AUTO SEAT DESIGN RECOMMENDATIONS

The two environments where we spend most time in constant sitting posture are at the workplace and in a motor vehicle. Most of the research findings concerning industrial and office chair design can be applied to auto seat design. However, additional considerations unique to the mobile environment are sight line requirements, extended knee posture and Safety concerns. These have important implications with regard to the orientation of the sitter's pelvis and lumbar spine. Additionally, vibration imposes tissue stresses that are not generally present in a stationary environment. Epidemiological studies show that the main problems that arise over prolonged sitting are lower-back pain and lumbar disc herniation risk. The aim of chair design should be to reduce or eliminate factors causing discomfort rather than to elicit feelings of well-being.

Hence the design parameters to be considered are divided into three categories:
1. Fit parameter: Anthropometry and Seat cushion.
2. Feel parameters: Pressure distribution and Upholstery properties.
3. Support parameters: Seat contours and Adjustments.

These parameters are very much in correlation with each other. For example, a change in backrest curvature (Support) will affect the pressure distribution (Feel) and also change the effective cushion length (Fit). However, this parameter categorization is useful because the knowledge required to specify parameter levels in each of these categories comes from distinct areas of research.

1. FIT PARAMETERS
The principle is ‘seat must fit the sitter’. However, in the passenger car market, where a single seat must accommodate a large percentage of the population, anthropometry is required. The procedure followed here to find out the parameters are (1) identify the members of the population who represent the extreme of the accommodation range (e.g., small women), (2) select the relevant anthropometric values, (3) determine appropriate values for the selected anthropometric measurements, and (4) provide for at least that level of accommodation. In general, fit parameter levels are specified by noting the constraining values among the set of 5th-percentile-female and 95th-percentile-male values for particular anthropometric dimensions.
1.1 Cushion width: The 95th-percentile-female hip width is used as a specification limit, as this exceeds the 95th-percentile-male hip width. However, a larger minimum cushion width would be desirable, because the anthropometric measure does not include clothing. Since an auto seat must generally be suitable for use in cold climates where heavy clothing is worn, a margin must be included for clothing thickness. Recommended minimum cushion width is 480 mm, including clothing and an allowance for leg splay. A good design practice would be to provide clearance for a width of 500 mm at the hips. Note that this does not mean that the cushion itself must be this wide, but only that the clearance at the hip point should meet or exceed this value.
1.2 Cushion length: an important determinant of comfort for several reasons. First, a cushion that is too long can put pressure on the back of the sitter's legs near the knee, an area that has many superficial nerves and blood vessels. Second, a cushion that is too long will pull sitters forward, away from the backrest, eliminating the possibility of providing appropriate lumbar support. Third, a long cushion can restrict leg splay by interfering with knee movement, and may impede posture changes that alter pressure distributions under the buttocks and upper thighs. Cushion length is constrained by the buttock-to-popliteal length. This dimension is measured on the seated subject from the rear most projection of the buttocks to the popliteal fold at the back of the knee. The cushion length, measured along the thigh line, should not exceed 440 mm from the depressed backrest, or 305 mm from the H-point. The 95th-percentile-male