10_Belecheanu_Riedel_Pawar

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The Proceedings of the 9th International Conference of Concurrent Enterprising, Espoo, Finland, 16-18 June 2003

A conceptualisation of design context to explain

design trade-offs in the automotive industry

Roxana Belecheanu1, Johann c.k.h. Riedel

1, Kulwant S Pawar

1

1 University of Nottingham, University Park, Nottingham, NG7 2RD, UK,

{Roxana.Belecheanu,Johann.Riedel, Kulwant.Pawar}@nottingham.ac.uk

Abstract was undertaken.

This paper presents a case study with a mass-market car manufacturer, aiming to improve the understanding of howdesign trade-offs are made in the context of a design organisation. The investigation covered designers, supervisors

and managers from body, powertrain and doors engineering departments, in small and medium size car programs.Through triangulation of data sources and data collection methods, a large and detailed set of design trade-off

examples was collected as part of the empirical evidence. The case study revealed how unformalised practices of

decision making in trade-off situations override the guidelines for decision making of the company. Based on these

findings, a framework for conceptualising the context of design decisions in trade-off situations was developed. Amethod for how to apply this framework to understanding the rationale of design trade-offs in different situations is

also provided, along with an illustration of its use on two real life trade-off examples.

Keywords

Decision making, design trade-offs, design context, automotive industry.

1 Introduction

Design trade-offs are a useful and fruitful means to study decision making. A design trade-off isa non-trivial type of a decision situation, when the choice is not obvious because no solution can

satisfy all the conflicting objectives. In these situations, an improvement in one performance

attribute of the design can only be achieved by damaging another performance attribute. Forexample, a cost/performance trade-off in which the two conflicting design objectives are“minimise cost” and “maximise performance”, has several solutions: a 20% improvement inperformance at the same cost, or a 20% reduction in cost at the same level of performance, or a

10% improvement in performance at 10% cost reduction. Therefore, trade-offs are potentiallymore challenging than simple decisions and can illustrate a decision making rationale which is

not self-evident.

In car design, making design trade-offs is particularly complex and challenging, due to the

existence of a large number of interacting parameters and the fact that interactions between theseparameters are often incompletely defined. In this case, customer satisfaction is delivered by a

complex set of performance characteristics (e.g. weight, style, ergonomics, efficiency), under

strict legislative requirements, safety and reliability concerns, while balancing subjective productattributes with objectively measured ones.

Making a design trade-off is a “very involved and iterative process and is not guaranteed to yielda desirable result” (Tate and Nordlund, 1996). As many design problems are complex and as

often there is no numerical analysis possible to support the decision making process, personalpreferences of the design engineers and constraints derived from the product development

process are the basis for the decisions they make (Ullman, 1997). Conflicts in preferences,resource requirements, etc. play an important role in the result of the design and analysing theseconflicts can provide insight into how certain design decisions are made (Rajan, 1997).




2 Existing research on design trade-offs

Design trade-offs have been addressed mostly in prescriptive design research, in the context of design evaluation (Scott and Antonsson, 1996; Thurston and Nogal, 2001) and design

optimisation (Sen and Yang, 1998). These are methods for decision support, originating in thedecision theory. The focus here is on determining the optimal design solution, rather than

analysing how decisions are made in order to reach an optimal design solution. These methodsrequire assigning precise values to weights and preferences for the decision criteria, which is notalways possible in the case of design problems of the complexity encountered in real life (e.g. car

design). For this reason, they are thought to address design trade-offs only in a rationalistic way,which resemble a scientific ideal of replicability and not how engineering decisions are made in

practice (Grudin, 1996). Therefore, they offer limited applicability to understanding how designtrade-offs are made in practice.

On the other hand descriptive design research, which investigates decision making in practice,pays insufficient attention to design trade-offs. Studies focusing on the decision maker (e.g.

Lindemann, 1999; Hansen and Andreasen, 2000) seek to understand design as a cognitive andsocial activity; however the setting here is usually experimental and little research is done in thenatural, real-life environment of a design organisation. Also, case studies on capturing design

decisions rationale in context are limited in number and do not address the particular case of design trade-offs. Moreover, while Design Rationale researchers (e.g. Grudin, 1996; Sharrock

and Anderson, 1996) acknowledge that tools for capturing design rationale must have amechanism for showing the impact of design context issues on design decisions, they are notable to show how such a tool would be applicable to different project contexts. The reason is that

a conceptualisation of the context of the design project does not exist.

Similarly existing research on design in context (Hales, 1993; Maffin, 1996; Birmingham et al.,

1997) fails to provide a coherent picture of design context. Firstly, there is no coherentunderstanding about what design context means. Secondly, its impact on design decisions is

addressed only at a general, broad level, without showing the influences or constraints imposedon design decisions in trade-off situations.

In summary, the review of literature identified two main knowledge gaps:

1. The need for an overarching framework for conceptualising the design context, as a

linking mechanism between the context factors which impact on design trade-offs.

2. The need for a systematic way to approach and understand the design context and itsimpact on design trade-offs, due to the complexity of the design context

3 Research approach

An in-depth case study was carried out with a volume car manufacturer (named here AutoVM1).

The company was chosen to represent a typical car manufacturer, i.e. a mass-market car maker.It was thus aimed to target typical design decisions taken in the development of mass-market car

models and to exclude exceptional decisions that can occur in specialist designs like luxury orsport cars.

The investigation involved semi-structured interviews, qualitative and quantitativequestionnaires, as well as non-participant observation. The respondents were designers,supervisors and managers from small and medium size car programs of family saloon type. They

were selected from body, powertrain and doors. The data collection process lasted for one year,in which a relationships with key contacts in the company was initially established, then two

series of semi-structured interviews were carried out, and then a qualitative questionnaire was

1AutoVM (Automotive Volume Manufacturer) is a fictitious name, used in this research for confidentiality reasons.




used to collect trade-off examples in a systematic way and with a high level of detail. Thequestionnaire was first piloted and filled in through interviews and then it was posted.

By using multiple sources of evidence and a triangulation of data collection methods, reliabilityof data was ensured. A large set of detailed examples of design trade-offs was collected andprovided evidence for the impact of several design context factors on design trade-offs.

The study exploratory, since a pre-defined agenda did not exist at the outset. This allowed the

researcher to identify gradually the important variables of the design context of AutoVM, in amanner in which data collection and interpretation were intertwined and iterative.

4 Case study findings

The analysis of the empirical data showed the decision making practices of designers,

supervisors and design managers in AutoVM differ from the prescribed form of decision making(e.g. guidelines and methods for making trade-offs) offered by the company. Thus, the followingdecision making patterns were found:

• The relative importance of decision criteria in trade-off situations:

1. changes over time, within the period of a program,

2. varies with the position of the decision maker within the organisational hierarchy

3. varies depending on the product part where the decision is made.

1. Time changes the importance that people give to decision criteria:

• While reducing variable cost and improving performance are equally important in theearly program stages, performance and quality are prioritised in the late program stages,

at the expense of product cost and development cost. Development cost suffers towardsthe end of the program mainly because meeting production deadlines is crucial: the

company must start selling cars, because revenues from selling can counteract theincrease in development cost. Exceptionally, saving product cost in the late stages can bea reason for not meeting the deadlines, only if the savings are significant enough to

account for the loss of market revenues (e.g. millions of dollars). It was thus found thatcost is one of the targets with most variable importance throughout the program.

2. A design trade-off is solved differently by different decision makers at differenthierarchical levels:

• design engineers tend to prioritise functionality and performance,

• design supervisors tend to treat equally performance, cost and time,

• design managers tend to prioritise cost, time, customer and competitiveness.

3. Thirdly, it was also observed that the product component or product system where thetrade-off is made affects the trade-off. For example, weight is often more important than

performance in trade-offs on car body, while weight is usually perceived as less important

than performance in trade-offs on doors.These findings reflect implicit, unformalised practices of decision making which overridecompany guidelines and which are simple principles to enable the researcher and practitioner toapproach a particular design trade-off in a particular design context.

It is therefore necessary that, in order to correctly capture the decision making rationale, a

conceptualisation framework for design trade-offs should accounts for:

1. when the trade-off is made

2. who makes the trade-off

3. the product part(s) where the trade-off is made.




A typical example to illustrate these patterns is the weight-performance-cost trade-off (Figure4.1):

Hierarchical

position

ProductSystem

Program

Stage

Design engineer

Design manager

Design supervisor

Powertrain

Doors

Reducing weight

while improving performance

and cost are sought

Performance is critical,

while weight and cost

lose importance

Body

Engineers tend to prioritise performance

Managers tend to prioritise cost

Weight loses against performance

Weight is more important than for

doors, due to impact on fuel economy

Weight is more important than for powertrain, due to its

impact on car weight

Early stages Late stages

Supervisors prioritise almost equally cost, performance and

weight

Figure 4.1: The weight-performance-cost trade-off positioned in the design context space

The variability of design targets’ relative importance with time is as follows:

performance

Importanceof target

weight

cost

Program

Stage (time)

Figure 4.2 Symbolic representation of the variance of importance of targets with time

• in the early stages, there is time to research means of lowering the weight at the sametime as lowering the cost and improving the performance;

• in the late stages, performance must be delivered at all costs, hence it becomes more

important than cost and weight.

• otherwise, the tendency is to allow a slight increase in weight, if performance is

improved significantly.Conflicts between the relative importance of the traded-off variables given at differenthierarchical levels make decisions go through several iterations or be reversed. The decision

making process taking into account these iterations was mapped on three hierarchical levels(Figure 4.3):




Assign values to

design variables,simulate and test

Are external

targets met?

Are cost, time,

customer affected?

iterations

Cross-functional trade-off:

Negotiate with Design Group B or

decide changes of own designs/targets

Cross-functional trade-off:

Decide which targets to meet or

ask for more iterations

Targets of own group

(internal) and of othergroups (external)

Decide what variables to change

in order to meet targets

No

Yes

Engineer level

Supervisor level

Manager level

Are internal

targets met?

OK!

Design

Group BYes

No

Yes

N o

DesignGroup A

Horizontal

(cross-functional)

Vertical(hierarchical)

Figure 4.3: Negotiation process for design trade-offs

5 A conceptualisation framework of design contextAs the design literature review had identified a need for a design context conceptualisation, these

three dimensions were seen as a useful and meaningful start to form this framework of the designcontext in AutoVM. A conceptualisation framework of design context was thus constructed,

having at the centre a 3-dimensional space Figure 5.1):

1. Program stage (the time),

2. Hierarchical position of the designer in the organisation,

3. Product system/ component.

Hierarchical position

of decision maker

Product

System

Program

Stage

Subjectivity ?

Customer

impact

Data availability

Engineering

complexity

Time

pressure

Carry over

?

??

?

Figure 5.1: A conceptualisation of the design context

Any design trade-off situation can be understood by mapping it firstly on this 3-dimensionalspace. The three dimensions must then be supplemented by a representation and analysis of othercontextual factors which explain, modify or reinforce the initial mapping (explanatory variables).

Some of the following variables investigated within this case study and their impact on designtrade-offs are outlined in Table 5.1 below:




Contextvariable

Impact on a design trade -off in AutoVM

Product

complexity

1. The complexity of a product in terms of the number of functionally interconnected

parts determines the number and difficulty of trade-offs inherent in design.

2. Component standardisation makes cost/performance trade-offs easier, by reducingthe cost.

3. The number of components is a concern for AutoVM’s designers due to its impact oncar weight.

4. Sometimes, the investment needed or the performance risks associated with reducing

product complexity can outweigh the cost and time benefits initially sought.

Reuse of components

(Carryover)

1. Functional constraints imposed by reused parts make achieving new design targetsmore difficult.

2. The number of conflicts between old and new components and hence the number of

cross-functional trade-offs are likely to increase.

3. In AutoVM, the engineering difficulties and the added development cost associated

with the use of carry over parts are less significant than the product cost and the timesaved.

Subjectivityof qualitative

producttargets

1. Subjectivity interferes in determining the relative importance of different productattributes

2. Qualitative product attributes which are not objectively quantified can lead tomisinterpretation and/or errors in design trade-offs.

3. The relative importance of design attributes can be underestimated or overestimated

Timepressure

and dataavailability

1. Design trade-offs made under time pressure are likely to be made with incomplete orinaccurate data.

2. As result of time pressure or insufficient data, design trade-offs are made under risk (the risk of higher manufacturing cost, manufacturing problems, quality problems, etc.).

Table 5.1: Some design context variables with impact on design trade-offs

The design context of each trade-off is different and depends on the situation. Therefore, toproduce a description of the context of a trade-off, a “pick and mix” approach must be used: only

the mix of the relevant variables must be investigated, to see their impact on the decision.

6 Example

The following example is a trade-off between NVH (Noise, Vibration, Harshness), durability andtime. The situation was generated by NVH problems at the right engine mount, problems whichare visible to the customer and which also affect the durability of the part. Designers wereconfronted with the choice between:

1. To fix the NVH problems completely, which would have meant a risk to meeting thedesign freeze deadline (here called Engineering Sign-Off, or ESO) in terms of testing the

durability. This would make the customer happy, but it would endanger safety.

2. To fix the NVH problem partially, which would give time to test durability completely,

thus meeting safety requirements.

3. Not to fix the NVH problem - with no risk to durability, but with negative impact on

customer.

Because of the need to solve a customer related problem in short time, a compromise had to bereached. The final decision was to implement only some of the design changes and solve theproblem partially. The changes which minimised the risk of negative impact on the customerwere implemented, while a secondary series of design changes was left for after the ESO.Durability testing was carried out only to allow for a minor risk to overrun the deadline.




In order to understand the rationale of this decision, the steps below must be followed:

1. Firstly, the NVH-durability-time trade-off situation must be mapped on the 3-

dimensional space, to understand how the three dimensions impact upon that trade-off (Figure 6.1). This generates the following assumptions in terms of the importance of the3 design targets:

• Program Stage: Confirmation Prototype (i.e. a late program stage)

The trade-off situation occurs 10 weeks before ESO (i.e. at a late program stage). Productiontooling has started and no design changes are allowed after ESO. Meeting ESO with all thetargets tested and all design functional problems solved is crucial.

Hence, customer, performance and meeting the deadline are more important than cost.

• Hierarchical Position of Decision Maker: Design Manager

The decision was taken by the NVH Manager and Vehicle Engineering Manager. Cost, time and

customer are expected to be the main concerns of these decision makers.

Hierarchical position

of decision maker

Product

System

Program

Stage

(time)

Design manager level

Powertrain

system

Trade-off

(NVH, durability)

Confirmation

Prototype

Pilot

Production

Engineering

Sign-Off (ESO)

10 weeks

before ESO

Figure 6.1: Mapping the NVH/durability trade-off in the 3-dimensional space

• Product System: the Powertrain System

The right engine mount is part of the powertrain system - which is highly complex, withhighly specialised engineering and with a strong interaction with the suppliers. It is expected

that cross-functional negotiation is needed due to interactions between engine mount andbody, hence several decision iterations are likely.

Furthermore, the product attributes involved in the trade-off determine the following patternsin decision making:

− Cost (variable cost and tooling cost) increases due to design changes, but the general

trend on cost is to lower its importance at this stage.− Weight increases due to adding material for fixing NVH problems. However, the general

trend on weight at this stage is to allow higher weight if performance gains.

2. Secondly, other explanatory variables from the design context applicable to this situation

must be identified and investigated. In this case, these were:

• Customer visibility:

− NVH is noticed by the customer and is of average importance to the customer.

− Durability is a measure of safety, legally crucial, but less noticeable by the customer.




• Engineering complexity: Design changes needed to solve NVH affect stiffness, hence thedecision must consider the impact on durability, and determining this impact takes time.Hence the decision needs to be made through several iterations (distributed in time) and

not completed at one moment.

• Data availability: Durability test data was missing for the decision and would have onlybeen available after ESO. Hence, making a decision required taking risks.

• Time to implement the change: Not enough time to solve all NVH issues before ESO,not enough time to test the durability impact before ESO.

The trade-off was solved through a compromise, by taking a risk in terms of durability, when

substituting missing data with engineering judgement. The decision maker took this risk,believing that there would be opportunities to fix potential problems even after the deadline.

7 Conclusions

This paper presented a conceptualisation framework for design context to explain decision

making in trade-off situations. The framework shows how to systematically investigate designcontext, thus helping designers to analyse the pos sible constraints imposed by design context ondesign trade-offs, depending on when, where and by whom the trade-off is made.

This framework is useful to designers because it brings simplicity and clarity to the complexlandscape of the design context. It is also practically important because it makes explicit a

phenomenon (i.e. the variance in the importance of decision criteria) of which decision makersmight not be aware or do not reflect upon, when making trade-offs.

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