The potential injury reducing benefits of pre-crash belt slack reduction of a motorized seat belt system was evaluated. The evaluation was carried out for 1 second pre-crash braking followed by a 56km/h full frontal rigid wall crash. For the evaluation a validated active human body model and a model of the THOR dummy were used. The active human body model is capable of, and validated for, predicting occupant kinematics during pre-crash braking and occupant response for crash loading. In the study substantial belt slack was introduced by adding 100mm thick foam pads between the occupant and belt. Pads were added between the chest portion of the seat belt and the chest and between the lap portion of the belt and the pelvis. The effect of 300N and 600N pre-crash pretensioning (pre-pretensioning) of the belt on occupant kinematics and chest deflection during 1 second braking followed by a 56km/h full frontal rigid wall crash was evaluated. In addition the effect of in-crash triggered pyrotechnic pretensioning of the belt was also evaluated.

It was found that pre-crash forward excursion of the occupant during braking was reduced by pre- pretensioning the belt. The forward excursion was reduced for both the occupant without slack and the occupant with 100mm slack. For pre-crash braking followed by a crash generally chest deflections were reduced with pre-crash pretensioning of the belt. Reductions were obtained for the occupant without slack as well as for the occupant with 100mm slack. However, greater reductions was obtained for the occupant with 100mm slack than for the occupant without slack. It was also generally found that additional reductions in chest deflection was obtained for the in crash activated pyrotechnic pretensioners.