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Cardboard Cars?

Exploring some potential ultra light-weight vehicle building materials

by Julian Edgar

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This article was first published in 2009.

In AutoSpeed we’ve recently covered a couple of techniques for building ultra light-weight vehicles.

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The University of South Australia’s ‘Trev’ uses composite fibreglass and aluminium honeycomb panels to form the monocoque...

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...with the chassis then covered in expanded polystyrene modelling foam, carved to shape and covered in fibreglass cloth and resin. (See Building an Ultra Light-Weight Car, Part 1 and Building an Ultra Light-Weight Car, Part 2.)

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The other technique we’ve described is using high strength chrome-moly steel tube, nickel-bronze brazed together. If well designed, the resulting space-frame can be very light and strong. (See Building Ultra Light-Weight Tubular Frame Vehicles, Part 1 and Building Ultra Light-Weight Tubular Frame Vehicles, Part 2.)

Both techniques are well within the realm of the home do-it-yourselfer, but neither approach is cheap. The fibreglass / aluminium honeycomb panels cost AUD$440 per 2400 x 1200mm sheet ($153 a square metre), and chrome moly tube can easily cost $80 a linear metre (obviously the actual cost depends on the tube wall thickness and diameter).

So are there any alternatives?

Yes, there’s a composite material that is very cheap, widely available in flat sheets, can be easily ‘worked’, has available excellent glues – and there’s also a lot of knowledge around on how to make things from it.

It’s wood – but not necessarily in the form you’d first think of.

Timber

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Wood has been widely used in the past to make light-weight, stiff and strong structures. The pictured WWII Mosquito aircraft was made using composite wood sandwich construction. High performance boats have also often been made from wood.

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The biggest flying boat ever made – the Howard Hughes’ ‘Spruce Goose’ – was structurally made entirely of timber.

Contrary to its nickname, the primary material was birch veneer. Members were built up from several plies of veneer, bonded together in multiple grain directions. These members were glued and steam-heated, often being shaped in huge moulds. In addition, some spruce, poplar, maple, and balsa were employed. The adhesive was ureaformaldehyde and until the adhesive set, the wooden parts were held together with nails – these were later removed.

In the case of this aircraft, wood was chosen to avoid the use of materials critical to the war effort (the aircraft was commissioned in 1942).

At least one car was made using a wooden chassis – the Marcos.

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First produced in the 1950s, the Marcos was one of the ‘specials’ so beloved of UK sports car producers. Developed by people with a good knowledge of WWII aircraft manufacturing techniques, the Marcos used spruce and plywood, glued with a synthetic adhesive. The wood was treated to make it fireproof; the car had a torsional stiffness of 3000 ft-lbs per degree. That stiffness is not fantastic in today’s terms, but for an ultra light-weight vehicle (especially if it has only three wheels!) it would be excellent.

The small car weighed only 400kg, and it was said that the body (complete with some fibreglass panels and the windscreen) weighed only 91kg.

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Here’s the plywood chassis...

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...and a cutaway of the whole car.

The pic of the chassis is worth a long look – it’s obvious that the structure has a lot of bending and torsional stiffness. It also looks like it would have good frontal crash performance.

The Marcos stayed in production for a long time, changing body styling, engines and suspension. It also dropped the plywood chassis for a tubular steel one, apparently because the plywood design took longer to build, and because of customer resistance (jokes about white ants...).

But the wooden chassis Marcos cars have shown to last very well – and that’s to be expected; think about plywood boats that spend a lot of their time half immersed in water.

Paper Honeycomb Core

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Another wood-based material worth investigating is paper honeycomb panelling. These panels have recently been released in forms that make themselves quite suitable for ultra light-weight vehicle construction. One brand of composite paper panel is X-Board, made by Xanita.

Available in a variety of core thicknesses (eg 10, 12, 16 and 30mm), the sheets can be factory clad in MDF, plywood or kraft paper.

So how light and how cheap? 10mm thick kraft clad paper board has a mass of 1.9kg per square metre and a 2400 x 1200mm sheets costs only about AUD$43 - so AUD$15 per square metre.

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Clearly, since this material is made solely of paper, weatherproofness becomes an issue. However, it can be treated with resin (that will be absorbed by the paper) or, if the material isn’t going to be used in all conditions, externally waxed.

The paper honeycomb panels can be routed, folded, curved and bent – obviously the thicker the external cladding, the more difficult these processes become.

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The core is made from recycled paper waste and once it reaches the end of its useful life, can be again recycled.

Conclusion

It may sound a bit down-market over the use of exotic materials like carbon fibre or Kevlar, but naturally occurring cellular composites (ie wood!) in either laminated, shaped form or as honeycomb panels, are cheaper, more easily ‘worked’, less energy intensive in production, and can be later recycled. As history shows, if it is used correctly, it can also be surprisingly strong and light.

If you’re building an ultra light-weight vehicle, wood is definitely a material to investigate.

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