Wilko wrote:

A helmet that is made of kevlar or some other composite material will
generally be stiffer, thereby spreading the force of the blow to your
head over a bigger area, and it will usually need (a lot) more force
before deforming. A plastic helmet is more likely to deform locally and
in that case it will pass the energy on the point of impact almost
directly to the skull below it.

This is a mistake most non engineers make. Composites absorb energy by
having the fibers break. If the impact isn't strong enough to break the
fibers, the impact in tranmitted into the lining. Plastic, whether ABS,
Poly or one of the newer polycarbonates, absorb energy by flexing,
which in reality spreads the force of the impact over a greater area
and seems to allow better absorbtion of the energy by the liner. While
there is nothing inherently wrong with using a composite as long as the
liner and fit are sufficient, plastic helmets actually start with an
advantage in absorbing impact. The real purpose of the shell is to
prevent physical trauma to the head, having a rock poke a big dent in
the skull, and even a Protec will do that much.

BTW, I took an old Protec Full Coverage that was old enough that I was
reluctant to trust it anymore and beat it with a large crowbar to see
if I could get it to break or even seriously deform. As you know, I'm a
rather large and stout guy, but I couldn't break it nor did I see if
ever deform enough to the point I thought it would contribute to an
injury.


A helmet that has a decent layer of foam inside will have more distance
between your skull and the rock and it might absorb more of the energy
of the impact (depending upon the kind of foam used).

This is what really seperates a good helmet from a poor one. The amount
and type of foam and how well that helmet fits your head is much more
important than the shell material. Sweets has one of the best foam
lining systems that I've seen, and the new WRSI helmet has the right
idea, although I think that is has several basic design flaws,
the biggest that it isn't full coverage.


A helmet shaped like a baseball cap might look very fashionable, but it
usually lacks adequate protection on the sides and back, as well as
adding a visor that will yank your head back if it hits on something.
For very little money you can add a flexible (velcro'd on) visor to
almost any helmet.


I'm a believer in full coverage helmets too. I might add that I've have
never heard of an injury that was the result of the bill on a helmet,
but a lot of this style helmet has so little side coverage that it may
be hard to tell if it was a factor. I've heard the same arguement about
a facemask, which both you and I wear, but I've never heard of an
incident with them either.

I've had five different helmets over the years, two Roemer, one Prijon
Corsica, a Shred Ready Full Mental Jacket and for the last five years or
so, I've used a Grateful Heads Dropzone helmet to which I added the
visor and a face guard.

My present helmet is a Predator Full Cut with a Hardnose, which I like
very well. I haven't got pounded into the river bottom with this one
yet, but it looks like it will handle the job fine. I still wear a
Wildwater Competition when I'm in the Culebra or a Raft.


Some pictures he http://kayaker.nl/tips.html#Helmets

HTH

--
Wilko van den Bergh wilkoa t)dse(d o tnl
Eindhoven The Netherlands Europe
---Look at the possibilities, don't worry about the limitations.---


Larry
http://kayaker.nl/