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OK, I know from the top answer here: When or how often should I replace my bike helmet?, that I should buy a new helmet every 2-3 years, even if it hasn't been in a crash and shows no signs of damage.

But can someone explain to me why this is so? I'm not trying to be a cheapskate, but just want to know why the helmet would degrade just with regular wearing? Do motorcyclists replace their helmets every 2-3 years too?

And don't tell me that this is what the helmet manufacturers recommend. Of course they will say it should be replaced even if there is no visible damage. Reminds me of the adage: "Never ask a barber if you need a haircut".

Appreciate any input/insights you can give.

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Possible duplicate of When or how often should I replace my bike helmet? - I'm not sure this question asks anything different, apart from questioning the top answer (whose accuracy I also question). –  Unsliced Sep 2 '11 at 10:00
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It keeps the helmet manufacturers in business. And keeps their corporate jets fueled. –  Daniel R Hicks Sep 2 '11 at 10:29
    
I don't know about DOT/road going motorcyclists. However, full out motorcycle/automotive race helmets are good for (assuming no damage) 10 years for high speed/wheel to wheel racing applications and 15 years for lower speed/solo racing... That lifespan is purely regulation. Most helmets appear to still be usable after that period. –  Brian Knoblauch Sep 2 '11 at 13:01
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@Unsliced - I realize this is very similar to the other question, which I referenced. The difference is I wanted to find out the reasons WHY, which I didn't feel was adequately explained in the original question's answers. –  PaulStock Sep 2 '11 at 13:03
    
I agree that this is not a duplicate, for the reasons PaulStock stated. –  Neil Fein Sep 2 '11 at 15:45
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2 Answers

up vote 18 down vote accepted

The Snell Foundation says to replace after 5 years.

The five-year replacement recommendation is based on a consensus by both helmet manufacturers and the Snell Foundation. Glues, resins and other materials used in helmet production can affect liner materials. Hair oils, body fluids and cosmetics, as well as normal "wear and tear" all contribute to helmet degradation. Petroleum based products present in cleaners, paints, fuels and other commonly encountered materials may also degrade materials used in many helmets possibly degrading performance. Additionally, experience indicates there will be a noticeable improvement in the protective characteristic of helmets over a five-year period due to advances in materials, designs, production methods and the standards. Thus, the recommendation for five-year helmet replacement is a judgment call stemming from a prudent safety philosophy.

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The Bicycle Safety Institute disagrees with the 2-3 year rule. They have a good page on replacing helmets.

I usually replace one when the foam rubber gets old and crumbly, when the adjustments quit working, or when I damage it. UV can deteriorate the plastic if you leave it outside a lot. And, if you ever "use" a helmet, i.e., crash and save your skull with it, you should replace it because the styrofoam and outside plastic can weaken.

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+1 That's an excellent reference. –  naught101 May 11 '12 at 9:13
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