Seat Design Principles to Reduce Neck Injuries in Rear Impacts

Abstract

Objectives: In the 1990s, research was conducted at General Motors R&D Center on seat safety in rear impacts. It led to the development of high retention seats and an active head restraint to improve occupant safety. This article provides an overview of the design principles found from that research and focuses on seat characteristics that lower whiplash risks.

Methods: Sled and quasistatic seat testing showed how occupants interact with the seat in rear impacts and what seat characteristics improve occupant retention, energy management, and support of the head-neck, lowering injury risks. Neck displacements, moments, and forces were used to assess whiplash and more severe injury risks. A QST test was developed to quasi-statically push a dummy rearward into the seat to determine seat stiffness (k), frame strength (j), and peak bending moment (M_Hpt). These parameters were related to neck displacements associated with whiplash. Sled tests were run with in-position and out-of-position male and female Hybrid III dummies to assess performance. A high retention seat and active head restraint were developed and put into production in 1997.

Results: High retention seats have 2.3 times greater moment, develop 2.2 times greater load, but have the same stiffness as earlier yielding seats. Seat stiffness was found to be a principle characteristic related to neck displacements associated with whiplash. The combination of a stronger frame, yielding seatback, and high-forward head restraint in the high retention seat provides early head support and low neck displacements in rear impacts. Larger reductions in neck displacement were obtained by adding an active head restraint that moves the head restraint forward and upward by occupant penetration into the seatback. This substantially reduces head contact time, neck displacements, and loads.

Conclusions: Whiplash risks are related to seat stiffness, the position of the head restraint, and frame strength. Low seat stiffness allows the occupant to move into the seatback without high loads on the torso until the head-neck is supported by the head restraint. A strong seat frame reduces early seatback rotation that increases the gap to the head restraint and drops it in relation to the occupant's head. A high and forward head restraint provides support of the head and neck. Large forces can be applied to the occupant once the head, neck, and torso are supported by the seat and head restraint without adverse loading of the spine. The addition of an active head restraint closes the gap behind the head before significant load develops on the neck. The movement provides a more upward trajectory of the head restraint. Low-speed rear crashes are not just a matter of whiplash; older occupants, some with cervical stenosis, are at risk for paralyzing spinal cord injury.

Keywords:

• Whiplash
• Seats
• Head restraints
• Spinal cord injury
• Rear crashes
• Injury biomechanics

ACKNOWLEDGMENTS

This article is based on a keynote lecture given at the World Congress on Neck Pain, January 20–22, 2008, at the Hyatt Regency Century Plaza, Los Angeles, California. The research reported here was primarily conducted from 1990 to 1997 at General Motors Research and Development Center in Warren, Michigan, and was undertaken in collaboration with the Interior Division of Delphi Corporation. In the late 1990s, Delphi sold its seating business to Lear Corporation. The sale included all intellectual property related to seating systems, including the patents for the high retention seat (vehicle seat with perimeter frame and pelvic catcher, U.S. Patent No. 5,509,716, April 23, 1996), active head restraint (vehicle pivotal headrest, U.S. Patent No. 5,378,043, January 3, 1995), and QST device (seatback load applying device, U.S. Patent No. 5,379,646, January 10, 1995). Lear is selling the high retention seat and SAHR today. While I was involved in the development of the QST test, high retention seat, and active head restraint, there were many people in Delphi, Saab, and General Motors who contributed to the successful efforts to bring to production seats and head restraints that increased the safety of occupants in rear impacts. For readers interested in the topic, two books have been published by the Society of Automotive Engineers (SAE) on seat safety. One describes seat benchmarking and the development of the high retention seat and active head restraint system (Viano, 2002). The other addresses the principles of seat design for occupant protection in rear impacts (Viano, 2003i). Much more study has been undertaken since the publication of these books, including seats and second–row children (Viano and Parenteau, 2008a). That research can be accessed at SAE (www.sae.org) and through PubMed (www.ncbi.nlm.nih.gov/pubmed/).