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 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
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
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.)
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
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.
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.
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/
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
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. :-)
