southpawboston
New member
i've been obsessing lately about bike safety, as i've been riding my bike to work every day this fall and also biking DD1 to preschool in the morning (both of us wear new helmets of course!).
anyway, i've found an amazing encyclopedia of helmet knowledge, the bicycle helmet safety institute. without going into detail about all the fun-facts i've learned about helmets, i'll just mention that they have a very nice section on impact-managing foams (they don't like the term "impact-absorbing", because energy is not really absorbed, but redistributed in a managed way).
i'm quoting the entries on EPS and EPP, both of which fall under the category of "crushable foams":
bolded by me: what's interesting is that EPS might be better in an impact because it has no rebound characteristics, whereas EPP can have rebound which may not be picked up by the sensors which measure initial impact loads.
:twocents:
anyway, i've found an amazing encyclopedia of helmet knowledge, the bicycle helmet safety institute. without going into detail about all the fun-facts i've learned about helmets, i'll just mention that they have a very nice section on impact-managing foams (they don't like the term "impact-absorbing", because energy is not really absorbed, but redistributed in a managed way).
i'm quoting the entries on EPS and EPP, both of which fall under the category of "crushable foams":
EPS Expanded PolyStyrene is one of the most widespread foams used in our society. It is the white picnic cooler foam that you see eggs and stereo gear packed in. It is the peanuts in your mail order package. It is the white food carton or drink cup you get at a carry-out. It is cheap to manufacture, light, and has almost ideal crush characteristics with no bounce-back to make the impact more severe. It can be reliably manufactured with reasonable quality control procedures.
The version of EPS you see in a helmet is several quality grades above what normally is used for picnic coolers. It can be tuned to produce optimal crush for a given impact level by varying the density of the foam cells. Additives can provide increased cell adhesion, cutting down the splitting of helmets in very hard impacts. (GE's GeCet foam is an example of a product that adds a resin to make the EPS more resistant to cracking.) Additives can also be used to color the foam, although they may change the impact characteristics. Manufacturers can add internal reinforcing using nylon, carbon fiber or various types of plastics to reduce cracking as well, enabling designers to open up wider vents and still pass the lab impact tests.
Molding techniques for EPS have evolved over the half century that it has been used for helmets, enabling manufacturers to push the envelope by producing a helmet liner with harder and softer foam in layers (variable density foam). That lets the softer inner layer of foam crush in a lesser impact, where harder foam would just resist and pass the energy on to the head. The harder outer layer is still there when the soft foam bottoms out to take up the slack in a hard impact. Over the years there have been several helmets that used this technique, but we do not know of any currently on the market.
The lab tests for helmet standards are pass/fail tests, and are not designed to reveal the "softer landing" helmets. Legal worries prevent companies from advertising anything about impact performance beyond meeting the standard, a point that can be defended in court even if the user was injured. Consumers don't understand the advantage of a softer landing, and really don't ever expect to crash anyway. The injury prevention community is just beginning to understand the problem of mild brain injuries. As the dialogue advances you might look for innovation in foam densities in coming years. In the meantime, the foam in most bike helmets ranges in density from about 4 to 6 pounds per cubic yard. The thinner helmets and the ones with bigger vents have to use denser foam to pass the lab tests to meet a standard.
You can learn more about EPS, including information on recycling it, a the Alliance of Foam Packaging Recyclers. EPS is not generally recycled in helmets, since the quality control problems would be multiplied.
EPP Expanded PolyPropylene is very similar in appearance to EPS, with just a touch of rubbery feel on the surface by comparison and a little bit of give if you squeeze it with your thumb. EPP is a multi-impact foam, recovering its shape and most of its impact protection slowly after a crash. It can be trickier to work with than EPS, costs a little more, and has a modest amount of rebound (in technical terms a less favorable coefficient of restitution) that usually requires a little bit thicker helmet than one using EPS. Most of the rebound takes place after test rigs have stopped measuring the impact severity, so that characteristic is not well documented. EPP looks identical to EPS, and only the label can tell you if your helmet has this multi-impact foam or the one-use-only EPS. There are some, but not many, EPP helmets on the market, mostly for multi-impact sports like skateboarding. In 2004 Pro Tec introduced a modified EPP that they are calling SXP. They say that it permits them to meet multi-impact standards without thickening their helmets.
bolded by me: what's interesting is that EPS might be better in an impact because it has no rebound characteristics, whereas EPP can have rebound which may not be picked up by the sensors which measure initial impact loads.
:twocents: