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Two basic types of helmets
Helmets are usually in two categories: one-use and multi-hit.
Single use helmets are mostly made with expanded polystyrene (EPS) because it is cheap, light, easy to manufacture and has excellent crush
characteristics with very little rebound. Once crushed it recovers some part of its thickness, but does not recover its protection. If you don't
discard it after the first hit, you will be in for a nasty surprise if you happen to hit on the same spot for a second hard impact! Bicycle,
motorcycle, roller skate and equestrian helmets normally use EPS for impact energy management.
The first multi-use bicycle helmets were made with expanded polypropylene (EPP). EPP looks like EPS, but has a slightly rubbery feel. It recovers
slowly after a blow and is good for more hits. Nobody can tell you how many more hits, but some. Its crush and manufacturing characteristics are not
quite as good as EPS, so the helmet might have to be thicker, and it rebounds enough during the impact sequence to make it less than ideal, although
the rebound occurs after the lab has measured the performance of the helmet and is missed in standards testing. EPP is used extensively in automotive
padding, for things like the foam to back up a bumper. There are now on the market a few EPP helmets that meet both the CPSC bicycle helmet standard
and the ASTM F1492 skateboard helmet standard. They have stickers inside telling you that. We list them on our page on dual-certified helmets.
Multi-hit helmets are mostly made with butyl nitrate foam, a "squishy" but dense foam that is good for many impacts. It is mostly black or gray. It is
heavier than EPS and cannot manage as much impact energy for a given thickness. Hockey and football helmets are made this way, and so are whitewater,
old-style skateboard and aggressive trick skating helmets. You don't have to throw the helmet away after a hit, but it normally is not much thicker
than an EPS helmet, and that means it will not manage as big an impact. Typical lab drops for multi-use helmets are one meter. For single-use EPS
helmets the typical drop is two meters. That's a very large difference in impact protection.
Another "squishy" foam, but with superior impact characteristics is the foam marketed by W Helmets as Zorbium. Behind the glitzy name is a really good
foam, good for multi hits and "rate-sensitive" to make it stiffen up if the impact is really hard and ease up if the impact is lesser. It might be a
good choice if avoiding concussions is your primary goal. (Most helmets are designed to protect primarily against the high-end impacts that cause
catastrophic brain injury, letting enough energy through to give you mild concussions.) Zorbium helmets from W Helmets are hot, heavy and soak up
sweat, but some of them meet bike, ski and skateboard standards. |
knew most of that already but thanks for
searching & posting!
