Hybrid 3-wheeler – structural panels

Eco-car structural bulkheads.

light and strong

light and strong

tube wrap
The concept sketch shows how aircraft grade birch plywood stuck onto an aluminium sheet has been CNC machined away to leave struts for load distribution and pockets for plug inserts. The real thing would be much more scientific with loads properly distributed by the remaining integral struts. A mirror image version will then be glued on top –wood to wood- and the outside aluminium edges peened over to leave a solid looking panel (but largely hollow inside) which will not only look good with complex curves but be very strong and light. Any big holes in the panel will have the edges peened over too. Peening can be largely automated with a CNC guided rotating roller head.
Heavy loads from components bolted to the panel are fed in via plugs inserted into machined pockets so that any through bolts can’t crush the wood structure when tightened. The plugs are fitted in pairs with a larger star washer in between so that the sharp edges crush into the plywood to give even more load transmission.
Being CNC machined, identical complex panels can be produced quickly and with minimal labour cost. It pays dividends to design complexity out of the car and into the panels. Padded leather panels, a headrest for example, can easily be bolted up to the bulkhead as can other trim items.
Front and rear bulkheads in our car will be connected by 3 of the tubes shown. They will be joined to the bulkhead by cast ally brackets (not shown) where the tube end is expanded and glued by a tapered collar being pressed into each end. The picture here shows how a side panel wraps the tube to make an immensely strong beam affording maximum side impact protection and, of course, a nice wide arm rest as well. The folding is achieved by machining ‘V’ grooves through the plywood leaving the aluminium skin to be a bent hinge line. A more complex shape to the groove can make rounder corners too.
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The inspiration for much of this comes from two great car designers. Colin Chapman made racing car bulkheads from opposed metal sheets with peened holes and edges for rigidity. Alec Issigonis with his Morris Minor design used a complex bulkhead to mount; suspension, shock absorbers, steering rack, battery box, pedals and more. We shall do the same.
No doubt the car will be strong, stiff and light but there is more to an eco-car than that; it must come at an affordable price and these techniques get us there all in one hit. In fact the car is so simple that anyone with a modest workshop and access to a CNC router could make one. Maybe a good base for a schools project?
Ultra strong and light complex panels like these will have many other uses. Paired frames for a motorcycle and a gyrocopter immediately spring to mind. In higher volume applications the plywood core can be precut with a waterjet cutter and then glued between the precut aluminium sheets.

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