A crash happens and then what? As an investigator, you may be called upon to evaluate the conditions of the crash, the forces and accelerations acting on the vehicles, and the forces and moments acting on the occupants. After you have completed this portion of the analysis, and you have studied the learned treatises and peer-reviewed literature relating to human tolerance, you discover that the severity and nature of the injuries may not be consistent with the level of forces generated in the particular crash.
Because crash injury protection is governed by the established tolerance of the human body to both inertial and impact injury mechanisms and vehicle forces are governed by the laws of physics, what can cause this disparity? One critical factor to consider is the design of the vehicle and its components, such as seats and seat belts. This paper focuses on collisions with a rear line of force relative to the occupant (i.e., rear-end crashes) and explores the design of the seating system as a cause of the disparity between the injuries to the occupants and the forces generated in the crash. In particular, this paper explores the risks versus the benefits of seat strength in the rear-impact direction to establish a roadmap for seating and restraint designers.
Review of the literature on automotive seating systems will show two opposing viewpoints on seat design. One view is commonly referred to as the "yielding seat"1,2,3,4 theory. This theory advocates a seat that "yields" rearward in a rear-line-of-force collision to allegedly offer benefit to the occupant by reducing the level of the crash impact force experienced by the occupant. The other view5,6,7,8,9,10,11,12 maintains that a stronger seat design which does not collapse provides superior occupant protection by ensuring that the occupant is fully supported and restrained during otherwise survivable rear line of force impacts.
A risk benefit analysis addresses the question of whether a foreseeable risk is deemed acceptable. This paper provides an analytical evaluation of the decision-making process for automotive seat design in terms of risks and benefits, highlighting the trade-offs between the two in relation to the injury potential during a rear-line-of-force crash. Through a series of tests, learned treatises, and supporting literature, this paper quantifies the hazards associated with each approach.
The published literature 13,14,15,16 reports that the primary benefit of a seat that "yields" rearward in a rear line of force impact is that the yielding or deformation of the seat structure is a method of energy absorption and will result in less force being applied to the seated occupant in a given crash. This design approach has never been subjected to a rigid or proper engineering hazard analysis; it has never been quantified in terms of the amount of force reduction achieved by the "yielding" of the seat, and it has never been tested with human subjects under crash conditions.