THE MOTOR WORLD

Otago Daily Times, Issue 18999, 23 October 1923, Page 5

THE MOTOR WORLD

By ACCELERATOR. SAFETY FIRST. Watch for someone else doing something silly-- and take care never to do it yourself. MAKING A OAR LAST LONGER. f “EXPENSIVE LUXURY” THAT IS ALL EXPENSE. Although the economies resulting from coasting on any given run of say, 50 miles or thereabouts may bn so slight as to be negligible fwrites Captain E. do Normauvillo, in the London Daily Chronicle), if the practice ho indulged in consistently throughout, a season’s riding?the story is quite different. A saving off 10 per cent, in the fuel consumed in a 50-mile run is not much, but the same proportion saved on the annua! -fuel bill is an appreciable “■discount.” There are very few give-and-take ICO-mile spins in England of which one-tenth may not bo covered with the gear in neutral and the engine stopped. The saving in engine oil consumption is negligible, but the economy in tyre costa is more important, although it will not bo so high as that in the fuel bill. The point is that when a car is coasting the tyres are subject only to rolling fricrtion, and to none of the slipping friction that causes nearly all their wear. The back tyres are being worn only f.t the same rate as the front, if we ignore for the moment braking stresses. An annual saving of five per cent, in tyre expenses is a conservative estimate of the benefit of coasting. The habit of indulging in violent and rapid increases in speed is one that, more than any other single driving factor, forces up all running expenses. A capacity for rapid acceleration is one of the charms of the really good modern car, but unfortunately there is a largo proportion _of ■ drivers who, while capable of appreciating this charm, do not know how to make proper use of the quality tha't provides it. You may see many a driver suddenly “standing on” his accelerator pedal when the car is proceeding at the most sedate of paces, and his smile of satisfaction and content as it jumps away is obviously based on an entire lack of knowledge of the stresses ho is unnecessarily imposing on eijery single component from the tyres to the crankshaft. Even when properly employed by intelligent use of the gear lever. Vapid accelerations take much out of the tyres and transmission system of cars. When improperly used, violent acceleration impose® stresses throughout the whole, chassis that may or may not make themselves immediately apparent. Most often the effects are seen only at some later date, when the car seems to need ■ a premature overhaul for quite inexplicable reasons! The same applies to violent and unnecessary application of the brakes. Nothing distinguishes the good driver from the bad more pointedly' than the way in which ho uses nis. brakes. Notice how one man, knowing that he has to stop at a certain point, will throw this gear lever into neutral, and. if the stop is to be a long one, will switch off his engine when he is 50yds or more from the point—the actual distance depending, of course, on his speed—-and will only require to use his brakes quite gently for about the last dozen yards or so of the car’s travelling. Notice how another driver will approach to within half the distance of the stopping point, without any reduction in his speed, and then will suddenly “jamb on everything,” as the saying goes. THE EXPLANATION OF HORSEPOWER. EXPLANATION OF A MUCH USED BUT LITTLE UNDERSTOOD TERM. This article is the answer of Captain lie Novmanville to a lady correspondent, who asks, “What—exactly—is horse-power, nnd what does the word moan in relation to a motor-car engine?” It is probable that_ many readers are not fully acquainted with the subject, so I will endeavour to explain it in a simple manner. The word horse-power was originallyadopted by engineers lo indicate —the power of a horse! That’s quite simple! But it was an unfortunate choice, in that the power developed by our equine friend varies enormously. Take, as examples, the extremes of a powerful cart horse straining up hill with a heavy load, and a hunter trotting lightly homo with, say, my fair correspondent astride its back. The former is probably exerting 10 times the “horse-power” of the latter. But in the mechanical world we have laid down an arbitrary quantity of work done to represent one horse-power. It is 11.0 quantity of work which is equivalent to raising 33,0001 b through one foot in one minute. When we speak of the horse-power of an engine we mean the rate at which it does its work. Power is the rate of doing work. * t You should know. however, that a normally strong horse doing sustained work can only accomplish about 22,000 footpounds per minute. The excess was adopted by the inventor of the term—tr.n famous engineer. Watt—and it has stayed with us ever since. A similar excess of .rating obtains throughout the world, though it varies a little in different countries. For example a French horse-power is only 93 per cent, of ours, the figure used in that country being 32.552 foot-pounds per minute. And now, of course, you want me to explain what a foot-pound really means. The “foot-pound” is the British Unit of Work. It is the equivalent of one pound weight avoirdupois raised one foot high. And new I must try and make you appreciate the difference between the word “power” and “horse-power.” The idea of “power’’ involves the element of time. For example. 53.000 foot-pounds of work may be accomplished in a ipinutc. an hour, or a day. In each case it is still 33,000 footpounds.

But note now that “horse-power” involves the performance of 33,000 foot-pounds in a minute—the time factor comes :’n. In calculating the amount, of -work done, therefore, try now to realise that the, time factor does not enter into our ealeu lalions. A certain quantity of work, done in a certain time, means the development of a certain amount of power. T.et me try to make this clearer. If you were six times as strong as a normally powerful man (a man can exert one-sixth of a horse-power), you could raise 33,0001 h of weight a foot high in one minute. And that would be the development of one norse power. But if half that weight xvere raised one foot high in half a minute it would still be one horse-power. It is a- difficult matter to explain this question to non-teehnienl readers, but, I hope I have made the general idea understandable. And whore does the 10 or 20 horso-powor of the average car go? Its two chief uses are to overcome the road resistance of the oar. and the air resistance. The former is normally' the more important factor. Air resistance is very important at nigh speed, however. Your scholastic days should enable you to understand me when I say' that air resistance increases as the square of the speed, and at high speeds, nearly ns the cube of the speed.