
|o o|
|/ \|

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.
--
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/

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

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

"Peter Clinch" wrote in message
...
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.


As I recall from my review of the literature, the 'helmets actually do
aggravate rotational injuries' argument seems to be have originated from
conjecture from A&E staff. I could be wrong, but it rings a bell.

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.

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 don't pretend to be either a materials scientist or somebody with
experience in analysing this kind of paper, so I'm not going to
critique their methodology. It seemed perfectly reasonable to me
froman 'outsider' perspective however.

They acknowledge that answering the question of whether the headform
stiffness makes a real difference to the measured outcomes will
require experiments with cadaver heads. Any volunteers?

Phil

--
http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt

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
--
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

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

--
http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt