Influence of Seating Position on Dummy Responses with ABTS Seats in Severe Rear Impacts 2009-01-0250

Objective: This study analyzes rear sled tests with a 95th% male and 5th% female Hybrid III dummy in various seating positions on ABTS (All Belt to Seat) seats in severe rear impact tests. Dummy interactions with the deforming seatback and upper body extension around the seat frame are considered.

Methods: The 1^st series involved an open sled fixture with a Sebring ABTS seat at 30 mph rear delta V. A 95th% Hybrid III dummy was placed in four different seating positions: 1) normal, 2) leaning inboard, 3) leaning forward and inboard, and 4) leaning forward and outboard. The 2^nd series used a 5th% female Hybrid III dummy in a Grand Voyager body buck at 25 mph rear delta V. The dummy was leaned forward and inboard on a LeSabre ABTS or Voyager seat. The 3^rd series used a 5th% female Hybrid III dummy in an Explorer body buck at 26 mph rear delta V. The dummy was leaned forward and inboard on a Sebring ABTS or Explorer seat. The upper and lower neck was instrumented in addition to the head, chest and pelvis. The dummies were belted. High-speed cameras documented occupant kinematics and seat deformation.

Results: For the 95th% male dummy in normal or leaning inboard position, there was more than 20^o dynamic rotation and twist of the ABTS seatback and the head was supported. Dummy responses were below 60% of IARVs. Leaning the dummy forward and inboard caused the greatest seatback rotation and twist. The upper body moved rearward and inboard of the seatback frame. This caused the upper body to wrap around the seat frame with high neck extension moments and head accelerations. HIC was 136% of tolerance and the lower neck extension moment was 143%. Leaning forward and outboard caused the dummy to move rearward into the strong side of the seatback and load the stanchion for the integrated shoulder belt. The head extended above the seatback and outboard of the head restraint with peak head acceleration 106% of tolerance and neck extension moment 133%. The seatbelt restrained the pelvis as the neck extended around the seatback. For the 5th% female dummy on a LeSabre ABTS seat, the change in seatback angle was 33^o on the inboard side of the seatback and there was 21^o of twist. The outboard side remained upright while the inboard rotated rearward. The inboard movement of the upper torso caused the neck to extend rearward around the seatback frame. Lower neck extension moment was 290% of tolerance. The comparable response with the Voyager seat was 110%. For the 5th% female dummy on the Sebring ABTS seat, the relatively upright seatback caused the neck to extend around the ABTS seatback. Lower neck extension moment was 261% of tolerance. The comparable response with the Explorer seat was 107%.

Conclusions: The 95th% male dummy tests show that occupant responses are well controlled when the head, neck and torso are supported by an ABTS seat in severe rear impacts. Seating positions that allow the head or upper back to displace outside the seat frame result in high biomechanical responses as the body wraps around the strong frame of the relatively upright seatback. The 5th% female dummy tests show that the stanchion-side of the ABTS seat remains relatively upright. Inboard movement of the dummy and seatback twist allow the head to extend around the seatback frame resulting in high neck extension moments. These tests show that occupants may be at risk for serious injury when the strength of the seat exceeds the extension tolerance of the spine and when their upper body, head or neck is unsupported. This occupant kinematic may occur in severe rear impacts that are offset, oblique or involve complex vehicle dynamics, or where the occupant is initially out-of-position. Heavier occupants may be more at risk due to proportionately higher mass of the unsupported upper body. Belted occupants may also be more vulnerable since the lap belt restrains the pelvis as the upper body displaces around the seatback frame.