Part of the cost event for IMechE Formula Student is a small sub-task based around the design and cost analysis of a bicycle rack for a Ford Focus. The brief was to produce a rack capable of carryingp to four bikes, based on sales figures of 125,000 units per annum for five years.
A model of a Focus was drawn on a CAD package so that CFD analysis could be carried out to establish the optimum location on the vehicle for the bike rack. The rack is made up of a two-piece aluminium anodised tubular structure, complimented with injected-moulded UV-stable ABS UL94-V0 bracketry. Material selection was based on ease of acquisition, ease of machining and low material cost.
The product operates in three main modes; mode one allows carriage of two adult bikes, mode two allows up to four bikes and mode three carries two adult bikes and two children’s bikes. The arms that carry the bikes are attached to the frame via brackets, which allow you to adjust the angle to customise for different bicycle frame types.
The attachment method to the car is very straight-forward; the loads are translated into the car via the bottom section of structure, which is all held in place through a system of straps which clip onto the cars body panels.
Analysis was carried out on the component parts and assemblies, which was validated using hand calculations to establish confidence prior to prototype manufacture.
Manufacture of the prototype differs from the production method due to costs incurred around tooling and practicalities of finding suppliers and manufacturers. To that end, the frame was constructed from a series of pre-bent tubing, which was TIG welded together. The polymer parts were rapid-prototyped via ALM-SLS from glass-filled nylon, chosen for its likeness to the mechanical properties of injection moulded ABS.
Two main tests were carried out on the finished prototype, namely a fit and function test and static and dynamic load testing. The load case was derived from the worst case operational load of the bike rack, based on data from popular bicycle suppliers. As only one prototype was produced, a destructive load test was not carried out.
Based on the brief from the IMechE, full cost analysis was carried out to establish initial capital outlay and the landed wholesale cost to the OEM. Costs were accurately established using industry contacts and so is a fair representation of the real world nature of the project.