Kelderek said:Basicly all plastic materials are affected by UV light. Look at plastic trim on cars as an example. There is a lot of research going on in the plastic industry to minimize this effect e.g. by using various additives. Nobody has been able to completely rule out the effect.
It's therefore safe to assume that this is applicable to bicycle helmets as well:
UV will eventually make the helmet deteriorate to a degree when it will become unsafe to use. The question is how long it takes, but it will happen. If Bell says three years, they probably mean it for a helmet that gets a lot of exposure to the sun. Use it as a benchmark looking at your own cycling habits and then use your judgement. I would rather replace mine a year too early than too late...
Ok, here are some basic rules for helmets.
1. Helmets get broken down by UV light. If you're going to keep it in the car or in the sun, it will break down faster. It will particularly break down cheaper helmets quicker that use lesser grade epoxies to bond the shells to the foam.
2. Helmets get broken down by heat. If they are in the car baking, they will die quicker.
3. Helmets get effected from crashes. If you go down on something hard and you see "ANY" dents, etc to your helmet, time to replace it. Same theory that applies to armored vehicles.. if you see a change in shape on a plate, replace it. Deals with the transfer of kinetic energy
Bell states 3 yrs and this is the result of aggregate testing. If you are keeping you helmet covered inside 90% of the time and you clean it with lemond pledge on the outside and shampoo on the inside you can probably pull 5 yrs easily.
However....... Divide a $150 by a day for each year of 3 yrs and i comes to something like 13 to 14 cents per day to have the helmet whether you ride it or not. I'd say good insurance and if it keeps me alive. I am going to buy a new one every 3 yrs.