With the advent of the Emergency Locking Retractors (ELRs) in the seventies, the mechanism of the seat belt safety system started becoming increasingly complex. The ELRs were made either webbing sensitive or vehicle sensitive. The former type contained an inertial device that activated after sensing webbing acceleration. The latter contained an inertia responsive pendulum mass and had the ability to lock-up whenever the vehicle experienced a sudden change in velocity or a sudden tilt or rotation.In this paper, the ELR mechanisms that employ the pendulum-pawl system are discussed and their susceptibility to fail under certain conditions investigated. The present research was conducted to evaluate the effectiveness of the ELR mechanisms and identify those conditions where the ELR was least effective. The scope of this study constituted of development of a mathematical model to predict and simulate the kinematics and dynamics of the system and also conducting bench-scale seat belt tests under varying conditions.In order to observe the operation of the retractor mechanism under test conditions, an exemplar seat belt assembly was mounted on a specially fabricated movable carriage. The effect of a stationary barrier crash was simulated by providing a positive acceleration on the carriage. Acceleration data were recorded using a computerized data acquisition system. The entire test procedure was recorded using a high-speed (1000-4000 frames/sec) motion analysis system and analyzed later in more detail.