THE INTERDEPENDENCE
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OF COMMERCIAL-MOTOR
AND PRI VAT :-CAR DESIGN
IT is a moot point whether private cars or commercial vehicles are more advanced in design—both have attained a degree of excellence undreamt of a decade ago—but development is so rapid these days that no one can afford to rest on laurels gained ; all must forge ahead with any new idea which has proved itself to be of advantage to either manufacturer or user. With these thoughts in mind we have made a tour of Inspection of the Motor Show at Olympia, and, although at first sight one is inclined to the belief that the various chassis and cars exhibited had little of interest from the point of view of our own branch of the industry, closer inspection shows detail refinements—only brought into vogue during recent months—which might well be considered by all interested in the transport of passengers or goods.
In the private-car field there is a notable change in frame design which has been brought aSout partly by the necessity of obtaining greater strength and partly because a new form of engine mounting, known under the various headings of "cushioned power," "floating power" and "silent power," etc., has been introduced. Broadly speaking, all these arrangements are similar in principle to one another, the whole secret lying in the fact that thick rubber inserts are placed between the engine feet and the frame, thereby "insulating" vibration.
Rubber Mountings for Engines.
We might divert from the subject of frame design for a moment and look into the question of the rubbermounted engines. It must be borne in mind that if an engine vibrates, it still continues to do so even if it be mounted on rubber, but that, due to the design of the structure, the characteristics of the vibration may be altered. The torque reaction at low engine speeds of a commercial engine would be registered by separate impulses and, due to the necessarily large forces at work, might well prove the cure to be worse than the malady. Probably some advantage could be gained by utilizing thinner rubber inserts than those used on private car chassis and it is quite likely that very nearly as good results would be obtained. Such a practiee is, of course, followed by several commercialchassis manufacturers.
Returning now to the question of frame design one is struck by the increase in the number of chassis utilizing a large cruciform cross-member amidships. This form of structure is unquestionably sound for it gives the greatest strength to the part of the chassis where it is in greatest demand. Full advantage of this construe n24 tion, however, does not seem always to be taken. The extremities of the cruciform could be extended to the mounting plates for the spring eyes, and, with other tubular members stretched athwart the chassis, would form a tremendously strong structure. Again, the amidships rigidity seems to suggest that all brake linkages should emanate from the centre of the chassis, so that the effect of distortion due to road shock would apply equally to each brake. This brings us to another feature—the brakes themselves. It is all very well to emphasize the fact that large drums and big frictional areas together tend to produce good brakes, but it is in the matter of detail application that effectiveness and at the same time Progressive action can be combined. To this end one notices that, where pressed-steel drums are employed, there is a growing tendency to introduce wide circumferential flanges to give enhanced rigidity ; even greater strength is being given to the cast ribbed-pattern drum.
In many cases the diameters of the drums are limited only by wheel diameters, and as it is necessary—for many reasons—to house the drums within the wheels, efficiency is dependent upon other things than just size. Servo shoes operated by cable seem to be gaining ground, but as the possible manual effort exerted by a private-car driver and a commercial-vehicle driver are obviously equal and yet the laden weights of the vehicles are so dissimilar, the large servo motors usually found on big commercial vehicles are likely to remain.
Suspension systems have received a certain amount of attention, not so much in the matter of design of the springs themselves, but in the method of control of the relative movement of axles and frames. A fairly general practice is to fit hydraulic recoil (tampers athwart the front axle with the arms operating on the part of the beam where greatest effect is obtained. The Humber models afford a good example of this, whilst the Talbot arrangement, whereby an additional frictiontype recoil absorber is employed, is interesting. In this case a control is taken from the Luvax hydraulic dampers to the steering column and the driver can produce conditions suitable for fast going on good roads or, on the other hand, for bumpy conditions at slow speeds.
Transmission systems seem to be the main subject for discussion this year. That we are on the eve of a great abandonment of existing ideas is indicated, but which way the cat will jump is still unknown. Armstrong Siddeley Motors, Ltd., pioneered the new easychange cult about four years ago when the self-changing gearbox was introduced on the larger cars produced by this Coventry concern ; since then the Daimler Co., Ltd. has introduced the Fluid Flywheel and combined with it the self-changing box. Now the Salerni coupling has come on the scene and synchro-mesh gearboxes have been standardized on Rolls Royce, Sunbeam and Vauxhall cars (to quote only three makes) whilst " silent " second and third speeds with constant-mesh helical p-nions are fitted to Singer cars.
Merits of the Fluid Flywheel.
For bus work the Fluid Flywheel unquestionably has many advantages over an ordinary friction type of clutch, for the driver is relieved of a certain amount of work and so can concentrate more fully on traffic negotiation;, moreover, the hydraulic action of the device is absolutely progressive, so that " finesse " in driving is not necessary. Daimler commercial chassis have for some time been equipped with a transmission system similar to that of the private cars, and we shall not be surprised if many other makers produce systems on somewhat similar lines in the near future.
Pioneered by Vauxhall Motors, Ltd., the synchromesh gearbox is very delightful to operate ; for the benefit of those readers of The Commercial Motor who do not know exactly how this mechanism works it might be mentioned that in addition to the ordinary clutch contained in the engine flywheel there are additional clutches in the direct and third-speed pinion clusters (sometimes also in the second-speed pinions) which in action ensure that the relative rotating speeds of the meshing elements are synchronised—thus, perfectly silent changes, upwards or downwards, can be effected by novices,
The Crossley method of isolating the gearbox by means of a free-wheel arrangement at the forward end of the propeller shaft works very well on the bus and coach chassis. It is operated in conjunction with the clutch and allows the rotating shafts to be slowed down quickly by a clutch stop, when the lever can be moved into any position with absolute silence.
From the early days, manufacturers have striven to produce lighter vehicles. Certainly great strides in this direction have been made of recent years, but there is still scope for further advancement by utilizing modern light alloys to a much greater extent. The new Armstrong Siddeley chassis rated at 30 h.p. is a workmanlike affair, thoroughly modern in conception, and, although possessing all essentials, is distinctly light in weight. The crankcase, cylinder block and head, the sump, pistons, connecting rods, etc., are made of Ilidutnium alloy—a material of great strength but of light weight. The cylinder walls are produced from centrifugally cast iron, machined all over, and in con
sequence an equal thickness throughout is ensured; the cooling water surrounds the liners. Finally one may mention the inlet and exhaust manifolding arrangements of some of the more prominent makes of private motorcar. There is still a certain amount of difference between the methods whereby distribution of the mixture is effected, especially on the six-cylindered models, but there does nevertheless seem to ben tendency towards the introduction of six-branch manifolds and three-branch pipes with siamesed ports. In this way the actual length and shape of the passage to each cylinder are more nearly equalized, with a resultant increase in power due to a better distribution of the charge. The =tackling arrangements on the Siddeley Special engine are well worthy of inspection, there being 12 ports—all in line—and a really efficient hot-spot arranged in the centre. Again, the new Alvis Firefly manifold is worthy of inspection, a semi-downdraught carburetter being utilized in conjunction with a falling V-shaped manifold.