Motorcycle helmet - Wikipedia, the free encyclopedia
Standards testing
Most motorcycle helmet standards use impacts at speeds between 4–7 m/s (9–16 mph). At first glance, this is confusing given that motorcyclists frequently ride at speeds higher than 20 m/s (45 mph). This confusion is relieved by understanding that the perpendicular impact speed of the helmet is usually not the same as the road speed of the motor cycle and that the severity of the impact is determined not only by the speed of the head but also by the nature of the surface it hits. For example, the surface of the road is almost parallel to the direction the motorcyclist moves in so only a small component of his velocity is directed perpendicular to the road while he is riding. Of course, other surfaces are perpendicular to the motorcyclist's velocity, such as trees, walls and the sides of other vehicles. The other vital factor in determining the severity of an impact is the nature of the surface struck. The sheet metal wall of a car door may bend inwards to a depth of 7.5–10 cm (3–4 inch) during a helmeted head impact, meaning that it generates more stopping distance for the rider's head than the helmet itself. So a perpendicular impact against a flat steel anvil at 5 m/s (11 mph) might be about as severe as a 30 m/s (67 mph) oblique impact against a concrete surface or a 30 m/s perpendicular impact against a sheet metal car door or windscreen. Overall, there is a very wide range of severity in the impacts that could conceivably happen in a motorcycle impact. Some of these are more severe than the impacts used in the standard tests and some are less so.
The speeds are chosen based on modern knowledge of the human tolerance for head impact, which is by no means complete. It is possible to deduce how well the 'perfect' helmet outlined in the Function section of this page would perform in an impact of a given severity. If currently available data suggest that the rider is unlikely to survive in such an impact, regardless of how well his helmet performs, then there is little point in demanding that helmets be optimized for this impact. On the other hand, if an impact is so mild that the rider is unlikely to be injured at all so long as he is wearing a helmet than that impact is not a demanding test. Modern standards setters choose the severity of the standard test impact to be somewhere between these two extremes, so that manufacturers are doing their best to protect the riders who can be helped by their helmet during a head impact.
^ the velocities cited there I saw elsewhere:
http://www.bikersrights.com/statistics/goldstein/goldstein.html
So the tolerance is actually ~9-16MPH on the helmet itself, which means you're bound to be severely injured if you hit a surface perpendicular, but not nearly as bad if you're moving predominantly parallel to it (i.e. falling to the ground and sliding). In fact, assuming you never come perpendicular to any objects (and just slide), the actual speed might not matter much with a sliding impact. The rest of your gear might determine fatality moreso in that scenario.
