Justifying a Material Change in a Bike Rack Design
A Year 12 team has designed a wall-mounted bicycle rack for the school bike shed. The original design uses 25 mm square mild-steel tube, welded, with a stated safe working load of 30 kg per hook and two hooks per rack. The school wants six racks. The team proposes switching to 25 mm square aluminium tube to cut mass and avoid welding, using bolted joint plates instead. The teacher asks the team to produce evidence that the change does not compromise safety, and to consider manufacturing, maintenance, and end-of-life implications, not just strength. The racks will be outdoors, near the coast, and used daily by students.
1. Outline the tests and calculations the team must complete to justify the material and joint change.
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Load case definition Each hook carries 30 kg → 294 N. A bicycle hung by one wheel can apply a dynamic factor when dropped or leaned on; a factor of 2 is a reasonable design assumption, giving ~590 N per hook, and the rack must also resist a horizontal pull (a student leaning or a bike swinging).
Structural checks
- Bending stress in the tube at the worst cantilever point, compared with the allowable stress for the chosen aluminium alloy (not generic 'aluminium').
- Deflection at the hook under service load - a rack that sags feels unsafe and may not hold the wheel.
- Bolted joint check: bolt shear and bearing on the joint plate, plus slip resistance. Bolt torque matters; a loose bolt redistributes load.
- Wall fixing check: pull-out and shear in the masonry, which is often the real weak link regardless of tube material.
Testing
- Proof load test at 1.5× the rated load on a prototype, held and inspected for permanent set.
- Cyclic test: repeated load/unload cycles to represent daily use over a school year.
- Salt-spray or accelerated corrosion exposure, since the racks are coastal and outdoors.
Judgement The team must show the aluminium design meets or exceeds the steel design's demonstrated performance, with the alloy, wall thickness, and bolt specification all stated. 'Aluminium is lighter' is not evidence.
2. Discuss the manufacturing, maintenance, and end-of-life trade-offs of the change, and recommend whether to proceed.
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Manufacturing Bolted joints remove the need for welding, which lowers skill and equipment requirements and lets the school fabricate and repair racks on site. That is a genuine educational and logistical advantage. The cost is more parts, more drilling, and more assembly time, plus a risk of inconsistent bolt torque if no procedure is followed.
Maintenance Aluminium avoids the rust problem of mild steel, which matters near the coast. But aluminium still corrodes, and it is prone to galvanic corrosion where steel bolts pass through aluminium plates, especially with salt spray. The design needs isolation washers or a compatible fastener choice, and a documented inspection interval for bolt tightness. Steel would need regular repainting; aluminium needs less cosmetic work but more attention to fasteners.
End of life Aluminium is readily recycled and has a good scrap value, which supports the change. Bolted construction also allows disassembly into clean material streams, unlike a welded steel frame. This is a real advantage for a school sustainability claim.
Recommendation Proceed conditionally: adopt aluminium only with a specified alloy, a stated bolt specification and torque, galvanic isolation at the joints, and a completed proof-load and cyclic test on a prototype. Document the inspection schedule. Without those conditions the change trades a known-safe welded design for an unverified one, and the safety case is not made.