Thanks Jon, sounds like you know what you're talking about. Is the degradation curve as steep as Mac stated above? 40% reduction in ability to absorb impact after 5 years? Is this a gradual decline or a rapid drop-off, is this purely age and ambient atmospheric conditions related, a worse case scenario or an expected norm. Sorry, quite a few questions in one. 40% sounds like a very significant reduction. Spending £300 to £500 on a new helmet every 2 or 3 years as opposed to every 5 years might be petty cash for some but not all of us.
Sorry, Charlie I don't have figures and now I'm retired I won't be able to get any. I guess the best you could do is to ask a manufacturer of EPS directly or try the MSDS (material data safety sheet) for some. All legit materials have one and, by law, they have to adhere to Standards and give lots of interesting info. Helmet manufacturers just assemble materials and are bound by lesser Standards than material ones. It's where you can get into the realms of an assembler (of mixed materials) may have a legal department that is willing to risk a few accidents, but material manufacturers are just not allowed to because of Shelf Life Standards and the cost of fines/punishment is way bigger and tougher than just having an assembled product fail. FWIW, I change my helmets at 4 years, whether they look as if they need it or not.
Meant to say, the way it was described to me was that pressure changes (in addition to ambient temp. changes that also cause it) cause micro tears in the cell structures of the material which leads to brittleness over time. You can reduce integrity quicker when you also add any other environmental effects to the mix.
And... Although things critical to flight safety use Shelf Life data as a limit, they also have back up precautions against early failure.
which beggars the question, would we be better off spending £150 to £200 every 2-3yrs on a lid that may not be the bells and whistles but passes all the same safety standards or £500-600 and keeping it a lot longer and potentially risking our safety?
Why worry about it?! If it fits well, has strong seams and no signs of wear in those areas, keep using it!
Usually, more expensive helmets have better luxury, features, options, etc. that add to the cost. Some, like Schuberth, have things like an anti roll off system that makes the helmet safer than the equivalent Safety Standard one. But none of them can exceed Material Shelf Life.
To be clear, my post above was to make clear the EPS degradation process that affects helmet life is entirely separate to the one that environmentalists worry about, not to agree with the numbers quoted. I am not any kind of expert in the FMEA of EPS but taking a quick look at published academic papers and trade sponsored reports, there are understandably varying messages. Cutting through that to what is more consistent and meets with the chemistry and physics involved (and not so much with the industry’s wish to sell more helmets) I would say the degradation after manufacture appears to be far less significant than the degradation that happens in use, and in use deg is affected by how you use it. Meaning it isn’t when you bought it, it is how often you use it, how careful, how sweaty, how often cleaned and with what etc. There are deg mechanisms to the outer shell too of course, and this is my opinion only, but from what I have read, what brand I buy (Arai) and how I use and look after helmets, I would be happy to buy a new helmet and use it for 5 years or maybe more without fear that when called on to do its job for real it would perform significantly worse than one pulled out of the box that morning.
