TENDENCIES IN AMERICAN BUS DESIGN.
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Many Bus Requirements are Diametrically Opposed, and in this Article We Give the Views of an American Engineer on How They Should be Balanced.
I NCREASING interest in the design of public-service passenger vehicles is being displayed in this country, and it is, therefore, of value to observe what is taking place elsewhere. In this connection some very interesting views on the subject of factors governing bus de7 sign have been put forward by the chief engineer of the International. Motor Co. in a long article recently published in cur American contemporary, Automotive Industries.
For the convenience of our readers we have picked out only the essential details, but these are sufficient to give a clear indication of general tendencies. It was pointed out that the main requirements are performance, comfort, appearance and economy; that is to 'say, the best-designed bus is that which gives the greatest satisfaction for the conditions of operation, carries its passengers with the maximum of comfort and convenience at the least operating cost and with the most pleasing appearance.
These factors are all' more ' or less opposed. The best performance can be secured only by sacrificing .cortifort,. disregarding appearance and running at an excessive cost. Luxurious riding can be secured only by forfeiting performance
and eccinomy. On the other hand, a pleasing appearance is not always compatible with the best performance, the
greatest. comfort and economy: The ideal -bus design must, therefore, be a mean between the extremes in all four directions. Earning capacity is necessarily of great importance, but, unless this be combined with comfort, passen: gels will not be attracted.
There are important differences between the various classes of 'bus service, hut, in the opinion of the gentleman with whose views we. are dealing, the best type of bus for the greatest preSent market is a chaSsis of from 2-ton to 2-1--ton weight with a frame not more than 30 ins. from the ground and with a floor laid directly on it, in order to take advantage of the low loading height. It should have a power unit slightly larger than its commercial prototype, and the chassis layout should be conventional, with final drive through a gear-driven axle. • The chassis must be robust and provided with liberal bearing areas, because of the very heavy duty imposed.
It will surprise many in this country to learn that a chassis weight greater than that of a lorry of a corresponding capacity is advocated, -but. it is pointed out that this can be compensated for by a corresponding elimination of useless body weight. The chassis should have springs longer than those used on lorries, dead flat under load,, as wide as practicable and built up of a large number of thin leaves.
In the majority of cases an engine of from 35 h.p. to 40 h.p. would be the most satisfactory, and this should be free from complicated carburetter controls or adjustments. Aceeleration and speed in intermediate gears are essential to a much more marked degree in buses than in .lorries, consequently a flatter torque curve and a power peak at high speeds are desirable. . There seems little justification, he says, for departing from the familiar poppet-valve cOnstriiction. •
The best form of final drive is, he considers, the double reduction, wherein a primary reduction at high speed is se-. cured through bevels, and a .secondary reduction at low speed through wide spur gears at the centre of the. axle ;concentrating the weight at this point reduces the extent to which it must be lifted when the wheels negotiate uneven roads.
The casing should he preferably a onepiece drop forging of the banjo type. So far as clearances are concerned, where the .operation , is over 'smooth streets, a road clearance' of as little as 4 ins, is quite safe, but on rougher mails it should certainly-. be not less than 8 ins.
It is pointed. out that the hotter the brakes the more quickly can a route he covered. Ability to arrest the vehicle rapidly reduces the .tirne be
tween stops, and also gives the driver increased confidence when accelerating. These requirements render it necessary to abandon all thought of double internal brakes of any description. Two sets of shoes cannot share' the flame drum surface without sacrificing much of the friction available, whilst concentric drums require one of them to be unduly small and, hence, lacking in power, and both are usually of cramped and flimsy construction. Where two sets operate on the same drum, it is difficult to secure adequate cooling, particularly when one set operates internally. For these reasons wheel brakes, whether front or rear, should be single, internal and fully enclosed, of large diameter, generous width, and provided with deep, stiff shoes. The friction material should extend only over the effective surface of the shoes Front-wheel brakes have been proved to add considerably to the effectiveness ofthe braking system when used in conjunction with rear-wheel brakes, as they multiply the points of road traction through which the braking effect can he applied. By themselves, however, these brakes offer little or no advantage and several disadvantages. They tend to produce skidding, owing to .the retarding -force being applied in front of the centre of gravity of the vehicle, and, unless great. care be taken in their design, they are apt to be uncertain in their action when the wheels are locked over.
At -present the cast-steel-spoked wheel appears to be best adapted for solid or cushioned tyres add the steel disc wheel for pneumatic &res. The use of cushion tyres is of very great importance. They, are somewhat more costly than solids, but they. are longer lived. They are certainly more economical (him the point of view that they. attract more patronage; they also cushion mechanical partsagainst shock, and so save maintenance costs. The " dough-nut" or " balloon" tyre, whilst still in the trial stage, promises to avoid the disadvantages of both the giant and the twin pneumatic.