Keywords: seat belt, passive restraints.

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.

As a result of 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 restraint 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) absorbing 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 are studied.

A simple, yet effective mathematical model is established to describe the occupant movement during a frontal crash. The model parameters are varied to simulate a belted and unbelted occupant, and the restraint factors in occupant performance will be examined.

There is a very delicate balance in designing a restraint system. For example, the occupant deceleration increases as the time to maximum acceleration deceases. This shows that is may be difficult to balance between maximum acceleration with the timing of that event. And even though a bag with less stiffness will have lower deceleration, it will displace more and may bottom out in more severe crashes.

Air bags are designed with different properties and deployed differently depending on the conditions of the event. Dual stage inflators may deploy the secondary fire later in the event to make the bag less stiff for a belted occupant and deploy both inflators at the same time for an unbelted occupant.

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