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THE MINI MAG. Volume 2 No.8 |  |
 August 2000Vol.2 Home Page | Index Page |
Paper of ALEC ISSIGONIS, Chief passenger Car Engineer of B.M.C. The Evolution of the Austin – Morris Baby Car. . THE ASSIGNMENT In leading the design which produced the Austin – Morris baby car I was fortunate in being given a free hand by Sir Leonard Lord, Chairman of the B.M.C. Much useful data had been accumulated in our previous experimental work on small cars and this simplified our decisions, but no detailed analyses were made of the benefits of different basic layouts. Once started on such a project I ignore what other people have done and go ahead on what I feel id the best approach to the problem. Although I only began serious work on ADO 15 (the works code name for the new car) in 1956, I had built an experimental transverse engine and front drive unit for installation in one of our existing cars as long ago as 1951. I knew it would be fractionally more expensive than a rear engine, rear drive unit, but felt this would not be a decisive factor in the price of the complete car. The brief was to create a small, light, inexpensive vehicle with interior space larger than that of the Austin A35. We considered that the “bubble” cars had failed to achieve real comfort for four people or real economy of operation or a satisfactory engine life and the aim was to produce something smaller, cheaper and lighter than anything we had in production, with quality, comfort, safety and stamina comparable with those of our larger models. There was a time when the small car was designed specifically for the man who could not afford to run a larger car, but many people now run small cars simply because they are so much more convenient in congested cities. From the start it was accepted that engine and transmission must be grouped at one end of the car of the other. Experiments to decide which end continued in conjunction with the experiments to decide the form the engine itself should take. TWO STROKE ENGINES We had accumulated a lot of experience with various types of two-stroke unit. Before the war I developed an opposed-piston two-stroke with twin crankshafts which ran up to 8,000 r.p.m. but developed very little power. More recently the B.M.C. had spent over three years experimenting with twin cylinder air cooled two-strokes but we finally decided that they were not acceptable on grounds of poor specific fuel consumption, and uneven running at idle or part load. TWO CYLINDER ENGINES Some 500 c.c. vertical twin four-strokes were designed, built and tested in a little over three months but they turned out to be an uneconomical use of labour and materials. Our production people could add two more cylinders and produce a much more satisfactory engine for 8 to 10 pound more. It also became apparent that air cooling adversely affected noise level and engine life, and so we concentrated on water cooling. We were running some rear-engined two-cylinder cars and I tried mounting our existing four-cylinder engine in one, but it spun off the first corner. It would have needed larger tyres at the back to secure satisfactory handling. The problem could no doubt have been solved by extensive redesign and a long programme of experiments but we wanted a quick result and we turned to the formula which we were convinced would give a good result in the shortest time. We built parts and prototypes quickly from rough sketches and diagrams and tried them out on the test bed or test track before handing them over into the intellectual atmosphere of the drawing office. The final answer was achieved almost accidentally. One day I revived my earlier scheme, mounting a 948 c.c. A35 engine and gearbox unit transversely at the front, with gearing to transit to the front wheels. The moment he tried it Sir Leonard Lord said “that’s it!” and there was no more discussion on the form the engine should take, or where we ought to mount it. The swept volume was subsequently reduced to 848 c.c. chiefly because the larger engine gave far more performance than the average driver could be expected to cope with in this size of car. By taking the gearbox off the end of the engine and putting the gears in the sump, we achieved a much more compact unit with longer drive shafts, which gave better working conditions for the universal joints, and bought another incidental advantage. One of the worst sources of noise and “boom” in unit construction bodies is dynamic bending of the long engine-gearbox unit. You cannot see it but it does transmit vibrations into the body which are difficult to suppress. The Mini Minor power unit, being short and rigid does not suffer these bending problems and so helps to produce a quiet running car. Probably the noisiest unit at the front is the fan and we are working hard to quieten this. VIBRATION PROBLEMS Vibration problems associated with a transverse engine were solved by rubber engine mountings which are stiff vertically and laterally but flexible fore and aft. Movement fore and aft is restrained by a short link between engine and body bulkhead and by using the exhaust pipe as the second link. Noise and vibration are further suppressed by mounting the whole engine-transmission unit on a separate sub-frame. COOLING PROBLEMS The radiator was originally in the conventional position in front of the car and as there was no convenient way of driving the fan from the transversely mounted crankshaft we began to think we might have to use an electric motor to drive it. However, careful investigation of air flow round the car showed us that there was a region of low pressure in the front wheel boxes which could be used to draw air through a suitably placed radiator. We therefore mounted the radiator sideways and helped the airflow with a fan driven in the normal way from a crankshaft pulley. Such a simple solution would not be possible with a transverse mounted rear engine. WHEELS AND TYRES Small 10 inch wheels were adopted because they cut down the size of the wheel arches and gave an important increase in passenger space. By putting them at the corners of the car, with the main structure joint lines sloping outwards towards them, we gave an illusion of length and an appearance of strength. The imported 4.80-10 tyres originally used had a very short life but the first experimental tyres produced by Dunlop gave 30 percent greater life. It is worth noting that although the 10-inch wheel is very small, the 5.20 tyre section is quite large for a small car with a kerb weight of only 1260 lb. FRONT WHEEL DRIVE On front drive cars the connections between the engine and the driving wheels is, of necessity, somewhat rigid and it is therefore not possible to operate these vehicles at very low speeds in top gear unless some design precautions are taken to increase drive flexibility. This has been done by a special universal joint which was worth its weight in gold. The spiders were encased in rubber bushes, which effectively damped out the unwanted snatch. The drive system was completed by outer universal joints of the Rzeppa type which transmit power through six hardened steel balls. These give smooth operation to the drive when the front wheels are turned to the full steering lock. SUSPENSION The very small wheels, which are naturally sensitive to road inequalities, and the relatively short wheelbase set the designers a special problem. Because the car is so light in relation to the load it can carry, ordinary springs were not suitable. Many different springing systems were tried and the present rubber “doughnuts” compressed between rubber cups finally provided the answer. The rubber is loaded partly in shear and partly in compression and keeps the frequency of the suspension practically constant irrespective of car loading. The progressive rate of this system, which gives reduced suspension movement as the load is increased, enabled us to produce a low built car running on small wheels, without fear of catching the underside on an obstruction when running fully loaded. Prolonged testing has failed to reveal any method by which the suspension units or the rubber universal joints can be destroyed. BODY CONSTRUCTION Structure weight received very careful attention and semi-automatic methods of assembly were adopted to cut production costs. The main pressings were flanged out-wards and welded together by moving wheels, the flanges being covered afterwards by strip mouldings. This produced a rigid, rattle free body structure to which two sub-frames, carrying front and rear suspension and the power unit, were attached by bolts through rubber bushes. ENDURANCE TESTS A low centre of gravity, good weight distribution and suspension which comes near to the theoretical ideal led us to hope for exceptional handling qualities but the results exceeded expectations. Two test cars each covered 50,000 miles at night on English roads at high average speeds and several more were sent away for prolonged endurance testing in France, Spain, Portugal, Switzerland, Norway, Sweden, Denmark, Germany and Italy. We had achieved a really compact baby car with four roomy seats, which had better acceleration than its competitors, had a maximum speed of 70 m.p.h. and would do 50 miles per gallon at a cruising speed of 50 m.p.h. Ed: This paper is truly a piece of history. Great to see such thing kept so that we can learn just how our great love began. |