Energy Transfer to an Occupant in Rear Crashes: Effect of Stiff and Yielding Seats 2003-01-0180

For several decades, there has been a debate on the safety merits of yielding and rigidized (stiff) seats. In 1995, GM adopted requirements for high retention seats and introduced a new generation of yielding seatbacks. These seats have the same stiffness as the yielding seats of the 1980s and early 1990s, but have a strong frame structure and recliners to substantially limit seatback rotation in severe rear crashes. The yielding behavior is given by compliance of the seat suspension across the side structures and an open perimeter frame, which allows the occupant to penetrate into the seatback.

The purpose of this study is to compare the energy transfer characteristics and occupant dynamics of yielding and stiff seats in 35 km/h and 16 km/h rear crashes. Based on benchmarking tests, the stiff seatback is defined as one having a 40 kN/m stiffness in rearward loading by a Hybrid dummy. The yielding seatback has a 20 kN/m stiffness, which is comparable to the 1997 W car seat, the first high retention seat introduced by GM. For this study, the vehicle acceleration was a constant 10 g or 4.5 g over 100 ms giving the delta V of 35 km/h or 16 km/h. An Excel spreadsheet analysis was conducted to determine occupant dynamics in a step-forward solution based on the differential displacement between the occupant and vehicle. Force and acceleration of the occupant are proportional to seatback stiffness and the differential displacement.

The yielding seatback developed 15%-16% lower forces on the occupant and acceleration with a delayed loading as compared to the stiff seat. The time delay improved the efficiency of the energy transfer and allowed force to be applied on the occupant after the head, neck and torso received uniform support by the seatback and head restraint. Even with an initial 15 cm gap behind the occupant or a higher initial vehicle acceleration pulse, the yielding seatback provided a more gradual acceleration and lower loads on the occupant with 68% greater occupant displacement with respect to the vehicle.

This study shows the benefits of a yielding seatback over a stiff seat on occupant dynamics in rear crashes. The more gradual ride-up of the crash delta V allows a later occupant loading during the time the head, neck and back have more uniform support from the seat and head restraint. In contrast, the stiff seat has a load of 4 kN when the relative penetration of the occupant is 10 cm, which is before the likely time of head restraint contact. The comparable load with the yielding seatback is only 2 kN indicating much lower forces on the back and base of the neck.