Real problems have limits
An engineer designing a bicycle helmet wants it to be as safe as possible. But it also has to be light, comfortable, stylish and affordable. Making it thicker might make it safer but heavier and more expensive. Engineering is about finding the best balance.
Criteria and constraints
- Criteria are what the design must do. They describe success.
- Constraints are the limits the design must stay within.
| Project | Criteria | Constraints |
|---|---|---|
| School water filter | Removes visible dirt; filters 1 L in under 5 minutes | Costs under $10; uses safe, local materials |
| Wind turbine model | Lights an LED in a fan breeze | Blades under 20 cm; built in two class periods |
| Bike helmet | Passes impact test; weighs under 400 g | Price under $60; meets safety standards |
Good criteria are measurable. "Strong" is vague; "holds 5 kg without bending more than 1 cm" can be tested.
Trade-offs
A trade-off happens when improving one feature makes another worse.
- A stronger bridge may need more material, which costs more.
- A bigger car battery gives more range but adds weight.
- A cheaper material may wear out sooner.
Engineers can’t usually have everything, so they decide which criteria matter most.
Comparing options with a decision matrix
A decision matrix helps choose between designs fairly. Give each criterion a weight based on its importance, score each design, then multiply and add.
| Criterion (weight) | Design A | Design B | Design C |
|---|---|---|---|
| Safety (×3) | 4 → 12 | 5 → 15 | 3 → 9 |
| Cost (×2) | 5 → 10 | 2 → 4 | 4 → 8 |
| Weight (×1) | 3 → 3 | 2 → 2 | 5 → 5 |
| Total | 25 | 21 | 22 |
Design A wins, even though Design B is safest, because A balances all criteria best.
Optimization
Optimization means improving a design to get the best performance possible within the constraints. Engineers optimize by:
- testing a prototype and collecting data
- finding the weakest part, called the point of failure
- changing one feature at a time
- testing again and comparing
Each cycle is called an iteration. A design might go through dozens of iterations.
| Iteration | Change | Load held before failing |
|---|---|---|
| 1 | Flat paper beam | 0.4 kg |
| 2 | Folded into a U shape | 1.8 kg |
| 3 | Folded into a triangle tube | 3.5 kg |
Thinking about impacts
Engineers also consider effects on people and the environment:
- Is it safe for all users, including children and people with disabilities?
- What happens to it at the end of its life? Can it be recycled?
- Does making it cause pollution?
In the real world
Aircraft designers trade off fuel efficiency, passenger space and cost. Phone makers balance battery life against thinness. City planners balance traffic flow, safety and cost when designing intersections.
Common mistakes
- Vague criteria that can’t be tested.
- Ignoring constraints until the end.
- Changing many features in one iteration, so you can’t tell which change helped.
Try it yourself
You must design a container to keep an ice cube from melting for as long as possible.
- Write two measurable criteria and three constraints.
- Sketch two designs and fill in a simple decision matrix.
- Describe one trade-off between them.
- Plan how you would test and improve your best design.
Quick recap
- Criteria define success; constraints set limits.
- Trade-offs mean improving one feature can worsen another.
- A decision matrix compares designs using weighted criteria.
- Optimization improves a design through repeated iterations.
- Good engineering considers people and the environment too.