The Effect of the Head to Head-Restraint Distance on Occupant Kinematics during Low-speed Rear-End Impacts

Paper #:
  • 2018-01-0537

  • 2018-04-03
The longitudinal motion of the head, thorax and lumbar spine of two test subjects was measured in low-speed rear-end impacts in order to understand the effect of the head to head-restraint distance (backset) on occupant kinematics. The two test subjects were exposed to three rear-end impacts at two different crash severities, changes in velocity (ΔV) of 3.9 km/h and 7.7 km/h. The struck vehicle was a 2006 700 series BMW sedan and the striking vehicle was a test buck. The independent variable in the crashes at a given ΔV was the backset, which was set at 0, 5 or 10 cm. The x and z-axis accelerations of the car and the anatomical x and z-axis accelerations of the test subject’s head, upper thorax and lower lumbar region were measured. The head accelerations were measured with a bite block that contained a rotational velocity sensor and two translational accelerometers. The bite block accelerations were transformed to the head center of gravity (CG). The thoracic accelerations were measured at the manubrium with two translational accelerometers and a rotational velocity sensor. The lumbar accelerations were measured at the L5 level with two translational accelerometers. The head CG, upper thoracic and L5 accelerations were transformed to an earth-based coordinate system in order to understand the forward accelerations and movement of each body region relative to the earth during the crash. Along the earth x-axis the vehicle accelerations increased first, followed by the accelerations of the lumbar region, then the thoracic region, and then the head CG. The peak earth x-axis accelerations of the lumbar region and the upper thoracic region were similar to the vehicle’s peak x-axis acceleration, regardless of the amount of the backset. The peak head CG earth x-axis acceleration was approximately 1.5 times greater than the peak vehicle x-axis acceleration when the backset was 0 cm. When the backset was 5 and 10 cm the peak head CG earth x-axis acceleration was approximately 2.0 times the peak vehicle x-axis acceleration. The amount of extension rotation, the peak rotational velocity and the peak rotational acceleration of the head CG increased as the backset increased. The peak rearward velocity of the head CG relative to the upper thorax increased as the backset increased. Head kinematics were found to be strongly influenced by the amount of backset and the manner in which the subject created the backset, either through cervical flexion or through a combination of cervical and upper thoracic flexion.
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