Trev’s main tub structure was made by folding and bonding honeycomb boards. Our new battery tray, which will fit under the floor of the car, was made using the same technique.
For the tub we used aluminium foil honeycomb. The battery tray is made from cheaper polyproplyene honeycomb, as shown in the images below.
The first step is to fold an edge. Secure the honeycomb sheet to a table with old lead acid batteries, and clamp a square steel tube along the fold line. To form a right angle fold in 10 mm thick honeycomb, heat a 10 × π / 2 = 16 mm strip using hot air guns.
When the polypropylene softens, fold and clamp the sheet. The softened honeycomb will crush, and the outside edge of the fold will have a nice 10 mm radius.
The fold will retain its shape when the honeycomb cools, after a few minutes.
Next, fold up the front of the tray. For a fold angle α (in radians) and a honeycomb thickness t, the strip that is heated and crushed has width α t.
Our honeycomb sheet was not long enough to include the rear end of the tray, so a separate rear panel was simply glued on. At this stage our 2300 × 540 × 80 mm tray had a mass of 2.4 kg.
Next, apply two layers of Kevlar to the inside of the tray. The first layer has the fibres at ±45° to the tray, to resist twisting. The second layer has the fibres at 0° and 90° to the tray, to resist bending. Kevlar is more expensive than fibreglass, but gives a low-mass structure with the stiffness and toughness we require.
Go home. The next day, when the resin has cured, trim the excess Kevlar, turn the tray over (you can use the same four old batteries to support it), and apply Kevlar to the outside of the tray.
A day later, trim the excess Kevlar and the tray is ready for a battery. The mass of the tray is 5.3 kg.