Lear First Year Project: The roles of Vehicle Seat Cushion Stiffness and Length in CRS Performance

Principal Investigator: Yun Seok Kang, PhD, The Ohio State University

Below is an executive summary of this project. Please note that this summary describes results and interpretation that may not be final. Final interpretation of results will be in the peer-reviewed literature.


Although over half of rear-seat occupants are children, evidence suggests that most rear seats are too long for children’s legs. However, shortening the seats to accommodate seat belt age children may pose a problem for younger children in large child restraint systems (CRS). It is unclear whether a CRS will perform adequately if its base extends over the edge of a vehicle seat. Additionally, the stiffness of vehicle seat cushions is often targeted toward adult occupants’ comfort preferences. It is unclear what role, if any, cushion stiffness has on CRS performance.


The objectives of this research are to identify whether short or long seats affect injury risk to pediatric occupants in several different types of CRS in frontal crashes, and to determine whether compliant, mid-range, or stiff cushions affect injury risk as well.

Vehicle seat "spacers" used to shorten the effective length of each seat. The seat length of the short condition measured 34.0 cm and the long condition was 43.5 cm.


Sled tests were conducted using the Federal Motor Vehicles Safety Standards (FMVSS) No. 213 frontal pulse. Vehicle seats from a full-size sports utility vehicle were modified and custom produced with three different levels of cushion stiffness: compliant, mid-range, and stiff. The length of each seat was manipulated using foam spacers provided by the manufacturer.

Two different seat lengths were examined in this study: short (34.0 cm) and long (43.5 cm). Three different types of CRS were tested with size-appropriate anthropomorphic test devices (ATDs): rear-facing (RF) CRS with 12-month-old CRABI, forward-facing (FF) CRS with Hybrid III 3-year-old, and high-back booster with Hybrid III 6-year-old. CRS with long base footprints were chosen so that the bases would protrude past the edge of the vehicle seat in the short length condition. The RF CRS also extended past the edge of the long seats.

Each CRS was tested on vehicle seats with each combination of length and stiffness for a total of 18 tests. Each CRS, vehicle seat (cushion and frame), seat belt webbing and buckle were replaced after every test. ATDs were instrumented with head accelerometers and angular rate sensors, 6-axis upper neck load cells, chest accelerometers, and chest linear potentiometers (Hybrid IIIs only).  Kinematics and kinetic data were compared across vehicle seat lengths and cushion stiffness levels to determine which seat configurations were the most beneficial to each type of CRS.


For RF CRS, the short vehicle seats allowed more y-axis rotation but reduced several injury metrics including HIC36 and neck flexion moment, and had slightly beneficial effects on chest resultant acceleration and neck tension force compared to the long seats. For FF CRS, the short seats resulted in slightly lower neck flexion moments but did not appear to affect most other injury metrics. For the booster, the long seats appeared beneficial in terms of chest resultant acceleration, and showed the potential to reduce neck tension and neck shear, but did not have a noticeable effect on other injury metrics. Variations in cushion stiffness did not have a consistent or relevant effect on any of the CRS or occupant responses. All CRS in all seat conditions produced HIC36 and chest resultant acceleration values at or below current FMVSS 213 limits.


The data suggest that short seats may be acceptable or even beneficial to occupants in RF and FF CRS. Boosters, however, may not follow this trend and more study should be conducted to determine if boosters should be fully supported by vehicle seats. CRS performance does not appear sensitive to the cushion stiffness ranges examined in this study.

Co-Principal Investigator

Julie Bing, MS, The Ohio State University


Laura Jurewicz, The Ohio State University; Yadetsie Zaragoza-Rivera, The Ohio State University

IAB Mentors

Russ Davidson, Lear Corporation; Mladen Humer, Lear Corporation; Arjun Yetukuri, Lear Corporation