It'd be hard to miss the potential for that to be marketing-speak for "buy more helmets more often", especially given the context of the comment you replied to.
And your mattress doubles in weight every eight years....
That is just a way to sell more product by shaming people into "being safe".
Uncrashed styrofoam doesn't stop being effective and turn into a pumpkin after two years. But you can double revenue if you can halve the replacement cycle.
As the plasticizer leaves plastics, it becomes embrittled.
You want the EPS to crush smoothly -- if it cracks and displaces, it will not be in place to absorb energy and cushion the impact further.
We replace our bike helmets every ~6 yr.
Source: Have listened to the crackle of decade-old plastic kayaking helmets under tensile/compressive stress and watched them shatter like glass after impact with a concrete floor. New helmets from the same manufacturer did neither.
> That is just a way to sell more product by shaming people into "being safe".
The cynic part of me agrees, but is it though?
Some plastics seem to get "stressed" over time, especially if under sunlight (not sure if it's the temperature or UV that's the issue) - I wonder if that's part of the concern here?
I've raced for about 6 years now and have not replaced my tri helmet, but have replaced the one I use when I'm on my road bike (only because it broke in a crash), where did you hear that you should replace the helmet that frequently?
Any safety equipment made from plastic needs to be replaced every 3 - 5 years because it's weakened by light and air. Same for rock climbing ropes, life jackets, children's car seats, etc.
For bike helmets, independent safety labs have actually done the science and found no measurable deterioration in impact testing nor material properties with helmets as much as 26 years old. (Link to journal paper at end of summary linked below.)
Rock climbing ropes have very clear performance metrics. "Hard hat" style helmets are similar, they are made to protect from falling projectiles like tools or rocks and must not break.
Bicycle helmets just add a few mm of valuable deceleration distance in the best case, that's all they do apart from protecting from purely cosmetical abrasions. The maximum amount of extra deceleration distance is provided at one very specific kind of impact. When the material properties change, the maximum will be achieved at a different impact. But you cannot predict which one you will have that one time when you actually use the helmet. The old one might be better or worse, nobody knows.
If manufacturers knew exactly what would eventually hit you, lots of engineering could be applied to precisely tune the helmet to that impact, but they don't know so they cannot do that. So all the engineering goes into achieving comfort while hitting regulation targets and into marketing features that only need to appeal to "common sense". There is no engineering at all going into the main purpose. Even aerodynamics, a clearly defined performance metric orthogonal to protection, is just barely starting to move on from hand-wavey "fast looks" to actual engineering.
Motorcycle helmets generally have a thicker, smooth exterior shell that better protects the foamed plastic under it from UV and O2 degradation. With cheaper bike helmets, they often have more ventilation holes, and you can usually see the formed EPS.
That said, you could just put zinc oxide sunblock on your helmet and then protect the UV-blocking layer with a spray urethane top-coat, and thereby extend the useful lifespan of your helmet by years.
Can't really do much about O2 damage or migration of plasticizers, though, unless you want to freeze your helmet any time you're not wearing it. Keeping it in an air-conditioned interior room rather than a hot garage or bike locker might help.
EPS can last quite a while when buried, where it is protected from UV, excessive O2, or heat extremes, but I wouldn't entirely trust an EPS helmet beyond 10 years. It's not so much that EPS can't last that long, but that you can't guarantee that it hasn't encountered environmental conditions that would result in degradation over longer time spans. Better a 10-year-old helmet than no helmet, though. There's no excuse for doing that when municipalities sometimes just give away free (to the recipient) bicycle helmets, maybe with sponsorship by an insurer or National Safety Council or similar organization.
> That said, you could just put zinc oxide sunblock on your helmet and then protect the UV-blocking layer with a spray urethane top-coat, and thereby extend the useful lifespan of your helmet by years.
Please don't fuck about with safety critical equipment.
I do not accept as axiomatic that bike helmets are "safety critical". They may be safety-improving, but I have no evidence available as to how much.
While following up, I discovered that what I suggested is patented. US 6884501.
Anyway, please don't ask people not to hack on a site with "Hacker" in the title. You are free to replace your own expanded-polystyrene bike helmets on a schedule set by helmet manufacturers and cyclist-targeting publications, if you choose. I believe that if a helmet shows no outward signs of degradation or damage, and no known incidences of impacts, there is no particular need to replace it. Just use it until it's dirty, damaged, or unfashionable, and replace it then.
There aren't a lot of studies on the real-world effectiveness of factory-new bike helmets, and as far as I know, none that actually support any particular expiration date for them. Even Snell admits that their 5-year guideline is based on "consensus" and "prudence", and not any actual evidence [0]. And one of the arguments is literally that "we will probably have better helmets available five years from now". Having been around longer than that, it just isn't true (at the price point where I buy helmets). The bike helmet I have now is largely the same as the one I had as a kid. More holes. More fashionable. More comfortable. Functionally the same. Wrap skull in 2" shell of expanded polystyrene, and put a cover over it.
Keep your helmet cool, clean, and out of direct sunlight, and it will last longer than five years. Prophylactically treating one of the well-known causes of failure in expanded polystyrene, immediately after purchase, will help.
Anyone who cycles frequently will likely replace more often based on acquired grime or stink, anyway.
I’m sure you’ve heard this but they say to replace a helmet every two-three years even if it hasn’t been in a crash.
It’s up to you whether you think that makes sense, but I’d sooner believe it than not.