From the January 1918 issue of Motorcyclist Magazine
If the next few years should see a great increase in the demand for motorcycles, as is confidently expected, we may hope to see also a great deal of improvement in design, partly to render the machine more serviceable as a conveyance and partly to facilitate its production on a large scale-in other words, to make it a better manufacturing proposition, says a motor car engineer in Automotive Industries. It is true that the present standard makes of motorcycles are very serviceable machines, but not even the designers themselves would contend that the zenith in engineering development has been approached within measurable distance. A widening market naturally encourages development work, for the greater the prize the greater the stakes that capitalists and engineers are willing to put up.
In regard to engines the situation in the motorcycle field is at present somewhat unsettled. Engines from one to four cylinders are used, though the two cylinder Vee type is distinctly in the lead. The selection of this type must be regarded as a compromise. Having two cylinders it is somewhat smoother running than a single cylinder, and it obviates some of the complication of the four cylinder. What undoubtedly decided the question in favor of this type of engine is the fact that it fits well into the conventional frame. Since the angle of Vee is always considerably lessthan 90 deg. the engine is not well balanced, and, besides, its explosions are unequally spaced.
The Pro and Con of the Horizontal Twin
From the standpoints of balance and uniform spacing of impulses the best type of engine with only two cylinders is undoubtedly the horizontal opposed type. This engine was successfully used for automobile work for many years. The objection to it is that it is of unwieldy length and very difficult to accommodate on a car. This objection might seem even more potent on a motorcycle, but practically it does not work out that way. The motorcycle industry probably has seen a greater development in England than in any other country and one of the best known English motorcycles, the Douglas, has a double opposed engine.
What may be called the displacement efficiency of the motorcycle engine has been increased to such a degree that for a light machine an engine of very small piston displacement now suffices. On the heavier machines the piston displacement is always between 60 and 61 cu. in., that is, just inside the limit set for racing machines. But for ordinary service over moderately good roads an engine with between 35 and 40 cu. in. piston displacement should deliver all the power required. To keep the length of the engine within bounds, a comparatively short stroke would be chosen, say very slightly greater than the bore. A bore of2 ¾in. with a stroke of 3 in., for instance, gives a piston displacement of about 36 cu. in. Such an engine can be built not to exceed 22 in. in length over all, and if the connecting rods are made short and the wristpins located as close to the top end of the pistons as possible-practices which do not conduce to long life of the engine-even this length can be materially cut. There is no difficulty in placing such an engine between the two wheels.
Light weight is certainly a desirable feature, and if it is advantageous to cast the cylinders of large touring cars of aluminum, the same practice applied to motorcycle engines should yield good results. There should be no need for liners of cast iron in an aluminum cylinder of this small size, for it stands to reason that if an aluminum piston in a cast iron cylinder has a satisfactory life, an aluminum cylinder having a cast iron piston working in it should leave little to be desired. In fact, the combination should be almost ideal because the lower coefficient of expansion of the cast iron would be compensated for by the higher temperature of the piston, so that a very slight clearance on the piston would be substantially the same fit in the cylinder whether the engine were hot or cold.