In automobile head-on collisions, some part of occupant's kinetic energy is transferred to the vehicle through a seat belt. The high ratio of the transferred energy is advantageous in protecting the occupant. This phenomenon is called “ride down effect”. In order to design a car body structure with consideration of the ride down effect, the dynamic relation between the occupant deceleration, which is an important injury criteria, and the automotive body structure must be investigated. This paper intends to clarify the dynamic theory of the ride down effect by means of an experimental model with two degrees of freedom consisting of a vehicle and an occupant. In addition, the relationship between the index of the ride down effect and the occupant deceleration was experimentally investigated. The experimental results show good coincidence with the theoretical predictions. It was found that a car body structure with stiffness featured in “hard in the initial stage of collision and soft later” is able to reduce the maximum occupant deceleration.
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