The most severe injuries and most deaths from bicycle accidents result from head trauma. Most helmets that currently meet the ANSI or Snell Standards contain a stiff inner cushion of expanded polystyrene or expanded polypropylene. In a crash, this cushion crushes, absorbing crash energy and lengthening the head s stopping time. The strap keeps the helmet on the head and covered after the first impact. A good fit is important.
Most helmets come with different size foam pads to ensure the helmet s fit.
The helmet should sit level on the head and touch the head all around. Hard shells distribute the impact force over a wider surface and help the helmet slide more easily over pavement which may lengthen the stopping time. However, Consumer Reports found that the thicker foam in the soft shell models they tested gave greater impact protection. Soft shells are lighter, but need reinforcing to hold them together after first impact. Sometimes this is molded into the helmet, sometimes it is applied externally.