On Thu, 05 Feb 2004 19:05:02 GMT, Simon Brooke <[email hidden]>
Quoted message said:Quoted message said:That would imply they have no effect on the response of the head in an impact. That seems
unlikely.
Quoted message said:I agree with you. I just can't buy that. We can clearly see ways in which a helmet must change the
physics of the impact. They do not do 'bugger all'. Consequently we have to conclude from the
overall statistics that they do at least as much actual harm as good.
from <url:http://www.cyclinghealth.org.nz/faq.html>
4. What kind of protection does a helmet provide?
Helmets are tested in the lab for straight line (linear) blows only. Test procedures set by
standards bodies like Snell, ANSI, and CPSC require a helmet containing a 5kg (11lbs) rigid headform
to be dropped onto a flat anvil from a height of 1.5 to 2.0 metres (5ft to 6ft 8in). If more than
300g's is imparted to the headform the helmet cannot be certified.
The helmet's outer shell is designed to mitigate the effects of an impact by spreading the force
over a greater area of the head and by reducing friction in a slide. The helmet's liner, however, is
made of stiff foam and requires a certain minimum force before it starts to crush. Until this
minimum is reached, the head must absorb the impact. Once the minimum is reached, the helmet's liner
absorbs energy until either all the remaining energy is absorbed or the liner has been crushed to
its minimum thickness. If the blow is of such severity that the liner gets crushed to its minimum
thickness, remaining energy is absorbed by the head and is likely to be enough to be lethal.
This means for a fall, whilst helmets are designed to limit the impact (deceleration) to a just sub-
lethal level, they won't reduce it much below that. In these cases, "sub-lethal" translates into
anything from a very bad concussion to a coma. Any impact of greater severity than received in a
typical fall will be lethal.
The medical profession now believes that even lesser accelerations can produce serious injury and
that the 300g level is too high. However, it is unlikely that helmet standards will be raised to
provide significant protection because the industry doesn't believe that consumers would buy the
resulting products.
The trend is in the opposite direction. In Australia, the standard was actually lowered because
helmets produced under the old standard did not meet with market acceptance. Manufacturers are
presently responding to market demand for helmets which improve air-flow inside the helmet and to
fashion by manufacturing helmets with more holes them. While these pass standard tests, they spread
impacts over a smaller area of the head, so when an impact occurs it will be more concentrated
around the center of the impact."
Guy
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May contain traces of irony. Contents liable to settle after posting.
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