Last week in Building an Ultra Light-Weight Car, Part 1 we looked at how the University of South Australia has built the monocoque tub of an ultra lightweight and innovative, three-wheel electric car. Rather than using a steel or aluminium tubular space-frame structure, the strength of the vehicle lies in a monocoque constructed from foldable, aluminium honeycomb/fibreglass sandwich panels.
That approach resulted in a complete tub that weighs just 32kg yet has room to carry two people. But yes you’re right, it doesn’t look very aerodynamic, does it... So in this story we look at how the monocoque was clothed in aerodynamic bodywork, a process that before paint, added just 13kg!
Making the Bodywork
The bodywork was formed by a deceptively simple approach, one that could be achieved by anyone working in their home workshop. It takes patience and care, but as University of South Australia’s Dr Peter Pudney said: “None of us had shaped a polystyrene body before!”
Adfoam ‘M’ grade modelling type expanded polystyrene blocks were cut and then epoxy’d to the fibreglass/aluminium sandwich panels. (Go to www.adfoam.com.au for more details on this foam.) While the epoxy was curing, the foam was held in place with an elastic trailer cargo net and various props.
The thickness of the foam was tailored to the final shape, with relatively small thicknesses used where the composite panels were close to the required shape...
... and greater thicknesses used where extensive curvature was needed. The individual foam blocks were shaped so that clearances between adjoining foam blocks were minimal.
The foam was then simply hand-carved to shape! Surform files, sandpaper, sanding blocks, surface levellers, hot wire cutters and other tools were used, aided by MDF templates and a marked-out grid on the work surface. This shows the body within 20mm of the final shape...
...and then nearly finished.
Low spots were filled with polyurethane expanding foam, before being reshaped. Peter Pudney points out that this is not a good approach to take: better to cut out the low spots and fill the area with a new block of the original foam. (This would have avoided sanding across foams with different hardness characteristics.)
The foam was then covered in a layer of fibreglass cloth and epoxy resin...
...and then a second layer of fibreglass and resin was applied, the latter complete with red pigment. Sanding then followed. The red pigment acted as an indicating coat, showing the depth to which the sanding had penetrated.
A carbon fibre reinforcement hoop was installed on the upper surface of body; the acrylic canopy (to be covered in a moment) rests on this. The canopy edge was reinforced with carbon fibre tape laid over plastic polypipe.
The openings for the rear lights and the rear suspension and wheel were cut out, followed by...
... the openings for the front lights, the latter being done with a jigsaw. These exposed foam areas were then fibreglassed.
The body then went to a professional spray shop where the body was further smoothed using body filler and then painted. The additional mass of the foam and fibreglass body was 13kg; painted and smoothed, this increased by another 9kg.
The plastic canopy was made from 3mm clear acrylic. It was free-blown but a template was developed that delineated the shape of the plastic blank; this was mathematically modelled so that the free-blown shape would result in a bubble from which the correctly shaped canopy could be cut.
The canopy was blown by Ian Linke of Aviation Acrylic Mouldings, a specialist in blowing canopies for gliders. The canopy weighs 10kg.
Final trimming of the canopy was made on the body of the car. The canopy is hinged at the rear but plans are afoot for a change to side hinging.
The most fascinating aspect about the construction of the University of South Australia’s car is that with the exception of the canopy blowing and final painting, the constructional techniques can all be done by amateurs working with just hand tools. There’s no need for welding gear, building moulds or panel beating. There’s not even a requirement for a large workspace.
The exterior body techniques lend themselves to custom, compound curve shapes, and the monocoque chassis can be as heavy duty (or as light duty) as the requirements demand.
For example, the 64kg total mass of the body and canopy could be halved if the vehicle was to carry only one person and be powered by a tiny electric or internal combustion engine. On the other hand, we can see a stiff and gusseted monocoque body of a small, open four wheel car coming in at only 150kg or so...