F02

International Master's Programme
Of ProdllCtion School of Technology Management andEconomics
Department of Master's
Consumer Technology Thesis nr 1996:l
An Experimental Pilot Study to
Evaluate Car Seat Side Support
Denis Alves Coelho
Gõteborg, February 1996 MoP 96:l
Master of Science Thesis
CHALMERSUNIVERSITYOFTECHNOLOGY • SWEDEN

1
An Experimental Pilot Study to Evaluate Car Seat Side Support Denis Alves Coelho
Master of Science Thesis 1996

Department of Master’s
Consumer Technology Thesis nr 1996:1
An Experimental Pilot Study to
Evaluate Car Seat Side Support
Denis Alves Coelho
Göteborg, February 1996 MoP
96:1
Master of Science Thesis

CTH / KT / EX-96 / 0001
AN EXPERIMENTAL PILOT STUDY TO
EVALUATE CAR SEAT SIDE SUPPORT
by
Denis Alves Coelho
A thesis submitted in partial fulfillment of
the requirements for the degree of
Master of Science
Management of Production International Master’s Programme
Department of Consumer Technology
School of Technology Management and Economics
Chalmers University of Technology
1996
ISSN 1401-081X

i
Foreword
This Master thesis was conducted as a final work for the degree of Master of Science,
in the International Masters Program Management of Production, of the School of
Technology Management and Economics, of Chalmers University of Technology,
Gothenburg, Sweden. My academic supervisor was Sven Dahlman, Professor in the
Department of Consumer Technology, integrated in the School of Technology Man-
agement and Economics, of Chalmers University of Technology.
Volvo Car Corporation’s Ergonomic Centre in Gothenburg was the problem provider
and all the automotive products used in the experiments included in this study origi-
nate from the Volvo 850 passenger car model. Solveig Nilsson was the contact person
at Volvo Car Corporation’s Ergonomic Centre in Gothenburg.
Sven Dahlman, was through out the whole thesis work (carried over a period of eight
months) an available person to discuss the evolution of the study and who provided
new ideas and suggested ways to tackle the problems at hand. Sven Dahlman provided
me with continuous and invaluable support, with meetings held every week where we
discussed the state of the project and found ways to overcome the obstacles in the
course.
Gothenburg, February 14th
, 1996
Denis Alves Coelho

ii
Table of Contents
Abstract ................................................................................................................................................... v
1. Introduction ........................................................................................................................................ 1
1.1 General problem............................................................................................................................ 1
1.2 Reasons for interest ....................................................................................................................... 1
1.2.1 Anthropometric variation....................................................................................................... 1
1.2.2 Adjustibility in the side support ............................................................................................. 3
1.2.3 Car utilisation type and purpose intended for the side support .............................................. 4
1.2.4 Concluding Remarks.............................................................................................................. 5
1.3 Demarcation of limits .................................................................................................................... 6
2. Problem Discussion............................................................................................................................. 8
2.1 Literature Review........................................................................................................................... 8
2.2 Discussion of the issues involved................................................................................................... 8
2.2.1 The phenomenon of lateral acceleration ................................................................................ 8
2.2.2 The securing function of the side support .............................................................................. 9
2.2.3 Perceived comfort and long term use of the seat with side support...................................... 10
2.2.4 Mechanical model................................................................................................................ 10
2.3 Objective, research questions and hypotheses............................................................................. 13
3. Experimental Design ........................................................................................................................ 15
3.1 Preliminary considerations.......................................................................................................... 15
3.1.1 An indicator of the functionality of the side support............................................................ 15
3.1.2 Indicators of (dis)comfort .................................................................................................... 15
3.1.3 Treatment of the side support’s adjustibility........................................................................ 16
3.1.4 Introduction to the description of the practical limitations encountered in setting the
experimental design ...................................................................................................................... 16
3.2 Trial persons (subjects)................................................................................................................ 17
3.2.1 Practical choices and limitations.......................................................................................... 17
3.2.2 Subjects for the experiments: With varied dimensions covering the accommodation range or
with similar dimensions?............................................................................................................... 17
3.2.3 Setting up of the actual subject recruiting criteria................................................................ 18
3.3 Test seats...................................................................................................................................... 19
3.3.2 Design of experiments using the fractional factorial approach ............................................ 20
3.3.3 Hypotheses to test and variation factors included in the experimental design...................... 21
3.4 Data collection ............................................................................................................................ 22
3.4.1 Questionnaire for subjective evaluation of support.............................................................. 22
3.4.2 Questionnaire for subjective evaluation of comfort ............................................................. 23
3.4.3 Questionnaire for satisfaction with seat characteristics (using the method of paired
comparisons)................................................................................................................................. 23
3.4.4 Video recording for evaluation of seat / occupant displacement.......................................... 24
3.4.5 Seat /occupant interfacial pressure readings......................................................................... 24
3.5 Driving sessions........................................................................................................................... 24
3.5.2 Plan for the execution of the driving experiments................................................................ 25
3.6 Readings of the pressure distribution over the seat / occupant contact with lateral acceleration25

iii
3.6.2 Validation of the tilting method for simulation of the lateral acceleration from a car / road
situation......................................................................................................................................... 27
3.6.3 Guidelines for the execution of the tilting sessions and delineation of the subsequent use of
the pressure readings..................................................................................................................... 28
3.7 Analysis of the data collected in the experiments ........................................................................ 28
3.8 Summary of the experimental design reached at ......................................................................... 29
4. Conduction of the experiments........................................................................................................ 30
4.1 Trial persons................................................................................................................................ 30
4.2 Test seats...................................................................................................................................... 30
4.3 Driving sessions........................................................................................................................... 34
4.3.1 Driving task, car, questionnaires and subjects..................................................................... .34
4.3.2 Duration and order of the seat evaluations........................................................................... 32
4.3.3 Video recordings.................................................................................................................. 33
4.3.4 Weather in the days of the evaluations................................................................................. 33
4.4 Readings of the pressure distribution over the seat / occupant contact ...................................... 35
4.4.1 Equipment characteristics and problems found in its operation........................................... 35
4.4.2 Setting in which the pressure measurements were done....................................................... 36
5. Results.................................................................................................................................................. 1
5.1 Description of the results............................................................................................................. 37
5.1.1 Results of the subjective evaluation of comfort ................................................................... 37
5.1.2 Results of the subjective evaluation of lateral support......................................................... 38
5.1.3 Results of the paired comparisons questionnaires................................................................ 38
5.1.4 Results of the video recording of the driving tasks .............................................................. 39
5.2 Analysis of the results .................................................................................................................. 40
5.2.1 Analysis of the subjective evaluations of comfort and support ............................................ 40
5.2.2 Analysis of the data from the paired comparisons................................................................ 41
5.2.3 Analysis of the data from the video recordings of the driving sessions................................ 41
5.2.4 Analysis of the data from the pressure measurements.......................................................... 42
5.2.5 Establishment of correlation among subjective evaluations, paired comparisons, pressure
data and video data........................................................................................................................ 46
5.2.6 Extrapolation of seat ranks in the aspects of comfort and support ....................................... 46
5.2.7 Interpretation of the results under the light of the theory of the design of experiments ....... 46
5.3 My own subjective impressions of the test seats .......................................................................... 46
5.3.1 Seat A................................................................................................................................... 47
5.3.2 Seat B................................................................................................................................... 47
5.3.3 Seat C................................................................................................................................... 47
5.3.4 Seat D................................................................................................................................... 47
5.3.5 Rankings of the seats............................................................................................................ 48
5.4 Concluding remarks..................................................................................................................... 48
6. Discussion.......................................................................................................................................... 49
6.1 Revision of problem discussion under the light of the results...................................................... 49
6.1.1 Formulation of new hypotheses............................................................................................ 49
6.2 Revision of the experimental design and the conduction of the experiments under the light of the
results................................................................................................................................................. 50
6.2.1 Test seat design and specification of the subjects’ dimensions............................................ 51
6.2.2 Driving task design .............................................................................................................. 51
6.2.3 Questionnaire design............................................................................................................ 52
6.2.4 Validity of tilting setting and pressure measurements.......................................................... 53

iv
6.2.5 Validity of the method of apprehension of displacements from the video recording in the
driving sessions............................................................................................................................. 53
6.2.6 Revised experimental design................................................................................................ 53
6.3 Confrontation of my subjective impressions of the test seats with the discussion under the light of
the results........................................................................................................................................... 54
7. Practical Design Advise.................................................................................................................... 57
7.1 Static and non-adjustable side support........................................................................................ 57
7.2 Introduction of adjustment in the side support ............................................................................ 58
7.3 Automatically and dynamically deploying side support .............................................................. 59
7.4 Summary of the findings and suggestions with practical relevance ............................................ 60
8. Conclusions ....................................................................................................................................... 61
9. Bibliography...................................................................................................................................... 62
10. Appendixes...................................................................................................................................... 63
10.1 Appendix A - Test seat sketches ................................................................................................. 63
10.1.1 Test seat A.......................................................................................................................... 63
10.1.2 Test seat B.......................................................................................................................... 64
10.1.3 Test seat C.......................................................................................................................... 65
10.1.4 Test seat D ......................................................................................................................... 66
10.2 Appendix B - Questionnaires..................................................................................................... 67
10.2.1 Support............................................................................................................................... 67
10.2.2 Comfort.............................................................................................................................. 68
10.2.3 Body map........................................................................................................................... 69
10.2.4 Paired comparisons ............................................................................................................ 70
10.3 Appendix C - Driving task description........................................................................................ 1
10.3.1 Map of the test circuit, trajectory and kinematics .............................................................. 71
10.3.2 Protocol.............................................................................................................................. 72
10.3.3 Photographic images of the driving sessions...................................................................... 73
10.4 Appendix D - Ordering of the prospective subjects................................................................... 74
10.5 Appendix E - Sample of pressure print ...................................................................................... 75

v
Abstract
Four seats were designed based on a reference standard seat in actual production.
Three design factors, thought to affect the support given by the seat, were manipulated
in the seats: The friction properties of the seat cover, the distance between the oppos-
ing side supports in the seat and the size of the side support at the hip-lower torso
level. Four subjects with similar body widths evaluated the seats in a standard driving
task in terms of support and comfort in different body regions. They were then pres-
sure mapped in the same seats, with varying levels of lateral acceleration. Video re-
cording of the driving task was done aiming at characterising the subjects’ shoulder
and hip displacements in the curves. The seats were also subjectively rated in terms of
overall comfort and support and in terms of egress / ingress characteristics and seat
preference. The data from the questionnaires was to be correlated with the data from
the pressure measurements. The goal was to grasp three of the design factors that were
thought to be more important to the support effect and to evaluate them at two levels
of variation seeking the best combination. Other factors were not included, in what is
a pilot study.
The results showed very low degree of concordance among the judgements of the sub-
jects. The data resulting from the video recordings was not useful in the characterisa-
tion of the support features of the seats, due to inconsistency with the assumptions un-
derlying the experimental design. The deficient conditions in which the preparation
and operation of the pressure measurement equipment occurred, reduced the validity
of the pressure distributions obtained. The analysis of the results was followed by the
theorising of the reasons for the inconsistency and low level of association of these,
resulting in the setting of recommendations for the experimental design in future stud-
ies of this nature.
The subjective impressions of the author from performing driving evaluations with the
test seats were confronted with the assumptions and hypotheses underlying their de-
sign, resulting in strong agreement. Considering these subjective impressions, the ini-
tial problem discussion and the author's accompanying of the conduction of the ex-
periments, practical design advises were outlined, and design concepts worth future
study are highlighted. A deeper understanding of how the side supports and the seat
provide lateral support, and how different lateral support modes are connected to dif-
ferent levels of perceived comfort was attained with this study.

An Experimental Pilot Study to Evaluate Car Seat Side Support
Introduction
1
1. Introduction
The initial question given as a starting point for the initiation of this study was raised
by a car manufacturer and worded as: “Where should the side support be located in the
seat and with which dimensions?” In the process of research this question has been
changed and restructured. Chapters 1 and 2 deal with my analysis of this initial ques-
tion, reaching for a definition of objectives and research questions.
1.1 General problem
Side support in a car seat is the region of the seat intended to hold the seat occupant
against lateral acceleration occurring when the car travels through a curve in its tra-
jectory. Side supports take the form of pads, or bolsters, positioned along the sides of
the seat backrest and seat cushion. The presence of this feature in car seats may denote
a sporty performance of the car, or at least a sporty image. The configuration of the
side support region of car seats shows many different solutions in seats of different
manufacturers and models. The side support in the seat could be located in the whole
side region of the seat, from the head restrain down to the seat cushion. However, in
this situation, problems, such as limitations in the reach, mobility, vision or ingress /
egress requirements would limit the utility of such a seat.
1.2 Reasons for interest
1.2.1 Anthropometric variation
The variety of human dimensions is a capital concern in car seat design. The in-
troduction of side support may aggravate the problem of designing a seat to fit every
human being in the range of accommodation targeted in the design, since the side
support is built in both sides of the seat and people of varying body widths must be
accommodated in between.
men women
1st%ile 50th%ile 99th%ile SD 1st%ile 50th%ile 99th%ile SD
Dim. mm mm mm mm mm mm mm mm
a 167 239 311 31 154 226 298 31
b 120 160 200 17 115 155 195 17
c 289 356 423 29 279 363 447 36
d 186 256 326 30 171 245 319 32
e 523 600 677 33 488 562 636 32
f 402 465 528 27 343 399 455 24
g 183 237 291 23 175 245 315 30
h
*
336 450 564 49 290 385 480 41
i
*
234 290 346 24 188 230 272 18
Table 1-1: Static anthropometric dimensions of international adult population (10% British,
25% estimate for the British population in the year 2000, 10% French, 10% German, 20% Swed-
ish, 5% Swiss, 20% USA). Sitting elbow height (a), thigh thickness (b), hip breadth (c), abdomi-
nal depth (d), sitting shoulder height (e), shoulder breadth (bideltoid) (f), chest (bust) depth (g),
elbow-elbow breadth (h) and waist breadth (i). (*)
British population only. Original data from
Pheasant, 1986.
In an effort to approach the problem from an anthropometric point of view, Pheasant,
1986, gave me a starting point by providing an extensive amount of data on the di-
mensions of the British population, as well as some from other nationalities (including
Comentário:

Introduction
2
Sweden, Germany, France, USA and Switzerland). From the dimensions presented in
Pheasant I found of primary interest to this investigation: Sitting elbow height (a),
thigh thickness (b), hip breadth (c), abdominal depth (d), sitting shoulder height (e),
shoulder breadth (bideltoid) (f), chest (bust) depth (g), elbow-elbow breadth (h) and
waist breadth (i). By combining the data for the adult population from the countries
previously mentioned, and attributing different weights to each country, I extrapolated
the data presented in Table 1-1.
The variation in the dimensions presented is quite remarkable, specially if one keeps
in mind the constraint of the width accommodation. Dimensions (c), (f) and (i) cor-
responding to the hip, shoulder and waist breadths are closely related to the side sup-
port configuration and accommodation problem, in terms of the distance between the
opposing side supports in the seat, and portray variations in the order of 150mm (con-
sidering the data presented in Table 1-1). This dimensional dependence lead me to
consider two conceptual alternatives for the inclusion of the side support in car seats,
each dealing with the accommodation problem in diverse ways. One is to design the
side support as non-adjustable, and thus with dimensions to fit the whole accommoda-
tion range of the design. The other is to include adjustment in the side support’s di-
mensions allowing different settings for different occupants in the range of accommo-
dation targeted. Whether the securing effect resulting from each of the two is homo-
geneous or not for the whole accommodation range is not known yet. In the first case,
the design should encompass the maximum breadths of the population, but also pro-
vide some support features to smaller individuals. Figure 1-1 is a sketch showing the
possible placement of non-adjustable side support in a car seat, where the choice of
the regions for the inclusion of side support (shoulder, torso, hip and thigh levels)
must take into consideration the problems previously mentioned (for example: the rear
vision, or the reach of controls).
shoulder support
torso support
hip support
thigh support
Figure 1-1: Sketch of a car seat with non-adjustable side support. There was no attempt made
when producing this figure to indicate a realistic and meaningful depth of the side support.

Introduction
3
A few drawbacks are foreseen from the first alternative (fixed side support), namely:
The side support might not perform with the maximum level of effectiveness for every
different occupant. Rather only for those whose dimensions are closer to the maxi-
mum widths the design can accommodate. People with smaller body widths might not
benefit as much from the side support, they might still be “bouncing’” from one side
to the other between the distant side supports or strongly holding on to the steering
wheel to avoid sliding on the seat when cornering or curving with considerable lateral
acceleration. The second alternative is also not foreseen as free of disadvantages,
namely: If the side support is made adjustable, assuring effective side support and low
discomfort impression for the wide range of accommodation, then the seat adjust-
ments would be more complex, and in terms of image of the seat, it might have disad-
vantages. It might also turn out to be more expensive in terms of production. The
greater complexity and higher cost of the adjustable solution, leads to the interest in
finding out if it holds considerable advantage. Whether it is possible or not to have a
fixed configuration deliver a satisfactory level of support for the whole accommoda-
tion range, has yet to be investigated.
1.2.2 Adjustibility in the side support
Some manufacturers have built adjustable side support into their seats, mainly at the
lower torso level, apart from the other more common seat adjustments. Still, the ma-
jority of seats available has fixed side support, with a constant distance between the
opposing side support segments. However, the inspection of Table 1-1, suggests that
adjustments in other dimensions of the side support, relatively to the rest of the seat,
might be helpful in tailoring the side support to the seat occupant (see Figure 1-2).
Other adjustments could be included, considering other dimensions of the side support
segments, besides their relative displacements in the seat.
Figure 1-2: Sketch of a car seat with adjustable side supports and indication of the degrees of
freedom of the adjustment.
The cost and complexity of developing a seat with many adjustments, possibly even
more than those presented, must be justified. A fixed configuration of the side support
would possibly be a proper reference point to evaluate the benefits of gradual intro-
duction of adjustments.

Introduction
4
1.2.3 Car utilisation type and purpose intended for the side support
Besides the accommodation issue, other reasons might justify the inclusion of adjust-
ment in the side support. Each particular seat occupant might, in different situations,
desire a different level of performance of the side support. Considering, for example, a
car seat with adjustable side support at the lower torso area, the seat occupant in a car
with such a seat would be able to regulate the slack1
in the side support according to
his / her desire. A possible utilisation of this feature is illustrated in Figure 1-3. This
speculation is based on the hypothesis that a tighter fit between the side support and
the occupant gives, on one hand, a higher securing effect, and, on the other, increased
discomfort.
With no slack With some slack With very much slack
Seat
Occupant
A little bit of fun driving,
very tight fit.
Mountain road or city
traffic, tight fit.
Several hours of relaxed
high-way driving, loose fit.
Figure 1-3: Different adjustments of adjustable side support according to driving condition.
By definition, the side support is intended to support the car seat occupant against
sliding or bending sidewards when curving or cornering with lateral acceleration.
However, this main purpose may not be the only one behind the introduction of side
support in car seats, and in some cases the side support may not even fulfil to a no-
ticeable level the securing aspect. In fact, among different car categories and models, a
different motivation can be behind the inclusion of side support in the seats. I have
thought of three categories of cars where side support in the driver’s seat fulfils the
several purposes mentioned so far in different combinations (see Table 1-2). The basic
function associated to the side support, securing the seat occupant against lateral ac-
celeration, can assume different levels of fulfilment or efficiency2
in different designs.
For instance, in a racing car the securing function should assume a higher degree of
fulfilment than in the sports family car, where a higher level of comfort is expected.
This is only a speculation, in fact another hypothesis to consider is that an increase in
seating comfort may be associated with the increase in the level of efficiency of the
securing function of the seat’s side support. Another reason for the introduction of this
feature may be the intention of conveying the image of a sports car. Finally, the moti-
vation behind the inclusion of this feature in car seats may be the will of transmitting
to the driver a sense of unity with the car. This sense of unity with the car may be rein-
forced by the securing effect of the side support, thus in that case, relying on physical
phenomena. In other cases, the sense of unity with the car may be reinforced by psy-
chological suggestion, due to the existence of the side support alone, even if the occu-
pant is not getting much lateral support.
1
The concept hereby identified as slack and used throughout this whole text, denotes the space between
the occupant’s side and the side support. It can be quantified as the measuremnt of the distance between
the side support of the seat and the closest point of the occupant’s body side.
2
Although a measurement of efficiency of the side support has not been defined yet, it seems
reasonable to admit that different design configurations of the side support and of the seat, hold the
occupant in different ways, and some securing manners might be better than others.

Introduction
5
Purposes of side support
Type of car Performing its
basic securing
function
Transmitting a
sense of unity
with the car
Increasing ride
quality and seating
comfort
Suggesting
a sports
image
Race car √ √ − −
Sporty family car √ √ √ √
Sporty looking car − √ − √
Table 1-2: Differences in the purposes of side support in different car types.
1.2.4 Concluding Remarks
The initial question given as a starting point for the initiation of this study was raised
by a car manufacturer, and worded as: “Where should the side support be located in
the seat and with which dimensions3
?” In this introduction I have exposed my reflec-
tions occurring after being given this question. It so appeared that a straightforward
answer, by the consideration of anthropometric dimensions would overlook the other
issues involved in this problem of the side support configuration.
The question of the location in the seat of the side support regions, in relation to the
human body, seems to fade in priority, when compared to the relevance of the distance
between the occupant and the side support. This is so because one might consider that
the relation of the regions of the side support to the human anatomy may well be less
important than the “fitness” of the side support to the occupant, given an acceptable
side support configuration. The anthropometric approach to the side support configu-
ration problem, along with the consideration of the ergonomic requirements such as
those of reach or vision, should not be a problematic task, and rather that of a design
problem with clear dimensional restrictions. However, when one brings into the analy-
sis the considerations of the physiology of the muscle and nerve tissues in contact with
the side support regions of the seat, the straightforwardness of the task seems to fade
away. Now, by looking around, one can see the body-hugging kind of seats in top
sports cars (see Figure 1-4 for an example). In this concept, side support is placed as
much as possible on the seats, and therefore the body-hugging designation. However,
the evaluation of the areas in the seats that are better candidates for the inclusion of
the side support, considering the ergonomic requirements identified, would eventually
favour some areas in detriment of others.
3
The direction I first pursued to find the answer to this question, as it was posed to me, was to
exclusively seak anthropometric and physiological considerations of the human body. The question
could then be rephrased as: “Which areas of the human body’s sides are better candidates for the
contact with the side support, considering the physiological underlying processes in a driving
situation?” This would lead to the study of the muscular tissues, bone structures, softness and hardness
of human body areas and issues alike. However, as it became clear after some insight into this area, the
research that links bio-mechanical engineering with medicine is not yet sufficiently developed to
support such a way of branching the problem at hand.

Introduction
6
Figure 1-4: Interior of Ferrari F355, with the body-hugging seats in evidence.
Source: (Internet address) http://www.autoscape.com/manu/ferrari/f355inte.html
As highlighted in the previous sections, the definition of the side support’s location
and dimensions must take into account the following issues:
− Adjustment of the side support;
− Purpose intended for the side support and car utilisation type;
− The design constraints from general car seat design;
− The anthropometric variations.
My judgement of this is that, for the definition of the side support’s location and di-
mensions, from the issues presented, of superior importance will be the anthropomet-
ric variations and the purpose intended for the side support and car utilisation type. As
to the adjustment of the side support, this issue will be brought up in the discussion,
because it relates to those other two. The design constraints have their role in the
process of adapting a side support concept to a particular car model, but should not be
taken deeply at this point, in order not to become caught in a process of too deep par-
ticularisation.
The reflections exposed so far in this text were, at an early stage, fed back to the car
manufacturer that originated the interest in the side support configuration problem.
Their concern focused upon defining a recommendation for the location and dimen-
sions of the side support. This would be done considering different degrees of adjust-
ment complexity and taking into account the efficiency of the support and the discom-
fort accruing from its presence. The utilisation of the seat and the side support’s pur-
pose were encompassed in the sports family car (see Table 1-2).
1.3 Demarcation of limits
The design of a car seat, and of the seat’s side support, must be done considering a
multitude of design restraints dealing with aspects such as the occupant packaging,
reach, comfort, safety or vision field (see Figure 1-5 for a diagram of the areas which
pose restraints on the design of the seat’s side support). As a practical example of such
a design restraint consider the seat belt path over the occupant in the seat, the side
support must not interfere with the path of the seat belt and its proper functionality.
This study does not aim at analysing the configuration of the side support under the
light of all these restraints, but rather to focus upon the fulfilment of the primary se-
curing function of the side support along with the interaction of the side support in
the occupant’s sense of comfort. The driver’s seat shall be the workstation considered
for the analysis of the side support.
Ergonomic Requirements Design Qualities in use

Introduction
7
Seat
Design
Safety (dis)Comfort
Design of Postural
Reach Seat Side Comfort Customer
Support Thermal Satisfaction
Comfort (seat / car
Vision Ride Quality preference)
Car Support
Mobility Utilisation
/ Purpose
Egress / Adjustment
Ingress
Static /
Anthro- Dynamic
pometrics Action
Figure 1-5: Areas involved and related to the problem of the side support configuration. The de-
sign of the seat side support, included in seat design, must respect a set of ergonomic require-
ments. It must also take into account the type of utilisation of the car it is meant to be fit into and
the purpose it is meat to serve. The design parameters of adjustment and static versus dynamic
action of the side support are important in the perception of the support provided to the user.
The level of support and discomfort sensed by the occupant, while using the seat with side sup-
ports, will contribute to the satisfaction provided with the car, and ultimately to the user’s
judgement of car preference.

Problem Discussion
8
2. Problem Discussion
2.1 Literature Review
An extensive literature search was performed for references on seat side support and
its relation to ergonomics. Many references were found covering research on automo-
tive seat comfort, but only one reference was found with insight into the problem of
interest, this was Gamache, 1994. A possible explanation for this lack of published
research in the side support issue, despite its growing dissemination in car seat design,
is the unwillingness of the car manufacturers who have conducted studies in this field
to share their knowledge with competitors.
In Gamache’s paper, a short description is given of an automatically articulating seat
which compensates for lateral acceleration. The focus is on the dynamics of the car
and on the technical solution for the layout of the innovation. Very little is said about
the ergonomic considerations (which are of prior concern in this study). There it is
stated: “The seat provides additional lateral support by moving seat torso bolsters to-
wards the occupant. The support is provided on the side needed and can accommodate
any size occupant. This additional lateral support during cornering reduces fatigue and
enhances comfort.”
In face of these literature survey results, the setting of the theoretical background and
frame of analysis had to be done in a very much self-constructed manner. The issues
found to be related and of interest to the problem under concern are presented and dis-
cussed in the points below.
2.2 Discussion of the issues involved
2.2.1 The phenomenon of lateral acceleration
Lateral acceleration occurs when the car’s trajectory evolves from a straight line to a
curved line. For a particle in plane circular motion, the normal acceleration (normal to
the circular path), throughout this study called lateral acceleration, takes the expres-
sion a
v
rl =
2
where v is the value of the velocity of the particle in m/s and r is the ra-
dius of the curve in m (as in Beer et Johnson, 1987). The occupant is subjected to a
form of inertial acceleration that has a direction pointing away from the centre of the
curve (see Figure 2-1).
v
ai
an
Figure 2-1: Direction of the inertial acceleration (ai) actuating the car occupant when cruising at
constant speed (v) in a circular path.
In order to cause noticeable effects on the occupants, the speed of the car and the ra-
dius of the curve must be such as to result in a remarkable value of lateral accelera-
tion. Gamache, 1994, reports typical maximum values of lateral acceleration in a left
turn in an urban ride of 0,35g (3,4ms-2
). Due to the inertial forces acting on the occu-
pants of the car, they tend to continue in the straight line trajectory bending or sliding

Problem Discussion
9
to the outer side of the curve. This may cause discomfort due to the resistance the oc-
cupant must provide to overcome this effect, or if such resistance is not provided, due
to the sliding and bending effect.
2.2.2 The securing function of the side support
In the introductory chapter, the issues concerned with the adjustibility and the differ-
ent purposes of the side support were presented. These will resurface in the following
discussion.
The side support may provide the resistance to the sliding and bending effects on the
occupants due to the lateral acceleration phenomenon. As to the driver, it may con-
tribute to transmit a greater sense of control over the car and a sense of unity with the
car. However, the level of functionality of the side support conditions the benefits it
might be responsible for. The several design factors related to the side support, and its
adjusting and static or dynamic features (see Figure 2-2), condition, I believe, the ef-
fectiveness of the securing function. Gamache, 1994, reports the development of a
seat with adjustable and dynamic side support positioned at the torso level of the oc-
cupant. The fixed configuration (non-adjustable and static) is nowadays quite spread
in various car models of various types. This is a case where we are lead to believe that,
due to the wide range of accommodation the seat must support, the efficiency of the
securing function and the associated increase in comfort might vary a lot for different
occupant sizes. Therefore, for many occupants in the range of accommodation for
which the seat with side support has been designed, the side support may not fulfil its
root purpose, being perhaps only through psychological suggestion able to suggest a
greater sense of unity of the driver with the car,.
Non-adjustable Static
Side support
Adjustable Dynamic
Figure 2-2: Conceptual alternatives for the principle of action of the side support.
As to the adjustable configurations, these might be seen as a step further towards over-
coming the limitations of the fixed configuration. The adjusting of the side support,
whether of static or dynamic nature (the distinction is made considering the absence or
presence of automatic deployment of the side support according to the level of lateral
acceleration) might give the occupants in the whole range of accommodation of the
design the possibility of benefiting from the full potential of the side support feature.
The combination of the conceptual alternatives for the principle of action of the side
support can assume four cases. The time and resources available for the execution of
this study have discouraged an extensive analysis of the whole range of possibilities.
Since there was no reference work as a starting point, it was done from scratch. The
interest underlying this study comes from a car manufacturer who wants the less com-
plex and inexpensive solution to the side support design problem, delivering an ac-
ceptable level of efficiency in the support and comfort dimensions. The experimental
design based on a non-adjustable and static configuration may reveal the necessity of
an adjustable configuration, and open up for a later discussion of the benefits of dy-
namic action.

Problem Discussion
10
2.2.3 Perceived comfort and long term use of the seat with side support
In the introductory chapter of this report, two opposite hypotheses were outlined about
the nature of the relationship between the support and comfort sensed by the occupant
in the seat with side support, possibly assuming the characteristics of either conflict or
symbiosis. The introduction into this discussion, about the level of comfort the occu-
pant perceives, of the issue of the long term use of the seat, will be helpful in clarify-
ing this ambiguity.
While providing its securing function, the side support is also interfering with the as-
pect of ride quality and seating comfort. The increase in ride quality may be directly
related to the effectiveness of the securing function, whereas the relation to the seating
comfort aspect might be rather more complicated, specially when considering the long
term effects. Reed et al., 1991 consider that a short-term evaluation of a seat is insuf-
ficient to predict its comfort performance in actual conditions of use. In a long term
driving situation, the pressure in the occupant’s contact surface against the side sup-
port, may be high enough to restrain the blood circulation in those regions of the oc-
cupant’s body. Inadequate blood circulation, compromises the ability of the cell tis-
sues affected by the compression forces to exchange nutritional input and metabolic
by-products with the circulatory system, resulting in neural stimulus of fatigue and
pain (Pywell, 1993). The “tightness” of the side support and the frequency with which
its securing effect is necessary may condition the levels of contact pressure we are re-
ferring to. Thus it seems the side support may have an ambiguous effect on the occu-
pant’s perception of comfort, reducing or eliminating the discomfort caused by the
phenomenon of lateral acceleration (increasing the ride quality), but in the long run,
eventually creating discomfort due to the contact pressure between the occupant and
the side support.
Reynolds, 1993 emphasises fatigue and comfort in seat design, stating as a primordial
design criterion for a driver’s seat: “The seat should be comfortable for extended peri-
ods.” As to the concept of comfort he states: “Comfort describes an occupant’s em-
pirical perception of being at ease. Hertzberg, 1972, defined comfort ‘...as the absence
of discomfort.' Comfortable seats require rigorous application of data of human ge-
ometry and [the evaluation of the] long term sitting effects on the occupant.” One of
the referenced ways to assess the level of perceived comfort, of a sitting person is to
count the number of times the person moves in the seat under the period of time of the
observation. The design of the side support, unequivocally affects the liberty of the
person to change position in the seat, especially if the slack between the seat occupant
and the side support assumes a low value.
2.2.4 Mechanical model
Let us consider a seat cushion modelled as a cubic body, with a person sitting on it
modelled as another cubic body. What kind of forces exists in this model? The weight
of the person is distributed along the contact area with the seat. If the seat is fitted into
a car and the car is curving with lateral acceleration, a distributed friction force4
, in a
plane perpendicular to the direction of the force of weight will exist, counteracting the
lateral acceleration force (see Figure 2-3). This lateral force is also actuating in the
4
There are several layers of contact between the seat and the occupant, according to the seat’s
construction and the clothing of the occupant. The friction forces between the seat cover and the top
layer of the occupant’s clothes may or may not be the decisive ones, but I shall assume, in an academic
way of dealing with the problem, that it is so.

Problem Discussion
11
seat, but since this is rigidly attached to the car body no movement of the seat in rela-
tion to the car should occur. The value of this distributed friction force, should equal
the value of the lateral acceleration force up to a level where the maximum static fric-
tion is overcome and the person slides on the seat or performs muscular effort to avoid
this. The deformation of the surfaces is neglected here as well as other changes in ge-
ometry due to muscular effort of the occupant.
m. al
m.g m.g
f p f p
fr
Figure 2-3: Schematic representation of forces involved in seat cushion / occupant interaction.
If the static friction maximum is overcome by the total lateral forces, the occupant will
then slide in the seat cushion, reaching contact with the side support. This could offer
enough resistance to stop the occupant and overcome the lateral acceleration forces,
specially if the seat cushion has some side support. In this situation, the friction forces
that counteract the lateral force will be added by more action-reaction forces from
these side support areas (see Figure 2-4).
m.al
m.g m.g
fp
fp
fr
Figure 2-4: Schematic representation of forces involved in seat cushion / occupant interaction,
when seat cushion has side support.
If we extend our analogy to the situation where the occupant is in contact with both
the seat cushion and backrest, and assuming that the same model also applies for the
seat backrest/occupant interface, we can represent the dynamics of this model in a sca-
lar expression. The following equation (1) is a simplified representation of the equilib-
rium in the seat / occupant model contact.
m a f dA f dA al p r
AA
. cos . . .sin . '+ + =∫∫ γ γ (1)
In this equation m is the mass of the occupant, A is the contact area between the occu-
pant and the seat, al is the lateral acceleration actuating the occupant’s body, γ is the
angle at each point of the contact surface between the perpendicular to the surface at
that point and the horizontal (see Figure 2-5), fp is the value of the pressure force in
the point of the contact considered, fr is the elemental friction force at each point of
contact and a’ is the resultant acceleration in the occupant. This equation is based on
the simplification involved in assuming that the contour curves of the contact surface
are parallel lines perpendicular to the horizontal / vertical planes of the referential.

Problem Discussion
12
fp
γ
fr
section of the contact surface
Figure 2-5: Illustration of the definition of the angular quantity γγγγ.
From the analysis of the forces involved it is possible to see that the lateral forces act-
ing in the contact areas of the occupant with the seat are opposed by friction forces
and pressure forces in the side support (if there is contact with this). The resultant ac-
celeration (a’) can thus be expressed as a function of the remaining factors involved in
the equation of equilibrium (see equation 2).
a f m a A f fl p r' ( , , , , , )= γ (2)
According to this model, the factors that control the equilibrium in the contact area are
then: The contact pressure, the mass of the occupant that is supported by the seat, the
value of the lateral acceleration, the area of the contact, the friction between occupant
and seat and the surface shape of the contact area. If the occupant is held on the seat,
the resulting force in the seat/occupant interaction is null. But the accomplishment of
equilibrium in the seat with the occupant in the normal position in the seat without
having to exert effort to attain this, might still assume different levels of comfort per-
ception by the occupant. Therefore, other factors, which may not influence the force
equilibrium, may be important for the perception of comfort by the occupant. These
may be the location of the side support in the seat relatively to the anatomy of the oc-
cupant, the shape of the contour between the side support and the seat’s backrest or
cushion, the distance between the occupant and the side support, the stiffness of the
side support and its ability to offer resistance to the lateral movement.
The mechanical model presented may be extended and further developed by the con-
sideration of the bending moments resulting in the occupant’s body by the action of
lateral acceleration. In a normal driving posture, a major part of the occupant’s weight
is supported by the seat cushion (60 to 80%). This is also where the friction forces are
higher. The inertial forces of lateral acceleration actuating in every point of the occu-
pant’s torso and head will generate a bending moment in relation to the buttocks area
which is subject to higher friction forces, because a high fraction of the load is sup-
ported through this body zone. In this way the resulting tendency for rotation of the
upper parts of the body is added to the linear translation given by the lateral accelera-
tion in the direction pointing away from the centre of the curve (see Figure 2-6). On
the lower parts of the legs, the rotation caused by the bending moment is also expected
to occur. According to this consideration, the urgency in building side support in the
seat is greater for the levels of the upper torso and head, being prioritised in relation to
the inclusion of side support in the lower torso area. In practical terms, the inclusion
of the side support for the higher torso can be problematic, due to the interference with
the freedom of movement required for the arms.

Problem Discussion
13
Areas with higher friction forces
Figure 2-6: Diagram of the bending moments occurring because of the uneven distribution of
friction forces in the occupant’s body, when subjected to lateral acceleration.
2.3 Objective, research questions and hypotheses
Since our focus in the analysis of the problem of the side support configuration is to-
wards the support and comfort aspects, we shall narrow our view in the diagram pre-
sented in Figure 1-5, and look at the previously identified factors of variation in the
side support design with foreseen influence on these aspects (see Figure 2-7).
Under the light of the present analysis what is then an effective side support? In the
balance between all the factors involved which factor combination giving a null resul-
tant acceleration has a lower discomfort effect? Friction and pressure in the side sup-
port and the seat were identified as the decisive factors, but dependent on others.
Since the evaluation of improvement must have some reference start point, an analysis
of different design configurations of the side support must be done taking into consid-
eration a basic design configuration and starting the changes from there. The direction
of the research would then be to perform changes on this design base and evaluate this
resulting change from the aspects of interest (support and comfort). Put into more op-
erational oriented terms the research questions to answer in this study would then be:
− In what direction must the design changes in the identified factors occur to provide
an improvement in the securing function and / or an increase in perceived comfort
of the occupant, taking into consideration the whole accommodation range?
− Can a fixed and static configuration offer a uniform level of support and perceived
comfort to different occupants distant among themselves in the accommodation
range?
− Does the use of adjustments in the side support cause differences in the perceptions
of support and comfort by the occupant?
− Is there a conflict between comfort and securing effect in the seat with side sup-
port?
Support Seat Side (dis)Comfort
Support
Factors with
influence

Problem Discussion
1
contact pres-
sure
value of
lateral accel-
eration
area of the
contact
mass of the
occupant
friction be-
tween occu-
pant and seat
surface shape
of the contact
area
location of the
side support in
the seat
stiffness of the
foam in the
side support
distance be-
tween the oc-
cupant and the
side support

Problem Discussion
14
Figure 2-7: Mapping of factors influencing the dimensions of support and (dis)comfort of the side
support.
The objective of this study becomes then to apprehend the levels at which selected
design factors of the side support in the seat of the car driver better contribute to the
utility of the side support in the dimensions of support and comfort. Which of the
identified factors to select, is a choice that is restricted by practical restraints and
trade-offs in the foreseen relevance of each factor. The formulation of the hypotheses
to guide the experimental design and analysis will be based on the factors for which I,
intuitively and based on the previous analysis, can foresee the effect of change in the
dimensions of interest (support and comfort). The hypotheses related to each factor
and considered relevant and practically testable are:
− The distance between the occupant and the side support (or rather the slack be-
tween these) conditions the support performance and the feeling of comfort.
− The greater the area of the side support that the occupant gets in contact with, the
higher the values of lateral acceleration permitted for the same support effective-
ness, and the less the discomfort in extended duration use.
− The higher the friction coefficient between the seat cover material and the occupant
the more support is sensed by the occupant, although the side support is called into
play at only higher levels of lateral acceleration, and can be effective for greater
values of acceleration. Less discomfort on the side areas of the occupant is sensed,
but with more discomfort on the areas in contact with the seat cushion and the seat
backrest.
− The response in support offered by the seat side support is not constant with the
variation of lateral acceleration. The side support ceases to be effective over a cer-
tain value of lateral acceleration, which depends on the specific design.
− The stiffness of the foam in the side support conditions the perception of support
and the feelings of discomfort.
− The location of the side support areas in the seat, relatively to the occupant’s anat-
omy, condition the perception of support and the feelings of discomfort.

Experimental Design
15
3. Experimental Design
3.1 Preliminary considerations
A few remarks are made before the presentation of the reasoning leading from the pre-
vious discussion and hypotheses established to the actual experimental design. Under
focus is the pursuit of valid indicators of the aspects of support and comfort related to
the side support. The manner in which the adjustability issue, concerned with the side
support, will be treated, regarding the experimental design at hand, is also presented.
Finally, the general contours of the practical limitations encountered in the setting of
this experimental design are introduced.
3.1.1 An indicator of the functionality of the side support
A valid indicator of the functionality of the side support should unequivocally show the
action of the side support. Once the action of the side support is securing the occupant
against lateral acceleration, and in this process a distributed action-reaction force exists
between occupant and side support, the monitoring of the value of this force viewed
against the level of lateral acceleration should be elucidating towards the degree of func-
tionality of the side support. The evaluation of the functionality aspect inherent to the
side support in the seats must necessarily occur under the presence of lateral accelera-
tion.
The measurement of contact pressure has been documented in several studies of sitting
comfort (ex.: Lee and Ferraiuolo, 1993) by the use of a pressure transducer net. A rela-
tively dense net of pressure transducers on the side support areas of the seat would elu-
cidate about the contact area’s size and shape and the value and distribution of the con-
tact pressure. The data originating from these would, under controlled conditions, char-
acterise the load in the contact area and therefore serve as indicators of the efficiency of
the securing function of the side support. However, the characterisation of friction is not
possible with an equally objective measurement, as the one available for the pressure.
Although the higher values of friction forces are likely to occur where the pressure val-
ues are higher, this is not a direst relationship, since areas of high pressure may register
lower values of friction forces than areas with lower pressure.
3.1.2 Indicators of (dis)comfort
The feelings of comfort and discomfort are seen as an aspect of underlying physiological
processes in the human being. However, the relation between the feelings of comfort
and discomfort and physical indicators of the underlying physiological processes has not
yet been firmly established. Still, some researchers have reported a significant correla-
tion between subjective assessment of discomfort and the measure of pressure in the
seat / occupant contact. Thakurta et al., 1995, designed an experiment involving thirty-
six subjects, who evaluated five cars from the same market segment by completing a
comfort assessment questionnaire and being pressure mapped before and after a 128 km
highway drive. As an outcome, they found that the effect of distribution of pressure to
subjective comfort is significant, with lumbar support and ischial support appearing to
be more significant than shoulder and thigh support. The seat rated most comfortable,
after the highway drive, for the lumbar area also showed the highest percentage of pres-
sure distribution in that area. On the other hand, the seat rated most comfortable, after

Experimental Design
16
the highway drive, for the ischial area showed the lowest pressure percentage in that
area. A significant difference between the five seats was also reported.
The evaluation of seat comfort has been performed in a combination of contact pressure
measurement (or other measurements such as EMG - electromyography) with subjective
evaluation of the perception of comfort (Reed et al., 1991, Lee et Ferraiuolo, 1993, and
Thakurta et al., 1995). However, there is no reference in literature to the inclusion of the
side support in this evaluation under realistic conditions of use, that is, under the effect
of lateral acceleration.
The availability of an equipment for pressure measurement caused the choice of an ob-
jective measure, in this experimental design. This objective indicator, was to be corre-
lated to subjective comfort, apprehended through subjective comfort evaluation ques-
tionnaires, enabling a characterisation of the seat occupants’ perceived comfort.
3.1.3 Treatment of the side support’s adjustability
The manipulation of the design configuration of the side support in the experimental
design is meant to test the hypotheses drawn in the previous chapter and is based on a
fixed configuration (non-adjustable and static side support, see Figure 1-1). Thus, the
adjustment of the side support was not provided in the experimental designs (therefore,
the recurrence to subjects with selected dimensions, as delineated further in this text),
but it was to be taken into consideration when extrapolating the results to the whole
range of accommodation targeted.
3.1.4 Introduction to the description of the practical limitations encountered in
setting the experimental design
It was quite clear, from the objective and hypotheses drawn earlier in this study, that the
experimental design to be reached at should include some sort of subjective evaluation
of seat characteristics during driving in the presence of remarkable lateral acceleration.
Simultaneously and under a period of some time, discussions with the other two parts
involved in this study besides me (my university supervisor and the car manufacturer’s
contact person) were carried out in terms of the possibilities and limitations regarding
the experimental design. In terms of costs and data complexity, limits were early estab-
lished in terms of the number of subjects, number of seats and time available at the
rented car track (with the name of Stora Holm) where driving sessions would be per-
formed. The number of seats was set to 4, the number of subjects was also set to 4 and
the net time at Stora Holm was limited to 2 periods of 8 hours. This dimension of the
experimental design may not be ideal, but it is within a reasonable minimum and realis-
tic opportunities. Since the subjects collaborating in the driving sessions were bound to
have close anthropometric dimensions, the quantity of four implies that the replication
of the experiment is done four times (assuming that the anthropometric similarity will
not be overruled by personal characteristics of other nature). As to the net driving time,
if each subject evaluates each of the 4 test seats, then there will be sixteen evaluations to
be performed in the 16 hours available (one evaluation at a time with 1 hour of dura-
tion). The evaluation of 1 hour of duration may be considered short, for the evaluation
of long term sitting effects. On the other hand, in the driving with recurrent and remark-
able lateral acceleration, extending this period to two or three hours might turn the driv-
ing sessions into a very tiring and hideous task for the subjects, taking away from them,
the necessary level of relaxation to perform true and objective evaluations.

Experimental Design
17
3.2 Trial persons (subjects)
3.2.1 Practical choices and limitations
As to the question of recruiting the necessary subjects, several choices had to be made.
First came the question of defining a criterion for recruiting of the subjects. It should be
decided if we would have subjects covering the variation of the accommodation range of
the seats, or if we would try to keep to a “standardised” subject dimension. It was also
necessary to define if every subject would evaluate every seat, or only some of the seats,
and how much time each trial person would be requested to spend with this study. It be-
came clear that subjects would have to be recruited and offered some monetary compen-
sation for the time they would spend conducting the driving sessions. We expected that
if a longer time was required, less persons would be interested and perhaps a higher
monetary compensation would have to be offered.
3.2.2 Subjects for the experiments: With varied dimensions covering the ac-
commodation range or with similar dimensions?
The choices and practical limitations related to the trial persons in the experiments have
been introduced. I shall now present the arguments that support these choices. The ad-
vantages of having trial persons in these experiments as persons of varied body dimen-
sions, covering the seat accommodation range were identified as:
− Providing a more representative sample of the user population;
− Making it easier to find the actual subjects to perform the experiments;
− A stronger validation of the results of the experiments would more likely occur;
In this case the disadvantages were seen as:
− A greater number of seat evaluations per subject would be necessary, compared to
the option of having closely dimensioned persons, because the repeatability of the
evaluations would be small if the subjects had very different dimensions;
− A greater complexity would be introduced in the data analysis;
− A greater number of subjects would be necessary, or the results would be very de-
pendent on the subjects’ characteristics, since they would not be evaluating the same
dimensional relation (anthropometric / seat dimensions).
On the other hand, recruiting middle sized persons with close dimensions amongst
themselves would bear the following advantages:
− Providing a greater repeatability of measurements than the one possible with subjects
of varied dimensions;
− Allowing the number of evaluations per subject to be smaller, compared to the previ-
ous alternative, because the repeatability of the evaluations would be higher;
− Providing a better chance of having consistent judgements between the trial persons,
since they would be judging approximately the same thing.
This concept for recruiting of the trial persons is, however, not free of disadvantages,
namely:
− Although the subjects would have approximate dimensions, their individual prefer-
ences (for example: their driving position) might jeopardize the repeatability of the
experiments between subjects;

Experimental Design
18
− Finding the actual subjects to perform the experiments would be a harder task than
the one foreseen if the subjects would have distinct dimensions covering the
anthropometric variation allowed in the range of accommodation of the seats;
− The validity of results is dependent on an extrapolation between groups with similar
slack but differing anthropometric dimensions.
The advantages and disadvantages of both alternatives are quite balanced, and none of
the alternatives is obviously superior, except from a practical resources point of view.
The interest in testing the effect of slack variation between occupant and side support
was the decisive argument in the choice for the second alternative - closely dimensioned
subjects. If this factor was to be tested with subjects of varied dimensions, with test
seats based on a fixed (non-adjustable) configuration of the side support, the number of
test seats necessary would increase beyond the quantity of 4, which had been set as a
limit. In fact, the choice of design factors of the side support to include in the experi-
mental design, is not dependent on the type of subjects considered, except for this par-
ticular design factor. I am particularly interested in including it in the experimental de-
sign, since it allows the extrapolations for the adjustibility question, even though a fixed
configuration is used.
By having “standard” dimensioned subjects and including in the experimental design
test seats with two seat widths, we get assessments of two cases of the dimensional
relation occupant / seat side support dimensions: “tight fit” and “loose fit”. We can
assume that an “undersized” fit is never good, not in terms of comfort neither in terms
of support. If we get a strong difference between the “tight” and “loose” test seats, it
indicates that this factor is of importance. If a small diference is found, or no difference
is found at all, then this factor is unimportant. With this procedure, we are able to make
an assessment of the criticality of the dimensional horizontal fit between the seat
occupant’s body and the side support.
Nakaya et Okiyama, 1993, present a series of cross section curves of the human back at
different levels. From a group of 31 subjects they divide them in three groups according
to anthropometric characteristics: small, average and big. They then plotted the average
curves for the back cross sections referred for each of the groups. The averaged curves
are similar in shape for the three groups. These facts presented in Nakaya et Okiyama’s
paper may support the pretension of extrapolating the results achieved with very simi-
larly dimensioned subjects to the whole accommodation range.
3.2.3 Setting up of the actual subject recruiting criteria
Once the choice of the conceptual alternative for the recruiting of subjects was settled it
was necessary to establish which anthropometric dimensions would be the control vari-
ables for the recruiting of the trial persons. I now recall that what I am looking for is 4
trial persons with similar body widths, weights and heights, for whom the physical phe-
nomena taking place when they are driving with lateral acceleration and with side sup-
port in the seat would be very much the same. The dimensions of weight and height do
not need to meet a predetermined value, as long as the four subjects are reasonably simi-
lar and normal in these dimensions. However, the hip width and shoulder width should
be assigned a control value, connected to the level of the experimental design factor:
slack between seat occupant and seat side support. Further in this chapter, under the
heading “Test seats”, the rationale behind the setting of the levels of this design factor in
the experimental design of the test seats is presented. For the purposes of subject recruit-
ing, the value of the hip breadth of the subjects should lie around 350 mm. The meas-

Experimental Design
19
urements of both the hip and shoulder widths of prospective subjects were to be taken.
However, the hip breadth was judged more important. For the shoulder breadth, a con-
trol value was not set, but, since the side support is present in the seats from the hip
level up to almost the shoulder level, the conicity of the subjects’ torso had to be kept to
a rather constant value. This is to ensure that, as much as possible and can be con-
trolled, the physical phenomena occurring between side support and trial persons is the
same. This problem of the conicity of the subjects hip-torso segments, lead to the choice
of having subjects of only one sex. Choosing among the male and female sex was a
straightforward decision, due to the reduced number of women interested in participat-
ing in the experiments, from the group inquired. The age of the trial persons was al-
lowed to vary between 20 and 40 years. Finally, another condition for the recruiting of
subjects was that they should have had a driving license and some driving experience
for a few years.
3.3 Test seats
The four test seats were based on the existing configuration of the reference seat. Each
seat was to be the result of the manipulation of this basic configuration in three combi-
nations of factors of variation at two levels (+/−): Slack between the occupant and the
side support at the hip level(A), friction of the cover of the seat(B) and relative size of
the side support in the hip-torso area(C). In all the test seats the foam in the side support
areas of the seat was to be replaced with harder than standard foam. This is a way of
overcoming the difficulty presented by the relative softness of the foam present in the
reference seat. It was assumed that a harder foam would highlight the differences in the
geometry of the side support of the four test seats. The seat cushions of the four seats
were to be kept at the reference configuration, but the cover material of each seat cush-
ion was to match the cover material of that seat’s backrest. Table 3-1 presents the con-
figuration of the test seats that was reached at, concerning the level of the three factors
of variation included. Appendix A presents the sketches of the four test seats as given to
the workshop that executed the seat modifications.
Factor A Factor B Factor C
Seat ref. A − − −
Seat ref. B + + −
Seat ref. C − + +
Seat ref. D + − +
Table 3-1: Configuration of the test seats according to the level of each factor of variation at two
levels (+/-): Slack between the occupant and the side support at the hip level(A), friction of the
cover of the seat(B) and relative size of the side support in the hip-torso area(C).
How may factors to include in this experimental design, which of the identified factors
to include and varying at what levels, are the choices that were made in the setting of the
characteristics of the test seats. Many design factors of the side support have been fore-
seen to affect the utility of the side support. The research questions and hypotheses for-
mulated in the Problem discussion stage of this study shall now be brought up to discus-
sion again. Here they shall be analysed in terms of the feasibility and interest in their
testing in these experiments. The inclusion in the test seats of the variation factors that
would allow the testing of these hypotheses, or the answer to the connected research
questions, is decided at this stage. But prior to this discussion, the practical choices and
limitations concerning the test seats are presented.

Experimental Design
20
Possibilities were explored in terms of using seats from different cars, portraying differ-
ent side support characteristics. However, this idea was abandoned due to the problem
of fixing the seats in the same car. If the seats originated from different car models, the
cars for the driving sessions would also have to be different, corresponding to the seats,
and this would lead to a great administrative complexity. In light of these considera-
tions, the seats were set as based on one seat model basis corresponding to a car model,
so that the driving sessions could be carried out only with one car model. In these terms,
it was under our control to set the differences among the seats in the design factors of
the side support, since the possibility of modifying the seats and having specially built
seats was feasible. The possibilities of factor modifications were also studied under this
initial period of the experimental design, and a group of design factors was prioritised in
terms of the interest in that group of factors and the possibilities of practical implemen-
tation of the modifications under the conditions available.
The adjusting of the side support was left out from the experimental design in an earlier
stage of this study, but the inclusion of the factor of slack between the occupant and the
side support had been confirmed when choosing the conceptual alternative for the defi-
nition of the subject recruiting criteria.
3.3.2 Design of experiments using the fractional factorial approach
Using a fractional factorial approach, as presented in Table 3-1, to the design of experi-
ments, three factors of variation at two levels can be included in the four test seats. The
full factorial approach of testing three factor variations at two levels would involve 23
(eight) different runs, alias test seats. However, using only half of the runs, correspond-
ing to 2(3-1)
(four) different test seats, allows the identification of the main effects, al-
though aliased with two factor interactions. The use of four subjects, would, translated
to the design of experiments terminology, imply having the runs replicated four times.
In terms of results, the main effects will be confused with two factor interactions, but
that is the price that has to be paid for having half the number of runs of the full factorial
design. According to Montgomery, 1991, the successful use of fractional factorial de-
signs is based on three key ideas:
1. The sparsity of effects principle. When there are several variables, the system or
process is likely to be driven primarily by some of the main effects and low-order in-
teractions.
2. The projective property. Fractional factorial designs can be projected into stronger
(larger) designs in the subset of significant factors.
3. Sequential experimentation. It is possible to combine the runs of two (or more)
fractional factorials to assemble sequentially a larger design to estimate the factor ef-
fects and interactions of interest.
This approach to an experimental design is recommended when the cost of including all
the factors in the experimental design is to high, or the number of runs necessary to test
all the factor combinations included is also great. In the present situation, both condi-
tions are present.

Experimental Design
21
3.3.3 Hypotheses to test and related independent variables included in the ex-
perimental design
Three factors of variation of the side support can be included in the experimental design
to have the related hypotheses tested. Hypothesis 1 can be tested by the inclusion in the
test seats of a variation of the slack between the occupant and the side support, or in
terms of seat design, the distance between the opposing side supports.
“The distance between the occupant and the side support (or rather the slack be-
tween these) conditions the support performance and the feeling of comfort.” (Hy-
pothesis 1)
The inclusion of this variation factor was confirmed earlier in this study. What has yet to
be defined is the two levels of variation it will assume. The reference seat holds the
value for this distance, measured at the lower level of the backrest and close to the back-
rest at 28 cm (see Appendix A - Test seat sketches: Test seat A), while the distance
measured at the same level but between the interior front edges of the side support
“wings” is 38 cm. This is chosen as the tight fit, or the lower level of this factor. The
higher level is defined as 7cm wider, so that a loose fit is provided without exaggerating
too much the total width of the seat (see Appendix A - Test seat sketches: Test seat B
and Test seat D). The adoption of a slack of 7 cm and a person with a hip breadth of
350mm would then be equivalent to a situation where a person with 280 mm of hip
breadth is sitting in the standard seat. This value of the hip breadth (280mm) is close to
the 1st
percentile male and female (see Table 1-1).
“The greater the side support area that the occupant gets in contact with, the
higher the values of lateral acceleration can be permitted for the same support ef-
fectiveness, and the less the discomfort in extended duration use.” (Hypothesis 2)
The factor implicit in the formulation of the previous hypothesis is considered to be very
important and therefore its inclusion in the experimental design is given priority. The
depth of the side support at the hip level in the backrest of the reference seat (see
Appendix A - Test seat sketches: Test seat A) is taken as the lower level of variation
and the higher level is set by adding 7 cm in the depth/height of the side support at this
level (see Appendix A - Test seat sketches: Test seat C and Test seat D).
The following hypothesis can be tested by the inclusion in the experimental design of a
factor of variation in the friction properties of the seat cover material.
“The higher the friction coefficient between the seat cover material and the occu-
pant the more support is sensed by the occupant, although the side support is
called into play at only higher levels of lateral acceleration, and can only be effec-
tive for greater values of acceleration.” (Hypothesis 3a)
“Less discomfort on the side areas of the occupant is sensed, but with more dis-
comfort on the areas in contact with the seat cushion and the seat backrest.” (Hy-
pothesis 3b)
In my literature and analytical studies, friction had been identified as of capital impor-
tance in the perception of support by the occupant and for the level of lateral accelera-
tion at which the side support is called into play. The variation of the friction factor is
materialised in the experiment design by having the lower factor of variation corre-
sponding to a leather seat cover (see Appendix A - Test seat sketches: Test seat A and
Test seat D) and the higher level corresponding to a high level fabric cover material

Experimental Design
22
(see Appendix A - Test seat sketches: Test seat B and Test seat C). The clothing of
the subjects should be kept to similar materials so that the variation of the friction coef-
ficient between seat and occupant is not significant for different subjects and the same
seat.
“The response in support offered by the seat side support is not constant with the
variation of lateral acceleration. The degree of effectiveness of the support de-
creases with higher lateral acceleration values.” (Hypothesis 4)
The testing of the previous hypothesis is not connected to a particular factor of variation,
but still is related to the analysis of the data collected in the experiments. Whether dif-
ferences between the test seats are evident will be discussed in the data analysis section
of chapter 5.
“The stiffness of the foam in the side support conditions the perception of support
and the feelings of discomfort.” (Hypothesis 5)
“The location of the side support areas in the seat, relatively to the occupant’s
anatomy condition the perception of support and the feelings of discomfort.” (Hy-
pothesis 6)
These two hypotheses are not included for testing in this experimental design, where
only three factors of variation can be included. The two design factors implicit in these
two hypotheses, although of foreseen importance to the performance of the side support
could not easily be varied in the test seats. The stiffness of the foam of the side support
is seen more as a factor for “fine tuning” of the side support configuration.
3.4 Data collection
How shall the aspects of support and comfort be apprehended from the subjects' interac-
tion with the test seats in the driving sessions? Two kinds of subjective evaluations are
completed by the trial persons to enable this apprehension. The first concerns the sub-
jective impressions of the aspects of securing effect and discomfort in the several body
regions in contact with the seat side support or with the whole seat (further described in
points 3.4.1 and 3.4.2). The second requires an overall judgement of the seats in terms
of support and comfort (see point 3.4.3).
The readings of interfacial pressure between seat and occupant are used to characterise
the support and comfort aspects from an objective point of view (see point 3.4.5 and
3.6). Video recordings of the subjects in the driving session are also made with the aim
of characterising the support aspect of the side support (see point 3.4.4).
3.4.1 Questionnaire for subjective evaluation of support
The subjective judgement of the trial persons about the support provided by each par-
ticular test seat is transmitted to the researcher through the filling in of a questionnaire
presented in Appendix B - Questionnaires. A line about 10 cm long with the words Low
and High anchored to the extremes is presented. The aim is to get to know in which ar-
eas of the occupant body’s side the support is given with more or less importance. A
map illustrating the body regions (adapted from Corlett et Bishop, 1976) for which the
judgement was requested is prepared to present to the subjects when performing the
evaluation. The graphical form of the questionnaire that was reached at is based on Reed
et al., 1991.

Experimental Design
23
3.4.2 Questionnaire for subjective evaluation of comfort
The subjective judgement of the trial persons about the level of discomfort perceived in
each particular test seat is transmitted to the researcher through the filling in of a ques-
tionnaire presented in appendix. A line about 10 cm long with the expressions “no dis-
comfort” and “unbearable discomfort” anchored at the extremes is positioned in front
of each body region. A map illustrating the body regions for which the judgement was
requested (adapted from Corlett et Bishop, 1976) is prepared to present to the subjects
when performing the evaluation. The graphical form of the questionnaire that was
reached at is based on Reed et al., 1991.
3.4.3 Questionnaire for satisfaction with seat characteristics (using the method
of paired comparisons)
The interest in apprehending the degree of satisfaction of the subjects with the seat char-
acteristics led to the using of the paired comparison method for rating the test seats from
different aspects. The ratings for which the previous two questionnaires have been for-
mulated (as described in points 3.4.1 and 3.4.2) do not allow the extrapolation of the
level of satisfaction of the trial subjects with each seat’s characteristics. My interest in
understanding if there is a conflict between the support and comfort aspects perceived
from the seat is not satisfied with the data generated from those two questionnaires.
Two aspects are involved in the choice of designing a third questionnaire as a means to
convey the level of satisfaction the trial persons have with different seats. One is the dif-
ferences in the subjective rating of discomfort between subjects. In other words, the
same physical conditions acting in two subjects will be perceived by each of them with
different levels of intensity, depending on their individually “calibrated” sensitivity.
This statement is the result of my extrapolation from the arguments presented by Borg,
1982, for the development of a category scale with ratio properties for intermodal and
interindividual comparisons. Therefore, the ratings of discomfort should not be com-
pared quantitatively among subjects, but rather in terms of the ratio between different
body areas of the same subject. Asking the subjects if they would like to have more sup-
port on the shoulder area for this one seat or that other, and to extend this question to all
the body areas for which the rating of the securing effect had already been done, might
overload them with questions. This would not contribute to have clear and unbiased an-
swers from the subjects, putting at risk the validity of the answers of both question-
naires, and therefore, jeopardising the utility of such responses.
The problem of satisfaction may also be related to the discomfort aspect. Although I be-
lieve that any person, when asked about satisfaction on comfort at different body areas,
would answer that the higher satisfaction occurs in body areas with the lower levels of
discomfort, there is still one pertinent question left unanswered. This is, if a person can
choose from the combination of levels of discomfort from different seats, in which areas
of the body would the person tolerate the discomfort, and in which other body areas
would the person rather reduce the discomfort, even if ideally the person would prefer to
reduce the discomfort in all the body areas. So the second aspect involved in the choice
of the method of paired comparisons for a third questionnaire is a question of trade-offs.
This stands besides the problem of the eventual confusion and unclearness about what is
asked if the subjects are given two questions that seem too similar. The same applies for
the comfort issue.
In the case of the trade-off between support and comfort, the choice of the rating of the
seats in order of overall preference, will have this trade-off implicit in the subjects’ de-

Experimental Design
24
cision process. Again the temptation of asking the subjects to point a level of preferred
compromise between the discomfort and support aspects for each body area was big.
However, this would probably be a quite difficult question to answer, even if it could be
understood unequivocally.
The method of paired comparisons between seats is the solution found to overcome all
these problems of possibly perceived redundancy and confusion from the trial persons
when presented the several different questions per body area. The subject is asked to
choose the best seat of a pair that has already been evaluated. This choice is asked for
overall characteristics of the seats, with no discrimination of body areas, because the
seat evaluations are spaced in time with intervals from one hour up to one day, therefore
the non discrimination of the ten body areas depicted in the body map prepared for the
subjective body area ratings of support and comfort.
Paired comparisons between seats for the characteristics of overall support, comfort,
preference and egress/ingress characteristics are answered by the trial persons as soon as
the relevant seat evaluations have been completed. The actual questionnaire is presented
in Appendix B.
3.4.4 Video recording for evaluation of seat / occupant displacement
Video recording of the trial persons along one complete lap of the standardised driving
task, is intended to provide a basis for the further understanding of the differences in
support offered by the different test seats, assuming a control function in the data analy-
sis.
3.4.5 Seat /occupant interfacial pressure readings
Pressure sensor equipment is used to record the pressure prints of the subjects in the
seats at simulated levels of lateral acceleration (see point 3.6).
3.5 Driving sessions
The driving with remarkable lateral acceleration was performed on a special car track
with many “interesting” curves. The track was secluded from other traffic and was rent.
The driving sessions could be seen as a standard task, thus resulting in a smaller number
of uncontrolled variables in the whole experimental design. In this way, the speeds and
trajectory could be precisely defined and repeated with each subject for each seat
evaluation.
As to the time length of the individual driving sessions and the timing of the question-
naires, our reference was a study by Reed et al., 1991. Other studies of sitting comfort,
namely by Grandjean, showed some relevance for the definition of the duration of the
seat evaluations. While Grandjean’s main concern was office seating, Reed’s study dealt
with automotive seating, presenting also interesting concepts for questionnaire design.
In that study, subjects conduct a laboratory simulated driving task over 3 hours in vari-
ous seats and performed subjective evaluations of comfort every 30 minutes. In our
study the emphasis is both on the support and comfort aspects. For the support aspect it
seems reasonable to accept that a subjective evaluation of the securing effect of the seats
does not have to be carried out over such an extended period of time. On the other hand,
the aspect of extended duration driving, which would influence the subjective comfort
evaluation, would require a longer time, as in the reference study.

Experimental Design
25
The decision was also based on the possibilities of renting the car track and the time that
would be required from the subjects to perform the evaluations.
3.5.2 Plan for the execution of the driving experiments
A part of the car track in Stora Holm, was to be used to conduct the driving experi-
ments. Four seats, configured according to the description given above in point 3.3, are
used in the driver’s position. Four subjects, of defined hip breadths and other body char-
acteristics, were to drive the car with pre-defined trajectory, speed and time duration.
Using a well defined trajectory (Figure 3-1) and pre-defined speeds, the driving sessions
are standardised and it is possible to estimate the lateral acceleration values for each
curve (from the speed of the car and the radius of the curve - see Appendix C).
Figure 3-1: Portion of “Stora Holm” track with the trajectory chosen to provide a balance be-
tween curves to the right and to the left, and having the length of about 2000m per lap.
Videotaping of the subjects’ right shoulder and right side of the hip was to be performed
after 15 minutes of initial driving during two laps of the circuit. After this video re-
cording there was to be a pause to allow the subject to answer a questionnaire covering
the sense of discomfort (over time) induced by the seat. The subject then would perform
another 30 minutes of driving in the pre-defined driving task, at the end of which he
would answer the questionnaires concerning the senses of discomfort and support. The
reason why the evaluation of support was to be done only at the end of each seat evalua-
tion was to allow the subject time to acknowledge the securing effect provided by the
seats in the curves. The evaluation of discomfort was to be done twice, to provide some
insight in to the evolution of discomfort over time in this comparatively short seat
evaluation (50 minutes of net driving time and about 1 hour of total seated time per
evaluation). Each subject was to evaluate each of the four seats. In order to avoid accu-
mulation of discomfort and biased subjective assessment, each subject was to perform
the maximum of two different tests per day, separated by at least one hour. Question-
naires in terms of paired comparisons of seats were to be answered for overall prefer-
ence, support, comfort and egress/ingress characteristics when the relevant seat evalua-
tions had been completed.
3.6 Readings of the pressure distribution over the seat / occupant contact
with lateral acceleration
The pressure distribution in the interface between occupant and side support was identi-
fied as valuable in the objective characterisation of the support and comfort utilities of
different side support configurations. Due to restrictions of the portability of the pres-
sure sensor equipment available, we were not able to perform the readings of the pres-
sure values in a dynamic car-road situation. A tilting device that enables the simulation
of lateral acceleration, but only statically, was the solution found to deal with the limita-
tions of the pressure measuring equipment (Figure 3-2).
lateral acceleration

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