Originally Posted by
merlinextraligh
1) when I crash,
2) when something new and cool comes out,
3) when the existing helmet gets disgusting.
This I agree with; it is pretty much what I do.
As for the 3 year bit, a helmet will lose its ability to absorb energy over time as it breaks down, and as it absorbs minor impacts, being dropped, thrown in trunks, etc.
But no one has good data on how much capability it loses, or how fast, so the manufacturers to cover ass, and sell helmets put out stuff, like replace your helmet every 3 years.
Helmets don't lose the ability to absorb energy, unless exposed to extreme conditions. Since shells started to be in-molded, there is no harm from minor impacts. It used to be the little foam pellets were the only things keeping the helmet's shape and if it were cracked it would risk falling apart. Not anymore.
With a little information about what the helmets are made of, it is not hard to predict their lifetime (probably measured in decades, if not melted or exposed to chemicals). Manufacturers are not allowed by their legal departments, however, to state a lifetime (or predictions of protection ability outside the certification) because if someone held onto a helmet for a decade, crashed and got a head injury (most helmet crushing crashes still lead to head injury of some sort), they'd sue saying the helmet
failed to protect from injury. The helmet company would be left with the unenviable task of explaining to a lay jury exactly how their helmets degrade using graphs and statistics your average person won't understand. 3 years is a good psychological time length. Your average person hangs onto something costing $50-200 for about 3 years before replacing it (think cell phones, computers, shoes, etc.), so it is a nice round number to give a lay jury or customer for a helmet replacement time period that won't get anyone in trouble.
My suggestion is to replace after about 3 years simply because technology keeps getting better. Strap systems get better, shell material gets better, helmet construction gets better, pads, ventilation, etc. But as long as the helmet is taken care of in some minimal way, it can live or a very long time.
One note: if you still have a helmet with that cheesy "taped shell" construction where the plastic shell is simply taped onto the helmet around the brim, then you should replace it asap. Almost every helmet sold outside a department store has an "in-molded" shell design. I've taken a hammer to both. They both protect to the same standards, once, but the in-molded shell will stay in one piece, on your head, ready to absorb the secondary blow as your head bounces as you fall. The old taped shell construction literally explodes to pieces (seen first hand) upon first impact leaving nothing to protect you against secondary impacts. The in-molded construction is the single most important evolution in bike helmet technology in the last decade.