TENDENCIES IN CHASSIS DESIGN
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The Last Instalment of Our Series Dealing with the Subject of Progress in the Design of Goods and Passenger chassis. This Week's Article Touches Upon Present-day Transmissions and What We May Expect in the Future
IN the past three issues of The Commercial Motor we have given a review of the main features in the layout of modern chassis, under the headings of frame design, engine improvements and the progress made in the suspension, steering and braking qualities of both freight and passenger-carrying vehicles. This instalment, the last in the series, deals with transmission systems.
If this review had been made a year ago there would have been very little to write about, but recent developments have indicated quite clearly that we may expect an almost complete break-away from the orthodox three or four-speed-and-reverse clashing-pinion gearbox as the means whereby engine power can be maintained at varying vehicle speeds. First the free wheel came along, then " silent " and "easy-change" third speeds, and now the Daimler system in which a Fluid Flywheel and automatically changing gearbox are combined in a system which is silent in action and absurdly simple to operate.
Unquestionably the one item in the transmission line which has done more than any other to add to the E24 refinement in operation of the gearbox and its attendant changing mechanism is the development of the singleplate type of clutch; few people realize what an unportant component this is and what an enormous bearing a good design can have on the handling of a vehicle. The chief factor in gear handling is to have a freely
operating mechanism and a light free-member in the clutch, so that when the friction surfaces are disengaged the spinning, member can quickly and easily adjust itself to a new rate of revolution speed. This necessary change in speed is, of,course, determined by the ratios of the gears themselves, and if the driven components (i.e., the disc of the clutch, the shaft upon which it is mounted, And the driven gears) have a relatively large weight, the momentum of the parts, when rotating, is correspondingly high.
It follows, then, that when changing gear the speeds of the meshing pinions in the gearbox have to be• gauged very accurately in order to avoid a heavy load on the teeth of the pinions if the free Member be rapidly accelerated or slowed down, as the case may be.
When the parts enumerated above are of small mass, the load is correspondingly reduced. (It is easy to appreciate, of course, that if the free member and the driven gears had no weight whatever, it would be possible to change at any speed without the slightest load.) This is an absurdity, but it illustrates the point.
One may be tempted to ask why the cone clutch has had to give place to the single-plate type. The answer is very simple and is mainly two-fold. In the first place it is possible to build a clutch having a single disc the moment of inertia of which is much lower than in the case of a cone. This latter type has to be stiffened up because the load is taken on one face only, and it follows that such stiffening must take place at the extreme diameter where the weight has the greatest effect upon inertia.
With a single plate we are concerned only with torsional propensities of the disc, and as the shape of the free member is ideally suited to the transmission of rotational power, the thickness of metal demanded even in designs where high torque figures are concerned is comparatively small.
The other reason is once more connected with mechanics than anything else. To obtain an easy gear change it is essential that the frictional contact between the driving and driven members of the clutch must be completely severed. Now, it is fairly obvious that in a single disc the end movement required to effect complete freedom is much smaller than in a design where the friction surfaces are of conical formation. The result is that the leverages between the pedal and the thrust race in the clutch itself can be so arranged that the pedal pressure is lower in the case of the singleplate type of mechanism.
In the private-car world the " silent-third " type of gearbox has proved to be a very popular feature. As most people know, the idea in most designs is not only to obtain silence in operation but to enable the gear to be struck as easily as possible. To this end most types employ internal teeth for engagement and by personal experience we are able to testify to the resultant effectiveness in obtaining en easy change.
This tendency will, no doubt, spread to the commercial-vehicle industry, as there are unquestionably many advantages in the system. The silence of the gears, in 3325 passenger chassis, at any rate, would be greatly appreciated, and if the ratios of top and third gears are carefully chosen there would be little need for long-distance coach drivers to employ the lower ratios except for starting purposes.
The Daimler Co. has pursued the matter much farther and developed a transmission that is practically silent on all its four gears and combines a Fluid 'Flywheel, which also acts as a clutch. Although the system has already been described in The Commercial Motor, it will be well-in this' review to recapitulate a fecv of the main features, in order that our readers may have a: mind picture of the great advantages, to both driver atid passengers, which the system gives.
Briefly described, the Clutch consists of two rotors placed in close . proximity* in -an oil-tight casing bolted to the flywheel. There is no mechanical connection between the two, but when relative rotation occurs, oil contained in the casing is forced to flow through a series of cups in the rotors. As the rotational speed is increased, so the oil increasingly resists being churned around, and, in consequence, the slip between the rotors is greatly decreased. One rotor is, of course, driven by the engine, whilst the other drives the shaft transmitting the power to the gearbox. From this description it will be seen that at low . engine speeds there is little tendency for the vehicle to move away from ri rest with a gear engaged; thus, for all normal conditions, top gear can be left in situ and the gear control need seldom be touched. As might be expected, the drive is taken up with absolute smoothness so soon as a sufficiently high engine speed is reached to create the necessary torque.
Used in conjunction with the hydraulic clutch is an epicyclic gearbox with a very interesting change-speed
mechanism attached to the steer_Mg column. This control is of the pre-selector type and is so arranged that any ratio can be pre-selected without the .gear being struck until a pedal is depressed; thus the driver proceeds to get into top gear and then immediately replaces the pre-selector control into the third position, so that at any moment when he may require a lower ratio he has Only to depress the clutch pedal and the alteration will. be effected without it being necessary for him to take his hands from the steering wheel.
There are-many advantages in the combination of the hydraulic clutch and the self-changing gearbox, amongst which the following points are, perhaps, the most e important :—(a) The Fluid Fly:wheel which replaces the ordinary clutch has only two parts and is entirely automatic in action. (b) The drive is transmitted from the engine by means of interchanges of energy In a rapidly moving fluid, with the result that starting from rest and the pick-up after gear-changing are very smooth indeed. (c) Over-running vibration is entirely eliminated. (d) There is an absence of gear noise when idling or running on any gear—a very important matter for passenger-carrying vehicles in particular. This has already been referred to in a test report.
We have purposely.dealt at length with this Daimler system because we feel that it is the prelude to a new era in transmission design. Although unorthodox in the sense that the mechanism employed is quite different from that of the general run of commercialvehicle chassis, the transmission of the power from the gearbox to the axle remains much the same—that is, by a propeller shaft of the usual type, an arrangement which has been proved to be thoroughly satisfactory and efficient.