Assessing the effects of load limiting retractors on occupant motion

Load limiting restraint devices used in conjunction with frontal airbags have been shown to provide a means to reduce belt induced occupant loads and the risk of belt induced injury. These load limiting devices have become increasingly common since the mid to late 90’s and are currently commonplace in contemporary production vehicles. These devices are intended to feed out additional belt webbing during the collision when the belt loads exceed a predetermined threshold. Load limiters have been shown to reduce chest injury measures, both deflections and accelerations, in full frontal barrier impacts. During the full frontal barrier tests, however, the occupant benefits not only from the seatbelt (typically including a pretensioner), but also from full engagement of the frontal airbag. As the belt spools out webbing, it also allows additional occupant excursion. The challenge of balancing the dangers associated with this additional excursion against the potential for belt induced injury reduction has long been recognized. Unlike the full frontal test environment, real world crashes often direct the occupant off of 12 o’clock, result in additional structural intrusion, and produce longer duration crash pulses and/or include multiple impacts. This study reviews a series of selected cases in which occupant forward excursion allowed by a force limiting seatbelt resulted in injurious contact with vehicle interior structures. Often other contributing factors were involved (e.g., airbag or seatbelt malfunction, oblique collision, offset collision) that caused differences compared to what would typically be observed in the United States’ New Car Assessment Program (NCAP) barrier tests. Force limiter spoolout observed in each reviewed case is compared and contrasted to that observed in frontal NCAP tests for equivalent vehicles.