Designing for seat comfort
Published on: 1st July 2019
The reason is that there is a myriad of often inter-related factors that influence our perception of how comfortable we are. The type of journey, or context, will play an important role: whether it’s commuting or for pleasure, the time of day, the duration and the activities that the passengers are likely to engage in (reading, sleeping, dining or working.)
The perceived seat comfort will differ for different passengers – based on their size and shape, their weight, their capabilities (at a sensory, cognitive and physical level), along with their previous experiences and expectations. These may be shaped by regional and international differences. Vehicles may have different size constraints, different regulations and standards, and different capacity challenges. A good seating experience will, of course, also be influenced by the design of the seat and the journey it has been designed for. The size and shape of the seat, its orientation, the level of adjustments and features, how firm or compliant it is (dependent upon the materials used, how it is constructed and how it compresses under load) and the way it looks.
But it’s not just about the seat. The wider environment can also impact the perceived level of comfort – the proximity of other passengers and staff, the temperature and airflow, air quality, light levels, acoustics, vibration through the seat, and even smells will all shape our perceived level of comfort.
Seat design for transportation environments is almost always constrained by a challenging range of factors including cost, efficiency, safety and durability. One aspect of efficiency is a desire to maximise the capacity of a given space. For aircraft, this simply means more seats. For trains and buses, it involves striking the right balance between standing and seating as well as different types of seats (longitudinal, lateral, tip-up).
The floor space that a seat consumes is determined by its width and its pitch. For uni-directional seats, the width is governed by the interior gauge of the vehicle and the requirement for movement along the aisle. The number of seats that can be accommodated will be based on a minimum seat width that will in turn be defined by the hip width of the occupant. Seat pitch will largely be constrained by the buttock-knee length of the largest occupant to be accommodated. This then has to be added to the depth of the seat back at the point when the knee contacts it. A larger pitch generally equates to more comfort (up to a point) as it allows different postures to be adopted, such as crossing legs, and more personal space.
Weight is another key efficiency consideration. Reduced seat weight contributes to lower energy consumption and better acceleration and deceleration. Given a large number of seats, small changes to seat weight can have a marked impact on the total vehicle weight and performance.
There are two core aspects that influence the safety of a seat: its behaviour in the event of a crash and its performance in the event of a fire. When it comes to a crash situation, the seat has a critical role in containing its occupant and protecting them from injury as a result of impacts. Fire regulations clearly describe the expected performance of seats in terms of spread of flame and smoke toxicity. Essentially this limits the type and amount of material that can be used within the seat. These requirements tend to be revisited periodically, vary by region and application, and have become more stringent over time.
The spread of fire can essentially be limited in three ways:
1. Reduce the amount of volatile elements (e.g. foam and fabrics)
2. Incorporate fire barriers to protect more flammable materials (such as foams)
3. Adopt less combustible materials (e.g. specialist foams and fabrics)
Each of these options comes with their own challenges. Reducing the amount of foam can have an impact on comfort, as can fire barriers materials due to their relative stiffness, while less flammable materials may be more costly and have reduced service performance.
Durability is another key factor that constrains seat design. Most mass transport systems are designed for a long service life (typically around 30+ years). Train seats, for example, are expected to last a minimum of five years without a refit. This tends to have a marked impact on material selection such as foams which break down over time with continual use. Airline seats, which tend to use much softer foams and are replaced far more frequently (typically every six months).
Unfortunately, passengers are understandably not aware of the challenges and constraints these regulations impose on designers. In any case this is quite reasonably irrelevant to them. All they want is a comfortable seat.
When a passenger tries a new seat for the first time, their assessment is likely to be biased by their experience of other seats. These may be from totally different environments and subject to very different regulations. That makes it even more challenging to match or exceed these referenced expectations with a seat that complies with the requirements of a particular transportation application.
Designing better seats
When it comes to designing a new seat, a sensible starting point is to specify the constraints that the seat must work within. This will typically involve:
1. The anthropometry (size) of the passenger population
2. The applicable regulations and standards defining accessibility (such as PRM standards) and safety standards (crash-worthiness and fire performance)
3. The demand in terms of passenger numbers (often derived from a business case, or needs of a particular route)
In reality, there is often very little flexibility in the size of a seat as this tends to be squeezed by the pressures of maximising occupancy, especially in standard or economy class. The seat width and pitch tends to end up being very close to the minimum as defined in accessibility standards and the anthropometric data for the target passenger population. For some vehicles, the opportunity exists to do some clever things with seating layouts, configurations and types of seats. However, in the majority of cases, seat comfort is governed by its design, in terms of its form and profile, and the selection of materials.
Anthropometric data and ergonomics theory are good starting points for defining the profile of the seat to ensure that it provides support in the right places. However, it is impractical to try to predict comfort for a diverse user population without some kind of physical evaluation. As such, the next step in the process is to build a model or test rig so that the seat comfort can be evaluated practically.
Fundamentally, there are two different ways of evaluating seat comfort:
1. Subjectively (what people think about it)
2. Objectively (what can be measured)
In many ways, subjective assessment is the most obvious option. Simply this involves asking people to sit in the seat (or a range of seats) and provide verbal feedback on how comfortable they find it. The great thing about subjective assessments is that they provide direct feedback. However, they also involve a number of inherent challenges:
1. User comments may be very specific to the individual – as such, a diverse range of participants should be used.
2. Participants are not always great at describing exactly what they do and don’t like about a seat in a way that allows the design to be improved.
3. Context is very important and a single seat in isolation can give a very unrealistic comfort score. Seat assessment environments should be as close to the vehicle environment as possible.
4. The duration of use is a key factor. A five-minute comfort study may yield very different results to a much longer one that is more representative of the journey time (sometimes 12+ hours).
5. A static assessment may be unrepresentative, as vibration, movement and even noise may all shape the perceived level of comfort.
When used in conjunction with subjective measures, objective evaluations have a lot to offer in balancing some of these weaknesses. One of the most effective objective evaluations available is the use of pressure pads fitted to the seat. These pads allow ‘contour-maps’ to be generated that illustrate how the occupant’s weight is distributed across the seat cushion and back. As a static image, these contour-maps can be used to identify pressure points that could be moved or redistributed. At a dynamic level, pressure pad readouts can also provide a description of how often the occupant moves to redistribute their weight.
Scoring it all
The outputs of these numerical objective data can be combined with subjective ratings on a five-point scale, for example, from very uncomfortable, to very comfortable, along with measurements of the seat width and depth at key points to derive a seat comfort score. These scores can then be used to compare designs from different suppliers, or alternative material options, construction approaches and forms.
While designing for seat comfort may seem like an art form, there is, in fact, considerable scope for a rigorous scientific approach. A structured, evidence-based process combining ergonomic theory with representative practical assessments offers the opportunity to ensure that passenger comfort is designed into the seat from the outset. Careful and considered iteration allows the design to be refined and optimised. It is a philosophy and approach that transfers across all transport types, from automotive to rail, bus/coach and airline. It can be applied to first-of-a-kind seat design just as well as to refining existing seat designs.
Watch this space
DCA has assembled a consortium of seat, foam and material suppliers to support our in-house transport design and human factors experts on a programme of work to explore the interaction between alternative seat construction approaches that can be adopted to meet rail fire performance regulations and the resulting perceived passenger comfort. Keep watching this space for the results of this study when they are released later this year.
Written by Dan Jenkins, Senior Skill Leader - Research & Human Factors & Paul Rutter, Sector Manager - Transport.