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Good Design Versus Bad Design

Objectives The difference between good and bad design.

Anyone who has taken a car in for repair recognizes the difference between a good mechanic and a bad mechanic. A good mechanic diagnoses your problem in a timely manner, fixes what's broken at a fair price, and makes repairs that last. A bad mechanic fails to find the real problem, masks the symptoms with expensive solutions that don't last, and charges too much money for needless repairs. Engineers are a bit like auto mechanics in this respect. The world is full of both good engineers and bad engineers. Just because an engineer has produced something does not mean that the product has been designed well. Just because the design works initially doesn't mean that the product will last over time. Although the criteria by which a product is judged varies with the nature of the product, the success of most design efforts can be judged by the general characteristics summarized in Table 2-1

Characteristics of Good Design Versus Bad Design

Good Design	Bad Design
1. Meets all technical requirements	1. Meets only some technical requirements
2. Works all the time	2. Works initially but stops working after a short time
3. Meets cost requirements	3. Costs more than it should
4. Requires little or no maintenance	4. Requires frequent maintenance
5. Is safe	5. Poses a hazard to users
6. Creates no ethical dilemma	6. Raises ethical questions

The contrast between good and bad design is readily illustrated by the catapult of Figure 5. Suppose that the Apex Catapult Corporation has been asked to produce this device (actually called a trebuchet) for a brigade intent on recapturing their castle. The buyers will judge the worthiness of the catapult based on the considerations outlined in Table 2-1, as illustrated by the following discussion.

5. Reproduction of a medieval catapult called the “trebuchet.” (Photo courtesy of Middelaltercentret.)

1. Does the Product Meet Technical Requirements?

It might seem a simple matter to decide whether or not a catapult meets its technical requirements. Either the stone hits its target, or it does not. But success can be judged in many ways. A well-designed catapult will accommodate a wide range of stone weights, textures, and sizes. It will require the efforts of only one or two people to operate, and will repeatedly hit its target, even in strong wind or rain. A poorly designed catapult may meet its launch specification under ideal conditions, but it may accommodate stones of only a single weight or require that only smooth, hard-to-find stones be used. It may not work in the rain, or it may not produce repeatable trajectories. When the arm of a poorly designed catapult is released, it may hit its own support structure, causing the stone to lose momentum and fall short of the target. The catapult might work fine for the first few launches, only to fail at a later time.

2. Does the Product Work?

During the development stage, a product need not be “bug”-free the very first time it is tested. However, it must work perfectly before it can be delivered to the customer. It must be durable and not fail after only a short time in the field. The catapult of Figure 5 provides an excellent example of this second principle. Even a bad designer could produce a catapult capable of meeting its specifications upon initial delivery. The Apex Corporation could make the catapult from whatever local timbers were available. It might use a simple trigger mechanism made from vines and twigs. The bad designer would build the catapult as he went along, adding new features on top of old ones without examining how each feature interacted with those before it. The catapult would likely pass inspection upon delivery and be able to hurl stones several times before fraying a line, cracking a timber, or breaking its trigger mechanism. After a short period of use, however, the ill-designed timbers of its launch arm might weaken, causing the projectile to fall short of its target.

A good designer would develop a robust catapult capable of many long hours of service. This conscientious engineer would test different building materials, carriage configurations, trigger mechanisms, and launch arms before choosing materials and design strategies. The catapult would be designed as a whole, with consideration given to how its various parts interacted. The process typically would require stronger and more expensive materials, but it would prove more reliable and enable the user to hit the target repeatedly.

3. Does the Product Meet Cost Requirements?

Some design problems can be approached without regard to cost, but in most cases, cost is a major factor in making design decisions. Often a trade-off exists between adding features and adding cost. A catapult made from cheap local wood will be much less expensive than one requiring stronger, imported wood. Will the consumer be willing to pay Apex a higher price for a stronger catapult? Durable leather thongs will last longer than links made of less expensive hemp rope. Will the consumer absorb the cost of the more durable thongs? Painting the catapult will make it visually more attractive but will not enhance performance. Will the customer want an attractive piece of machinery at a higher price? An engineer must face questions such as these in just about every design situation.

4. Will the Product Require Extensive Maintenance?

A durable product will provide many years of flawless service. Durability is something that must be planned for as part of the design process, even when the cost of the final product is important. At each step, the designer must decide whether cutting corners to save money or time will lead to component failure later on. A good designer will eliminate as many latent weaknesses as possible. A bad designer will ignore them as long as the product can pass its initial inspection tests. If the Apex Catapult Corporation wishes to make a long-lasting product worthy of its company name, then it will design durability into its catapult from the very beginning of the design process.

5. Is the Product Safe?

Safety is a quality measured only in relative terms. No product can be made completely hazard free, so when we say that a product is “safe,” we mean that it has a significantly smaller probability of causing injury than does a product that is “unsafe.” Assigning a safety value to a product is one of the harder aspects of engineering design, because adding safety features usually requires adding cost. Also, accidents are subject to chance, and it can be difficult to identify a potential hazard until an accident occurs. An unsafe product may never cause harm to any one user, while statistically, some fraction of a large group of users is likely to sustain injury. The catapult provides an example of the trade-off between safety versus cost. Can a catapult be designed that provides a strategic advantage without injuring people? When a stone is thrown at the door of a castle, a probability exists that it will hit a person instead. Designing a device that can throw, say, large bags of water instead of stones would reduce the potential for human injury, but at the added cost of producing water bags. Features also could be added to the catapult to protect its users. Guards, safety shields, and interlocks would prevent accidental misfirings, but would add cost and inconvenience to the finished product.

6. Does the Product Create an Ethical Dilemma?

The catapult has been chosen as an example for this section because it poses a common ethical dilemma faced by engineers: Should a device be built simply because it can be built? A catapult, for example, can be a lethal device. When asked to build a catapult, is Apex obligated to build it? Is Apex responsible for suggesting alternatives to the rescue brigade? A less destructive battering ram might help recapture the castle while sparing innocent lives. Quiet diplomacy in lieu of force may lead to resolution and peaceful cooperation. As contrived as this fictitious example may be, it exemplifies the ethical dilemmas that may confront you as an engineer. If asked by a future employer to design offensive military weapons, will you find it personally objectionable? If your boss asks you to use cheaper materials but bill the customer for more expensive ones, will you comply with these instructions or defy your employer? If you discover a serious safety flaw in your company's product that might lead to human injury, will you insist on costly revisions that will reduce the profitability of the product? Or will you say nothing and hope for the best? Questions of these sorts are never simple to answer, but engineers face them regularly. As part of your training as an engineer, you must learn to apply your own ethical standards, whatever they may be, to problems that you encounter on the job. This aspect of design will be one of the hardest to learn, but it is one that you must master if you wish to be an engineer.

 

Professional Success: Choose a Good Designer to Be Your Mentor

There is a difference between good designers and bad designers. Practicing engineers of both types can be found in the engineering profession, and it's up to you to learn to distinguish between the two. As you make the transition from student to professional engineer, you are likely to seek a mentor at some point in your career. Be certain that the individual you choose follows good design practices. Seek an engineer who has an intrinsic feeling for why and how things work. Find someone who adheres to ethical standards that are consistent with your own. Avoid “formula pluggers” who memorize equations and blindly plug in numbers to arrive at design decisions but have little feeling for what the formulas actually mean. Avoid engineers who lack vision and perspective. Likewise, shun engineers who take irresponsible shortcuts, ignore safety concerns, or choose design solutions without thorough testing. In contrast, do emulate engineers who are well respected, experienced, and practiced at design.