helmet subtext

Adrian Godwin said:

Ok, so I suppose I'm just slow ..

I have a Specialised helmet sitting in its box in the garage (it came
free with the bike, and no I wasn't allowed to have anything else
instead - I asked). I thought it might come in handy one day.

I see that it is made in accordance with their <em>PRO</em>fit
'retention system'. Professional fit, presumably. Or perhaps a
freudian slip.

On a related note, and since the other thread has long since gone beyond
reasonable debate, can someone point me to any documentation on what happens
when a helmet cracks ? Cyclehelmets.org suggest that energy is not absorbed
by the helmet and this seems largely reasonable. But it's the most common
reason for insisting they work ("my helmet cracked but it saved me"😉 and
features heavily in helmets.org's list of personal tributes. So I'd
really like to read a paper on just how much energy is absorbed /
elastically recovered / transmitted when a helmet is used well above
the magic 15mph threshold, even if we don't know for sure how the
head is affected by the remaining energy.

I don't know about the particular values for a direct head impact but
a material cracking absorbs virtually no energy whereas a material
being crushed can absorb rather a lot. A good introduction to
materials and their failure modes can be found in

http://www.amazon.co.uk/Science-Materials-Through-Princeton-Library/dp/0691125481/ref=sr_1_2/202-3794026-8621443?ie=UTF8&s=books&qid=1177619624&sr=8-2

Chris
--
Chris Eilbeck

Chris Eilbeck said:

Adrian Godwin said:

Ok, so I suppose I'm just slow ..

Quoted message said:

I have a Specialised helmet sitting in its box in the garage (it came
free with the bike, and no I wasn't allowed to have anything else
instead - I asked). I thought it might come in handy one day.

Quoted message said:

I see that it is made in accordance with their <em>PRO</em>fit
'retention system'. Professional fit, presumably. Or perhaps a
freudian slip.

Quoted message said:

On a related note, and since the other thread has long since gone beyond
reasonable debate, can someone point me to any documentation on what happens
when a helmet cracks ? Cyclehelmets.org suggest that energy is not absorbed
by the helmet and this seems largely reasonable. But it's the most common
reason for insisting they work ("my helmet cracked but it saved me"😉 and
features heavily in helmets.org's list of personal tributes. So I'd
really like to read a paper on just how much energy is absorbed /
elastically recovered / transmitted when a helmet is used well above
the magic 15mph threshold, even if we don't know for sure how the
head is affected by the remaining energy.

I don't know about the particular values for a direct head impact but
a material cracking absorbs virtually no energy whereas a material
being crushed can absorb rather a lot. A good introduction to
materials and their failure modes can be found in

http://www.amazon.co.uk/Science-Materials-Through-Princeton-Library/d...

Of course it is possible to absorb energy both by crushing and
cracking. Just cracking doesn't absorb much energy. Cracking is not
indication of a helmet failing to crush. But it is not indicative on
its own of a large impact. (I can readily crack a helmet in my hands,
I can't do that to a skull.)

...d

David Martin wrote on 26/04/2007 23:24 +0100:

Quoted message said:

Of course it is possible to absorb energy both by crushing and
cracking. Just cracking doesn't absorb much energy. Cracking is not
indication of a helmet failing to crush. But it is not indicative on
its own of a large impact. (I can readily crack a helmet in my hands,
I can't do that to a skull.)

Get a reasonable sized piece of 1" thick polystyrene foam sheet. Try to
crush part of it to half its original thickness. Now hold it in your
hands and try to snap it. Which requires more effort?

--
Tony

"The most savage controversies are those about matters as to which there
is no good evidence either way."
- Bertrand Russell

Chris Eilbeck said:

I don't know about the particular values for a direct head impact but
a material cracking absorbs virtually no energy whereas a material
being crushed can absorb rather a lot. A good introduction to
materials and their failure modes can be found in

Yes, I appreciate it's generally true, which is why the usual response
makes sense. But if helmets are only characterised up to a limited
speed of impact, someone must have done accelerometer tests to determine
that they meet 12 or 15 mph (and don't crack at 5mph, for instance).

David's point that they can crack under much lower non-impact
stresses applied in the 'wrong' direction (unsupported bend in
particular) is also good. This could easily crack the side off a
a helmet in a light glancing blow.

But I'd like to see an actual measurement of what happens to the head
in the straightforward (and perhaps oversimplified) case of a straight
fall onto a test anvil, as the speed approaches and exceeds the point
where it cracks. This ought to show some absorption due to
crushing. At higher speeds (and I'd like to know how high) the
absorption due to crushing will not only become less significant as
the total energy grows, but may also be reduced if the rate at which
the foam is able to crush may be limited : i.e. the shock wave from
the initial impact might travel through the material faster than it
can crush, resulting in no attenuation at all.

-adrian

Adrian Godwin said:

But I'd like to see an actual measurement of what happens to the head
in the straightforward (and perhaps oversimplified) case of a straight
fall onto a test anvil, as the speed approaches and exceeds the point
where it cracks.

If someone's done this it's likely to be Head Protection Evaluation, who
do the UK's testing against standards.

See http://www.hpe.eu.com/

I see there's a contact form there, you could try asking directly.

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 [email hidden] http://www.dundee.ac.uk/~pjclinch/

Adrian Godwin said:

Ok, so I suppose I'm just slow ..

I have a Specialised helmet sitting in its box in the garage (it came
free with the bike, and no I wasn't allowed to have anything else
instead - I asked). I thought it might come in handy one day.

I see that it is made in accordance with their <em>PRO</em>fit
'retention system'. Professional fit, presumably. Or perhaps a
freudian slip.

On a related note, and since the other thread has long since gone beyond
reasonable debate, can someone point me to any documentation on what happens
when a helmet cracks ? Cyclehelmets.org suggest that energy is not absorbed
by the helmet and this seems largely reasonable. But it's the most common
reason for insisting they work ("my helmet cracked but it saved me"😉 and
features heavily in helmets.org's list of personal tributes. So I'd
really like to read a paper on just how much energy is absorbed /
elastically recovered / transmitted when a helmet is used well above
the magic 15mph threshold, even if we don't know for sure how the
head is affected by the remaining energy.

-adrian

Perhaps this will help

Mills, N. J., Protective capability of bicycle helmets. Br J Sports
Med, 1990, 24, 55-60

John Kane, Kingston ON Canada

In article <[email hidden]>, Adrian Godwin
[email hidden] says...
<snip>

Quoted message said:

On a related note, and since the other thread has long since gone beyond
reasonable debate, can someone point me to any documentation on what happens
when a helmet cracks ? Cyclehelmets.org suggest that energy is not absorbed
by the helmet and this seems largely reasonable. But it's the most common
reason for insisting they work ("my helmet cracked but it saved me"😉

Maybe we should fill our pockets with eggs to prevent other injuries.
:-)