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The Design of Commercial Vehicles.*

8th February 1912
Page 15
Page 15, 8th February 1912 — The Design of Commercial Vehicles.*
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Which of the following most accurately describes the problem?

Can we have our frames too rigid? I think we can. One school of designers would build the frame as rigid as possible, making the whole structure as nearly as may be akin to a solid bed-plate, a construction which betokens meat weight, and one, I. am inclined to think, giving none too much satisfaction in use. For myself, I hold that the first consideration of frame strength should be the carrying of the load without undue distortion, and I think it will generally be found that, if it will do this, it will carry the machinery of the car without, difficulty, if the said machinery is mounted within it separately and flexibly connected. I should not be at all surprised to see further development along these lines, as time goes on, especially where cars are concerned which are intended for use over very rough and uneven country.


It, is possible we may see a strung development, in the future, along the two-stroke idea. On the score of simplicity and fewness of working parts, there is nothing, of course, to touch this type of engine, but at present it lacks something in fuel efficiency. Designers, however, will be well advised not to overlook its possibilities, for, if once the efficiency of the four-stroke type is equalled, or very nearly approached, there will be " only one in it." A fair amount of work, too, has been done in America in the direction of air-cooling, and here again efficiency falls off, particularly in regard to the inability to respond to forcing. If this difficulty could be surmounted, we should at once be able to abolish the radiator and connections (we have already largely abolished the pump in favour of thermo-syphon circulation), and very little thought will show what a great advance would be made on the score of lightness, simplicity and low first cost. if a satisfactory air-cooled two-stroke motor were devised. Up to the present, except for the new series of wagons which the Daimler Co. is at work upon, the sleeve-valve engine has not as yet been adapted for commercial work. In great part, up to now, touring-car engines have been employed, many with practically no alteration, but I cannot help thinking that, a heavier construction and slower speed than are found in the modern touring-car engine are preferable for commercial work. In view of the fact that nuieli-lower gearratios are employed on wagons than on cars, and that consequently there is very much more tendency for the driver to race his engine, the employment, of a governor is certainly desirable. It is true that very few trade cars are so provided, but that is, I fancy, rather on account of the difficulty of designing a really-satisfactory system than because of any lack of need for its employment.


Upon the score of transmission, will the future see the survival of the shaft or the chain drive, and what form will our gear-changing system ultimately take? There can be no gainsaying the fact that, both for strength and simplicity, the chain drive scores, as well as for lowness of first cost, and where the chains are properly encased in dust proof cases it will be very hard to beat them for efficiency. Unfortunately, good chain cases add so much to the first cost as snore than to counterbalance the cheapness of chain-drive construction, and unless a car is designed to take them they are difficult to fit. Several firms have lately gone over to the shaft drive. It is in regard to the change-speed mechanism that perhaps the greatest difference in conditions exist. The commercialcar driver has not generally that delicateness of touch and "feeling " for his car which is characteristic of the touringcar driver, and we all know that there are very many of these latter who are terribly rough on their gears. We also know that gear repair and replacement of gear wheels is one of the principal items the repair shop has to deal with. Now. when we remember that, since the touring car is working mainly on open country roads, that it gets out of the traffic as fast, as it can, that it rarely stops between start and destination. and that most cars to-day are capable in average country of doing " everything on the top," it will be seen that touring-car motoring calls for the minimum of gear changing. On the other hand, look at the commercial vehicle. Not only is practically all its work done in the traffic of our large towns, but it is constantly stopping to set down passeneers. or to make deliveries, so that it is plain the exact opposite of conditions holds good, and the maximum of gear changing takes place. Not long since, I travelled in a motorbus from Liverpool Street to the Law Courts, a distance of less than a mile and a half, and I counted no fewer than 30 geer changes made during tile run. I think, therefore, it may ue safely said that the trade vehicle makes from a score to tiity changes to the touring car's one. Now, even for the touraig ear, the sliding gear is held by some to be a mechanical monstrosity. How much more unsuitable, therefore, must It be for the work of the trade car? The marvel is that such gears last a month in a motorvan or motorbus. It is now a recognized fact amongst commercial-car designers that some form ot even-meshed gear is necessary, if successful results and long life are to be obtained. Chain and toothed-wheel gearing connected up with dog clutches and planetary gearing frictionally controlled are lines to be followed, and were it not for the limitations of the latter in the way of number of speed variations and cost of production there is no doubt it would have long since ousted other types, for, except for wear on brake shoes, it is practically indestructible, and gives a marvellous power of control over a car. A high top or direct-drive ratio, such as is customary in a touring car, is entirely unsuitable for commercial work, because high speed on solid tires is very trying on the vehicle, and is even more trying when the car is empty than when it is full, owing to the very great difference in the empty and loaded weights on the springs, and to the fact that with a high top gear the driver will inevitably drive home fast when empty. The ideal gear for commercial week, to be sought for by designers, is one in which all variation, as well as starting from rest, shall be made smoothly and without shock, and one in which the range of variation shall be ample. A gear with infinite variation front zero to maximum is desirable, and one of that class will probably provide the WUmate transmission for commercial work. Such an ideal is obtainable with a petrol-electric system, with friction disc gears and with hydraulic devices. In America, quite a lot has been done with friction drive, and in one case at, least this class of transmission is employed— apparently with satisfaction—in vehicles as large as five-ton trucks. This class of transmission, if the surfaces can be made to stand up to the work, presents attractions for trade cars, by reason of cheapness and simplicity. So far as hydraulic transmission is concerned, to my mind it possesses great possibilities, and students of design will do well to at least keep in touch with developments. Such gears—if they may be so called—have all the advantages of infinite variation ana smoothness of operation desirable, and with oil as the fluid medium they are practically indestructible, though hardly cheap to produce. If we once got to quantity production with them, the costs could be brought within practical figures.

Wheel Diameters.

. Although, in the pleasure-car world, any increase in wheel 817,es meets with but little encouragement, the use of large wheels Is already recognized as essential in commercial vehicles. I need scarcely point out the advantages from a theoretical point. of view, and I am able to say that, in practice, the teachings of theory are consistently upheld. Were it not for the heavy cost of rubber tires in past years (and we have yet to discover a better material), wheels would have increased in, size more rapidly than they have done, and I look for a still further increase when the tire makers are in a position to supply their portion of the construction. For commercial work, with dead loads exceeding half a ton, pneumatic tires are not a commercial proposition.


The only other point I need refer to is the springing, and here there is a large field for investigation. Designing for touring-car work is a vastly different thing, so far as springing is concerned, from designing for goods-wagon work, yet, so far, we have made but little advance upon touring-car practice, and it is generally admitted that, in that much-investigated field of work, the springing of the car has been, until lately, sadly neglected. Springs must, of course, be equal to the war]: of carrying the maximum loads which will be put upon them, and so far as this is concerned we have arrived at a fair amount of practicality. But we are still far from the ideal spring system which shall automatically adjust itself to varying load conditions, and that is what is wanted. If wagons were always run under full load and never under any other conditions, the task would be easy, but, when, say, there is five tons difference in the weight the springs have to carry. The difference, of course, ranges through an the range of variation of load from full to empty.


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