Seated Weight Distribution of Adults and Children in Normal and Non-Normal Positions

AIRBAGS have been reported to be effective in reducing the number of fatalities due to car accidents. Nonetheless, severe injuries induced by airbag deployments have become a sensitive issue in the field of crash safety. In July 1998, NHTSA reported 56 cases where a child was severely injured in a low speed accident due to an airbag deployment. Out of the 56 cases, 50 were fatal. Thirty-six of the 50 fatal cases involved a child who weighed less than 30 kg. Another group prone to serious airbag injury is small female drivers. Studies (De Leonardis et al, 1998; Parkin et al., 1993) have shown that small drivers tend to sit closer to airbag module for driving, due to packaging and posture constraints. Industrial consensus believes that small female drivers whose body weight is less than 105 lbs have high probability of airbag injury because of both the short distance between chest to steering wheel and the light body weight. In response to these issues, new airbag technologies are currently being developed to minimize the number of severe or even fatal injuries due to airbag deployment. Those technologies may include either suppression of deployment and/or reduction of deployment power. Technology implementation depends upon accurate detection and discrimination of occupants by using techniques such as occupant weight sensing, seat position sensing, and chest to steering wheel distance sensing. A focus is now being placed on weight-sensing technologies due to NHTSA’s proposal (1998a) that airbag should not deploy if the occupant weighs less than 30 kg. Various weight-sensing technologies are available, including bladder system for seat cushion and frame based system for the cushion and back. Cushion based bladder system is less costly and more easily implemented in a vehicle than the frame based, but can only measure the occupant’s weight distributed through the seat cushion. Frame based system provides a more accurate estimate of the occupant’s weight, but it may require seat design modifications. Prior to assess the optimal sensing technology, a better understanding of occupant’s weight distribution must first be achieved. When an occupant sits in a vehicle, his/her weight can be distributed through the seat cushion, seatback, footrest, armrests, door, instrument panel, and other vehicle interior components. In a normal sitting posture, body weight is largely distributed up to 78% on cushion, 13% on seatback, and 18% on the floor (Zacharkov, 1988). However, since these reported percentages are gross estimates, the results may not be used as guidelines for weight-sensing developments. First, the data reported so far in the literature is typically empirical. While the data is based on some testing or experimental results, the testing is most probably carried out for the purpose of body pressure measurement, with very little control over, and interest in, the load bearing and transfer mechanism. Due to the vast variety of seat design and occupant anthropometry, the obtained data may not represent the mean range of different seats and the mean range of the population weight distribution. Problems such as difference in the actual angles of seat cushion and seat back also exist. Secondly, the estimate is based on normal posture, assuming that the occupant sits symmetrically and that the seat cushion and back are effectively used. In real situation however, occupants may sit differently from lab conditions, causing different distribution of body weight. This study was carried out to determine weight distribution of occupants seated in normal and non-normal positions on a test seat. The study consists of three experiments, adults in normal sitting positions, adults in non-normal sitting positions, and children in various sitting positions. In the first experiment, adult occupant weight distribution on seat cushion, footrest, and armrests was assessed for various cushion angles, back angles, and seat height in normal driving positions. In the second experiment, the weight distribution on the cushion, back, armrest, floor, instrument panel (IP) and door was assessed for normal and non-normal sitting positions. In the third experiment, the weight applied to the cushion, back, floor and IP was determined for children sitting in normal and non-normal sitting positions.