I have always had an interest in sports racing cars, so when the opportunity arose after obtaining a set of CAD drawings, which enabled one to be built, I decided to go ahead.
My requirements of the car differed in that the CAD drawings were based around the 750 Motor Club rules for a new formula, Sports 1000. This allowed modern suspension design with cast aluminium uprights and billet callipers but a BMC solid axle at the rear. I wanted something light weight with independent suspension at the rear and with the ability to carry two people and as well as being used for competition could also be used for track days but which could still utilise the bodywork from the Sports 1000 car. This was styled after the BMW LMP sports racing car, which competed at Le Mans.
How
was I to achieve my objective? Well I remembered that some time ago a chap
called Alan Staniforth designed and built a car called the Terrapin, a
variation of which was the Monopin. The Monopin was built using an aluminium
monocoque to which was fixed front and rear subframes to carry suspension
and engine. The engine was a Honda and the complete car weighed about 272kg.
My plan was to use the same idea; an aluminium monocoque with lightweight subframes and my target weight was 300kg. After taking basic but essential dimensions, I transferred these onto paper and then reproduced them on plywood. Plywood is easy to cut, fix together and alterations can be made easily. One half of the monocoque was made this way and when positions were finalised, the dimensions were then set out onto NS4 grade aluminium sheet. For those who may not know NS4 is a reasonably hard grade of aluminium and contributes more to the ultimate stiffness when formed.
To begin with the centre section resembled a wide U shape and was folded using two pieces of angle iron and various clamps, not very high tech but effective. At this early stage I wondered if this was the right way to go as the whole structure was wobbling about but when the sections that made up the seat backs and bases were added that made quite a difference. Each flange was drilled, deburred, bonded and riveted to its adjacent section, hopefully never to come apart. The tunnel section was added in a similar fashion and by this time the assembly was extremely rigid. You can see
from
the photograph above how the monocoque was built. It still has a few pieces
to be fitted and also the side pontoons; these won’t be fitted until all
other fixings are complete. These will eventually be filled with two-part
foam, which will add to the stiffness and also provide crash protection.
The subframes are fabricated from 1”, ¾” and ½” tube of 1.5mm wall thickness. Part of the front and rear subframes has a section that fits exactly into either end of the monocoque and will be bonded and riveted in position when needed.
The rear subframe has to carry the engine, Kawasaki ZX9, but any engine could be fitted, the diff is a Ford Sierra unit, again other diffs could be used including the Quaife unit designed for bike power but is much more costly than the £50 paid for the Ford diff and drive shafts.
The photographs show the bare rear frame and also with the diff and engine in place which is located in a north south
position
and will be linked to the diff via a short prop shaft.
Previously I mentioned my target weight of 300kg, so is the car going to be…. heavy or light, well having weighed the following bits, front sub, rear sub, diff, driveshafts, uprights, discs, callipers, exhaust, wishbones, pushrods, spring/dampers, wheels/tyres, rollbar, steering rack, prop shaft, engine, the weight amounts to 228kg, so that leaves 72kg for bodywork, rear wing and assorted small components. I think it should be possible to achieve this. Assuming the weight target is achieved and with about 150 or so bhp, that would give a power to weight ratio of 500bhp per tonne.