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In the early days of the automobile, the main thing was to keep going without a breakdown. There was no question of "performance" and the " chauffeur", as he was then called, reckoned himself very fortunate if he was able to cover a distance such as the seventy-five miles from Paris to Rouen, without running into trouble-and troubles were of all kinds, not only mechanical ones. Indeed, up to 1914, although notable progress had been made towards achieving reliability, an average speed of 35 m.p.h.-9 hours for ParisGeneva or 15-16 hours for Paris-Nice, stops excludedwas regarded as quite an achievement and was by no means possible for everyone.
Certainly, there were even in those far off times enthusiasts who demanded faster cars with a more brilliant performance. They could not be provided, first because higher speeds demand more efficient brakes and front-wheel brakes had not yet been evolved, and secondly, because we did not have at our disposal all the information which has since made it possible to reduce the weight while increasing the safety factor. This has been achieved by new high-grade steels, by light alloys, by engines which run faster and by factorybuilt bodywork. Here as in everything, the constructors needed time.
AN ESSENTIAL QUESTION
In fact it was only after the first world war, between 1921 and 1925, that the" sports-car" began to appear in the catalogues. It then attracted the attention of a clientele which was going to grow continuously. Improvement of the roads, the possibility of safely using much higher speeds and the appearance of a new conception, that of rapid acceleration (both from a standing start and after a temporary check) soon produced a demand for a light type of vehicle capable of a performance comparable with that of a racing car.
The creation of the type led to the modification of certain traditional ideas and started a whole range of new studies. Up to then no great importance had been given to aero-dynamic research. Road speeds before 1914 were rarely higher than 55 m.p.h. and it was quite exceptional to reach 75 m.p.h. Even in the factory design offices, one could hear the astonishing claim that" It is only above 90 m.p.h. that the shape of the car has any importance". We are now better informed and we know that the design of the right bodylines for a car is a paying proposition from about 45 m.p.h. upwards, or let us say from 20 m/sec. In principle, one may assume that even with present production touring-cars which are in fact better shaped than the racing-cars of 30 years ago, the two elements which make up the resistance to movement, i.e. rolling resistance on the one hand and air resistance on the other hand, are practically equal at a speed of 20 m/sec.
Let us look at this question a little more closely.
If P is the horsepower applied at the periphery of the road wheels, K is the coefficient of resistance of the vehicle, V its speed in m/sec. and 25 kg. its rolling resistance for a weight of, say 1,250 kg. -corresponding to a resistance of 20 kg. per ton-one finds that these diverse elements are related by the formula: P = 25 V + KVs For a car with K equal to 0.06 (an average figure for recent touring-cars of the smaller type) we can therefore write: P = 500 + 480 = 980 kgm/sec. That is to say, rolling resistance takes 500 kgm/sec., air resistance absorbs 480 kgm/sec. This total of 980 kgm/sec. represents about 13 h.p., actually utilized, or 15 h.p. at the engine, allowing for losses in the transmission. So at 44.7 m.p.h. we use 15 h.p. divided equally between rolling resistance and air resistance. Now consider the same car travelling at 84.5 m.p.h. (40 m/sec), which often happens with a sports-car. How many h.p. will it need to maintain this speed, still running on the level, of course.
The rolling resistance will have doubled, as it is proportional to the speed; the first figure will therefore become 1,000 kgm/sec. instead of 500, but the second figure, which is proportional to the cube of the speed, will be multiplied by 8, and so becomes:
8 X 480 = 3,840 kgm/sec.
So having doubled the speed we need :
1,000 + 3,840 = 4,840 kgm/sec., or
64.5 effective h.p., which means about 70 at the engine.
Quite simply we may say that at present day speeds, with modern sports-cars such as JAGUAR, FERRARI, ALFA ROMEO, PEGASO ••• and many others, air resistance absorbs six to eight times more power than rolling resistance.
A very simple little sum would have shown us what to expect.
Going back to the equation for a moving car:
Power = 25 V + KV3
we can take out tho figure 25, fixed by the weight of this particular car (25 kg. rolling resistance, or 20 kg. per ton), and for general application we can write:
Power = 20 P V + KVS,
where p is the weight of the car in tons.
A reduction of one tenth in the total weight, which would be something enormous, that is, taking 125 kg. off a vehicle weighing 1,250 kg., would allow us to save about 26 kgm/see. in power. Thus at the same speed of 20 m/see. this very important weight reduction will only result in a saving of about one third of a horsepower.
On the other hand, imagine that a new design of coach work had allowed the air resistance coefficient K to be reduced from 0.06 to 0.04 (which is not at all exceptional, for many sports-cars can claim a K factor of 0.04) ; one can see immediately that, at 20 m/sec., one has saved 2 h.p. and at double the speed, i.e. 40 m/scc., one will ha.ve saved more than 16 h.p. Thus, as soon a.s one is concerned with high speeds, it is much more important to improve the shape than to reduce the weight of the car.
This conclusion, which admits of no argument, remained for a long time incomprehensible to car-users, although they were the people most concerned. However, as a logical consequence of the facts briefly expounded above, the lay-out of the sports.car has now been modified particularly as regards the height of the centre of gravity, weight distribution and refinement of the external lines of the coachwork. In some cases the constructor has even been induced to put the engine at the rear in order to obtain a better shape. This was the case before1940 with racing-cars, MERCEDES of AUTO UNION conceived by Dr. Porsche who was also responsible for the present PORSCHE sports-car which is now produced by his son Ferry in Stuttgart.
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