Seating Comfort:
For occupant’s comfort and health, good seat design should be applied by considering sitting postures. Static and dynamic anthropometry data are considered for proper design of a comfortable and safe seat.
Some factors to be considered for driver’s seat:
• The seat should position the driver with unobstructed vision and within reach of all vehicle controls. For this purpose appropriate seat adjustment features should be there.
• Proper back support, headrest, thigh support should be provided but there should not be and obstruction/ hindrance during arm or leg movement.
• Seat must accommodate the driver’s size and shape.
• Seat should be comfortable for extended period.
• Seat should provide a shape zone to the driver in a crash.
Passengers in the front and rear seats need comfortable supporting surfaces for a variety of postures unconstrained by the vehicle operation. Postural stress, vibration, muscular effort, impact and shock are the causes of backache and lower back pain in drivers. Safety should be taken into account while considering the design of seats without compromising the comfort.
Different Factors Considered for Seat Design:
Human geometry both in static and dynamic are considered for designing seats. The static geometry describes the physical size to be accommodated in the seat and dynamic geometry describes the functional position to be accommodated in the seat.
• Body Size:
Seats are mostly designed as per the body weight and anthropometry of the targeted user population to fit at least 90 percent of population. The 95th percentile of male and 5th percentile female anthropometric data is generally considered for accommodation on seats.
Human linkage system: Rigid human body can be specified according to the joint centre position and the angle between adjacent links. The movements and dimensions of human linkage system help to define the curvature of seats and comfortable position for sitting.
• Position of the Body:
Driver’s seat position is dependent on the vision and reach of the driver. Clear view and comfortable sitting posture are the factors considered for designing seat. The dimensioning is mostly depends on eye, hand and foot positioning. For different body verticals, back angle adjustments are provided.
• Posture of the Body:
Seats should reduce postural stress and optimize muscular effort. Postural stress occurs due to adopting one posture for long period of time, so comfortable support for many postures is essential and this can be accommodated by manipulation of anthropometric data and the linkage system.
• Vibration and Ride Comfort:
Vibration, shock and impacts are major factors for judgments of comfortability according to most users. Thus, the seat design also must consider the vehicle suspension system and the vibration transmitted to the seated user.
Geometric Features of Seat Design:
Seat design can be divided into accommodation and comfort. Accommodation refers to seat size and adjustments for horizontal distance from controls, height and back angle. Comfort, however, refers to stiffness, contour, climate and vehicle features that promote the user's comfort.
The seat height, width and back angles are based on the human anthropometry data collected from the research. It is important to provide sufficient space for physical and psychological
comfort.
• Cushion’s length from seat back to the waterfall line is 440-550mm is recommended (Grandjean, 1980).
• The breadth of the cushion is recommended at 480mm (Grandjean, 1980) for clothing and leg splay. The measurement is based on 95th percentile of female hip breadth and additional space for comfort since female hips are greater than male hip breadth.
• Seat back height is recommended at 509mm (Grandjean, 1980) by considering the small female, sitting shoulder height.
• Seat back breadth may be divided into lower and upper regions. The lower must accommodate a tapered shape from 432mm at the hip to 367mm at the chest (Grandjeans, 1980). 480mm is recommended for seat back breadth (Grandjean, 1980).
• Horizontal adjustments accommodate differences in leg length that are associated with seat height and preferred knee angle. Grandjean (1980) recommended a minimum of 150mm horizontal adjustment. The joint angles in automobiles are typically between 95 and 120 degrees for the hip, and 95 and 135 degrees for the knee (Rebiffe, 1969).
• Horizontal seat travel is a function of seat height and body size. Average seat travel was investigated at 148mm approx. (Schneider et al., 1979).
• Vertical adjustments accommodate differences in sitting eye height between the fifth percentile female and 95th percentile male. A simple trigonometric relation can be established with link length and joint angles to compute the amount of seat adjustment needed in the vertical direction.
• Adjusting a flat, non-deformable surface over a range of 163mm maintains a constant eye height. Seat cushion compression and suspension deflection are no-linear functions of applied force; as a result the vertical displacement needed in a soft seat is poorly calculated from anthropometric data. Grandjean (1980) recommended a seat height between 250 and 300 mm.
• Seat back angle adjustments accommodate differences in arm length and occupant preferred hip angle. Grandjean (1980) recommended a seat cushion angle of 19 degrees with a range from 10 to 22 degrees. d on multiple joints or overall body posture (Krist 1994).
All the seat design criteria and dimensions mentioned above are for general understanding of the subject. Presently, various SAE standards are followed in automobile industries all over the world.
Both subjective and objective methods of discomfort measurements are used to analyze and rate the level of discomfort. Among the various rating scales ‘Visual Analogue Discomfort Scale’ or ‘Verbal Numerical Rating Scale’ for assessment of intensity; ‘Body Map’ or specific instruction for assessment of discomfort location and repeated measurement for the assessment of temporal pattern of discomfort are generally used (Van der Grinten 1991). Empirical studies of various scientists provide comfort data for quantitative estimation of sitting comfort of vehicle occupants or drivers based on single joint postural analysis (Porter and Gyi 1998, Grandjean 1980, Henry Dreyfuss Associates 1993, Rebiffe 1969) and some based on multiple joints or overall body posture (Krist 1994).
