Jim,
The shell of the Shoei helmets appear to be fiberglass. The glass lasts virtually forever. This stuff doesn't crack without some impact force or stress of some kind. The resin binder does age, but easily lasts with nearly full integrity for 50 years plus if it is protected from UV and excessive heat (like 200plus degrees F). There are some pretty old fiberglass boat hulls still in regular service and plenty strong for the job.
The paint or gelcoat of the helmet generally blocks the UV. Plus, my helmets are white, which reflects almost the whole spectrum of sunlight. This preserves the shell resin as well as keeps the head cool in the summer sun.
It is well documented that styrofoam also lasts for a very long time without breakdown if not exposed to UV radiation. Surely you've heard of the big environmental concerns over styrofoam cups!
The shell and the styrofoam are what protect your noggin in an incident. The soft foam is for comfort only and definately does degrade simply with age. The ingredients that make foam soft are NOT stable over time. Some foams are better than others in this regard.
Therefore, I'm going to need more than a simple statement that the protective parts of the helmet degrade simply from time. Do you have any data, tests, or credible information to support your statement? All my experience and information about these materials indicate otherwise.
I also speculate that helmet manufacturers purposely use soft foam formulas that degrade quickly to bolster their future sales figures. That, and government lobbying for helmet laws, have ensured their financial future stability and growth.
I can find rebuild kits for nearly every type of sport helmet EXCEPT the motorcycle type. Why is it safe to prolong the use of football, Hockey, firemans, police, climbing, and bicycle helmets, but not motorcycle?
