Basic ergonomics in automotive design

• Outline of Introduction
• Importance of course
• What is ergonomics?
• Human Characteristics, Capabilities and Limitations
• Ergonomics Applications in Vehicle Design
• Vehicle design Process
• SAE Standard J1100

Vehicle Assumptions:
- Body Style/ Size/ Weight/Powertrain
- Market Segment/ Driver/User Characteristics
- Features
Exterior Design:
- Wheelbase, Tread width
- Shape, Proportions
- Cowl Point, Deck Point
- Overhangs, Clearances, etc.
- Windshield, Backlite and Tumblehome Angles
Seating Package Layout:
- Firewall, Floor, Wheel wells, Tunnel, Roof
- Pedals, AHP, PRP, SgRP, H30, L53, A40, L50
- Eyellipse, Head Clearance Contours
- Steering Wheel: H17, L11, A18, W9
- Rim, Spokes and Hub design
Entry/ Exit and Cargo Loading/Unloading
Evaluations:
- Door Openings, Seat locations
- Storage Spaces, Load Heights, Widths, etc.
Seat Design
- SgRPs
- Occupant Accommodation
- Seating Comfort
Controls & Displays Space:
- Min and Max Reach Zones
- Visibility through SW
- 30-35 deg. Down angles Zone
Instrument Panel Design:
- Controls and Displays Layout
- Findability, Visibility, Interpretation, Operability
- Movements, Feedback, Effort, Feel
- Graphics, Legibility
Field of View Analyses:
- Window Openings, Wiper Zones
- Indirect (Mirror) Fields of Views
- Obscurations, Glare, Reflections
Night Visibility and Lighting
- Forward Visibility - Headlamps
- Signal Lamps - Position/Tail, Stop, Turn
- Safety, Security and Convenience Lighting
Package and Ergonomics Evaluations
Body Design:
- Closures- Doors, Trunk, Hood
Chassis and
Powertain Systems
Design
Craftsmanship Evaluations
- Perception of Quality-- Visual, Auditory, Tactile
and Olfactory Sensation
- Harmony
- Vehicle Image, Brand Perception

• Increasing Customer/User Satisfaction
• Ergonomics integrated with “Quality” Definition
• Safety Improvements and Product Liability Reduction
• Increasing Competitiveness
• Reduce product development time
• Do (Design) it right the first time
• Increase Productivity and Mobility of People
• Ergonomics is a key “vehicle” attribute
• Systems Engineering Approach in Vehicle Design

While the author and publisher believe that the information and guidance given in this work are correct, all parties
must rely upon their own skill and judgement when making use of them. Neither the author nor publisher assumes
any liability to anyone for any loss or damage caused by any error or omission in the work, whether such an error or
omission is the result of negligence or any other cause. Any and all such liability is disclaimed.

Flow diagram showing tasks
involved in occupant packaging
and ergonomic evaluations.

While each automotive company most
probably has specific requirements to cover for
their targeted users, this is a generic occupant
packaging process

The word “Ergonomics” originated from two Greek words “Ergon” means
“work” and “Nomos” means “natural laws”
International Ergonomics Association (IEA) defined Ergonomics (or human
factors) as the scientific discipline concerned with the understanding of
interactions among humans and other elements of a system, and the
profession that applies theory, principles, data and methods to design in
order to optimize human well-being and overall system performance. An
ergonomist is an individual whose knowledge and skills concern the
analysis of human-system interaction and the design of the system in order
to optimize human well-being and overall system performance (IEA, 2000).
Illustration of a vehicle package layout

Introduction to Ergonomics:
Introduction to Ergonomics module is divided into following:
• Ergonomics
• Domains of Specialization
• Applications and Benefits
• Aspects of Ergonomics

Ergonomics is concerned with the health of the people and the productivity of the system. It is to get proper fit
between people and their technological tools and environments. It takes account of the user's capabilities and
limitations in seeking to ensure that tasks, equipment, information and the environment suit each user.
Simply expressed we can say that Ergonomics is fitting the task to the person rather than fitting the person to the
task.

Domains of Specialization :
According to international Ergonomics Association (IEA) ergonomics can be broadly classified into:
• Physical Ergonomics
• Cognitive Ergonomics
• Organizational Ergonomics
Physical Ergonomics:
It is concerned with human anatomical, anthropometric, physiological and biomechanical characteristics as they related to
physical activity. Relevant topics may include working postures, material handling, repetitive movements, work related
musculoskeletal disorders, workplace layout, health and safety.
Cognitive Ergonomics:
A proper fit of a product to a user does not end with physical interfaces. Cognitive / perceptual ergonomics is concerned with
mental processes, such as perception, memory, reasoning, and motor response, as they affect interactions among humans and
other elements of a system. Relevant topics include mental workload, decision-making, skilled performance, human-computer
interaction, human reliability, work stress and training as these may relate to human-system and Human computer
interaction design.
Organizational Ergonomics:
It is concerned with the optimization of socio technical systems, including their organizational structures, policies, and processes.
Relevant topics include communication, crew resource management, work design, design of working times, teamwork, community
ergonomics, cooperative work, new work programs, virtual organizations, telework, and quality management.

Applications and Benefits:
Applications:
Ergonomics continues to be successfully applied in the fields of workplace design, occupational health, safety,
product design, aerospace engineering, mechanical engineering, health care, IT sectors, transportation, training,
nuclear power plant, virtual environments, industrial design and so on.
Benefits:
Application of ergonomic principles in various fields provides to better man-machine interaction, healthy and
comfortable working environments, enhancement of human performance and efficiency and thus ultimately leads
to overall improvement of system’s (man-machine-environment) productivity with reduction of error and accidents.
Key benefits of application of ergonomics are listed below:
• Human fatigue and error can be reduced.
• increase productivity and safety
• Increase work quality
• Decrease risk of accidents
• Improve people attitude
• More user satisfaction
• Less absenteeism
• Reduced lost time , etc.

Aspects of Ergonomics:
Study of compatibility issues for proper man-machine interface is very important in ergonomics. Here, focus is
generally made on user’s requirement, user’s characteristics and user’s capabilities/limitations for user friendly
design. Human compatibility with machine/instrument/work elements are discussed in terms of anthropometric,
biomechanical, physiological and cognitive/ psychological aspects.
Anthropometry:
Anthropometry is the subject which deals with the measurements of the human external body dimensions in static
and dynamic conditions. Anthropometric data is used for product and workplace design.
Anthropometry is of two types:
• Static Anthropometry
• Dynamic Anthropometry

Static Anthropometry:
External human body dimensional measurement taken when a man is placed in a rigid static position i.e. standing,
sitting, or other adopted postures.
Dynamic Anthropometry:
The dimensional measurement of human body with various movements taken into consideration in different
adopted postures which the work context demands are termed dynamic anthropometry.

To understand anthropometry, knowledge of body planes and somato-types are essential. These imaginary planes
are used for the identification of relationship between the position of things and postural configuration, and for
description of any location.
Somatotypes:
The human body types are classified according to the contents of fat in the body. These are
• Ectomorphs,
• Mesomorphs and
• Endomorphs.

Ectomorphs:
Due to low fat storage the full body appears to be skinny, lean and thin. Abnormal postures are adopted by the
people of this category while working, standing, and sitting.

Mesomorphs:
This type of body contains less fat but well balanced and firmed; usually referred to as muscular. Movements are
well co-ordinated in all the limbs and in the body as a whole..

Endomorphs:
This body type has increased fat storage, a wide waist and a large bone structure, usually referred to as fat.

Measuring Procedures:
Direct and indirect measuring techniques are followed to collect anthropometry data. In direct measurement
method, body dimensions are measured with standard anthropometric tools/kits. In indirect measurements method
relies on steal photographs and pictures. Presently, 3D Laser Body Scanner is used for getting detailed data.
For the larger survey and sample size, statistical treatment of anthropometric data is done to get the standard
measurements for whole population. An extremely useful statistic for designers is the percentile.
Percentile are the statistical values of a distribution of variables transferred into a hundred scale. The population is
divided into 100 percentage categories, ranked from least to highest, with respect to some specific types of body
measurement. The first percentile of any height indicates that 99 percent of the population would have heights of
greater dimensions than that. Similarly, 95th percentile height would indicate that only 5 percent of the study
population would have greater height and the 95 percent of the study population would have the same or less
height. The 50th percentile, or median, is one kind of average which divides the whole population into two similar
halves.

Biomechanics:
Biomechanics is the study of the structure and function of biological systems by means of the methods of
mechanics.
It is the application of mechanical principles to biological systems of human-beings.

Factors to be considered in Biomechanics:
Newton three laws of motion can be used to solve most biomechanical problems.
Force:
Forces are key to understand mechanics. A force is any influence that causes a free body to undergo a change in
speed, a change in direction, or a change in shape. The unit of force is kgms-2. Force can be internal or external
when we consider biomechanical problems. We generally consider the body is acting within the environment.
Internal forces are the forces that act within the body, such as muscle forces, joint reaction forces, load that act on
the various body tissues.
To move relative to the outside world the body need to be subjected to external forces. These are often the result
of internal forces causing a change in the body conformation but can also be due to some other external forces
such as gravity or other applied forces from contact with the object.

Joint:
A joint is the location at which two or more bones make contact. They are constructed to allow movement and
provide mechanical support, and are classified structurally and functionally. Structural classification is determined
by how the bones connect to each other, while functional classification is determined by the degree of movement
between the articulating bones.
Joints can also be classified according to their anatomy or their mechanical properties.
• Simple Joint: 2 articulation surfaces, e.g. shoulder joint, hip joint etc.
• Compound Joint: 3 or more articulation surfaces, e.g. radio carpal joint
• Complex Joint: 2 or more articulation surfaces and an articular disc or meniscus, e.g. Knee joint
Shoulder joint Radio carpal joint Knee joint

Range of Motion:
Generally speaking, range of motion refers to the distance and
direction a joint can move to its full potential. Each specific joint
has a normal range of motion that is expressed in degrees after
being measured with a goniometer (i.e., an instrument that
measures angles from axis of the joint). Rage of movement for
different body joints are shown
Movements of the human body

Physiology:
Human physiology is the science of the mechanical, physical and biochemical functions of humans in good health,
and how to apply that information in the evaluation and design of work. Various physiological aspects which are
studied to evaluate work performance of human being are as follows:
• Cardiovascular response (HR, BP, Cardiac output).
• Respiratory response (O2 uptake, CO2 out put).
• Metabolic response (Energy expenditure).
• Static & dynamic muscle loading.
• Tissue compression etc.
Increase of physiological demand for performing activity leads to physiological stress which in turn create various
health complications or disorders.

Psychology:
Psychology in ergonomics is concerned with adapting the equipment and environment to people, based upon
their psychological capacities and limitations with the objective of improving overall system performance.
The objectives of psychology in ergonomics are to optimize the effectiveness and efficiency with which human
activities are conducted as well as to improve the general quality of life through "increased safety, reduced fatigue
and stress, increased comfort and satisfaction.
For performing any activity, human receives various information through different sensory organs (e.g. eye, ear,
nose, tongue, skin etc.), processes those information in brain and then execute neuromuscular actions. Processing
more information in shorter duration leads to increase in cognitive work load. Hence, in man-machine compatibility
evaluation, analysis of cognitive work load is very essential.

Automotive Ergonomics:
Automotive Ergonomics Module is divided into following:
• Introduction to Automotive Ergonomics
• Spatial Arrangement
• Reachability
• Strength Capability
• Visual Field & Visual Obstruction
• Seat Design & Seating Comfort

Introduction to Automotive Ergonomics:
Automotive ergonomics focuses on the role of human factors in the design and use of automobiles. This includes
analysis of accommodation of driver and/or passengers; their comfort; vision inside and outside vehicle;
control and display design; pedal behaviour , information processing and cognitive load during driving etc.
In the present module attempt will be made to discuss various physical aspect of occupant packaging for
providing comfortable driving posture, clearance dimensions, proper view field, easy reach of the controls etc. to
the driver.
This module highlights the following:
• Spatial accommodation
• Seating Position
• Leg Room
• Head Clearance
• Lateral Clearance
• Sitting comfort /discomfort
• Reach and limitations of human
• Visual field and Visual Obstruction.

To establish the required interior space, and arranging the interior and structural components, the design methods
relies on the human factors data base through years of research and practical applications.
The anthropometry for automotive design is consistent with the driver and passenger safety, comfort, convenience
and accommodation. The study of human capabilities and limitations gives the measurements for designing
automobiles.
The anthropometry for automotive design is consistent with the driver and passenger safety, comfort, convenience
and accommodation. The study of human capabilities and limitations gives the measurements for designing
automobiles.

Anthropometric Measurements for Automotive Ergonomics:
Automobile is designed as per the anthropometry of the targeted
user population. Measurement process can be broadly classified
into two categories.
Conventional Static Measurements:
The measurements taken on human body with the subjects in
rigid, standardized position (fig.10). They are typically length,
width, height and circumferences. These measurement includes
standing height, seated height, seated eye height, upper leg
length, knee height, seat length, upper and lower arm length,
reach (total arm length), shoulder width, hip or seat width, weight,
etc.
These measurements are referenced to non-deflecting horizontal
or vertical surfaces supporting the subject.
Conventional static measurements

Functional Task Oriented
Measurements:
The measurements are taken with the
human body dimensional co-ordinates
x, y, z with respect to body land marks
as reference points. at work or motion
in the workspace (fig. 11). Typically they
are represented in three dimensional
co-ordinates x, y, z with respect to body
land marks as reference points.
Functional task oriented measurements

Functional Task Oriented Measurements:
Few reference points e.g. H-point, BOF, AHP etc. are used as standard
practice to define driver’s position while SRP, NSRP and SgRP are generally
used to define seat position in relation to driver.
• H-point ( Hip pivot): Mid point of the line connecting two hip joints.
• BOF (Ball of Foot): Ball joint of Foot.
AHP (Accelerator Heel Point): position of the heel while placed on the
accelerator.
• SRP (Seat Reference Point): Intersection point between midline of
compressed seat back and compressed seat pan.
• NSRP (Neutral Seat Reference Point): 50th percentile person selected SRP.
• SgRP: 95th percentile person selected SRP.
These landmarks relate the occupant to components in the vehicle interior
such as foot controls, seat and floor. For example, the foot is related to the ball
of foot and accelerator heel point, where as hip, elbow and shoulder width
are related to the h-point location.
To accommodate wide range of target population, 5th and 95th percentile
anthropometric data are used in general.
Landmarks for measurements

Seat Track Travel Limit:
Seat track travel limit is decided in such
a way so that individuals with smaller
body dimensions as well as larger body
dimensions can seat comfortably on the
seat and can access all the controls
including accelerator, break and
clutch. Seat track travel limits in
forward-backward and upward-down
ward direction are decided as per
operational requirement.
Figure depicts forward-backward
movement of the seat as per the
different percentile driver selected seat
position (SAE- J1517).
SAE recommended occupant packaging

Spatial Arrangement:
After defining the position of the driver on the seat, all other interior and structural components inside the vehicle
are arranged accordingly with the intension to provide sufficient clearance dimensions around him/her. This
process relies on human factor database. Larger anthropometric data (95th percentile value) are generally
considered for this purpose.
Spatial arrangement includes the positioning of driver’s seat and passenger’s seat in the allocated space in side,
arrangement of various controls/components according to seating arrangements.
In this module leg room, head room and lateral space are to be described in brief.
Legroom:
The sufficient space for keeping legs of the driver/passenger in a comfortable position in an automobile. Proper
legroom enables drivers to access structural component with ease. There should not be any obstacle to keep feet
comfortably and at the same time for accessing controls like pedals (break/accelerator/clutch).

Measurement of horizontal distance between H-Point and AHP is useful for this purpose. Care should be taken to
ensure that any parts of lower body like thighs/knees should not touch with steering wheel or dash board or any
other component.
Headroom:
The height. It is the vertical clearance space above the head of driver/passenger in an automobile. A minimum 5.0
cm head clearance for jolt in a vehicle is recommended (Galer 1987, Woodson et al. 1992). In vehicular
workstation, available head clearance must be sufficient for wearing and removing the helmet in seated posture in
seat.
Lateral Space:
Lateral space is the space pertaining to the side of driver/passenger. Lateral space is important for physical or
psychological comfort.
Conventionally, 95th percentile bi-deltoid breadth of the population with an additional allowance of 10% on each
side can be considered adequate for lateral clearance during normal sitting side by side.

Lateral clearance for sitting side by side

Top view of a car to show lateral spaces for the front seats
Dimensional Consideration of
Lateral Space:
The overall distance
between the inner part of
doors “W” is the out put of
manipulated anthropometric
data of lateral between two
seats, and the distance from
seat’s edge to the door.
spaces required for comfort.
This ‘W’ is composed of the
width of seats, distance.

Reach and Limitation of Human:
In many work situations, individuals
perform their activity within a
specified 3D space of fixed location
which is sometimes referred to as
‘work-space envelope’ (Sanders and
McCormick 1993). This envelope
preferably should be circumscribed
by the functional arm reach of the
operator and most of the things they
need to handle should be arranged
within this envelope.
In figure describe human capabilities
and limits in terms of reach on
horizontal work-surface with their
measurements.
Normal and maximum horizontal reach areas

Normal and maximum horizontal
arm reach does not correlate with
reach capabilities in actual vehicle
workstation. Factors such as seat
position, seat deflection, shoulder
articulation, and lean allowed by
slack in a shoulder harness (if one is
worn) affects a driver’s reach
capabilities.
Forward arm reach of the driver
according to anthropometry and
seat track travel as described in
SAE J287 shown in figure.

Strength Capability
Strength for Control Operation:
Strength is one type of human performance limiting factor and concerns the application of force in the operation of controls
and in other physical tasks. Often, limitation of strength imposes a one-way constraint and it is sufficient to determine the level
of force that is acceptable for a weak limiting user.
The capabilities of human body is considered to make the operational components in the vehicle while driving. For example,
the force is required for the ease of operation of clutch, steering, opening and closing of doors etc.
Actuating force limits for some important tractor controls for Indian male agricultural workers (CIAE, Bhopal, 2009) are given
below:
Brake Pedal:
 5th p Rt leg strength (male)=261 N.
 Maximum actuating force for break operation should be less than 260 N.
Clutch Pedal:
 5th p Lt leg strength (male)=247 N.
 Frequently operated compared to break pedal.
 50% of 5th p Lt leg strength (male)= 123.5 N.
 Maximum actuating force for Clutch operation should be less than 124 N.

Accelerator Pedal:
5th p Rt foot strength (male)=163 N.
Continuously operated, 30% of 5th p Rt foot strength (male)=49 N (upper limit).
Maximum actuating force for accelerator operation should be less than 49 N.
Weight of leg = 9%= .09 of body wt., part of this wt. is supported by heel.
Lower limit of force exertion for accelerator= 54.7kg x9.81x.09x0.5=24N.
Steering Wheel:
5th p torque strength with both hands, sitting (male)=36 Nm (force 171 N with lever arm of 0.21 m).
Frequently operated, 30% of 5th p = 51 N.
Maximum actuating force for steering wheel operation should be less than 51 N.
Gear Selection/ Speed Selection Lever:
5th p Rt hand push strength = 49 N, limiting force for operation.
Maximum actuating force for gear operation should be less than 49 N.

Visual Field and Visual Obstruction:
Limits of Visual Field:
Driver can turn both eyes and head to gain a wider field of view, and moreover can make use of peripheral vision
to see objects or movements even without turning eyes.
In the horizontal plane, the binocular field of view extends some 120 degrees, as in figure given below. Vision is
sharp only over a fairly small area directly ahead. So, eyes need to be turned to focus on objects outside the foveal
area. According to SAE J985 eyes generally only turn by about 30 degrees before the head is turned, which can
comfortably give a further 45 degrees view to either side.
Binocular field view in horizontal plane

In the vertical plane eye movement is comfortable within 15 degrees above or below the horizontal, although the
eye can see upto 45 degrees upward or 65 degrees downward if necessary.
On the other hand, head can easily incline 30 degrees upward or downward. Thus, by movement of head and
eye, the driver can have extended direct field view. The driver has to concentrate on direct view, that is on road.
So glancing away from the road for a short period is possible. Mirror and other instruments should be close to the
driver, so that driver does not require a much head and eye turn to have a look.
vertical views of human eye

Driver’s Eye Location:
Variation of eye positions inside the vehicle for any driving population is considerable due to variation of seat
locations and variable anthropometry of the drivers. In order to address this problem, the SAE J941 ‘Eyellipse’
concept was developed as a drafting tool to define the range of eye positions within the driving population. It is
based on the position of eyes of drivers in space. The distribution of eye position in space closely approximated an
ellipsoid.
During automobile design, care should be taken to provide maximum view all around either through direct vision or
with the help of devices like mirror or camera. It is also important to ensure minimum visual obstruction either by
vehicle components or by driver’s own body parts. This is particularly important for allowing unobstructed view of
the displays on the dash board.
Internal and External View from Driver Seat:
The vision is a crucial factor in the driving task as most of the information received by the driver come through the
visual sense. The clear view of road (front and rear) enables the driver a safe driving. Poor visibility conditions are
stressful for the drivers and results in a significantly increased risk of accident.

• Visual Needs:
The view ahead through the wind shield has to
be sufficient and clear for the driver. It enables
driver to stop in emergency and necessary
conditions. Similarly, rear and side views are
important for maintaining speed, taking turn,
exerting break or during parking.
On the road driver need much longer view to
anticipate and prepare for avoiding actions.
Views close to the vehicle is equally important
when turning left or right and maintain proper
distance to avoid accidents. Fig. 16 shows the
view inside the vehicle, forward and side views
through glasses and rear view through mirror.
View during driving

Specifying the Field of View:
Direct View:
The views observed by the driver directly through eyes are considered as direct views. The visual field of human eye
is complex, limited by anatomical and optical factors. However, it can be represented by sightlines drawn from the
eye to all the points which can be seen, collectively defining the visible field of view.
The view of driver can be represented in two dimensional geometry by considering a imaginary sight line (Horizon)
passing through the driver’s eye.
The viewing angle above the horizon can be considered for traffic signals and signs.
The downwards view can be considered for road. Height of the dashboard and curvature of the bonnet are the
two determining factor for downward view through front windshield. Upper edge of the dashboard should be at
least 15 degree below that horizontal eye line of driver with smallest (5th percentile) sitting eye height.
The far distance view is based on the horizon, the sightline passing through the driver’s eye.

Indirect View:
The views to the rear of the vehicle mainly obtained through the mirrors. This view provides information
on passing vehicle, vehicle close to the rear when the driver proposing to change the lane.
The reflected view of mirror can be represented in the same way as in direct view with the viewing
angles.
The view of image is bounded by the frame of mirror.
The image boundaries can be determined by the mirror dimensions, locations of the mirror with
respect to driver’s eye and optical characteristic of mirror. By adjusting the mirror the field of view of
rear can be adjusted.
Rear-ward field view in the interior and side mirror

Seat Design and Seating Comfort:
Seating Comfort:
For occupant’s comfort and health, good seat design should be applied by considering sitting postures . Static and
dynamic anthropometry data are considered for proper design of a comfortable and safe seat.
Some factors to be considered for driver’s seat:
• The seat should position the driver with unobstructed vision and within reach of all vehicle controls. For this
purpose appropriate seat adjustment features should be there.
• Proper back support, head rest, thigh support should be provided but there should not be and obstruction/
hindrance during arm or leg movement.
• Seat must accommodate the driver’s size and shape.
• Seat should be comfortable for extended period.
• Seat should provide a shape zone to the driver in a crash.
Passenger in the front and rear seats need comfortable supporting surfaces for a variety of postures unconstrained
by the vehicle operation. Postural stress, vibration, muscular effort, impact and shock are the causes for backache
and lower back pain in drivers. Safety should be taken into account while considering the design of seats without
compromising the comfort.

Different Factors considered for Seat Design:
Human geometry both in static and dynamic are considered for designing seats. The static geometry describes the
physical size to be accommodated in the seat and dynamic geometry describes the functional position to be
accommodated in the seat.
Body Size:
Seats are mostly designed as per the body weight and anthropometry of the targeted user population to fit at least
90 percent of population. The 95th percentile of male and 5th percentile female anthropometric data is
generally considered for accommodation on seats.
Human linkage system: Rigid human body can be specified according to the joint centre position and the angle
between adjacent links. The movements and dimensions of human linkage system helps to define the curvature of
seats and comfortable position for sitting.
Position of the Body:
Driver’s seat position is dependent on the vision and reach of the driver. Clear view and comfortable sitting posture
are the factors considered for designing seat. The dimensioning is mostly depends on eye, hand and foot
positioning. For different body vertical, back angle adjustments are provided.

Posture of the Body:
Seat should reduce postural stress and optimize muscular effort. Postural stress occurs due to adopting one posture
for long period of time, so comfortable support for many postures is essential and this can be accommodated by
manipulation of anthropometric data and the linkage system.
Vibration and Ride Comfort:
Vibration, shock and impacts are major factors for judgments of comfort ability according to most users. Thus, the
seat design also must consider the vehicle suspension system and the vibration transmitted to the seated user.
Geometric Features of Seat Design:
Seat design can be divided into accommodation and comfort. Accommodation refers to seat size and
adjustments for horizontal distance from controls, height and back angle. Comfort, however refers to stiffness,
contour, climate and vehicle features that promote users comfort.

The seat height, width and back angles are based on the human anthropometry data collected from the research.
its important to provide sufficient space for physical and psychological comfort.
• Cushion’s length from seat back to the waterfall line is 440-550mm is recommended (Grandjean, 1980).
• The breadth of the cushion is recommended 480mm (Grandjean, 1980) for clothing and leg splay. The
measurement is based on 95th percentile of female hip breadth and additional space for comfort since female
hips are greater than male hip breadth.
• Seat back height is recommended 509mm (Grandjean, 1980) by considering the small female, sitting shoulder
height.
• Seat back breadth may be divided into lower and upper regions. The lower must accommodate a tapered
shape from 432mm at the hip to 367mm at the chest (Grandjeans, 1980). 480mm is recommended for seat back
breadth (Grandjean, 1980).
• Horizontal adjustments accommodate differences in leg length that are associated with seat height and
preferred knee angle. Grandjean (1980) recommended a minimum of 150mm horizontal adjustment. The joint
angles in automobile are typically between 95 and 120 degrees for the hip, and 95 and 135 degrees for the
knee (Rebiffe, 1969).

• Horizontal seat travel is a function of seat height and body size. Average seat travel was investigated 148mm
approx. (Schneider et al., 1979).
• Vertical adjustments accommodate differences in sitting eye height between the fifth percentile female and
95th percentile male. A simple trigonometric relation can be established with link length and joint angles to
compute the amount of seat adjustment needed in the vertical direction.
• Adjusting a flat, non-deformable surface over a range of 163mm maintains a constant eye height. Seat cushion
compression and suspension deflection are no-linear function of applied force; as a result the vertical
displacement needed in a soft seat is poorly calculated from anthropometric data. Grandjean (1980)
recommended a seat height between 250 and 300 mm.
• Seat back angle adjustments accommodate differences in arm length and occupant preferred hip angle.
Grandjean (1980) recommended a seat cushion angle of 19 degrees with a range from10 to 22 degrees. d on
multiple joints or overall body posture (Krist 1994).

All the seat design criteria and dimensions mentioned above are for general understanding of the subject.
Presently, various SAE standards are followed in automobile industries all over the world.
Both subjective and objective methods of discomfort measurements are used to analyse and rate the level of
discomfort. Among the various rating scales ‘Visual analogue Discomfort Scale’ or ‘Verbal Numerical Rating Scale’
for assessment of intensity; ‘Body Map’ or specific instruction for assessment of discomfort location and repeated
measurement for the assessment of temporal pattern of discomfort are generally used (Van der Grinten
1991). Empirical studies of various scientists provide comfort data for quantitative estimation of sitting comfort of
vehicle occupants or drivers based on single joint postural analysis (Porter and Gyi 1998, Grandjean 1980, Henry
Dreyfuss Associates 1993, Rebiffe 1969) and some based on multiple joints or overall body posture (Krist 1994).

The reference points used for location of the driver and their relevant dimensions are described below.
1. The accelerator heel point (AHP) is the heel point of the driver’s shoe that is on the depressed floor covering
(carpet) on the vehicle floor when the driver’s foot is in contact with the undepressed accelerator (gas) pedal .
SAE standard J1100 defines it as “a point on the shoe located at the intersection of the heel of shoe and the
depressed floor covering, when the shoe tool (specified in SAE J826 or J4002) is properly positioned (essentially,
with the ball of foot (BOF) contacting the lateral centreline of the undepressed accelerator pedal, while the
bottom of shoe is maintained on the pedal plane)
2. The pedal plane angle (A47) is defined as the angle of the accelerator pedal plane in the side view measured
in degrees from the horizontal. The pedal plane is not the plane of the accelerator pedal, but it is the plane
representing the bottom of the manikin’s shoe defined in SAE J826 or J4002. (As described later in this chapter,
A47 can be computed by using equations provided in SAE J1516 or J4004. Or, it can be measured by using the
manikin tools described in SAE J 826 or J4002.)
3. Ball of Foot (BOF) on the accelerator pedal is the point on the top portion of the driver’s foot that is normally in
contact with the accelerator pedal. The BOF is located 200 mm from the AHP measured along the pedal plane
(SAE J4004, SAE 2009).

4. The pedal reference point (PRP) is on the accelerator pedal lateral centreline where the BOF contacts the pedal
when the shoe is properly positioned (i.e., heel of shoe at AHP and bottom of shoe on the pedal plane). SAE
standard J4004 provides a procedure for locating PRP for curved and flat accelerator pedals using SAE J4002 shoe
tool. If the pedal plane is based on SAE standards J826 and J1516, the BOF point should be taken as the PRP.
5. The seating reference point (SgRP) is the location of a special hip point (H-point) designated by the vehicle
manufacturer as a key reference point to define the seating location for each designated seating position. Thus,
there is a unique SgRP for each designated seating position (e.g., the driver’s seating position, front passenger’s
seating position, left rear passenger’s seating position). An H-point simulates the hip joint (in the side view as a hinge
point) between the torso and the thighs, and thus, it provides a reference for locating a seating position. In the
plan view, the H-point is located on the centreline of the occupant.
The SgRP for the driver’s position is specified as follows:
a. It is designated by the vehicle manufacturer.
b. It is located near or at the rearmost point of the seat track travel.
c. The SAE (in standards J1517 or J4004) recommends that the SgRP should be placed at the 95th percentile location
of the H-point distribution obtained by a seat position model (called the SgRP curve) at an H-point height (H30 from
the AHP specified by the vehicle manufacturer).
6. The seat track length is defined as the horizontal distance between the foremost and rearmost location of the H-
point of the seated drivers.

A number of interior package dimensions shown in the previous slide are described in this section. The dimensions
are defined using the nomenclature specified in SAE standard J1100.
1. AHP to SgRP location: The horizontal and the vertical distances between the AHP and the SgRP are defined as
L53 and H30, respectively.
2. Posture angles: The driver’s posture is defined by the angles of the HPM or the HPD. The angles shown in the
previous slide are defined as follows:
a. Torso angle (A40). It is the angle between the torso line (also called the backline) and the vertical. It is
also called the seat back angle or back angle.
b. Hip angle (A42). It is the angle between the thigh line and the torso line.
c. Knee angle (A44). It is the angle between the thigh line and the lower leg line. It is measured on the right
leg (on the accelerator pedal).
d. Ankle angle (A46). It is the angle between the (lower) leg line and the bare-foot flesh line, measured on
the right leg.
e. Pedal plane angle (A47). It is the angle between the accelerator pedal plane and the horizontal.
3. Steering wheel: The centre of the steering is specified by locating its centre by dimensions L11 and H17 in the
side view. The steering wheel centre is located on the top plane of the steering wheel rim (see previous slide).
The lateral distance between the centre of the steering wheel and the vehicle centreline is defined as W7. The
diameter of the steering wheel is defined as W9. The angle of the steering wheel plane with respect to the
vertical is defined as A18 (see previous slide).

4. Entrance height (H11): It is the vertical distance from the driver’s SgRP to the upper trimmed body opening (see
Figure below). The trimmed body opening is defined as the vehicle body opening with all plastic trim (covering)
components installed. This dimension is used to evaluate head clearance as the driver enters the vehicle and
slides over the seat during entry and egress.
Entrance height (H11)

5. Belt height (H25): It is the vertical distance between the driver’s SgRP and the bottom of the side window
daylight opening at the SgRP X-plane (plane perpendicular to the longitudinal X-axis and passing through the
SgRP; (see Figure below). The belt height is important to determine the driver’s visibility to the sides. It is
especially important in tall vehicles such as heavy trucks and buses to evaluate if the driver can see vehicles in
the adjacent lanes, especially on the right-hand side. The belt height is also an important exterior styling
characteristic (e.g., some luxury sedans have high belt height from the ground as compared with their overall
vehicle height).
Belt height (H25)

6. Effective headroom (H61): It is the distance along a line 8° rear of the vertical from the SgRP to the headlining,
plus 102 mm (to account for SgRP to bottom of buttocks distance; (see Figure below). It is one of the commonly
reported interior dimensions and is usually included in vehicle brochures and websites.
Effective head room (H61).

7. Leg room (L33): It is the maximum distance along a line from the ankle pivot centre to the farthest H-point in the
travel path, plus 254 mm (to account for the ankle point to accelerator pedal distance), measured with the
right foot on the undepressed accelerator pedal (see Figure below). It is also one of the commonly reported
interior dimensions and is usually included in vehicle brochures and websites.
Leg room (L33)

8. Shoulder room (W3; minimum cross-car width at beltline zone): It is the minimum cross-car distance between the
trimmed doors within the measurement zone. The measurement zone lies between the beltline and 254 mm
above SgRP, in the X-plane through SgRP (see Figure below. It shows a cross-sectional front view of the vehicle.)
It is also one of the commonly reported interior dimensions and is usually included in vehicle brochures and
websites.
Shoulder room (W3)

9. Elbow room (W31; cross-car width at armrest): It is the cross-car distance between the trimmed doors, measured
in the X-plane through the SgRP, at a height of 30 mm above the highest point on the flat surface of the
armrest. If no armrest is provided, it is measured at 180 mm above the SgRP (see Figure below).
Elbow room (W31)

10. Hip room (W5; minimum cross-car width at SgRP zone): It is the minimum cross-car distance between the
trimmed doors within the measurement zone. The measurement zone extends 25 mm below and 76 mm above
SgRP, and 76 mm fore and aft of the SgRP (see Figure below).
Hip room (W5)

11. Knee clearance (L62; minimum knee clearance—front): It is the minimum distance between the right leg K-point
(knee pivot point) and the nearest interference, minus 51 mm (to account for the knee point to front of the
knee distance) measured in the side view, on the same Y-plane as the K-point, with the heel of shoe at FRP
(floor reference point; see Figure below).
Knee clearance (L62)

12. Thigh Room (H13; steering wheel to thigh line): It is the minimum distance from the bottom of the steering wheel
rim to the thigh line (see Figure below).
Thigh room (H13).

SAE Recommended practices
Although each company has many in-house procedures and guidelines to design and evaluate vehicles and seats,
SAE recommended practices form the basis for many common design procedures. Table below lists the SAE
Practices that are used for vehicle interior packaging.
Practice Title
J182 Motor Vehicle Fiducial Marks
J287 Driver Hand Control Reach
J826 Devices for Use in Defining and Measuring Vehicle Seating Accommodation
J941 Motor Vehicle Driver’s Eye Range
J1052 Motor Vehicle Driver and Passenger Head Position
J1100 Motor Vehicle Dimensions
J1516 Accommodation Tool Reference Point
J1517 Driver Selected Seat Position

SAE Recommended practices
The design process will be illustrated using the packaging of a hypothetical vehicle.
For this illustration, a seat height of 220 mm and some predefined pedal geometry are assumed. Figure in next slide
shows the tools on a side view of the package.
First, the pedal plane angle is calculated using the equation in J1516, in the process defining the BOF and AHP
locations. Next, using the 95th-percentile driver-selected seat position curve, an SgRP location is established.
The 2.5th percentile and 97.5th-percentile curves from J1517 are laid in, along with a seat-track (H-point) travel line,
to define the seat track travel length necessary to accommodate preferred seat positions of 95 percent of a U.S.
driver population with an equal gender mix. With the SgRP and pedal reference points established, the other
accommodation models may be positioned.
The eyellipse (defined in J941 for a U.S. driving population with an even gender mix) is positioned relative to the SgRP
using the design seatback angle (defined in J1100 as L40). Head contours from J1052 are positioned in a similar
manner. Positioning the hand-control reach envelopes requires calculation of the “G” factor from a range of
vehicle interior dimensions, including H30, L11, and others.

Accommodation tools defined in
SAE recommended practices

SchematicofrelationshipsamongSAE
recommendedpractices.

The resulting accommodation tool
locations can then be used to assess the
vehicle interior design. Display locations
and steering wheel obscuration can be
evaluated using sight lines or planes
constructed tangent to the eyellipse.
Head clearance can be quantified using
procedures defined in J1100 based on
translation of the head contours toward
the roof.
In addition, many other design guidelines
developed by individual companies that
reference the SAE accommodation
models can then be applied..

Developing the Occupant Package: Design Considerations
In developing a vehicle package, a number of design considerations related to the functioning of various vehicle
systems and interfaces between the systems and occupant comfort, convenience, and safety issues are
considered. The occupant packaging considerations can be grouped into the following areas:
1. Entry and egress space: Location of the seats, seat shape, clearances required during entry and exit with
various vehicle components (i.e., space available for movements of head, torso, knees, thighs, feet, hands, and
torso), walk-through in the centre (in vans or multi-passenger vehicles), locations of grasp handles, and so forth.
Illustrationofavehicle
packagelayout.

2. Comfortable seated posture: Seat height and leg space, head and shoulder room, torso angle (between torso and upper
leg), neck angle (between head and torso), knee angle (between upper leg and lower leg), ankle angle (between foot
and lower leg), lengths and widths of seat cushion, seat back, head rests, stresses (forces and pressures) in the spinal
column, shape of seat support surfaces in the lumbar region and thigh/buttocks region, and so forth with respect to the
steering wheel and pedal locations.
3. Operating controls (hand and foot controls): Locations of controls and displays; head, eye, and ear positions (for acquiring
information); body movements and postures (hand, foot, head, and torso) during reaching, grasping, and operating
controls; natural versus awkward postures; and use of other in-vehicle items (e.g., cup holder, map pockets, entertainment
and information systems).
4. Visibility of interior and exterior areas: Eye locations; movements of eyes, head, neck, and torso during gathering of visual
information from the road and inside the vehicle (e.g., visibility of displays); and available fields of view (obstructions caused
by vehicle structures and components and in-direct fields from mirrors).
5. Storage spaces: Providing convenient and safe storage spaces to accommodate items brought into the vehicle during
trips.
6. Vehicle service: Providing convenient access and space for performing vehicle service and maintenance tasks (e.g.,
refueling, checking engine oil, replacing bulbs, flat tires).
The challenge of the occupant package engineer is to assure that the largest percentage of the user population is
accommodated in performing all tasks involved in the above areas during vehicle usages—while driving and not driving.

Driver Package Development Procedures
In this section, we cover basic steps involved in positioning the driver, determining the seat track length, positioning
eyellipse and head clearance envelopes, determining maximum and minimum reach envelopes, and positioning the steering
wheel.
1. Determine H30 = height of the SgRP from the AHP.
The H30 value is usually selected by the package engineer based on the type of vehicle to be designed. The H30
dimension is one of the dimensions used in the SAE standards to define class A vehicles (passenger cars and light trucks) and class B
vehicles (medium and heavy trucks).
The values of H30 for class A vehicles range between 127 and 405 mm. It should be noted that smaller values of H30
will allow lower roof height (measured from the vehicle floor) and will require longer horizontal space (dimension L53 and X95) to
accommodate the driver—like in a sports car. Conversely, if a large value of H30 is selected, the taller cab height and shorter
horizontal space (dimension L53 and X95) will be required to accommodate the driver.
The class B vehicles (medium and heavy trucks) will have large values of H30 (typically 350 mm and above) so that
less horizontal cab space is used to accommodate the driver, and thus, longer longitudinal space is available for the cargo area.
The BOF-to-SgRP dimension is usually determined by computing the X95 value (i.e., 95% of the drivers will have their H-
point forward of the SgRP; measured in mm) from the following equation given in SAE J1517. (This equation is called the SgRP curve
in SAE J4004.)
X95 = 913.7 + (0.672316z − 0.00195530z2) where z = H30 in millimetres.

2. Determine pedal plane angle (A47).
The value of the pedal plane angle in degrees is obtained by using the following equation from SAE standard
J1516.
A47 = 78.96 − 0.15z − 0.0173z2 where z = H30 in centimetres (note: this z value is in centimetres—for the above
equation only).
In SAE standard J4004, the pedal plane angle is defined as alpha (α), where
α = 77 − 0.08 (H30) (degrees from horizontal) (note: H30 is specified in millimetres).
3. The vertical height (H) between the BOF and AHP can be computed as follows:
H = 203 × sin(A47)
It should be noted that distance between AHP to BOF is specified as 203 mm in SAE standard J1517 and 200 mm in
SAE standard J4004.
4. The horizontal length (L) between the BOF and AHP can be computed as follows:
L = 203 × cos(A47)
5. The horizontal distance between the AHP and SgRP (L53) can be computed as follows:
L53 = X95 − L

6. The seat track length is defined by the total horizontal distance of the fore and aft movement of the H-point (for
a seat that does not have vertical movement of the H-point).
The foremost H-point and rearmost H-point on the seat track are defined by the vehicle manufacturer.
To accommodate 95% of the drivers (with 50% males and 50% females), the foremost point is defined as at X2.5
horizontal distance from the rearward of the BOF and the rearmost point is defined as at X97.5 horizontal distance
from the rearward of the BOF. SAE standard J1517 defines X2.5 and X97.5 distances as follows:
X 2.5 = 687.1 + 0.895336z − 0.00210494 z2
X 97.5 = 936.6 + 0.613879z − 0.00186247 z2 where z = H30 in millimetres.
TL23 = X 95 − X 2.5
= horizontal distance between the SgRP and the foremost H-point
TL2 = X 97.5 − X 95
= horizontal distance between the SgRP and the rearmost H-point
Total seat track length to accommodate 95% of the drivers = TL1where TL1 = TL23 + TL2 = X 97.5 − X 2.5.

If SAE standard J4004 is used to
locate the seat track, then the X distance of the
H-point reference point aft the PRP is computed
as follows:
Xref = 718 − 0.24(H30) + 0.41 (L6) − 18.2t
where L6 is the horizontal distance from the PRP
to the steering wheel centre (see Figure) and t is
the transmission type (t = 1 if clutch pedal is
present and t = 0 if no clutch pedal is present).
The foremost and rearmost points
on the seat track are obtained from data
presented in Figure. It should be noted that the
X-axis of Figure below presents distances of the
foremost and rearmost points with respect to
Xref. From Figure, for 95% accommodation, the
TL1 would be 240 mm.

7. The seat back angle (or what is also called the torso angle) is defined by dimension A40 (measured in degrees
with respect to the vertical). With the reclinable seat back feature, a driver can adjust the angle to his or her
preferred seat back angle. The seat back angle in the 1960s and 1970s was defined as 24° or 25°by many
manufacturers (due to bench seats that were not reclinable). However, with the reclinable seat back features,
most drivers prefer to sit more upright with angles of about 18°–22° in most passenger cars and about 15–18
degrees for pickups and SUVs. The seat back angles selected by class B (medium and large commercial trucks)
drivers are generally more upright—about 10°–15°.
8. The driver’s eyes are located in the vehicle space by positioning eyellipses in the CAD model (or a drawing) of
the vehicle package. The “eyellipse” is a concocted word created by the SAE by joining the two words “eye”
and “ellipse” (using only one “e” in the middle for the joint word). The eyellipse is a statistical representation of
the locations of drivers’ eyes used in visibility analyses.
SAE standard J941 defines these eyellipses, which are actually two ellipsoidal surfaces (one for each
eye) in three dimensions (they look like two footballs fused together at average interocular distance of 65 mm; see
Figure in next slide, in plan view and rear view). The eyellipses are defined based on the tangent cutoff principle,
that is, any tangent drawn to the ellipse in two dimensions (or a tangent plane to an ellipsoid in three dimensions)
divides the population of eyes above and below the tangent in proportions defined by the percentile value of the
eyellipse. Sight lines are constructed as tangents to the ellipsoids.

Locationofeyellipsesandhead
clearanceenvelope.

SAE standard 941 has defined four eyellipsoids by combinations of two percentile values (95th and 99th)
and two seat track lengths (shorter than 133 mm and greater than 133 mm).
The eyellipsoids are defined by the lengths of their three axes (X, Y, and Z directions; shown in Figure
3.18 as EX, EY, and EZ). The values of EX, EY, and EZ for the 95th percentile eyellipse with TL23 > 133 mm are 206.4,
60.3, and 93.4 mm, respectively. (The values of EX, EY, and EZ for other combinations for percentile and seat track
travel are available in SAE standard J941.)
The eyellipses are located by specifying X, Y, and Z coordinates of their centroids. The ellipsoids are also
tilted downward in the forward direction by β = 12 degrees (i.e., the horizontal axes of the ellipsoids are rotated
counter clockwise by 12 degrees; see Figure in previous slide).
The coordinates of the left and right eyellipse centroids [(Xc, Ycl, Zc) and (Xc, Ycr, Zc), respectively] with
respect to the body zero are defined in SAE standard J941 as follows (see Figure in previous slide):
Xc = L1 + 664 + 0.587 (L6) − 0.178(H30) − 12.5t.
Ycl = W20 − 32.5 Ycr = W20 + 32.5 Zc = 638 + H30 + H8
where (L1, W1, H1) = coordinates of the PRP, L6 = horizontal distance between the BOF (or PRP) and the steering
wheel centre, and t = 0 for vehicle equipped with automatic transmission and t = 1 for vehicle with clutch pedal
(manual transmission). [Note: The SgRP coordinates with respect to the body zero are (L31, W20, H8 + H30). See
Figure 3.18. L1 = L31 − X95.]

The following design considerations will improve the driver’s accommodation and comfort
Adjustable seat:
To accommodate the largest percentage of the drivers at their preferred driving posture, it is very important to
allow them to adjust
1) seat height,
2) seat cushion angle,
3) seatback angle (reclining seatback),
4) height and protruding fore–aft length of the lumbar support,
5) headrest height and fore–aft location,
6) seat cushion length,
7) armrest height and length and its lateral location from the driver centreline, and
8) seat cushion and seatback bolster heights and/or angles.
Power seats (which allow easy adjustments of many of the above-mentioned parameters with rocker or
multifunction switches)

Seat height:
The seat height in vehicle package is measured using dimension H30, which is defined as the vertical height of the
seating reference point from the accelerator heel point. The H30 dimension defines the driver’s seated posture
(defined by ankle angle, knee angle, torso angle, and seatback angle).
Vehicles with overall low height (e.g., sports cars) typically have very low H30 (about 150–250 mm), whereas heavy
commercial trucks have large H30 (more than 405 mm).
If the seat is too high, the short driver’s feet will dangle, and if the driver is unable to rest his or her heels on the
vehicle floor/carpet (or on a foot rest), the driver will find the seating posture to be very uncomfortable. Therefore,
based on the comfort of the fifth percentile female seated popliteal height of 351 mm, the top of the seat from the
vehicle floor should not be more than about 320 mm. Power seats generally allow adjustment of the seat height so
that drivers with different leg lengths can be accommodated.
The horizontal distance between the accelerator heel point and seating reference point (defined as L53) increases
as the H30 value is decreased. Thus, to minimize the horizontal space required to accommodate the driver in
commercial vehicles (truck products), the seat height is increased (as compared with the passenger cars) and the
driver sits more erect (seatback angle typically is more vertical, around 12°–18° from the vertical). In sports cars, the
seatback angle will be more reclined to about 22°–28° from the vertical.

Seat cushion length:
The seat cushion length should not be longer than the driver’s buttock-to-popliteal (back of knee) distance. Thus, if this
length is restricted to the fifth percentile female buttock-to-popliteal distance (about 440 mm), then most drivers can use
the seat and still use the back rest. Drivers with longer upper legs would prefer longer seat cushion lengths, but shorter
females will not be able to use the seatback without a pillow on the seatback. Further, in case of longer seat cushion
lengths, shorter females will find operation of the pedals difficult as they will be compressing the seat cushions with their
thighs while depressing the pedals. Thus, an adjustable cushion length will reduce such problems and accommodate a
larger percentage of the drivers.
Seat cushion angle:
The seat cushion should slope backward by about 5–15°. This will allow the user to slide back and allow the transferring of
torso weight on to the seatback. Provision of an adjustable seat cushion angle will allow the user to find his or her preferred
seat cushion angle.
Seat width:
Since females have larger hip widths (breadths), the seat cushion width should be greater than 95th percentile female
sitting hip width (about 432 mm; see measurement no. 25 in Table Static Body Dimensions of United States Adults (Values
Are in Millimetres) at the end). In addition, clearance should be provided for clothing (especially thick winter coats); thus, a
width of 500–525 mm at the hips can be recommended.

Seatback angle:
The seatback angle (called A40 in SAE J1100) in automotive seating is defined by the angle of the torso line (back
line) of the SAE H-point machine or the two-dimensional (manikin) template (refer to SAE standards J826 and J4002
[SAE 2009]) with respect to the vertical. The seatback angle (seat recline angle) should allow drivers to assume their
preferred back angles. For passenger cars, drivers generally prefer to set the seatback angle between about 20°
and 26°. In trucks, due to the higher seat height (H30), drivers prefer to sit more erect with seatback angles between
about 12° and 18°.
Seatback height:
From an anthropometric accommodation viewpoint, the maximum seatback height can be selected as the fifth
percentile female acromial height, which is about 509 mm above the seat surface. However, considering the
Federal Motor Vehicle Safety requirements on head restraints, the seatback height is dictated by the headrest
design.
Lumbar area:
The seat contour in the lumbar area affects the shape of the seated person’s spinal column. The most important
characteristic of the seat contour in the lumbar region is that it should maintain the natural curvature (bulging
forward, i.e., convex, called lordosis) of the spinal column in the lower back region of the seated person. An
adjustable lumbar support that allows setting its height (i.e., up and down adjustment) and protrusion location (i.e.,
fore–aft adjustment) would allow accommodation of different individuals while maintaining their natural lordosis.

Lateral location of the seat:
The dimension W20-1 defines the lateral distance between the vehicle centreline and the driver’s seating reference point. It should
be designed so that the driver will have sufficient elbow clearance from the driver’s door trim panel between the shoulder-and-
elbow heights. This lateral distance from the driver centerline to the door trim panel should be larger than half of 95th percentile
elbow-to-elbow width of males plus elbow clearance to avoid elbows rubbing against the door trim panel while grasping the
steering wheel.
Armrest height:
A properly designed and adjusted armrest can reduce the load on the driver’s spinal column and thus increase the perception of
comfort and reduce driver fatigue. The preferred height of the armrest will depend on the lateral location of the armrest from the
driver centerline. Since it is difficult to position an armrest that can be perceived to be optimal by most drivers, the armrest height
and lateral distance from the driver centerline should be adjustable. If the armrests are provided on both sides (i.e., on the door trim
panel and on the seat or on the center console), both the armrests should be at the same height to reduce discomfort (due to
leaning on one side).
Bolster height:
The bolsters on the sides of the seat cushion and seat back can provide the driver feeling of sitting “snug or cuddled” (like in a
contoured seat) in the seat and provide a sense of stability and security while negotiating curves and driving on winding roads. The
bolsters restrict the seated person’s movements in the seat, and, therefore, especially on long trips, such seats will be perceived to
be less comfortable. (Smaller postural movements can increase the comfort of seated persons especially during longer trips.) The
taller bolsters on the seatback may also move the minimum reach distance to controls and door handles more forward (due to
forward shifting of driver’s elbows when touching the bolsters; Further, taller bolsters will increase the difficulty in “sliding” on the seat
during entry and egress.

Padding:
Cushioning/padding is desirable because it reduces pressure by increasing support area (Konz and Johnson 2004). Seats should be
covered with padded material to allow a deflection of about 25 mm and distribute the pressure under the buttocks and thighs. In
general, the seat should be designed to allow higher pressure under the ischialtuberosities (i.e., the sitting bones—the lower
protruding parts of the pelvic bones) and gradually decrease in outward directions. For long-term comfort, the pressure on the body
tissues should not be constant. Changes in the pressures (due to deliberate massaging actions or postural movements) will reduce
discomfort and fatigue. The padding also helps in reducing discomfort caused by vehicle body vibrations under dynamic driving
conditions.
Seat track length:
The locations of hip points of different drivers as they adjust the seat fore and aft define the length of the seat track. The foremost
and rearmost hip points on the seat track define the seat track length. It should be long enough and placed at a horizontal
distance from the ball of foot on the accelerator pedal of 2.5 percentile to 97.5 percentile hip point locations (defined as X2.5 and
X97.5 in SAE J1517 and J4004). Based on the SAE J4004, a seat track length of about 240 mm would be needed to accommodate
95% of the drivers in passenger cars.

Basic ergonomics in automotive design

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