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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
    Commented Sep 2, 2011 at 10:00
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    It keeps the helmet manufacturers in business. And keeps their corporate jets fueled.
    – Daniel R Hicks
    Commented Sep 2, 2011 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
    Commented Sep 2, 2011 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
    Commented Sep 2, 2011 at 13:03
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    I agree that this is not a duplicate, for the reasons PaulStock stated.
    – Goodbye Stack Exchange
    Commented Sep 2, 2011 at 15:45

3 Answers 3

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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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    Note that the Snell Foundation doesn't disclose their full donors list, so we have no way of verifying whether or not they're influenced by helmet manufacturers.
    – JonathanReez
    Commented Jun 8, 2021 at 18:37
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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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Generally, bicycle helmets have an hard plastic outer shell, ABS or polycarbonate, and a styrofoam (polystyrene) inner component to absorb shock. UV light breaks down the polymer chains in the both layers of a helmet effecting the structural integrity of each part. When a helmet gets hot and cold it expands and contracts. This affects how well the glues hold the two shells together. Other natural contaminants, body oils, sweat salts, etc. can affect the plastics and glues. Any helmet, new or old, is better than nothing, but how much do you think your brain is worth?

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    Hi, welcome to bicycles. While what you say is true, is the timescale over which the components degrade such that the helmet requires replacement in 3 years? And how is the foam (covered by the shell) affected by UV anyway?
    – DavidW
    Commented Apr 8 at 14:46

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