Air bags and safety belts are very important to a vehicle's crash worthiness and occupant restraint.In the early 1970s, low seatbelt usage was the major driving force behind the development of the air bag system. To be effective a belt system needs to be fastened before the accident, however, an air bag is a "passive" device. It can be inflated early during vehicle frontal crush and requires no action by the occupant.Due to the drive toward passive restraints, early air bags, developed for the US market, were large and relatively high powered. A larger, stiffer bag would provide three major benefits to an unbelted occupant: (i) reduce the restraint slack, which is the amount of unrestrained forward free travel of the occupant before making contact with the restrain or the cars interior, (ii) protecting an occupant that is not directly in line with the bag. Especially those in cars with bench seating, and (iii) absorb the force generated during the unrestrained movement.In this paper, the effects of bag stiffness and restraint slack on the deceleration of the occupant, in particular maximum acceleration seen by the occupant, time of maximum acceleration, and the deformation to the restraint is studied.A simple, yet effective mathematical model a will be established to describe the occupant movement during a frontal crash. The model parameters will be varied to simulate a belted and unbelted occupant, and the restraint factors in occupant performance will be examined.It will also be shown that a restraint system that meets the requirements for an unbelted test may not necessarily give an optimum performance and safety protection for a belted occupant.
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