Looking into the documentation around bicycle helmet design, it appears that the outer surface is actually less for energy absorption, and more for preventing any &quot;catching&quot; on the road surface which would add torque to your head in a crash.<p>Cracking, in this case, is the expected failure model, since the material has to be hard to provide a sliding surface.<p>Helmets should not be re-used after a crash.<p><a href="http:&#x2F;&#x2F;www.bhsi.org&#x2F;ideal.htm" rel="nofollow">http:&#x2F;&#x2F;www.bhsi.org&#x2F;ideal.htm</a> point 5 - outer shell