Building an Ultra Light-Weight Car, Part 1

This article was first published in 2009.

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?

Contacts Ayres Lightweight Panel Systems: www.ayrescom.com UniSA Two-seater Renewable Energy Vehicle: www.unisa.edu.au/solarcar/trev/

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