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This article was first published in 2009.
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Lots of people would love to make their own
vehicle – especially a light-weight design that requires less power and fuel for
the same performance. But it’s a lot harder to do than it sounds! The greatest
problem isn’t the mechanicals but instead the bodywork and frame.
Designs
For the home constructor, there are basically two
approaches that can be taken: a frame made from small diameter tubing that’s
then (optionally) covered in non-structural body panels, or a monocoque.
Despite being an old technique, multi-tubular
frames remain very popular for one-offs and even small production runs. Cars
like the Skelta, a high performance road/racing car, use a spaceframe of small
diameter tubes.
Bicycles and other human-powered vehicles (like
the air suspension recumbent trike shown here) are also made from steel tubes.
However, nearly all production cars are made in a
monocoque manner, where the pressed steel (or aluminium) panels are welded
together to become both the framework and the body.
Fibreglass cars often use a chassis of sheet steel
(red in this exploded diagram of a 1960s Lotus Elise) enveloped in a
semi-structural fibreglass body.
But whatever approach is taken, there are
problems. A light-weight vehicle made from a steel tube frame clad in body
panels is usually heavier than a pure monocoque. Shaping the external panels is
also difficult. (And if a low aero drag is a requirement, those panels must be
shaped well!)
A monocoque is even more difficult as the panels
must not only be shaped but also provide the structural strength.
So is there an answer? The University of South
Australia thinks there is. They’ve built a very interesting ultra-lightweight,
two-seat, electric-powered car, using a constructional approach that is
relatively cheap, straightforward, and utilises off-the-shelf materials. It’s
also strong. Furthermore, the external shape of the car can be produced without
the need for moulds, or panel-beating or pressing metal panels to shape.
The construction makes use of lightweight
composite fibreglass and aluminium honeycomb panels, fibreglass cloth, Kevlar
cloth, a small amount of carbon fibre cloth, epoxy resin and expanded
polystyrene.
It’s a vehicle building technique that could
literally be done in your home workshop with just hand tools.
We’ll cover the mechanical details of the car in a
later article, but let’s look now at how they made the vehicle monocoque tub.
It’s a process that opens up enormous possibilities in DIY cars across a range
of efficient vehicle types – from human-powered, to electric-powered, to road
machines with ultra-efficient internal combustion engines.
Planning and Design
We’ll concentrate here on how the university built
the car, rather than its design. But the starting point was to decide on such
basics as the number and location of the wheels, the track and wheelbase, and
the body length and width. As can be seen here, a ‘tadpole’ trike configuration
was chosen. In addition to reducing rolling resistance (three wheels as opposed
to four), the tadpole configuration allows for boat-tailing of the body at the
rear, reducing aerodynamic drag. The one-behind-the-other seating arrangement
reduces width (and so frontal area) and is again good for reduced aero drag.
Project co-ordinator Dr Peter Pudney can be seen at right.
A mock-up cabin was then built. MDF (pressed wood
panel) was used to form part of the bodywork, while the approximate shape of the
clear canopy was replicated by plastic tube. At this stage, entrance and exit
strategies could be evaluated, especially for the rear passenger. Foot-well
depth, seat height and shape could also all be easily altered. It is critical
that a lot of time be spent in this stage: once the unique constructional
approach is embarked upon, major design aspects cannot be altered. (Note: that’s
very different to a tubular space-frame construction, where design changes are
possible even after the vehicle is ostensibly finished!)
Rolling MDF Prototype
A prototype was then made from MDF. It was
originally intended just as a full-size test-bed for seating, visibility, canopy
design and so on, but in fact it ended up being fitted with suspension (not the
same front system as was finally adopted), steering, wheels and an electric
motor. However, not being a structural prototype, this vehicle actually broke in
half when it was driven over a bump.
Making the Tub
The material used to form the primary structure of
the car is 20mm thick composite sandwich panel, comprising a 18mm honeycomb core
faced with 1mm fibreglass reinforced with epoxy resin. The material is produced
in Australia by Ayres Lightweight Panel Systems and is their Ayrelite 2016
panel. In 20mm thickness it has a mass of 1.7kg per square metre. Go to www.ayrescom.com for more details.
The panels can be cut with normal woodworking
tools. When a panel is to be folded, a plunge router is used to cut a shallow
(~1mm) groove through one of the external fibreglass facings. This groove
becomes the weakness around which the panel folds, with the width of the routed
groove determining the final fold angle. The router removes only the external
fibreglass - the internal honeycomb crushes as the material is folded.
The beginning of the tub comprised a flat floor
with two sides folded vertically. Here the sides are being kept vertical by the
steel channel sections (green arrows) clamped to the bench each side. The
internal join is being filled with epoxy resin and micro-spheres, giving a
smooth internal radius. The joins were then covered with two layers of
fibreglass tape, again epoxy’d into place.
The way in which the internal honeycomb is
distorted at the corner can be seen here (arrowed). The white tape line down the
middle of the flat floor marks the centreline. Note how the inner skin has been
routed along two lines in the rear side panel, to allow for further folding to
occur. As should be obvious, all the routing needs to occur before the sheet
folding is started!
This view, from the other end of the assembly,
more clearly shows the routing that has been done to allow further folding of
the panel closest to the camera. Note also the lateral routing (arrowed) to
allow the floor to be later folded-up.
To form twin strong triangular-section
longitudinal beams, the tub sides were folded as shown here. Note the way that
the base of the fold curves right around (arrowed). This achieves two outcomes –
it makes the inner finished neater and it also gives much greater connecting
strength to the floor as the join area is larger (ie it isn’t an “end-grain”
butt joint).
This shows the base of the tub, with what will
become the rear of the vehicle closest to the camera. The triangular fold is
being held in place by long steel angle sections clamped to the steel beams.
Generally, the epoxy was left to harden overnight.
Here the end fold can be seen. This completes the
‘open tray’ and gives this section of the floor torsional stiffness. This is the
view from the inside...
...and here is the view from the outside. The
revealed aluminium honeycomb was subsequently covered with two layers of
fibreglass cloth, epoxy’d into place.
At the other end of the tub, extensive additions
have now been made. (1) the driver’s footwell, (2) the transverse bulkhead that
provides great torsional stiffness and forms the dash panel support and rear of
front wheel arch, (3) the panel that forms the inner of the wheel well.
This view is looking at the front, (2) and (3) are
as described above. (4) shows the forward side of the footwell; the steering
rack is mounted to this panel. The design is strong in this area to cope with
front suspension and braking loads.
Positioned next to a Holden Commodore, the overall
size of the tub can be seen.
Viewed from the rear, the ‘cabin’ additions are
now being made. The tub that we saw constructed above has been covered inside
and out with two layers of Kevlar, epoxy’d into place. This both dramatically
increased strength and also gave penetration resistance from stones (outside)
and sharp-edged objects within the passenger compartment.
The view looking at the front-three-quarters of
the car. The arrowed cylinder indicates the size of the electric motor,
positioned below the squab of the passenger seat. The large opening in the side
of the car is for the single side door that’s hinged ‘suicide style’ from its
rear edge.
The arrowed parts are vacuum-bagged carbon-fibre
components that support a steel roll-bar hoop and also provide mounting points
for the front seatbelt.
The completed tub. Note that the seat back that is
visible is for the rear passenger. Also note the small amount of room left for
the front suspension, something that (I think) creates a major limitation in the
finished car.
The completed tub, showing the rear-hinged door.
Note the lower width of the door – the floor is cut away, allowing the person
entering to stand within the wheel track before sitting down. This approach is
very effective at improving access. As shown here, the completed tub weighs just
32kg.
Costs
So how much does it cost to take this approach?
The aluminium honeycomb / fibreglass panels are 2400 x 1200mm in size and a have
retail price of AUD$440 each. Five of these panels were used in the construction
of the car. In addition, the budget needs to include the epoxy resin, fibreglass
tape, micro-spheres filler balls and incidentals.
Conclusion
The beauty of this technique is the ease with
which a one-off monocoque tub can be constructed. Such an approach can result in
a very stiff, ultra light-weight foundation for a vehicle – all without the need
for welding or metal-working!
Next week: so that’s the monocoque tub
produced, but how do you easily and cheaply cloak it in shaped, lightweight
panels?
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