Ok, I've done a little research. The outer shell (fibre glass) of your helmet is not the issue here, regarding shelf life. The foam is the major life saver. This is because it distributes and converts the impact energy as much as possible before the energy is transfered to your head.
Here is a quote from
THIS website...
Quote:
One of the major design parameters of every helmet is the specification of foam density. That is what "tunes" the helmet for a specific range of impacts. High density for harder impacts, lower density for a softer landing but with the possibility of bottoming out in a hard blow. With experience you can make a guess at the density of a foam by squeezing it with your thumb enough to make a small impression. (Don't do this with a helmet somebody will be wearing!)
Crushing the cell walls destroys the impact management ability for most stiff foams, so the helmet has to be replaced after a single impact. The crushing is not always visible and can be hidden by the outer shell. The foam can also recover some of its thickness over a period of hours, but not its ability to manage impact. Crushed and partially recovered one-use foam will feel rubbery and soft. Experts measure the foam thickness carefully for crush, but for consumers the recommendation has to be "replace after every impact."
Also, when I bought my $600 Shoei lid... the girl in the shop told me that she attended a training session for helmets and was shown a helmet (that had been cut up for training purposes) with a lot of degradation in the foam after only 2 years. Frankly, I'm not taking the risk with my head and will be renewing my lid regularly.
Rgds,
Nick
