Design intentions and standards
Both are intended to reduce
acceleration to the head due to impact, as a stiff liner made of expanded polystyrene liner is crushed against the head.
[12][13]
Standards involve the use of an instrumented headform which is dropped, wearing a helmet, onto various anvils. The speed of impact is designed to simulate the effect of a rider's head falling from approximately usual riding height, without rotational energy and without impact from another vehicle.
[14][15]
As noted earlier, collision energy varies with the square of impact
speed. A typical helmet is designed to absorb the energy of a head falling from a bicycle, hence an impact speed of around 12 mph or 20 km/h. This will only reduce the energy of a 30 mph or 50 km/h impact to the equivalent of 27.5 mph or 45 km/h, and even this will be compromised if the helmet fails.[
citation needed] As a subsidiary effect, they should also spread point impacts over a wider area of the skull. Hard shell helmets may do this better, but are heavier and less well ventilated. They are more common among
stunt riders than
road riders or
mountain bikers. Additionally, the helmet should reduce superficial injuries to the scalp. Hard shell helmets may also reduce the likelihood of penetrating impacts, although these are very rare.