Re: My helmet saved me, and broke
I submit that on or about Sat, 27 Aug 2005 10:06:59 -0500, the person
known to the court as “steveyo”
<steveyo@NoEmail.Message.Poster.at.Unicyclist.com> made a statement
(<steveyo.1ufhrd@NoEmail.Message.Poster.at.Unicyclist.com> in Your
Honour’s bundle) to the following effect:
>With all due respect, and perhaps more so, you’re wrong.
Please point out where in the standards it says they are supposed to
fracture.
>Bicycle helmets are designed to protect one’s head from a SINGLE impact,
>and it did. That’s a success, not a failure.
They are designed to do so without breaking.
>It absorbed a huge amount of energy, not “very little”. If it had
>absorbed “very little” energy, I’d be in the hospital right now awaiting
>facial surgery, or worse.
Here is what is supposed to happen:
The foam liner is supposed to crush, reducing the linear acceleration
of the head. There are standards for the amount of energy they are
supposed to absorb without breaking, and these are equivalent to a
fall from about 1.5m onto a flat surface, discounting the weight of
the body.
There are two main failure modes: one is that the foam is compressed
beyond its capacity, in which case it has absorbed as much energy as
it can, and any further energy is transferred to the head. The other
is that it breaks, in which case an unknown proportion of the design
energy is absorbed beforehand. Think here about a sheet of glass. You
can put something quite heavy on a sheet of glass and it is fine, but
drop a much lighter weight from even a few inches and the glass
shatters. If the energy rating of the foam is exceeded, it can fail
almost instantaneously. You can verify this for yourself using some
blocks cut form computer packaging (the same stuff, essentially).
So: we do not actually know what proportion of the design energy was
absorbed before the helmet broke. Some, sure. But not as much as it
was supposed to. It did not work as designed.
In my view the chances of falling are sufficiently high with
unicycling, and the direction of falling so unpredictable, that a
standard bicycle helmet is not a good choice for yiking. I use (when
I do) an old Bell lid which covers the back of my head much better
than my modern bike helmet. BMX helmets are probably the best choice
for unicyclists.
Sorry, I’m really not trying to be contentious here (honest!); it’s
just that “my broken helmet worked like a dream” is a bit of a red rag
for me. I don’t think we do ourselves any favours by deluding
ourselves on these matters. When you replace the lid, I strongly
suggest a hard shell, preferably Snell certified.
And - I am ashamed to say - I forgot to say I’m glad you;re fine. I
am, truly. I have had a couple of moderately serious head and face
injuries in my life, and they are no fun.
Guy
http://www.chapmancentral.co.uk
“To every complex problem there is a solution which is
simple, neat and wrong” - HL Mencken
