Chassis design

Can something be learned just from a book? Probably not, you need to be a copy cat as well.

The thinking bit

The best design advice came from the Constructors Car Club and from plundering the UK Locost Builders site.

The essence of a space frame chassis seems to be:

• Position your suspension points in space.
• Join them with straight lines.
• Add diagonal braces.
• At a minimum, have a least one fully braced box somewhere in the structure to give it rigidity.
• All tubes should be in push/pull and none subject to bending.
• A true space frame should remain rigid even if the welded joints are replaced with hinges.
• Every joint (node) should have tubes coming into it from three dimensions.
• In side view it should be all triangles – or sheet panel infill.
• Loaded components (engine mounts, suspension, springs) should attach at a node.
• Amateur engineers should over-engineer because it keeps them alive to design their next 'more perfect' car.

Major issues included how to access and remove the engine and tranny; the twisting caused by a narrow mounted VW torsion beam; and the old problem of squeezing in the driver and passenger. And the New Zealand LVVTA regs: seat belt mounts (four point harness), side impact, frontal impact, firewalls and more.

By trial and error the front and rear were fully triangulated albeit at the expense of the spare wheel, luggage space and doors. There is no transmission tunnel to brace the cockpit area so the side intrusion bars were designed as a pyramid (another option was a wide sill).

The CAD produced a highly detailed materials list that said the chassis would weigh 104kg, a bit porky compared to a ‘7’ but still on for a weight of only 600kg.

Even then there were questions about how the area around the driver’s knees was braced that were only addressed when metal was cut.

The Constructors Car Club design scrutiny helped avoid stupid mistakes - 50X25 is not the same as 25X50 when it comes to carrying a bending load.

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