Helmet ******s

On Sun, 08 Feb 2004 21:37:59 +0000, "Just zis Guy, you know?"
wrote:

Interesting. I was recently told that the mechanism by which helmets
absorb energy is plastic deformation, which is why the Snell standards
specify the helmet must not break. Apparently if a helmet breaks this
is an indication that it has moved from plastic deformation to brittle
failure, a mode in which it absorbs virtually no energy. So all those
cracked helmets which "saved people's lives" actually simply failed!

Guy, do you know of a clear reference to this? IOW a concise
authoritative statement that a shattered helmet is a failed helmet?

I've come across plenty of photos of broken or chipped helmets along
with descriptions of how the helmet saved someone's life. If the
helmets actually failed that suggests that not only did they not save
the lives in question, but that they failed in a relatively trivial
incident.

--
Dave...

Get a bicycle. You will not regret it. If you live. - Mark Twain

In article , junk@raven-
family.com says...
David Kerber wrote:

That's an interesting study, but based on their descriptions, I'm not
sure it applies to most bicycle helmets that people actually wear. They
didn't describe in much detail what a "non-shell" helmet is, which they
noted was the only one which gave significant rotational force to the
head. Would that be the leather style ones which you used to see on
racers? Their description of "ribbed hard-shell" helmets seems to be
consistent with the description of the ones most riders wear these days.

Hardshell is one like a motorbike helmet, microshell is what most of of are
used to with the thin glossy plastic outer layer and non-shell is the old
style bare polystyrene. Another interesting paper is

For hard shell, I was also thinking of the ones BMX riders wear. I
don't remember ever seeing a bare polystyrene helmet. The slick outer
covering on current helmet designs seems unlikely to "catch" on pavement
unless some kind of object (sewer grate, curb edge, car mirror?) grabs
one of the ventilation holes.

http://www.bhsi.org/hodgstud.htm. They say the 4500r/s/s is not exceeded on
any of the helmets but also their maximum speed is 6.4mph. If you look at the
traces near the end they are not that much below the limit to consider you
would stay within the limits at not much higher speeds. There is no control
data though on the bare human head. Its also noticeable that the vented
helmets they show have virtually no vents compared with today. It could do
with an updated study with current helmet designs.

Sounds like it.

--
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"Dave Kahn" wrote in message
...

I was recently told that the mechanism by which helmets
absorb energy is plastic deformation, which is why the Snell standards
specify the helmet must not break. Apparently if a helmet breaks this
is an indication that it has moved from plastic deformation to brittle
failure, a mode in which it absorbs virtually no energy. So all those
cracked helmets which "saved people's lives" actually simply failed!

Guy, do you know of a clear reference to this? IOW a concise
authoritative statement that a shattered helmet is a failed helmet?

I am trying to get one. The source was not a public one, but there must be
some public-domain references I can quote. My contacts have been, er,
contacted. Like I said, it is something I was only recently told.

--
Guy
===

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"Nick Maclaren" wrote in message
...
In article ,
David Kerber wrote:

Do you have any cites for the claim that helmets "amplify the rotational
component" of a head impact to any significant degree? I don't need to
see them for the fact that rotational acclerations are more damaging to
the brain; that is well-known, and has been for many years.

I haven't seen any analyses of experimental data, though I have heard
that there may have been a little for motorcyclists and/or horse
riders. It is, however, immediate from the physics involved that they
are very LIKELY to do that.

Most accidents involving reasonably cautious cyclists have the cyclist
coming off sideways - even being hit from behind at a fairly low
relative speed will do that. If someone comes off sideways, the impact
is on hip, shoulder and sometimes knee and elbow. Because a helmet
increases both the width and the moment of the head by a significant
factor, it is very likely to cause head/helmet contact where it would
not otherwise have happened. Q.E.D.

Only by about 20% and there is a far less friction between shell/road and
head/road.
So not QED.

David Kerber wrote:

Do you have any cites for the claim that helmets "amplify the rotational
component" of a head impact to any significant degree?

2PiR ?


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Apparently if a helmet breaks this
is an indication that it has moved from plastic deformation to brittle
failure, a mode in which it absorbs virtually no energy. So all those
cracked helmets which "saved people's lives" actually simply failed!


This "conclusion" conveniently ignores the energy expended during the
plastic deformation phase prior to the breakup. The aim would be to absorb
as much energy as possible b4 breakup - if the energy level exceeds that of
course it WILL break. It has still reduced the impact energy on the head.

One might as well suggest that since your suspension "bottoms out" on really
big bumps once in a while, you might as well have no suspension at all.....

I used to avoid wearing a helmet, but my intelligence finally overtook my
ego.

"Steve" wrote in message
...

I used to avoid wearing a helmet, but my intelligence finally overtook my
ego.

um, no.

On Tue, 10 Feb 2004 13:01:44 +1100, Steve wrote:
Apparently if a helmet breaks this
is an indication that it has moved from plastic deformation to brittle
failure, a mode in which it absorbs virtually no energy. So all those
cracked helmets which "saved people's lives" actually simply failed!


This "conclusion" conveniently ignores the energy expended during the
plastic deformation phase prior to the breakup. The aim would be to absorb
as much energy as possible b4 breakup - if the energy level exceeds that of
course it WILL break. It has still reduced the impact energy on the head.

No, because had there been any energy in _PLASTIC_ deformation, the
bits picked up afterwards would be crushed, and the the anecdotes
typically describe helmets cracked _without_ significant plastic
distortion.

regards, Ian SMith
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"Steve" wrote in message
...
Apparently if a helmet breaks this
is an indication that it has moved from plastic deformation to brittle
failure, a mode in which it absorbs virtually no energy. So all those
cracked helmets which "saved people's lives" actually simply failed!

This "conclusion" conveniently ignores the energy expended during the
plastic deformation phase prior to the breakup.

Maybe you missed the part above where it said "brittle failure, a mode in
which it absorbs virtually no energy." If so you will note that breaking a
helmet doesn't absorb very much energy. This is a COMMON mode of helmet
failure and contrary to your visualization, it doesn't absorb very much
energy BEFORE it starts to break up.

Think about this - when a helmet is working under perfect conditions it
hardly works at all. Reducing it's ability by 60 or 70 percent sure as hell
isn't going to improve matters even when you do mention that it is better
than nothing.

I used to avoid wearing a helmet, but my intelligence finally overtook my
ego.

It ain't a matter of ego. I suggest that you're the one exhibiting ego if
you think that your choice to wear a helmet is any better, intelligent or
more effective than the guy who chooses otherwise.

That's the whole point here. Maybe there's some small collection of accident
types in which helmets offer an effective solution to saving your head from
getting knocked around. But there is pretty obviously another spectrum of
accidents in which wearing a helmet leads to more accidents, more serious
accidents or redirects one type of accident into another type of accident in
which a person is injured or killed. The statistics are pretty plain that
helmet wearing doesn't change the numbers or severity of head injuries.

That's the whole point here. Maybe there's some small collection of accident
types in which helmets offer an effective solution to saving your head from
getting knocked around.

A large collection - they'll be oodles and scroodles of minor knocks and
headbutts that the helmet helps with. Only problem is that these don't produce
KSIs, which is what the legislation is seeking to prevent.


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