CHANGING GEAR

Press, Volume LXXI, Issue 21417, 8 March 1935, Page 18

CHANGING GEAR

THE CORRECT WAYS SYXCHROMESH AND WILSON BOXES The task of the driver has undoubtedly been simplified by the easychange transmissions which have become so widely used during the last few years. For this very reason pre-sent-day drivers are apt to become careless in handling the gear lever; furthermore, they do not always realise the limitations of the various mechanisms employed. Most of these devices enable a gear to be selected without clash or shock, but the re-engagement of the clutch i or gear-change pedal) which follows can result in a most unpleasant jerk if the driver is careless. Consequently, it is true to say that skilful handling is still highly desirable if a smooth transmission from gear to gear is to be achieved. Two Leading Systems At the present time the two leading easy-change transmissions are synchromesh and the "Wilson preselective epicycle gearbox. In the synchromesh system the toothed parts in the box. which must be meshed to effect a change of gear, are guarded by friction cones which come into contact as the gear lever is moved. In changing down from top to third, for example, the driver declutches, and should then push the gear lever through neutral to a point where a slight resistance will be felt. This represents the contact of the cones, and an instant later the movement can be completed to bring the toothed parts into mesh. The cones will by then have synchronised the speeds of the teeth so that they may engage without clashing. In the old-fashioned gearbox the driver had to see to the synchronising by a skilful manipulation of the clutch and accelerator. When the gearbox does this for him, he is apt to forget all ab»ut the speed of the engine, so that this may have dropped to an idle by the time that the clutch is once again engaged. How Shock May Occur A car moving at speed is equivalent to a big flywheel mounted at the tail end of the transmission, and the rotating parts of the engine resemble a smaller flywheel in front of the fJutch. The careless driver is therefore trying to couple these flywheels by a friction clutch when the big one :s turning fast and the small one is ■turning quite slowly. Before these speeds can be balanced, the big flywheel must lose energy to the smaller one. The result, on the road, is to produce a retarding effect on the car which is so abrupt as to be unpleasant. It also has the effect of pressing the whole transmission system. .. A .y this can very easily be avoided it the driver will remember to accelerate the engine before re-engag-ing the clutch. The extent to which engine speed must be increased will, 2.ti c « be determined by the gear Xh» *JI J ? n actual fact > so lone as we speed is somewhere near the klv- Lu° od c,utch win take UP the Wive quite smoothly. • point fn connexion with the syn-

chromesh is that, with the exception of a few well-known cars, it is applied only to the top and third gears of a four-speed box. Consequently, when changing down into second gear on a stiff gradient no assistance is derived from the synchromesh mechanism. It is then that the skilful driver will find the benefit of having learned how to control engine speed to a nicety. As so little has been heard about it lately, it seems advisable to give a brief account of the double declutching method which must then be employed. After moving the lever from third into neutral, with the clutch out, the clutch pedal is released momentarily and the engine accelerated to the speed at which the driver estimates that it will have to run after a change is completed. The usual "step-up" is in the neighbourhood of 50 per cent. This should have the effect of synchronising the gear speeds within the box, so that, upon again declutching, the gears can be meshed without difficulty. Changing; Down on a Steep Hill The advantage of being able to carry out this change smartly lies in the fact that it is usually required on an exceptionally steep ascent. Consequently, the change should be made before the road speed becomes unduly low, and the quicker the engagement can be effected the less is the drop in the speed of the car which occurs while the change is in progress. In the Wilson system all the epicycle gearing is continually meshed and the various ratios are selected by applying one or other of a series of brake bands. To effect a change of speed, the driver first moves the preselecting lever to the position required, and then depresses the gearchange pedal with his left foot. The mechanism is then in neutral, and upon releasing the pedal, the appropriate brake band is engaged. The action of the brake band is to couple the engine to the back axle, although it does not perform this function as directly as does an ordinary friction clutch. The fact remains that if the engine speed is not increased when changing down energy must be subtracted from the car to "rev." the power unit, and this can only result in a sudden decrease in car speed. How to Change Up Although only downward changes of gear have been described, it will be realised that, when changing up (with either the preselective gearbox or the synchromesh system), the speed of the engine should be allowed to drop. Some drivers keep the engine turning fast by resting the right foot on the accelerator, so that when the clutch (or gear change pedal) is re-engaged energy is subtracted from the engine and put into the car. The result is a forward surge which may be exhilarating, but Certainly adds to wear and tear. In conclusion, mention must be made of the free wheel from the easy change aspect. This device is a oneway drive which allows the engine to convey power to the back axle, but will not permit the back axle to turn the power unit. Consequently, whenever the accelerator is released the car coasts with the engine idling. Under these conditions, the speed of the gears is so low that changes can be made in many cases without even declutching. After the change has been made the car continues to coast until the driver uses the accelerator to make the engine catch up with the transmission, as it were. Consequently, even if one attempts a change from, say, top to second gear at 50 miles an hour, th-? transmission cannot very ■well be over-emphasised, although