A motorcycle helmet
has two major parts: the outer shell and the energy absorbing inner
liner. The inner lining is made of expanded polystyrene or EPS, the same
stuff used in beer coolers, foam coffee cups, and packing material.
Outer shells come in two basic flavors: a resin/fiber composite, such as
fiberglass, carbon fiber and Kevlar, or a molded thermoplastic such as
ABS or polycarbonate, the same basic stuff used in face shields and F-16
canopies.
The shell is there for a number of reasons. First,
it's supposed to protect against pointy things trying to penetrate the
EPS-though that almost never happens in a real accident. Second, the
shell protects against abrasion, which is a good thing when you're
sliding into the chicane at Daytona. Third, it gives Troy Lee a nice,
smooth surface to paint dragons on. Riders and helmet marketers pay a
lot of attention to the outer shell and its material. But the part of
the helmet that absorbs most of the energy in a crash is actually the
inner liner.
When a helmet hits the road or a curb, the
outer shell stops instantly. Inside, your head keeps going until it
collides with the liner. When this happens, the liner's job is to bring
the head to a gentle stop-if you want your brain to keep working like it
does now, that is.
The great thing about EPS is that as it
crushes, it absorbs lots of energy at a predictable rate. It doesn't
store energy and rebound like a spring, which would be a bad thing
because your head would bounce back up, shaking your brain not just
once, but twice. EPS actually absorbs the kinetic energy of your moving
head, creating a very small amount of heat as the foam collapses.
The
helmet's shell also absorbs energy as it flexes in the case of a
polycarbonate helmet, or flexes, crushes and delaminates in the case of a
fiberglass composite helmet.
Almost everybody designing
serious helmets seems to know exactly how to get what they want-the only
variable is deciding what they want. And for the most part, the
standards make that decision for them, not flashes of genius on the
parts of the helmet designers themselves.
All the helmets
we tested performed exactly as the standards they were designed to meet
predicted. And they seemed to exceed those standards-that is, the DOT
-only helmets were better at high-energy impacts than they had to be
just to pass the DOT standard, and the Snell helmets were better at
absorbing low energy impacts than they had to be to pass DOT or Snell.
So choosing a helmet, at least in terms of safety, is not a question of
choosing high or low quality, it's one of choosing what degree of
stiffness you prefer, finding a helmet in that range by choosing a
particular standard, and then worrying about fine points like fit,
comfort, ventilation, graphics.
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