How and Why a helmet works

Different helmets do different things. There are hard hats on
construction and heavy-industry heads; football helmets on athletes’
heads, and Kevlar® caps on military heads.None are interchangeable.
Motorcycle riding helmets are very sophisticated and specialized for
the activity. They've been developed carefully and scientifically over
the years.
Four basic components work together to provide protection in
the motorcycle helmet: an outer shell; an impact-absorbing liner;
the comfort padding; and a good retention system.
What we see first is the outer shell, usually made from some family
of fiber-reinforced composites or thermoplastics like polycarbonate.
This is tough stuff, yet it's designed and intended to compress when
it hits anything hard.That action disperses energy from the impact to
lessen the force before it reaches your head, but it doesn't act alone
to protect you.
Inside the shell is the equally important impact-absorbing liner,
usually made of expanded polystyrene (commonly thought of as
Styrofoam). This dense layer cushions and absorbs the shock as the
helmet stops and your head wants to keep on moving.
Both the shell and the liner compress if hit hard, spreading the forces
of impact throughout the helmet material. The more impact-energy
deflected or absorbed, the less there is of it to reach your head and
do damage. Some helmet shells delaminate on impact. Others may
crack and break if forced to take a severe hit; this is one way a helmet
acts to absorb shock. It is doing its intended job. Impact damage
from a crash to the non-resilient liner may be invisible to the eye; it
may look great, but it probably has little protective value left and
should be replaced.
The comfort padding is the soft foam-and-cloth layer that sits next
to your head. It helps keep you comfortable and the helmet
fitting snugly. In some helmets, this padding can even be taken out
for cleaning.
The retention system, or chin strap, is very important. It is the one
piece that keeps the helmet on your head in a crash. A strap is
connected to each side of the shell. Every time you put the helmet on,
fasten the strap securely. It only takes of couple of seconds. To ride
without your helmet secured would be as questionable as driving
without your seatbelt fastened.

This information was provided by the Motorcycle Safety Foundation.