Construction Techniques

Author Bob Carter

The techniques used to construct the CAUC cars have been fairly varied – they are often “experimental”!

Brian Chassis

The structural frame of Brian is made from the carbon fibre honeycomb composite sheeting used for commercial aircraft flooring. The team acquired a large 8 x 4 ft sheet. The main chassis can be seen in the photos in the gallery & is effectively a single large C section beam. This structure is glued together with araldite with a “glass bubble” filler and has stayed ‘rock solid’ since the day it was made. Wheel mountings and motor/gearbox mountings are on separate steel (front) and aluminium (rear) sub frames which typically attach to the honeycomb structure by clamping 50mm x 70mm steel plates either side of the honeycomb and tightening the bolts onto 10mm steel crush tubes.

The steering uprights are 20mm round steel bar, bolting onto 12mm rose joints for the steering pivots. The stub axles are long M12 bolts. All steering rods and tie rods joints pivot on rose joints. This produces a strong reliable, adjustable system which is commendably accurate and free from play or lost motion. The tie rod is right hand thread on one end and left hand thread on the other so that toe in can be adjusted to sub-millimetre precision without unbolting anything!

Brian Body

The body of Brian was designed to follow low- drag profiles generated by NASA software (!) – with subtle bulges for the wheels. It was necessary that the body should accurately reproduce the design, so we cut out a series of 6mm MDF profiles, (one every 80mm – 33 in all) and bolted these together with M8 studding. The club was organised into a production line with 4 workstations for this job: -

1) Cut out basic rectangle

2) Drill studding index holes

3) Draft profile onto the MDF with pencil

4) Cut out with band saw and jigsaw

This was a great introduction to production techniques, and got a time consuming job done in only 2 evenings.

We used the negative profiles, filled in between with builders foam (the squirty stuff from cans) then cut off the excess between profiles when it was hard.

The surface was then roughly levelled with a mixture of Polyfiller and sand – this gives a roughish surface which is easily sanded to shape. (Incidentally don’t use sand intended for concrete which has some big lumps in it… oops, guess who bought the wrong stuff at Homebase) Finally a finishing surface of ????? filler was applied & sanded smooth to the final contours. This surface was painted with “high build” primer paint and sanded smooth again before being given a final coat of emulsion paint and several coats of floor wax and some PVA mould release agent.

This procedure worked OK I think but was only possible because of the shape of Brian, it was good in being a 2 stage process instead of the normal 3 stages, but bad in that a lot of time was spent bending into a large concave mould & cleaning out the dust with a hoover.

The body was created by laying one layer of carbon fibre weave then two layers of glass fibre weave, brush applying the epoxy resin. When this had cured it was prised out of the mould (with difficulty!), cleaned up and then put back in to apply strengthening ribs to the inside. These were made by gluing on foam in the required positions and rosining 2 layers of glass tape round the outside of the foam. This technique was worked exceedingly well – Brian’s body is VERY rigid in spite of its large size and relative lightness.

Again the steps in these procedures are illustrated in the build gallery for Brian.

Raptor Chassis

Raptor is an altogether more conventional construction compared to Brian. A steel ladder chassis was built (main rails 25mm x 50mm x 1.6mm) with cross members, a battery tray and an aerofoil section rollover bar all MIG welded on. The result is massively strong and reasonably light – because there is no need for local reinforcements at high stress points. The floor, which the driver sits/lies on, is bonded in place using TDK VHB tape and pop-riveted at intervals.

The aerofoil section of the roll bar was produced by a friendly blacksmith (OK it was me at my brother’s forge…). Normal round mild steel exhaust pipe (1.5inch diameter) was rolled flat (~1.2” x 1.8”) then one side of the oval hot forged to a sharp ‘blade’. The whole was then filled with sand and bent to the required radius in the rollers. It turned out pretty well!

Because of the relative narrowness of Raptor, the brake disc mounting face on the wheel hubs was machined back 6mm – this actually gives us a useful increase in steering lock where the brake discs used to interfere with the chassis. The wheels themselves are built in house because we needed a pronounced ‘dish’ to achieve the flat outside our aerodynamics call for.

The steering system is the same as Brian’s except we used smaller 10mm rose joints for the steering pivots.

The drive train is mounted directly to the chassis (previous cars have always had a big 6mm aluminium plate for the drive system). This is already clearly superior to, and lighter than, the old system.

Raptor Body

Again the Raptor body is being constructed in a more conventional 3-step process. A positive “buck” is being constructed from MDF and foam and the body mould (in 3 pieces will be pulled from this. The final body panels will then be made from the moulds.

Once again we need total accuracy in the buck production and were very fortunate to find that a colleague had just completed a home made 3d CNC routing machine and was looking to ‘try it out’. So, all our complex body profiles were cut out, with fantastic accuracy, free of charge. And we re-used the MDF from Brian’s mould – so that was free too! We now have a lot of sanding to do on the buck. Various stages of buck production are shown in the build gallery. We are using an epoxy/ glass bubble mix for filler – Mr B loves this stuff…. It doesn’t apply as well or sand as easily as normal filler but it is very hard & rigid when set!

I propose to completely cover the finished buck in glass fibre, then saw the whole thing up into the 3 mould/body sections, - that should make the mould creation a whole lot simpler and quicker!

To get good looks and low friction drag, it is important to have an excellent finish on the final body panels, and that requires close attention to detail every step of the way.