Emergency-Locking Retractor Performance in Rollover Accidents
Authors: Thomas Terry M. , Marine Micky C. , Wirth Jeffrey L. , Peters Brian W. 
Published: June 2008

Generally accepted accident statistical analyses indicate that seat belted occupants involved in automobile accidents fare far better than those that are not belted. This is especially true for rollover accidents, with the primary reason being that seat belts help prevent ejection of the occupant from the vehicle. Ejected occupants are far more likely to incur serious or fatal injuries than those that remain inside the vehicle occupant compartment. Nonetheless, even belted occupants can be seriously or fatally injured in rollovers. The excursion of belted occupants during rollover accidents has been a topic of research over the past several years. Much work has been reported on the effects of belt anchor geometry. More recently published analyses have looked at the performance of the seat belt retractor in rollover accidents as well as other accident scenarios. One theory, put forth by various analysts, is that the seat belt webbing can “spool-out” from vehicle-sensitive emergency-locking retractors (ELR’s). According to this theory, the “spool-out” mechanism occurs because the retractor may cycle between a locked condition to an unlocked condition as the vehicle is overturning. Seat belt webbing can then be spooled-out from the retractor if the occupant engages the seat belt at a time that the retractor is in an unlocked condition. The added webbing introduced into the seat belt system mitigates the effectiveness of the seat belt during the subsequent roll motion. In this paper, we specifically address the performance of ELR’s in rollover accidents. A detailed analysis of the various phases of a multiple-roll rollover sequence, with an emphasis on vehicle dynamics and occupant kinematics as they relate to the physics of the sensing mass and operation of the retractor spool and locking mechanism(s), is presented. Additionally, the results of full-scale rollover testing are analyzed. The conditions to effect a retractor “spool-out” require that the sensing mass of the ELR must move to a neutral position, and the occupant must move in such a way to release tension in the seat belt thereby allowing webbing to retract back onto the spool. This retraction motion is necessary to release the ELR lockup components from a locked position. After conditions have been achieved, the sensing mass must then remain in a neutral position while occupant moves sufficiently, relative to the vehicle, to withdraw seat belt webbing from the The analysis presented in this paper and the results of testing indicate that the circumstances necessary for retractor spool-out to occur are not present in rollover accidents. A condition where sensing mass of the ELR will remain in a neutral position long enough and coincident with the occupant moving relative to the vehicle in such a manner to withdraw appreciable webbing from the does not occur. The external inputs to the vehicle that induce occupant motion also induce mass motion. The sensing mass need only move fractions of an inch to activate the retractor mechanisms. As a result, the retractor will be locked before webbing can be extracted from webbing spool.