High-backed booster seats provide good protection to child occupants primarily by promoting good posture and positioning the adult seatbelt correctly across the torso and pelvis. Sash belt-positioning features (also known as sash guides) assist with this. The position of the upper seat belt anchorage is known to affect static sash belt geometry in booster seats. However dynamic testing is commonly performed using a single standard location, not representative of the wide variability seen in the rear seat of the vehicle fleet.

This study investigates the effectiveness of three booster seat sash guide designs during moderatespeed frontal impacts across a range of upper seat belt anchorage locations seen in Australia. On the basis of previous static studies, it was hypothesised that more outboard located anchorages would produce the most challenging belt geometry for the sash guides to overcome.

34 frontal crash tests (Δv=31.5 km/hr, 16.9 g) using the Hybrid III 6 year old test dummy were conducted. The tests were filmed using a high speed camera and head excursions were determined using Phantom software. Seat belt forces, head accelerations and neck loads were measured. The upper D-Ring position was varied over five vertical and horizontal (inboard/outboard) conditions, representing maximum and minimum anchorage height and distance between upper and lower inboard anchorage points. Two different booster seat models incorporating three different sash guide designs were tested with and without the sash guides engaged. The influence of lap-belt placement on dynamic sash belt fit was minimised by use of an anti-submarining feature.

Head excursions with all sash guides at the standard anchorage position, and for standardised belt geometry were comparable. Excursions were substantially lower when no sash guide was used for the integrated head restraint type sash guide. Wide variation in excursion was seen between the minimal and maximal combinations of anchorage position. The integrated sash guide outperformed both variations of strap type in the lower anchorage positions, but produced substantially greater head excursion in the highest, most outboard anchorage position. The strap type sash guides performed worse in the lower positions. The highest, most outboard position yielded comparable excursions for both strap type guides which were similar to excursion at the standard position.

These results suggest that the sash guides were not uniformly effective in maintaining dynamic sash belt position across the range of anchorage positions tested.

This study is the first to demonstrate that both sash guide design and upper belt anchorage position interact to control head excursion in booster seats. While the sash guides produced comparable excursions in the standard anchorage position and for standardized belt geometry, large variations are observed when tested over the range of anchorage positions seen commonly in the rear seat.