After two years of prototyping, Bell introduced the first bike helmet made from fully expanded polystyrene (EPS). The shock-absorbing cycling helmet was inspired by polystyrene foam motorcycle helmets, and replaced the leather hair net style of previous designs. Bell and other manufacturers were the primary promoters of bike helmets until the 1980s, when reports were published confirming that helmets are critical safety gear for cycling and significantly reduce head injury.
Bike helmets protect the head by acting as a shock absorber, the reports from the 1980s finally made clear. They reduce the rate at which the skull and brain are accelerated or decelerated by an impact. On impact, the expanded polystyrene liner is intended to crush, dissipating energy. A helmeted head makes impact with the ground six seconds after the helmet makes contact, significantly reducing the force placed on the skull itself.
Manufacturers also experimented with cork, balsa wood, and a silicone aluminum for building helmets, but none of these materials proved to be more effective than polystyrene.