Body
From Trevipedia
The Trev body is non-structural, so can be made from low-mass materials. The UniSA prototype used hand-shaped polystyrene foam with a skin of fibreglass; see Building an Ultra Light-Weight Car, Part 2.
This technique had some disadvantages:
- carving the body shape was slow
- it is difficult for novices to stay accurate
- elbows leave dents.
If several cars of the same design are to be made, it is probably worth making moulded body panels. These can be easily replaced if damaged.
The UniSA prototype was not particularly air tight or water tight.
CAD model
The UniSA prototype body design, designed by John Packer, is available as a zipped IGES file.
A wireframe version of the model is also available as the SketchUp model Body_1.skp, shown in the image below.
Aerodynamics
Minimising aerodynamic drag is important. We estimate that at 60km/h, about 45% of the power at the wheels will be used to overcome aerodynamic drag; at 100km/h aerodynamic drag will account for about 70% of the drive power.
We have not done any serious Computational Fluid Dynamics (CFD) calculations or testing. Instead, we have simply followed some simple principles of drag reduction:
- where the width of the body reduces, we have kept the slant angle to less than 15 degrees so that the likelihood of attached flow is maximised
- smooth curves, once again to maximise the likelihood of attached flow
- the bottom of the car is smooth
- the rear of the car is a truncated tear-drop shape (Kamm back).
References:
- Barnard, R. H. 2001, Road Vehicle Aerodyamic Design: An Introduction, second edition, MechAero.
Body construction
A common approach taken to reduce denting of foam/fiberglass constructions is to add a layer of higher density foam around the outside of the softer foam before skinning with fiberglass. Sailboards use this method to avoid heel dents under the rider's feet. The higher density foam can be applied as a flat sheet and then formed to the shape of the product using a vacuum bagging process. Keeping the high density foam layer thin helps keep the mass down though it does add another step to the body build process. In boat building, most attempts at using polystyrene cores have had longer term issues with shear failure of the core just below the surface adhered to the skin because the strength of the foam is so low. This may not be as much of an issue in this vehicle because unlike a boat, the skin is not as highly loaded as it is in a marine application.
