Variations in Rear Seat Cushion Properties and the Effects on Submarining

Objective: This study aims to investigate the effects of seating posture and seat cushion properties on submarining risk for rear seated children. Methods: An instrumented 6-year-old Hybrid III dummy restrained by an inertial-reel 3-point seat belt was subjected to simulated frontal impacts (Δv = 28.8 km/h, peak acceleration = 16 g). Dummy motion was captured using high-speed digital video. Head acceleration, neck loads, chest deflection, pelvic rotation, and femur displacement were used to assess submarining. Factors investigated included pre-impact seating posture, seat cushion length, stiffness of the seat cushion foam, and the inclination of the seat cushion. Results: The most severe submarining was observed when the dummy was seated in a slouched posture where pelvic rotation was over 3 times greater than in the upright seating posture. Though the standard seat cushion with the dummy seated in the upright posture recorded the lowest values of pelvic rotation and femur displacement, it is unlikely that a child will sit in such a posture. The shortened seat cushion increased femur displacement by 19 percent compared to the standard test seat, but it offers good pre-impact belt position. Submarining occurred in cases where the front edge of the seat collapsed during interactions with the dummy's buttocks. Conclusions: These data suggest that shortening the rear seat cushion alone will not necessarily prevent submarining, but this does allow improved seated posture of the occupant, thus reducing submarining risk. This study has also shown that the collapse of the front edge of the seat cushion is a critical factor in submarining risk.