Analysis of performance degradation of an active pretensioning seatbelt in a real-world vehicle braking test

Previous research has established the effectiveness of active pretensioning seatbelts (APS), also termed motorized pretensioning seatbelts, in mitigating forward leaning and out-of-position displacement during pre-crash scenarios. In the Chinese market, APS trigger times are typically set later than those reported in the literature. This study investigates the real-world performance of APS systems with delayed trigger times under emergency braking conditions. Static pretensioning tests were conducted without the influence of braking deceleration pulses, using a THOR-50M ATD and seatbelt pretensioning parameters recommended by the APS supplier and vehicle manufacturer. The time histories of seatbelt tension force and webbing displacement were recorded and compared with data from real-world vehicle braking volunteer tests. To examine the causes of pretensioning performance degradation, a simulation restraint model incorporating APS was developed in the MADYMO platform. The simulation results were analyzed to elucidate discrepancies between the supplier’s claims and the performance observed in real vehicles. During static sled tests, the APS demonstrated a stable pretensioning force of 200 N and a webbing pull-in distance of 3–4 cm. In contrast, real-world vehicle braking tests revealed significant fluctuations in seatbelt tension force, which ranged from below to exceeding 200 N, with no webbing retraction into the retractor observed during braking. Simulations further suggested that a functional conflict between the inertia reel retractor and the active pretensioning motor could cause performance degradation, aligning with experimental findings. When this functional conflict was removed in the simulation model, the pretensioning behavior corresponded with results from previous studies. This study identified that active pretensioning seatbelt (APS) systems installed in vehicles may exhibit inadequate performance during real-world emergency braking scenarios. It is hypothesized that if the active pretensioning function is triggered after braking has initiated, the retractor may already be locked, restricting the spool rotation and preventing the pretensioning motor from further retracting. These findings highlight the necessity for APS suppliers and vehicle manufacturers to carefully design pretensioning configurations and ensure that the system can effectively retract seatbelt webbing during emergency braking.