Motors and Motoring
The Modern Motor Car. Some Special Features.
Changing
Simpler Gear- IN previous articles a number of special features in the modern motor car have been described; as, for instance, servo assistance for the braking, thermostatic control of the water temperature, crankcase ventilation, semi-automatic chassis lubrication, and so forth ; and the next subject for reference is that of simpler gear-changing. This is a matter which is receiving much attention, and rightly so, at the , present time, both at the hands of motor car manufacturers, , especially British ones, and of the motoring public. There , is little doubt but that gear-changing is the bugbear in the driving of the modern motorist, and any arrangement which tends to simplify the operation while assisting more economical running, is worthy of serious consideration. The combination is such that it makes the topic of very widespread interest, for-, on the one hand, any simplification in the handling of a car tends to popularize and widen the appeal of the motor, while, on the other, greater economy is of importance to-day even to the man who keeps a paid driver. The principle which I am going to discuss applies to the existing construction., and does not entail a new form of gear-box. Most people are familiar with the effect of a free wheel on an ordinary pedal bicycle, and it is on the same broad lines that the free-wheel devices now available on cars are based.
In the question of changing gear, the main difficulty on the ordinary car arises from the fact that two gear wheels are brbught together for engagement, that is, without special - treatment, at widely differing speeds. It is not hard to see that in such cases noise or grating of the wheels is almost inevitable, while the time allowed for getting the teeth on one wheel -into the spaces on the other is far too short The wider the difference of the speeds of the two wheels the harder is engagement, and it is for this reason that one can make straight-through changes on' a car at low speed much more easily with quietness than at higher rates. For example, if the driver on a hill allows his car, which, for the sake of argument, we will assume has a four-speed gear-box, to drop down to,rsay, 12 or 15 miles an `hour" on' top before changing straight into third, he will probably make a quiet change. On the other hand, he will lose efficiency, which means that his travel will cost him more, for to get the best results with the highest average speed he should change down at 30 or 35 miles an hour, or even more with a high-powered car, in order to keep up his engine revolutions and the vehicle speed. This (in all probability) he will not be able to do, unless he resorts to what is known as double-clutching. This means that while the speed lever is in neutral he will , accelerate the engine with the clutch disengaged, and so speed tip the gear-wheel on the lay shaft to be engaged with the wheel on the main shaft which is, at the moment, being driven by the back wheels of the car. This calls for a certain skill, which may well delight the heart of the experienced driver but which is apt to bother the inexperienced one. The free-wheel device is primarily intended to obviate the need of any special operation.
There are various forms of free wheel, and one which is on the market replaces the ordinary differential gear in the back axle ; but whether it is so situated, or is incorporated as a small separate unit behind the gear-box and in front of the propellor, shaft, the main principle is the same. While the engine is under power, that is, the driver is depressing his accelerator pedal, the car is driven forward in the normal way. When, however, the accelerator is released, the free wheel operates, with the results that the engine, being no longer called upon to work, drops down in speed to the limit of the hand-throttle setting and idles, while the car coasts freely. With the ordinary form of solid drive, when the accelerator pedal is eased up, the back wheels drive the engine, turning it into an air corn-, pressor as the vehicle speed tends to be greater in relation' to • engine speed. With the free-wheel arrangement the main gear-box shaft, which carries the gear wheels moved by the gear lever, is not permanently coupled to the back wheels, with the outcome that directly the engine -has died down after the power is taken off, the main shaft is also allowed to slow up. Thus, with the engine turning the lay shaft wheels slowly and the wheels on the main shaft also slightly turning or even stationary, it will be seen that engagement is easy, and can be made without clash since the time allow- ance for getting teeth on the one wheel into spaces on the other is relatively large. This is so much the case that with' a "ear fitted with a free-wheel device it is not necessary to , use the Clutch'at fin when making changes of gear, provided the engin' is allowed a second or two to sldw up after the. accelerator has been released. Here, then, is one distinct. simplification in gear-changing. . The matter, however, doe&. not end there by a long way, for- not only is it possible, but easy, to make silent changes, but these can be made by straight-through movements of the hand, both up and down and from one forward gear into any other. Thus, if a driver wishes to change from second into top, or, having rushed a very steep hill on top, lie desires to go straight into second; missing third, he can do so with equal ease and assurance of quietness.
Changing gears with a free wheel calls for slightly different methods than those recommended for ordinary changing, and this has been shown above, while an Cher point in connexion with changing up is that a slight hesitation should be made with the clutch out and before the speed lever is moved. With an ordinary car a slight pause is often good practice between the speeds, that is, when the gear level is in neutral. Again, it will be readily understood that with the car coasting whenever the power is eased off the engine, it will not do for a driver to ji dge matters quite by the same standard as he would in the usual way, that is, if he wishes to drive with economy and without excessive braking. It is therefore best to let off the accelerator more in advance where a free wheel is fitted than otherwise, because he may not have his engine acting as a brake. I have said " may " on purpose, and will refer to this matter again. A drawback is the slight pause which must occur after coasting and before the drive is taken up by the engine. Additionally, there is more noise, since the engine is being speeded up without any load.
It has been shown above that, when the accelerator is eased up, the engine is free and drops in speed, and the car coasts. The operation of connecting the drive again merely entails depressing the accelerator, but before the engine can take up a load it is obvious that it must pick up speed equivalent to that of the vehicle. The same effect can be seen on a free-wheel bicycle. I do not, however, look upon the delay and additional noise mentioned as serious deterrents to the use of the free wheel, because most modern engines are readily responsive and quiet, while if it has the effect of making designers construct engines to be run with greater silence at all normal speeds, it will have done good service. The first time a motorist tries a free-wheel car, perhaps even more so if he is experienced, he may well feel a sense of insecurity, especially on greasy surface, owing to the fact that his engine is not braking when his accelerator is let up. Here, again, I do not consider the point more than a minor drawback, because the efficiency of the ordinary up-to-date four-wheel braking, if kept in reasonable order, is so high. Moreover, and here we come to a very important item, these free-wheel devices, whether fitted to old cars or incorporated on a new model, are offered with a convenient hand control, by which the driver can at will lock the arrange- ment so that the free wheel is inoperative and the car is drivable in the old way. I would assuredly recommend no owner to have a free-wheel device without such control, for there are occasions, such as in descending really steep hills, when the engine should without doubt be used as a brake, and as a forcible brake, by letting the back wheels drive it through one of the low gears. Without this additional retardation force it would probably mean that brakes were so used that they would get excessively hot if they did not burn out, and grave risk would be incurred. Means of reversing are of course provided. On the other hand, with the free wheel it is possible for a driver to get into a low speed preparatory to moving his control and using his engine as a brake while descending a steep hill, when it would be impossible for him to do this, owing to the speed of the car being too high, without the free wheel. Directly the low gear is engaged the free wheel should, of course, be locked, and the clutch may be let in with care and gentleness to avoid excessive jerk. It has been sometimes stated, and in my opinion with a certain amount of truth, that owing to the free-wheel action being in general employ- ment the brakes must be used a good, deal more. Never- theless, against this drawback may be set the fact that less force is required to pull up the car when coasting than when the engine has to be retarded also. So much for the principle of the free wheel and the manage- ment of a car so fitted. We come now to the other side; of equal interest, namely, economy. Any coasting clearly saves petrol and oil consumption, since the engine will be doing far fewer revolutions for a given distance than if it were being driven. For the same-reason it is equally obvious that wear throughout the unit will be less. Transmission Maintenance is also effected, and for two reasons. One is that the take-up of the drive with the free wheel in operation
is progressive and free of jerk, which is, of course, not always the case with a solid drive, and the second is that a reversal of stresses, which is the factor that causes fatigue in metals, is largely avoided. It is hardly necessary to point out that this last is because the main drive is in one direction only, and that when the accelerator is released neither gear- box, clutch, nor engine are driving. As about 20 per cent. of a car's mileage may well be done by free-wheeling or coasting, there should be a saving in petrol of some 1,5 per cent. or more. Here, then, we have a means .which is not expensive for the results that it gives, by which gear-changing is considerably simplified, by which an appreciable proportion of a car's running when in open and undulating country is as silent as it could well be, and by which definite econoir y in maintenance can be effected.
YOUR MOTORING CORRESPONDENT.
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