Tub

Trev is built around a structural tub chassis fabricated from composite sandwich panels. Low-mass non-structural body panels are fitted to the tub to form an aesthetic and aerodynamic body.

The technique used to build the UniSA prototype is summarised in three articles in AutoSpeed magazine:
The construction technique is simple and effective.

Tub design

The original design had the battery in the front of the car. Team Trev made some modifications for the 2010 Zero Race:
  • suspended a battery tray underneath the car to accommodate the 13 kWh battery and most of the high voltage system
  • moved the front lower wishbone pivots under the original floor, alongside the new battery tray, to accommodate longer wishbones
  • lowered the rear seat (not enough) and put a swing arm pivot under the rear seat.

The 2001 tub model (Tub_2011.skp below) makes the following improvements to the design:
  • the stiffening edges of the floor extend downwards instead of upwards to protect the battery tray and to reduce the height of the interior sill
  • battery trays of different heights can be accommodated
  • body height adjusted to take into account the underfloor battery
  • more shoulder room in the rear seat
  • an external pivot point for the rear swing arm (not shown in model, but a folded plate extends under the floor and behind the rear seat).

The tub design is almost finished. Still to be done:
  • close the leading and trailing edges of the battery tray
  • design side intrusion beams folded into the side panels?
  • design the chassis structure behind rear seat, with parcel shelf
  • hole under rear seat for access to motor controller in battery tray.

Materials

The original Trev tub was made from Ayrlite 2016 20 mm thick aluminium honeycomb panels, with 290 gsm fibreglass skins. The floor and front compartment were reinforced with two layers of 170 gsm Kevlar, at 0 and 45 degree angles, to give the tub sufficient stiffness and toughness.

Polypropylene honeycomb is a much cheaper alternative, is non-conductive and perhaps quieter, but is slightly heavier. Team Trev is using polypropylene honeycomb for their African Solar Taxi.

Folding templates

The diagram below illustrates a simple right-angle fold.



The desired final shape has two panels at right angles to each other. The two panels are marked out on a board with a gap between them. The width of the gap is a t, where a is the desired angle, in radians, and t is the thickness of the board. For a right angle in 20 mm thick board the gap is pi/2 x 20 mm = 31.4 mm. Folding the panel will close the gap on the inside of the fold.

The process for more complicated shapes is similar. Use the dimensions from the inside surfaces of each panel, and draw the panels onto the board with gaps between them corresponding to the fold angles.

The following images show the the tub templates. The corresponding SketchUp models can be downloaded from the bottom of this page. You can export DXF drawings from the SketchUp models using a DXF and STL export plug-in.




Each of these template sheets fits on a 1200 x 2400 board. The folds are designed for 20 mm thick board. The red strips indicate regions where the skin must be removed on the  bottom of the board; the blue strips indicate regions where the skin must be removed on the top of the board. (For those that remember Origami: the red strips are mountain folds and the blue strips are valley folds.) Dimensions can be determined from the SketchUp models.

We have not yet finalised the design of the rear of the car, behind the rear seat. This will partly depend on the design of the rear swing-arm.

The vertical walls to the side of the front seat are not parallel to to centreline of the car. When these panels are folded, the inner face of each wall will be in the correct location but the outer face of the wall will have a slight gap at the front and a slight overhang at the rear. The gap can be filled, and the overhang can be trimmed off.

Fold and bond

Once the panels have been marked out:

  • Cut around the outside edge of the design using a jig saw with a straight-edge guide.
  • Use a router to remove the skin from the inside of the fold gap, as indicated in the diagram. The router depth should be set to about 2 mm, to remove the skin and any glue.
  • Clamp one side of the fold to a bench, then fold the other panel. The aluminium foil in the fold will crush. Use blocks or clamps (or old car batteries) to hold the panels at the correct angle.
  • Wipe the inside of the fold with a clean cloth and acetone. Brush epoxy resin along the fold, place a strip of 50 mm wide fibreglass tape along the fold, then stipple and roll epoxy resin into the tape. Lay up another layer of fibreglass tape, 75 mm wide, over the first. Apply peel-ply to the tape to absorb up any excess resin and to give a matt finish.

For tight angles, applying a fillet of microballoons into the corner can make it easier to apply the fibreglass tape.

Fixing components to the tub

There are several possible techniques for fixing components to the tub:
  • For light fixings, nylon plasterboard anchor screws can be simply screwed into the board. Trim the anchor screw to to the correct depth, drill a small pilot hole through the skin, apply 5-minute epoxy to the anchor screw, and screw it in.
  • For medium fixings, an aluminium ferrule can be bonded into the core.
  • For heavy fixings, such as suspension components, plates are required on either side of the panel to distribute the load, and ferrules are used inside the core between the plates to prevent the plates from crushing the core.

The photo below shows the mounting for the rear swing arm.

[photo to be added]


In the next version, the mounting for the rear suspension will be lower. We also need to incorporate some means of isolating road and motor noise from the chassis.

The front suspension is mounted to vertical steel plates which also fold underneath the tub.

[photo to be added]
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floor_template.skp
(54k)
Peter Pudney,
7 Jul 2013, 02:07
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side_B_template.skp
(77k)
Peter Pudney,
7 Jul 2013, 02:07
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side_C_template.skp
(53k)
Peter Pudney,
7 Jul 2013, 02:08
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tub_2011.skp
(7182k)
Peter Pudney,
7 Jul 2013, 02:06
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tub_templates.skp
(62k)
Peter Pudney,
7 Jul 2013, 02:08
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