From Our U.S.A. Correspondent.
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So far as American van chaseis can be said to have assumed a " type" at all, the study of all the makes at present on the market reveals a tendency to copy a pattern which has been tried and, generally speaking, found wanting in Europe, namely, that of the short, hoelbase with the driver's seat perched above the bonnet. 1 have endeavoured to ascertain the reasons for this, and it appears that no very definite ones can be assigned. It is not, apparently, due to any expressed wish on the part of the customers, for no heavy vans have until quite recently--been in use in sufficient numbers to enable generulizatious of this kind to have been arrived at by experience; but, it seems to be due more to some notion that the very-short wheelbase will render the vehicle more handy in traffic and congested situations. On the face of it, of course, this is true; but, in all my experience, have known only three vases where a very-short wheelbase on a four-ton lorry was a necessity, and, I believe that, ill those cases, the neeessity, which was met by the builder of the cars making specially-short wheelbases, has since been found to be an evil as well. Many times, in England, I encountered people who thought they wanted to turn in a very-small radius, but this opinion was in nearly every case dispelled by an actual demonstration of the mameuviing capabilities of a four-ton wagon with a wheelbase of approximately 13 ft. A motor wagon is not primarily required for pirouetting between champagne bottles placed a yard apart, which feat may with propriety be left to the trick bicyclist. In all the European countries, it has been abundantly shown that, for general purposes, a four-ton wagon with a 13 ft. wheelbase—assuming proper attention has been paid to the correct design of the steering gear and swivel axles—can be handled with surprising ease, and that its flexibility of steering is more than sufficient for all practical purposes. This is one of the points of design which must receive early attention if the American motor-wagon is to develop as rapidly as has the English one. The crowding of the unfortunate driver on to the top of the bonnet, which greatly complicates the design of clutch gear and gearbox control, while rendering examination of the engine a thing of the utmost difficulty, is of course the direct outcome of the attempt to " shorten up " the chassis and the wheelbase while still providing adequate body area for large and useful loads. The front axle is moved backwards, and the body forwards, until a very-considerable portion of the live load is carried on the front wheels.
There is, however, another aspect of this wheelbase question, and one that is very important in giving a useful lessen to English designers and users. I refer to its relation to adhesion. I have put together data, obtained from various American vehicles, and compared them with several types of English wagons which have been successfully used for some years, and the result seems to be of sufficient general interest to warrant reproduction here.
Axle-weights of four-ton wagons:— English.—F.A.W. 4,256 lb. = 27 per cent, of total weight; R.A.W. 11,536 lb. = 73 per cent, of total weight. American.—F.A.W. 5,527 lb. 35 per cent. of total weight; R.A.W. 10,265 lb. = 65 per cent, of total weight.
Assuming 3.5 .h.p. for the engine, and a transmission efficiency of 70 per eent. between engine and road wheels, the power delivered at the wheels will be 24.5 h.p. The co-efficient of friction between a rubber tire and a dry macadamized road is from .625 as a maximum to .4 as a minimum, and the figure .5 is very nearly correct on an average. Consider the vehicle moving at 1 m.p.hr.; the figures are, of course, applicable to any speed by simple mnitiplication. At I nephr.. the peripheral velocity of the
1,760 x 3 tires will be = 88 ft. per min. Each wheel carries 60 half the hack-axle load, in the English example half of 11,330 lb., or 5,706' lb., and the co-efficient of friction allows 5,768 x .5 = 2,884 lb. to be usefully employed in. the transmission of power to the road. At the speed named (l in.p.hr.), the possible horse-power transmitted will be: 2,884 x 88 e. 7.7 h.p. per wheel, 33,000 or a total of 13.4 up. Now, taking an average of American trucks, the axle weights are: rear, 65 per cent. front, 35 per cent. Thus, on the same total weight as the English example, viz., 15,792 lb., we have: R.A.W., 10,265 lb.; F.A.W., 3,527 lb. The load per rear wheel will be half of 10,263 lb., or 5,132 lb. Multiplied by the coefficient of friction, this gives 5,132 x .5 = 2,566 lb. for adhesion per wheel. At 1 in.p.hr., the possible horse-power transmitted will be : 2,560 x 88 33,000 6.85 hap. per wheel, or a total of 13.70 h.p. This is just 89 per cent, of the useful power available in the English model, and it must be borne in mind that this is quite irrespective of the actual power of the engine. Thus, when starting away, the English car has 11 per cent. more power available. The question as to which vehicle will be of the more use in general practice hardly needs discussion. When starting away on a hill, or getting out of soft ground, all possible power is required. From many thousands of miles of actual driving experience in England, including omnibus work in London, I have found that a heavy wagon must possess three supreme qualities: in London and other close traffic, acceleration; in the country, hill-dimbiny pourer; and everywhere brake power. As the brake power is directly dependent on adhesion, it follows that, with an insufficiently-loaded rear axle, this all-important quality will be seriously impaired. The subject of acceleration has received very-little attention in America, but, since it is directly proportioned to the power applied, the car with the heavy rear-axle load must shine; so we find adhesion, and the tendency to stability, power to start quickly, and ability to stop, simplicity of control, to say nothing of the driver's comfort, all sacrificed to the imaginary advantage of being able to turn much more easily than one ever requires to do in practice!
The next point is that of tires. At first sight, it would appear that to effect a more-equal distribution of the load would be to save wear on tires, but it is easy to show that this is the reverse of what actually happens, for, if the hack axle is relieved of, say, 2,000 lb., since there are four tires to share this, the saving per tire is only 500 lb. — an inconsiderable amount, whereas the front tires have each to carry an extra load of 1,000 lb., which menus the use of heavier and, consequently, more-expensive tires. The short-wheelbase touring ears have all been discarded, because, on the American country roads, they were instruments of torture; obviously, on these same roads, to lengthen the wheelbase of a heavy chassis is to lessen vibration and strains generally, and this will tell in the repair bill. Loss of adhesion means racing engines and waste of petrol, which, along with the increased cost of tires, add immediately to the cog of operation per tonmile. One firm, as I reported sonic months ago, The White Co., has recognized all these points, and its new 30-cwt. van chassis hae a 12 ft. wheelbase. That company's customers will undoubtedly reap the benefits of the foresight displayed. The question is not one of criticism or personal opinion, but of simple arithmetic, and it remains to he seen how soon the other important builder will recognize the mistake, and follow the example of the Cleveland firm. H.K.T.