Motion sequence criteria for favorable occupant kinematics in rear impacts

Rear-impact restraint guidelines have not developed to the same degree as for frontal crashes. This study provides criteria for favorable occupant kinematics in rear impacts.

Rear criteria were developed as an extension of Adomeit and Heger (1975) and Adomeit (1977) motion sequence criteria (MSC) for favorable occupant kinematics in frontal crashes. In this study, occupant kinematics in rear sled tests were studied to develop motion sequence criteria for favorable and unfavorable occupant kinematics in rear impacts with containment of the hip on the seat and no ramping up the seatback.

Rear MSC limit the angle of the torso (α) rearward of vertical to less than the critical angle (α_c) for ramping up the seatback and H-pt displacement rearward and downward, so z_Hpt < z_Hpt0, where z_Hpt0 is the initial height of the H-pt. The lateral displacement of the occupant is limited to less than the critical lateral displacement y_T1c, where the head becomes unsupported by the head restraint or the chest by the seatback. The rear MSC contain the pelvis on the seat and provide uniform support of the torso, head and neck. Most front seats in production provide reasonably favorable occupant kinematics in rear impacts up to 40 km/h (25 mph) delta V with the 50^th Hybrid III. Kinematics become unfavorable in testing at higher severities and with heavier occupants. The amount of energy that the seat needs to transfer to the occupant in a rear impact depends on the delta V (ΔV or change in velocity) of the vehicle and the mass of the occupant (m) among other variables. The seat provides an interface with the occupant and transfers energy (E), which can be approximated by E = ½*0.7*m(ΔV)² using 70% of the occupant mass (m) and delta V. Rear MSC provide performance guidelines to advance seat designs with favorable occupant kinematics at higher energy transfer levels in rear impacts. Sled testing at 40 km/h (25 mph) involves an energy transfer of 3,421 J with the 50^th Hybrid III generally gives favorable kinematics. A 56.3 km/h (35 mph) test involves 6,704 J, double the energy transfer and often unfavorable kinematics. A target energy needs to be set, and there are practical limits because the energy transfer is 12,858 J with a 150 kg (330 lb) occupant in a 56.3 km/h (35 mph) delta V rear crash.

Rear motion sequence criteria (MSC) define favorable kinematics in rear impacts. MSC complement the assessment of biomechanical responses in sled and crash testing to ensure an overall evaluation of occupant restraint in rear impacts.