Overview of Seat Design Changes and Performance

<div class="section abstract"><div class="htmlview paragraph">The National Highway Traffic Safety Administration (NHTSA) published an Advance Notice of Proposed Rulemaking (ANPRM) to update the Federal Motor Vehicle Safety Standard (FMVSS) 207. Part of the ANPRM is to assess the merit of conducting quasi static body block seat pull tests and conducting FMVSS 301 rear crash tests at 80 km/h or higher with a 95<sup>th</sup> percentile ATD lap-shoulder belted in the front seats and limiting seatback deflection to 15 to 25 degrees. Prior to updating regulations, it is important to understand the seating design history and implications.</div><div class="htmlview paragraph">This study was conducted to provide a historical background on seat design and performance using literature and test data. One objective was to first define the terminology used to describe occupant kinematics in rear crashes. Secondly, seat design evolution is then discussed. Third, test methods and test results were summarized, and fourth, the field performance are synopsized and discussed with respect to 2<sup>nd</sup> row occupant protection.</div><div class="htmlview paragraph"><i>Seat design evolution:</i> Seat designs have continuously evolved for the last 70 years, including changes in seat structure and seat/head restraint geometry. Over the past decades, seatbacks have become taller and include open perimeter frames and dual recliners. Head restraints have become larger and/or more forward. Seat properties have also changed over time, resulting in better performance. These changes resulted in an increase in strength. The changes in design and properties allow the occupant to pocket while offering load-limiting and controlled head and neck support.</div><div class="htmlview paragraph"><i>Test method summary:</i> Various methods are used to evaluate seat performance, including quasi-static pull tests and dynamic sled tests. Pull tests include applying a rearward load at the upper cross member of the seatback frame, which does not account for the interaction between the occupant and the seat. Other test methods include using a body block as suggested in the ANPRM. Many tests have been conducted with the body block representative of the upper torso geometry and center of gravity (cg) of a 50<sup>th</sup> percentile occupant. The data show a continuous increase in rearward loading strength, averaging 1,232 Nm (10,902 in-lbs) in seats with model year (MY) 1989 and older and 3,244 Nm (28,716 in-lbs) in MY 2010+, decreasing dynamic seatback rotation. FRED (Ford Rear-Impact Energy Device) is another type of device used in pull testing. FRED provides a more biofidelic test for occupant loading and interaction with the seat; the load location coincides with the cg of a 50<sup>th</sup> percentile occupant. However, there is less FRED data available for historical comparisons.</div><div class="htmlview paragraph">Sled tests are conducted at low-to-moderate speeds and at high-speed. For example, low-to-moderate speed sled tests are conducted at 16 km/h as part of the Insurance Institute Highway Safety (IIHS) head restraint evaluation program, terminated in 2022, and at 17.3 km/h as part of FMVSS 202a.</div><div class="htmlview paragraph"><i>Testing and field performance:</i> The result of this study suggests that the newer seat designs are performing well. Modern seats (2010+ MY) exceed the FMVSS 207 static strength requirement by a factor greater than 8 on average. Dynamic sled tests, conducted with the BioRID and with the 50<sup>th</sup> Hybrid III, show good performance. The occupant biomechanical responses obtained from 40 km/h rear sled tests remain well below injury thresholds. There was a decrease in neck extension. Chest g’s 3ms however remined similar irrespective of MY group; it averaged 14.6 ± 4.6g with 1990-1999 MY- and 15.7 ± 2.4g in 2010+ MY seats. In either the earlier or more recent MY vehicles, these peak chest accelerations occurred prior to maximum seatback deflection.</div><div class="htmlview paragraph"><i>Conclusion:</i> These results of this study provide background for consideration of future test requirements. Overall, conducting dynamic sled tests with a FMVSS 301R impact energy and 50th percentile male ATD may be valuable. However, selecting a maximum dynamic deflection limit would require additional work, as it may affect seat yielding performance. Yielding is beneficial for overall crash safety, particularly for older and/or more vulnerable occupants. Protecting rear seat occupants from front seat interaction may be the reasoning behind limiting the seatback deflection. Care must be taken in understanding the cause of injury risk to rear occupants when considering design changes that may influence the risk to front seat occupants, especially since the front seats are occupied at a higher rate than rear seats. For example, the literature review indicates that factors outside of front seat design such as intrusion were a significant factor on second-row occupant injury outcomes.</div><div class="htmlview paragraph">The size of the BioRID and 50<sup>th</sup> Hybrid III used in sled testing is representative of an average driver involved in tow-away crashes. Conducting tests with 95<sup>th</sup> percentile ATD may thus bias the data and could have unintended consequences for smaller or vulnerable occupants.</div></div>