Occupant Responses in High-Speed Rear Sled Tests: Focus of Initial Position, Seat Strength and ATD Size

<div class="section abstract"><div class="htmlview paragraph">The National Highway Safety Administration (NHTSA) recently published an Advanced Notice of Proposed Rulemaking (ANPRM) to evaluate seat performance in rear impacts [<span class="xref">1</span>]. The ANPRM was issued partially in response to two petitions requesting an increase in seatback strength requirements and high-speed testing with various size Anthropometric Test Devices (ATDs). To better understand the effect of these requests, this study evaluates ATD responses with two high-speed rear sled conditions, three occupant sizes and various seat designs. Seat designs varied from modern conventional seats with yielding properties to stronger and stiffer seats represented by seat integrated restraint (SIR) designs, and rigidized SIR seats.</div><div class="htmlview paragraph">Twenty-four rear sled tests were analyzed. The tests were matched by crash severity, seat designs (strength), ATD sizes and initial postures (nominal/in-position, leaned forward and leaned outboard). The test data and videos were reviewed to identify time coinciding with maximum seatback rotation.</div><div class="htmlview paragraph">Sixteen tests were conducted with the lap-shoulder belted 50th male Hybrid III ATD at 40 km/h, 10 with nominal position and 6 with the ATD leaned forward. In the nominal position, the biomechanical responses were below Injury Assessment Reference Values (IARV); the lower neck Nij was however higher with SIR than non-SIR seats. The gap between the head/upper torso and seat/head restraint increased when the ATD was leaned forward. Compared to nominal position, the responses were higher due to the increase in differential velocity between the ATD and the seat/head restraint. The head, lower neck and chest responses were higher in the SIR than in the non-SIR seats. However, the responses were below IARV except for the lower neck extension in the SIR seat, highlighting the need to support the occupant early in the crash event and the need for yielding properties.</div><div class="htmlview paragraph">Six tests were conducted at 56 km/h with the 5th female Hybrid III, 4 in nominal position and 2 leaned outboard. The biomechanical responses were higher with SIR than non-SIR seats in the nominal position. When the ATD was leaned outboard, the head engaged the SIR rigid structures, resulting in high head responses. Two tests were also conducted with the 95th male Hybrid III at 56 km/h in two SIR seats. The seatback deflected more than 60 degrees, and the normalized biomechanical responses were below IARV.</div><div class="htmlview paragraph">The results from this study indicated that the ATDs’ kinematics were well controlled when the head, neck and torso were centered on the head restraint and when they were supported early in the 40 and 56 km/h rear sled tests. Seatback rotation increased with occupant size. It was higher in non-SIR seats than in SIR seats. The results also showed similar responses with the 5th ATD in a conventional seat and for the 95th in a stronger and stiffer SIR at 56 km/h.</div><div class="htmlview paragraph">In conclusion, seat and occupant responses were favorable with the lap-shoulder belted 50th Hybrid III placed in position in a 40 km/h delta V test, regardless of seat design. The responses were also favorable with the 95<sup>th</sup> male Hybrid III placed in position at 56 km/h in a stronger seat. However, the responses were unfavorable with the stronger seat with the 5<sup>th</sup> female Hybrid III at 56 km/h, and/or when the ATD was placed out-of-position. These results highlight concerns with respect to smaller occupants when recommending stronger and stiffer seats, higher test speeds and heavier ATDs.</div></div>