22-Mec-B5 Product Design and Development · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, May 2013 — 07-Mec-B5, Product Design & Development. Three hours; open book; no calculator permitted. Question 1 is compulsory and carries 40 % of the paper; four of the remaining six questions are chosen, each worth 15 %, for 100 % in total, and only the first five questions appearing in the answer book are marked. Note 5 of the paper states that most questions require an essay answer or the use of tables, figures and charts, and that clarity and organisation of the answer carry marks; Note 1 invites the candidate to state any assumption made where a question is open to interpretation, and that licence is used several times below with each use flagged. All seven printed questions are worked here — 130 marks of material against the 100 marks a candidate would actually attempt — so that the set serves as a complete study resource.
Reference texts. Ulrich & Eppinger, Product Design and Development (McGraw-Hill) — the framework text for this exam code, and the source of the generic development process, the needs-to-metrics translation, concept screening and concept scoring used throughout; Dieter & Schmidt, Engineering Design (McGraw-Hill) for the specification, materials and process-selection material; Pahl & Beitz, Engineering Design: A Systematic Approach (Springer) for systematic concept generation and the function structure; Boothroyd, Dewhurst & Knight, Product Design for Manufacture and Assembly (CRC) for the design-for-assembly and design-for-manufacture rules; Ashby, Materials Selection in Mechanical Design (Butterworth-Heinemann) and Kalpakjian & Schmid, Manufacturing Engineering and Technology (Pearson) for the process-selection charts and cost models. Canadian context is taken from the Patent Act, Industrial Design Act, Trademarks Act and Copyright Act (Canadian Intellectual Property Office), from CSA standards (notably CSA B651 Accessible design for the built environment), from the Canada Consumer Product Safety Act, and from Engineers Canada / EGBC guidance on professional practice and on equity, diversity and inclusion in the profession.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Product selected: the bicycle trailer. It is chosen because it is a genuine system rather than a single part — a structure, a hitch, a running gear, a load-carrying body and a set of user interfaces, all of which must be reconciled — and because it is sold into a Canadian market whose climate, regulations and storage habits impose real constraints. The design direction carried consistently through parts A to F is: a two-wheel, tow-behind trailer for a family cyclist who carries either a child or a week of groceries, who stores the trailer in an apartment locker or a car trunk, and who rides on urban streets and gravel paths between April and November. The whole answer follows the generic development process of Ulrich & Eppinger — mission statement, needs, metrics, target specifications, concept generation, concept selection — and stops at the point where one concept is carried into system-level design.
Method 1: voice-of-the-customer research translated through a needs-to-metrics matrix. The design team gathers raw statements of need directly from users and from the people around them, groups those statements into a hierarchy, assigns importance weights, and then translates each need into one or more measurable metrics with units. The translation is the engineering act: the need “the trailer is easy to get into the car” becomes the metrics folded envelope in millimetres, folding time in seconds and tools required to fold as a count. A House of Quality (quality function deployment) is the conventional vehicle for this, because it records both the strength of the link between each need and each metric and the trade-offs between metrics themselves. The output is a target specification with a marginal and an ideal value for every metric. This method is the only one of the three that can discover a requirement nobody has written down yet, and it is therefore the default for a consumer product.
Method 2: competitive benchmarking and reverse engineering. The team buys the leading trailers on the market, measures them, tests them against a common protocol, and tears them down to a costed bill of materials. Each metric identified in method 1 is then populated with the values achieved by each competitor, and the specification is set relative to that competitive frontier — at parity on the metrics customers do not shop on, and clearly ahead on the two or three metrics that decide the purchase. Warranty and dealer service data, product reviews and returns data are folded in to expose where the incumbents fail in service. This method produces defensible numbers quickly and protects against setting a target the market has already exceeded, but it is inherently backward-looking: it cannot originate a specification for a feature nobody sells yet.
Method 3: derivation from regulation, standards and a physics-based mission profile. A third family of specifications is not negotiable and does not come from customers at all. It is derived from the statutes and standards that apply to the product — for a bicycle trailer sold in Canada, the Canada Consumer Product Safety Act and its regulations on small parts, lead and phthalates in children's products, ASTM F1975 Standard Specification for Non-Motorized Bicycle Trailers for the hitch retention, roll-over and tip tests, and CSA/ISO reflector and conspicuity practice — and from the duty cycle the product must survive, which is turned into loads by engineering analysis. A stated mission of 8 000 km over five seasons on mixed pavement and gravel, with a 45 kg payload and a 2.5 g vertical shock over a curb, yields fatigue loads on the hitch arm and the axle that become specification lines in their own right. This method sets the floor beneath the other two, and it is the method whose violation is a professional and legal failure rather than a commercial one.
Method 1, voice-of-the-customer research, is carried forward. The purpose of the data collection is not to ask users what the trailer should be; it is to observe and record what users are trying to accomplish, in what circumstances, and what defeats them today. The information falls into six kinds.
Who the users are, and they are not one group. The buyer, the tower, the passenger and the person who stores the trailer are frequently different people, and a child passenger is a user who cannot report a problem. The data collected are demographic and, more usefully, behavioural: how often the trailer is towed, over what distance, in what months, by a rider of what strength and height, on what bicycle, with a child of what age and mass or with what cargo. Anthropometric data covering the intended range — from a fifth-percentile adult female tower to a ninety-fifth-percentile adult male, and children from one to six years — are gathered from published Canadian and North American anthropometric tables rather than from the interviews.
The context of use, gathered by observation rather than by questioning. Users are watched hitching the trailer in a stairwell, in a parking garage and on a driveway; the video record consistently reveals what the interview does not, which is that the hitching operation happens in poor light, with gloves on, with a child on one arm, and that the user does not read the label. Contexts recorded include storage location and its clearances, the transport mode when the trailer is not towed, the surfaces ridden, the weather, and the presence of curbs, streetcar tracks and bike-lane bollards.
Latent and unstated needs, captured as verbatim statements. Statements are recorded in the customer's own language and only then rewritten as need statements in the standard form — what the product must do, not how. “I gave up and left it hitched all winter” is raw data; the need statement is the trailer is quick to remove and to store; the metrics are unhitching time, folded envelope and mass. Lead users — the bicycle courier, the parent who tows daily through a Winnipeg March — are sampled deliberately, because their needs today are the mainstream needs of five years' time.
Importance and satisfaction ratings. Each consolidated need is rated by a larger sample for importance on a five-point scale and for satisfaction with the products they own now. The gap between the two, need by need, is the opportunity map, and the importance ratings become the weights used in part F. Roughly fifty interviews are enough to surface some ninety per cent of the needs of a product of this complexity; the larger rating sample can be run online.
Interface and compatibility data. Because the trailer attaches to a bicycle the team does not control, the survey records what the users actually own: rear-dropout geometry, quick-release skewer diameters against through-axle sizes, disc-brake caliper clearance, chainstay lengths, seat-post diameters and rack presence. This is objective data collected by measurement in bike shops, not by asking.
Failure and incident experience, and price behaviour. Users are asked what has broken, what has come loose, what has been repaired and what has caused a fall; retailers are asked what is returned and why. Finally, what users paid, what they compared, and where they bought is recorded, because the manufacturing cost target is a specification like any other and it is set from the retail price the market will bear, working backwards through the channel margins.
Each need from part B is now expressed as a metric with a unit, a marginal value (below which the product is not viable) and an ideal value (beyond which further improvement earns nothing). The list is deliberately at a high level, as the question directs: it describes the overall size, mass, capacity and interfaces of the trailer and its main features, not the dimensions of individual parts.
| # | Metric | Unit | Marginal | Ideal | Need it serves |
|---|---|---|---|---|---|
| 1 | Payload (child plus gear, or cargo) | kg | 34 | 45 | Carries a child to age six, or a week of groceries |
| 2 | Usable cargo volume | L | 90 | 120 | Bulk, not just mass |
| 3 | Tare mass, ready to tow | kg | 13 | 10 | Lifted into a car trunk by one adult |
| 4 | Overall track width | mm | 860 | 760 | Stays inside a painted bike lane; at the ideal width it also passes an 810 mm doorway |
| 5 | Folded envelope (L × W × H) | mm | 900 × 700 × 350 | 800 × 700 × 250 | Apartment locker and sedan trunk |
| 6 | Fold or unfold time, no tools | s | 60 | 25 | Done one-handed, in poor light |
| 7 | Hitch and unhitch time, no tools | s | 30 | 10 | Observed to be the abandonment point |
| 8 | Bicycle fitment coverage | % of surveyed bikes | 85 | 97 | Quick-release and through-axle, disc clearance |
| 9 | Static tip angle, laden | degrees | 40 | 50 | Roll-over resistance per ASTM F1975 |
| 10 | Hitch retention with secondary strap | — | No separation under the standard's tension and impact tests | Non-negotiable safety line | |
| 11 | Added towing drag at 20 km/h, laden | N | 12 | 8 | The trailer must not defeat the rider |
| 12 | Operating temperature range | °C | −20 to +35 | −30 to +40 | Canadian shoulder-season use |
| 13 | Corrosion resistance, salt spray | h | 96 | 240 | Road salt and coastal humidity |
| 14 | Conspicuity | — | Reflectors on three sides plus a 1.5 m flag; high-contrast body colour | Seen by drivers at night | |
| 15 | Design life | km / seasons | 6 000 / 4 | 10 000 / 8 | Resale and hand-down expectation |
| 16 | Manufacturing cost at 20 000 units/yr | CAD | 165 | 135 | Supports a CAD 499 retail price |
Two entries deserve comment because they drive the architecture. Metric 4, track width, and metric 5, folded envelope, together decide whether the trailer can be a rigid welded frame at all; and metric 16, manufacturing cost, is what will eventually punish the concept with the most joints and fasteners.
The three concepts differ in architecture, not in styling: a single-track trailer, a wide rigid two-track trailer, and a folding two-track trailer that converts to a stroller. Each addresses the same need set from part B, and each buys one group of needs at the expense of another.
The three are genuinely different answers to the need set, and each can be argued for. Concept 1 is the lightest and the narrowest: with one wheel it is no wider than the bicycle, it tracks perfectly through gaps and it has the fewest parts, but it can only be loaded to the point where the bicycle can still support the tongue mass, it puts that mass on the rider's rear wheel, and it is unusable for a child seat because it leans. Concept 2 is the most stable and the cheapest to build per unit of capacity — a welded frame, large wheels, a low centre of gravity, a wide track and a big flat deck — but it does not fold, so it fails metrics 5 and 6 outright and cannot be stored in an apartment or carried in a trunk. Concept 3 keeps the two-track stability at a slightly narrower track, folds flat in seconds, and earns a second use as a stroller, at the cost of more parts, more joints, more mass and a materially higher manufacturing cost.
The comparison is done in two passes, following the concept-selection method of Ulrich & Eppinger, because the two passes answer different questions and mixing them produces a false precision. The first pass is concept screening — a Pugh matrix. Every concept is compared, criterion by criterion, against a single reference datum (normally the best incumbent product or the simplest concept), and scored only as better (+), same (0) or worse (−). The criteria are unweighted at this stage. The purpose is not to rank but to expose which concepts are dominated, which are strong in a way worth preserving, and which can be improved by borrowing a feature from another — the matrix is a generator as much as a filter. Any concept that violates a non-negotiable constraint from part C is eliminated here, before it is scored, because a weighted average must never be allowed to buy back a safety or regulatory failure.
The second pass is concept scoring. The surviving concepts are rated against weighted criteria drawn directly from the specification of part C, with the weights taken from the customer importance ratings collected in part B, and the ratings placed on a five-point anchored scale where the anchors are stated in engineering terms so that different assessors mean the same thing by a “4”. The total score for concept j is the weighted sum
$$S_j=\sum_{i=1}^{n} w_i\,r_{ij}, \qquad \sum_{i=1}^{n} w_i = 1$$where $w_i$ is the weight of criterion $i$ and $r_{ij}$ the rating of concept $j$ against it. Six disciplines make the method trustworthy rather than decorative. The weights are fixed before any concept is scored, so that the weighting cannot be tuned to produce a preferred winner. Each criterion is rated by more than one assessor independently, and disagreements are discussed rather than averaged away, because a disagreement usually means the criterion is ambiguous. Ratings are anchored to the marginal and ideal values of the specification, so a rating of 3 means “meets the marginal value” and 5 means “reaches or beats the ideal value”. Criteria are kept independent, so that the same underlying property is not counted twice under two names, which would silently double its weight. A sensitivity analysis is run by re-weighting the two or three criteria the team is least certain about and observing whether the ranking survives. And the decision, the weights, the scores and the sensitivity result are recorded in the design record, so that when the project is challenged six months later the reasoning can be re-examined rather than reconstructed from memory.
Screening pass. Concept 2, the wide rigid trailer, is taken as the datum because it is closest to the incumbent products on the market.
| Criterion | Concept 1 (single-wheel) | Concept 2 (rigid, datum) | Concept 3 (folding convertible) |
|---|---|---|---|
| Stability and roll-over margin | − | 0 | 0 |
| Payload and versatility | − | 0 | + |
| Towing effort and mass | + | 0 | 0 |
| Storage and transport | + | 0 | + |
| Manufacturing cost | + | 0 | − |
| Bicycle compatibility | − | 0 | 0 |
| Net (plus minus minus) | 0 | 0 | +1 |
No concept is dominated and none violates a hard constraint, so all three survive to scoring. The screening pass does, however, produce one useful combination: concept 1's single-wheel narrowness cannot be transplanted, but its habit of carrying the load directly over the axle can be, and it is adopted into concept 3's load-floor layout to reduce tongue mass.
Given. Six weighted criteria derived from the part C specification, with weights taken from the part B importance ratings and normalised to unity: stability and safety 0.25, payload and versatility 0.20, mass and towing effort 0.15, storage and folding 0.15, manufacturing cost 0.15, bicycle compatibility 0.10. Ratings on a five-point anchored scale, where 3 meets the marginal value and 5 reaches the ideal value.
Find. The weighted score of each concept, the resulting rank, and whether that rank survives a re-weighting of the criterion the team is least sure about.
| Criterion | Weight w | Concept 1 | Concept 2 | Concept 3 | |||
|---|---|---|---|---|---|---|---|
| r | w·r | r | w·r | r | w·r | ||
| Stability and safety | 0.25 | 3 | 0.75 | 5 | 1.25 | 4 | 1.00 |
| Payload and versatility | 0.20 | 2 | 0.40 | 4 | 0.80 | 5 | 1.00 |
| Mass and towing effort | 0.15 | 5 | 0.75 | 3 | 0.45 | 3 | 0.45 |
| Storage and folding | 0.15 | 4 | 0.60 | 2 | 0.30 | 5 | 0.75 |
| Manufacturing cost | 0.15 | 4 | 0.60 | 3 | 0.45 | 2 | 0.30 |
| Bicycle compatibility | 0.10 | 3 | 0.30 | 4 | 0.40 | 4 | 0.40 |
| Total score | 1.00 | 3.40 | 3.65 | 3.90 | |||
| Rank | 3 | 2 | 1 | ||||
Taking concept 3 as an example of the arithmetic, the weighted sum is $0.25(4)+0.20(5)+0.15(3)+0.15(5)+0.15(2)+0.10(4)$, which gives
$$\boxed{S_3 = 1.00+1.00+0.45+0.75+0.30+0.40 = 3.90}$$and the same computation on the other two columns gives $S_1 = 3.40$ and $S_2 = 3.65$. The ranking is therefore concept 3 first, concept 2 second, concept 1 third.
Sensitivity. The weight the team is least confident about is manufacturing cost, because the cost estimate for the folding frame is the least mature number in the study. Doubling that weight from 0.15 to 0.30 and taking the increase out of payload and versatility (0.20 down to 0.05) gives $S_1 = 3.70$, $S_2 = 3.50$ and $S_3 = 3.45$ — the ranking inverts completely, and the cheapest concept wins. The selection is therefore not robust: it holds only so long as the folding frame's cost premium stays within the gap the specification allows.
| Result | Value |
|---|---|
| Concept 1 — single-wheel in-line, weighted score | 3.40 (rank 3) |
| Concept 2 — wide rigid two-wheel, weighted score | 3.65 (rank 2) |
| Concept 3 — folding convertible, weighted score | 3.90 (rank 1) |
| Scores if the cost weight is doubled to 0.30 | 3.70 / 3.50 / 3.45 — ranking inverts |
| Concept selected | Concept 3, the folding convertible trailer |
| Condition attached to the selection | A cost-reduction study on the folding frame before the system-level design gate; concept 2 retained as the fallback architecture |
Selection. Concept 3 is carried forward, because it is the only concept that satisfies both of the architecture-driving metrics — the folded envelope and the payload — and because its second use as a stroller addresses a need that no incumbent serves. The selection is made conditional: the folding frame must be re-costed against a target of CAD 165 before the system-level design gate, and if it cannot be met, the sensitivity result says plainly that the team should return to concept 2 and solve storage some other way rather than ship an expensive trailer. This is the honest reading of the matrix, and it is the reason the sensitivity pass is part of the method rather than an optional extra.
