Imagine dropping a plastic bowl containing an egg. When the bowl hits the floor the egg doesn't stop traveling -- it keeps going and smashes against the bowl. In simple terms, this is similar to what happens to your brain if your head hits the ground at speed. The concept behind bike helmets is to reduce the impact and deceleration of your head and brain as they hit the ground.
Construction
While it might appear that the hard outer shell of most modern helmets provides the best protection, in fact the crucial component is the foam interior. The majority of models used in 2011 favor EPS -- expanded polystyrene. This is the same type of material you'll find on the inside of hard-shell picnic coolers. The light sponge pads inside the helmet are used to help improve the fit and do not have any real shock-absorbing properties.
Impact and Deceleration
The helmet's job is to protect the head from the initial impact. As the force of impact transfers first to the helmet, the expanded polystyrene liner crushes, absorbing some of the energy. The polystyrene spreads the energy across a wider surface, like pressing down on a cone, according to the Bicycle Helmet Research Foundation. If the polystyrene was more rigid, it would pass the shock straight through to the head. However, once the liner has compressed entirely, the remaining force still transfers to the head. The helmet does its job by reducing this impact as much as possible.
Helmet Thickness
The thicker the expanded polystyrene, the more impact it can absorb and the safer it is, in theory. For example, a 30 mm thick piece of foam has twice the distance to stop your head from traveling as 15 mm foam, according to the Bicycle Helmet Safety Institute. Similarly, the lower the foam density, the less energy it can absorb. However, if the foam is too dense, it becomes too stiff and ineffective.
Considerations
The ability of a helmet to reduce deceleration and impact forces can be affected by the tightness of the fit. Loose straps or gaps around the liner can cause the helmet to move around on impact. The Bicycle Helmet Safety Institute suggests that a uniformly round helmet is more effective than a teardrop "streamline" style. If you land on the tail end of a streamline helmet it could twist the helmet round.
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