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This article was first published in 2009.
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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.
Canopy
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.
Conclusion
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...
And
the Car as a Whole?
As
we’ve indicated, the most impressive feature of the University of South
Australia’s car (they call it ‘Trev’) is its body design and construction. But
what about the rest of the car?
In
short, it’s largely a build in progress. The car is currently powered by a 6000
rpm, brushed DC electric motor driving the rear wheel through a two-stage drive
belt reduction. The motor is powered by a 45kg lithium ion battery pack, giving
a range of 120 kilometres. A commercial power controller is used.
In
this form, the car has completed the Darwin to Adelaide World Solar Challenge,
running in the Greenfleet Technology Class.
We
were able to drive the car around the roads of the Mawson Lakes campus.
Unfortunately (or fortunately?) for our drive, the car could not be equipped
with its canopy.
The
car shows enormous promise. The steering is light and direct and the performance
excellent. (The claim is for a top speed of 120 km/h and 0-100 km/h in around 10
seconds, but with the instant electric motor torque, it feels
faster-accelerating than that.) But the low-speed throttle control is dreadful,
completely destroying the refined feel that would otherwise be associated with
the electric motor.
The
ride quality from the front wishbones and rear trailing arm (sprung and damped
with motorcycle units) is fine on smooth surfaces but on rough surfaces, the
suspension shows severely inadequate travel. Corner hard and then pass over a
bump and the result is simply awful. The brakes are so bad it’s easy to assume
they’re not connected.
The
interior of the car is more of that ‘work in progress’ without a proper
dashboard or interior trimming. To our way of thinking, the current dashboard
LCD is over-kill – although the intention is to add a rear-view camera, which
would give a better reason for its presence. Other on-board electronics are
overly complex - including a whole computer system to control just the
lights!
But
what works well, works very well.
Considering
the overall dimensions, interior space is excellent. The rear-hinged, wide door
and the heavily cutaway floor combine in a design master-stroke – an adult can
easily get into the back seat (or where the back seat would go if the car
currently had a back seat!) and access to the driver’s seat is straightforward.
There’s a huge amount of room in the doors and the dash panel is spacious,
although the storage space in front of the dash is inaccessible. Luggage room is
just a little tight – not much is needed in a car of this type but it could
probably be better optimised. Incidentally, program director Peter Pudney is
well aware of these deficiencies.
But
instead of thinking of what the car is like now, think of what a car
could be like! Start by fitting decent, long-travel suspension (perhaps
air springs would be best) and a stiff anti-roll bar (remember, the back wheel
provides zero roll stiffness!).
Add
a decent motor speed controller driving a three-phase brushless motor with regen
braking, and you’re starting to talk about a car that would be economical (it
currently costs 1.1 cents per kilometre in electricity power cost), practical
for city use, and simply a helluva lot of fun to drive...
A
real car of the future.
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