Seatbelt performance in rollovers has come under increased scrutiny in recent years. This is due, in part, to growing popularity of sport utility vehicles which have a demonstrated inferior rollover resistance when compared to passenger cars [1]. In the United States (U.S.) the National Highway Traffic Safety Administration (NHTSA) has stated an intent to mandate an increase in the roof strength safety standard. Such an improvement in roof strength will undoubtedly bring an increased focus on the performance of seatbelts in rollovers. Many contemporary seatbelt retractors are equipped with both a vehicle crash sensor as well as a secondary, or backup, webbing sensor. The webbing sensor is intended as a backup locking device in the event of a failure of the primary inertially sensitive vehicle sensor. The crash modes presenting the most potential for the inertial sensor’s failure include nonplanar crashes, multiple impacts, and rollovers [2]. It follows, therefore, that to ensure reliable seatbelt retractor lockup in these modes, the redundant webbing sensor must be tuned with a lockup threshold consistent with expected occupant motions and webbing extraction rates seen during these events.

Rollover tests conducted by NHTSA wherein the belt systems were instrumented for both load and webbing payout were analyzed. This analysis provides insight for determining a baseline lockup threshold for the webbing sensor required to ensure activation in the rollover crash mode. Additionally, multiple retractors designed for both European and U.S. markets have been tested on a bench-top sled. These tests were conducted to include out-of-plane accelerations similar to those observed in rollover crashes.

The retractor sled test results, along with the analysis of the NHTSA rollover tests, are then discussed and used to develop a suggested webbing sensor lockup threshold necessary to ensure the effectiveness of the redundant and backup webbing crash sensor in realworld events.