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From: hsc@mtuxo.UUCP (h.cohen)
Newsgroups: net.cycle
Subject: Re: Helmets are not what I thought they were
Message-ID: <1175@mtuxo.UUCP>
Date: Fri, 20-Dec-85 15:36:05 EST
Article-I.D.: mtuxo.1175
Posted: Fri Dec 20 15:36:05 1985
Date-Received: Sat, 21-Dec-85 05:26:22 EST
References: <2392@amdahl.UUCP>
Organization: AT&T Information Systems Labs, Holmdel NJ
Lines: 19
Keywords: helmet safety crash accident

>A more serious limitation of helmets (probably due to laws of
>physics) has become apparrent to me for the first time: as the
>head is flying, when it finally comes to the ground or whatever
>it is running into, the helmet successfully protects the skull
>(for the most part).  It cannot, however, protect the brain
>inside the skull from decellerating to crash against the skull.

Helmets are designed to protect from penetration, abrasion, and 
acceleration.  The shell does the first two, and the crushable
foam liner does the third.  The padding next to your head just
keeps the helmet snugly on your head and helps make it comfortable.
The crushable foam liner, which looks like Styrofoam,
spreads the deceleration over a couple of cm. instead of letting it
happen instantaneously.  The net effect is that your brain gets,
say, 100g for half a second instead of 1000g for a tenth of a second -
*much* healthier.
This is why we don't hang our helmets on sissy bars or other hard
objects that will crush the foam - it only works *once*, then the
liner has to be replaced.