MOTORS & MOTORING

Dominion, Volume 27, Issue 34, 3 November 1933, Page 15

MOTORS & MOTORING

(By

SPARE WHEEL.)

Series of tests in America have shown that traffic-actuated light signals save some 45 per cent, of halting time as compared with pre-timed signals.

After considerable use, the windows of a closed car will often develop rattles owing to the glasses becoming a loose fit in the felt-lined channels In which they run. In some designs it is an easy matter to bend the edges of the channels so that the glass is again held firmly enough to silence it. Iu other cases thin strips of rubber , can be wedged at the side of the channel, having the same effect, but if neither can be applied it is usually not an expensive matter to have new channels fitted. • ♦' * There was a rare occurrence at Geurnsey (Channel Islands) recently. The Magistrate, Mr. H. J. Casey, had to fine himself for a motoring offence. He said that the dimmer lights on his car, which he left outside an hotel, must have gone out, a fact which was courteously pointed out to him as he was about to drive away. CARBON FORMATION » What It Depends On All oils and fuels, when burned in the combustion chamber of the engine, are likely to leave a residue which may not be entirely expelled through the exhaust and becomes carbon. Carbon deposit in automotive engines is largely due to imperfect combustion. Correct carburettor adjustment and the constant use of the most volatile fuel obtainable are of major importance. The carbon deposit from lubricating oil in the combustion chamber will depend largely upon four things: 1. The character of the oil. —Some oils are clean-burning; others burn to gummy, sticky masses; and still others “coke” to hard and dry but abundant carbon formation. 2. The body of the oil. —-Usually, oils of heavy body tend to leave more carbon than the lighter-bodied lubricants under the same operating conditions. 3. Engine operation temperatures.— With any oil, there is a certain critical temperature above which the carbon formed is consumed and below which the carbon formation will vary in quantity and character, depending upon the actual temperature. 4. Oil reaching the combustion chamber.—The more oil that, reaches the combustion chambers the more possibility for carbon. The amount of this oil depends largely upon engine design,, construction and use. Before an oil of correct body and character can be selected, every feature of engine design affecting carbon formation must be carefully considered. The tendency toward carbon formation in any engine is analysed from three distinct standpoints,:— 1. Do operating conditions promote carbon accumulation? —If so, as in average car service in contrast to tractor or aircraft service, such carbonising tendencies must be carefully studied.

2. Is the engine construction such that an excessive amount of oil will work past the piston rings into the combustion chamber?—-Where too much oil reaches the combustion chamber, due to features of design which control the oil supply, it is essential that the oil used be of clean-burning character and preferably of light or medium body. 3. Is the engine of a type in which a slight amount of carbon interferes seriously with its performance?—Engines developing high compression tend to “knock” when carbon is present io agreater extent than in moderate and low compression types. Engines with small combustion chambers act in a similar manner. For these types, therefore, the greatest care must be exercised in the selection of the lubricating oil. SPEED AND TIRE WEAR What Tests Have Revealed The manner in which speed increases tire wear was recently revealed by tests conducted by a well-known rubber company. A set of tires ran 12,000 miles on a car averaging 35 miles an hour, but another set of identically the same quality wore out after 6000 miles in a car doing 50. The additional 15 miles an hour doubled the rate of tire wear. The increase when the extra 10 miles-from 40 to 50 miles an hour is registered is considerably larger than when the speed is raised from 20 to 30 miles.

DOUBLE DECLUTCHING

Saving the Life of the Gears METHOD OF OPERATION “Double declutching” is a term, applied to a method employed in changing gears of the type ’in common use in modern motor-cars. Its employment is not as common as one might expect, but the driver who is ’keen to develop “good hands” very early acquires the knack of using this method and nothing else. It can be taken for granted that double declutching means a very much easier task for the gears .throughout the life of the car because they will not be damaged in any way through bad changing ’once the driver has learnt the succession of events that go to make up the complete movement. Actually what occurs is:—, (1) The clutch pedal is depressed and held down while (2) The gear shift lever is moved into neutral, then (3) The clutch pedal is released. (4) The engine is “revved up” by depressing the accelerator’until its speed is comparatively high. (5) The accelerator pedal is released and the clutch pedal depressed and held down while (6) The gear shift! lever is moved into the next lower gear, then (7) The clutch pedal is released and car speed controlled, as usual, by the accelerator. This is a simple operation, and if desired can quite easily be practised upon a ’stationary car before applying

it with the car in motion. ’ It should be borne in mind, however, that the reason why some drivers find it rather difficult at first to acquire skill in double declutching is simply the result of a natural hesitancy ’about speeding up the engine while none of the gears are engaged. Others, again,, object to a "racing” engine, but’the experimenter in commencing to learn double declutching must side-track such objections until practice teaches just how much ’acceleration. of the “free” engine is required. He’can derive a lot of help from.the recollection throughout his practising that, firstly, the speed at which the car is travelling when the change is made does not matter’at all, and, secondly, the movements must be gently but deliberately made. Naturally, a practised driver in Changing down, at high speed will go through tho motions in a shorter time than he would were he to change down at a low car speed, but the movements will never ba hurried or jerky.

The novice cannot do better than to get his,’car on to preferably a level road after getting a thorough grasp of the succession of events mentioned above, and there applying what he remembers. Do not have more than 15 m.p.li. on the speedometer when the initial attempt 1 is made; do nfot hurry the motions, and. do not be afraid of. “revving up” the engine for the short period that the gear shift lever is actually in neutral. . • , The main thing to strive for is a knowledge of just when to release the accelerator before engaging the lower gear.

Changing down, then, from second to low will, require more engine speed prior to engagement of low gear than will be the case in changing down from high to second. The “key” to a quiet change is engine speed, ■which fact will become apparent very early in practising double declutching. When the rudiments have been mastered and slow changes have been made speed up the car a little, but bear in mind that, as the speed at which the change is to be made increases, tho shorter must be the interval between the depression of the clutch pedal as m (5) above and the movement of the gear shift lever into second, or low, as in (G). When confidence is acquired after practising on the flat, take the car into the neighbourhood of a hill. Approach the hill at, say, 20 m.p.h., in high gear, and as the car rises on the gradient, l.e„ without waiting for momentum to be lost, practise the change down. This will increase the driver’s confidence and will demonstrate that he is capable of changing down by double declutching on a climb—if he can do it at low speed at the bottom of the hill, he can do it at any other point on the hill, and low car speed will result as the neutral effect of the grade (which makes it necessary to change-down). After this a-good finishing off can be obtained by driving to the crest of a hill and changing down, by exactly the same procedure, as the car dips down the gradient.

In this case a little more determination will be required in voluntarily revving up the engine while the car is being pulled down the grade by gravity, but If the driver can do it on the flat and on a .climb, he can do it just as well going downhill. This method as a habit in changing down has its advantages. It applies over quite a wide range of cars, and enables the driver to get more satisfaction from the use of low horse-power, because it entirely eradicates that which to so many motorists is a bug- ■ bear, i.e., jerky and uneven gear changing.