24-11-2012, 12:21 PM
SURVEY OF AUTO SEAT DESIGN RECOMMENDATIONS FOR IMPROVED COMFORT
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INTRODUCTION
There is a large body of literature devoted to the study of seating comfort. herblom is
widely credited with beginning the modern, scientific study of seating with his 1948
monograph on posture and chair design (herblom 1948), although he cited over 70
previous publications related to his work. Since 1948, hundreds of papers on topics
related to seating comfort have been published, many of which include recommendations
for seat design to enhance comfort.
The seating literature contains more papers concerned with office and industrial seating
than with automotive seating, probably because of the economic costs associated with
discomfort and injury in the office and factory. However, the motor-vehicle environment
is also a workplace, with the difference between the situations of a commuter and a
professional driver being primarily the length of time in the seat, both cumulative and at a
single sitting. Epidemiological studies have shown that low-back pain and lumbar disc
herniation risk increase with the amount of time spent driving (Kelsey and Hardy 1975).
The presence of vibration in the motor-vehicle environment has been suggested as a
potentiating factor (Troup 1978).
Most of the research findings concerning industrial and office chair design can be applied
to auto seat design. However, there are several important considerations unique to the
mobile environment that should influence design recommendations. In particular, the
control locations and sight line requirements serve to constrain postures to a greater
extent than in most other seated environments. Safety concerns dictate that the driver be
alert and continually responding to changing road conditions, and be positioned in such a
way that the occupant restraint systems offer maximal protection in a crash. Passenger
cars generally require a more extended knee posture than is necessary in other types of
seating. This has 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.
FIT PARAMETERS
The principle that the seat should fit the sitter is the most univers;lly employed concept in
seating ergonomics. People had used chairs for centuries before Akerblom's 1948
monograph, relying on the experience of furniture makers to produce a match between
the sitter and the seat. If a chair is to be used by only one sitter, careful measurements of
that person's body will yield appropriate dimensional specifications for the seat.
However, in the passenger car market, where a single seat must accommodate a large
percentage of the population, knowledge of population anthropometry is required.
The constraints on Fit parameter design values are usually imposed by the desire to
accommodate a sufficient range of the population on one anthropometric measure. A
widely used design criterion is that the seat should accommodate the members of the
population who lie between the 5th-percentile-female and 95th-percentile-male values on
some anthropometric measure of interest. Note that it is not meaningful to refer to
accommodating, for example, a 5th-percentile female, without specifying the dimension
that is accommodated. For example, a woman who is 5th percentile in sitting height
might have thighs that are shorter than 5th percentile for thigh length, so that she might
be accommodated with respect to her view of the instrument panel, but experience
uncomfortable pressure on the back of her knees from a seat cushion that is too long.
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. In the case of cushion width, the 95th-percentile-female hip width is used as
a specification limit, since this measure exceeds the 95th-percentile-male hip width. The
case of cushion width is a good example of how parameter levels might appropriately be
selected in practice. Using the methodology described above, the minimum cushion
width would be chosen to be greater than the 95th-percentile-female seated hip breadth of
432 mm (Gordon et al. 1989). However, a larger minimum cushion width would be
desirable, mainly 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. Grandjean (1 980) recommended
a minimum cushion width of 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.
Cushion Width
Cushion width is specified to accommodate the largest sitting hip breadths in the
population, with additional clearance for clothing and movement. The constraining
population segment is large females, who have a 95th-percentile seated hip breadth of
432 mrn in the Army data. However, Chaffin and Andersson (1991) cite a study of 143
women aged 50-64 years who had a 95th-percentile hip breadth of 457 mm. The higher
number might be more representative of the driving population than the military data
taken from younger subjects. Schneider et al. (1985), in a study of driver anthropometry,
reported an average seated hip breadth of 439 mm in 25 males who were approximately
95th-percentile by stature and weight. This is slightly larger than the 95th-percentilefemale
hip breadth in the Army data. Grandjean (1980) recommends 480 mm as a
minimum clearance at the hips to accommodate large females with an allowance for
clothing. The 480-mm value should be considered to be a minimum to accommodate the
population at a single position on the seat. Maertens (1993) recommends a minimum
overall cushion width of 500 mm, but does not specify the position at which this
dimension is to be measured. Chaffin and Andersson (1991) cite recommendations
from a variety of sources for office chair widths between 400 and 480 mm. Since
freedom of movement is desired to allow for posture changes, 500 mm is recommended
as the minimum clearance at the hips.
Cushion Length
Cushion length is 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. Pressure in this area will lead to
local discomfort and restricted blood flow to the legs. 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 of the small-female
segment of the population. This dimension is measured on the seated subject from the
rearmost projection of the buttocks to the popliteal fold at the back of the knee. Gordon
et al. (1989) report a 5th-percentile-female buttock-to-popliteal length of 440 mm. For
general chair design, Chaffin and Andersson (1991) cite recommendations for cushion
length, measured from the furthest forward contact point on the backrest to the front edge
of the chair, of 330 to 470 mm. Grandjean (1980) recommends 440 to 550 mm, while
Keegan (1964) recommends 432 mm. Maertens (1993) specifies that the cushion should
not extend more than 380 mm forward of the H-point (hip point), which is a seating
reference point approximating the hip-joint center location of a male sitter who is 50th
percentile by height and weight (see SAE 5826). Some calculations are necessary to
compare the Maertens recommendations with those of other researchers.