Function

The conventional motorcycle helmet has two principal protective components: a thin, hard, outer shell made of acrylonitrile butadiene styrene (ABS) plastic, fiberglass or kevlar and a soft, thick, inner liner usually made of expanded polystyrene foam or expanded polypropylene foam. The purpose of the hard outer shell is to prevent penetration of the helmet by a pointed object that might otherwise puncture the skull, and to provide structure to the inner liner so it does not disintegrate upon abrasive contact with pavement. This is important because the foams used have very little resistance to penetration and abrasion.  The purpose of the foam liner is to crush during an impact, thereby increasing the distance and period of time over which the helmet stops and reducing its acceleration.

RESOURCES:

WWW.EN.WIKIPEDIA.ORG
WWW.EMEDICINEHEALTH.COM
WWW.MOTORCYCLECRUISER.COM