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Seventeen rear-end impacts with a nominal 8 km/hr change in velocity to five human subjects in four positions were conducted. The four seated positions consisted of the Normal position, with the torso against the seat back, looking straight ahead, hands on the steering wheel, and feet on the floor; the Torso Lean position, with the torso leaned forward approximately 10 degrees away from the seat back; the Head Flex position, with the head flexed forward approximately 20 degrees from normal; and the Head Flex / Torso Lean position, with the head flexed forward approximately 20 degrees from normal and the torso leaned forward approximately 10 degrees from normal position. Relative to the Normal position, it was found that in both positions involving the torso lean, the peak head acceleration for the subject's head was reduced during the head-restraint impact.

The equations of rotational motion used to calculate pre-impact vehicle speeds using the rotational displacement of the vehicles following a collision are well known. The technique uses the rotational momentum exchange during impact and the principle of conservation of rotational energy to calculate the post impact vehicle angular velocity from the energy dissipated during the vehicle's rotation to a stop (product of torque and rotational displacement). Integral to the calculation of the stopping torque on the vehicle is the determination of the effective rotational coefficient of friction (fr) between the tires and the roadway. The interactions of the road with the tires to produce the rotational coefficient of friction (fr) are more complex and less understood than those of linear coefficient of friction (deceleration factor). A derivation of the post impact equations of motion and the kinematics of vehicles in rotation are examined.