BUILDING A 12 SEATER CHUMMY

By Tim Griffiths and Alan Billington

This year's winter project has been to build the vehicle that the world has been waiting for: The stretched 1925 Austin Seven AC Tourer.

Our new stretched Chummy

The technical problems that have needed to be overcome are massive because in order to seat 12 people the car has had to be specially strengthened and the engine arrangement dramatically modified.

The first problem we encountered was the issue of chassis strength. With a total projected weight of 1450Kg when fully laden the members that go into forming the normal Seven chassis are totally unusable but modern metal technology came to the rescue with a special honeycomb titanium floor pan that allowed us to build in impressive strength and rigidity without compromising the look of the finished car or adding excessive weight.

Forming the honeycomb titanium chassis

The next problem was the wheels. Standard Austin Seven tyres and spokes would just not support the anticipated loads so we had to call upon the expertise of Rex Grogan who directed us to the Special Research Unit at Avon Tyres (SRU-AT) the same team responsible for the tyres on the famous Alfa Romeo Velo with its ill fated tandem wheel arrangement.

An early prototype of the tyre undergoing testing.

Working purely for the enjoyment as our budget was very limited they managed to fabricate tyres that give the outward appearance of standard Austin Seven tyres but with reinforced cross fins that help to brace the side walls and give extra compressive stiffness. The only unfortunate side effect of this arrangement is a soft squeaking noise that is made as air passes from one compartment to the next. Rex is hoping that substituting a vertical slot for the circular pressure balancing holes will cure this on the next batch.

After successfully constructing the bodywork we were faced with installing our radical new engine arrangement.

Forming the brake enclosure

Since starting this project we had realised that the engine compartment in the Seven simply wouldn't be big enough to enclose the size of engine needed to pull the projected load so a radical new solution was called for.

The solution was simple; the engine was removed and four powerful inductive electric motors were installed inside the brake drums. The brakes would have been insufficient on this weight of vehicle but by reversing the polarity of the motors they would offer a variable braking force with the added advantage of generating electricity from the energy that would otherwise be dissipated as heat.

The inductive electric motor assembly ready for mounting in the brake drums.

The four electric motors were tuned to provide about 185 bhp (as tested on the rolling road) and would achieve a top speed close to our 75mph target. The energy for the motors comes from Lithium Ion batteries arranged in banks below each of the seats and alongside the electronics under the bonnet.

The inverter and cut-out switchgear

From an early stage we had realised that the batteries, even with the energy recouped from braking, wouldn't allow a range of more than 25 miles fully laden and so a novel source of auxiliary power was found in the form of 'Somnium'.

The Somnium fabric illustrated in an early research paper.

Research on the internet had turned up a research project in Anaheim that had successfully produced a photovoltaic film called Somnium that when bonded with a cotton substrate gave a reasonable facsimile of Rexine but with the potential to generate 0.3KwH of electricity per square metre per mile from the hood if deployed on a sunny day.

The hood headlining power transmission ribbon.

Tests are currently underway but we are confident that we will be able to achieve a full day's motoring without having to recharge before sunset.

The final hurdle we now face is getting permission to run her legally on the road. She has passed the MOT tests for braking and emissions with flying colours but their computer won't accept more than 6 seats! Oh well, back to the drawing board.