When the pedal cycle with a three-speed hub-gear, and the motor car with a three or four speed gear-box were in common use, the motorcycle was still devoid of change-speed gears, and was driven by a single belt. There were, of course, exceptions, one of which was the two speed chain-driven P. & M., first produced in 1902. Engine power on the earliest machines was transferred to the front or back wheel by means of a twisted leather belt. This soon proved in adequate for the increasing power of new engines, and "V" sec Lion belts were employed, the angle formed by the two driving surfaces being 28°. These "V" belts were built up either of leather or of rubber surrounding a fabric core. Leather belts were harsh and wore the driving pulleys quickly and rubber belts slipped badly when they became wet. The failings of both were thus very marked, and since they connected the engine direct to the back wheel, the former could not run unless the motorcycle also were in motion. This meant that the only way to start the machine was for the rider to push it at a speed of 6-8 miles per hour and to jump on as soon as the engine fired, an athletic procedure which could only be indulged in by a small section of the community. To understand the motorcycle's lack of develop ment in this respect, it is necessary to consider the type of engine used. This was in most cases a four-stroke of either single or twin-cylinder pattern. (See INTERNAL COMBUSTION ENGINES.)' As the power impulses of the engines were too harsh to be trans mitted to the rear wheel by chain or other such appliance, it was necessary to have a driving element capable of absorbing the shock of the explosions, and a belt, which provided a certain amount of elasticity, proved the easiest solution of the difficulty. With the single belt, an unpleasant but inevitable component, the only place to fit change-speed gears was the hub of the rear wheel. Various two- and three-speed hub gears were made during the early part of the 2oth century, each of these in corporating a clutch, a slipping device for allowing the power of the engine to be applied gradually to the rear wheel, so that the machine would start under its own power. Hub gears, however, proved to be very unsuitable, since their internal parts could not be made large enough to withstand the strain. Designers, there fore, applied themselves to the production of counter-shaft gear boxes—so called because the gear wheels work on more than one shaft, as opposed to epicyclic gears which employ a common shaft —usually placed just behind the engine. With a few exceptions, the early counter-shaft gear-box was driven by a chain from the engine, an ordinary "V" belt connecting it to the rear wheel. This type of transmission was much in favour in 1914, when the out break of war stopped further experiments in motorcycles. Belt trouble was still existent, but, since the belt ran in larger pulleys than was the case with a single-belt drive, it was not so marked. Shortly after the war, however, machines with all-chain drive became almost universal, a device to absorb the shock of the engine being fitted on the engine shaft, in the gear-box or in the rear wheel. This type of transmission or one which replaces the primary chain by gear-wheels, together with many other improve ments, has made the modern motorcycle a most efficient piece of engineering and suitable for riders of either sex. It can be ridden by anyone who is in possession of normal physical and mental powers, and it is a cheap form of mechanical transport.
As has already been mentioned, most motorcycles employ single- or twin-cylinder en gines. This rule may be said to apply to machines all over the world, except in the U.S.A. where a few four-cylinder motor
cycles are made. Apart from this small number of cylinders, most motorcycle and motor car engines are similar in general design (see INTERNAL COMBUSTION ENGINES), and are of the four stroke type. Many motorcycles, however, are fitted with a type of engine little used for cars, the two-stroke. The two-stroke en gine fires on every revolution, instead of every alternate revolu tion as does the four-stroke, and its power impulse is thus more smooth. It is extremely simple and reliable, in that it has only three moving parts, but it has certain defects, and it does not develop more power than the four-stroke. The earliest two-stroke to be placed on the British market was the twin-cylinder water cooled Scott (1909), the first air-cooled two-stroke being the Levis which made its appearance a year later. Races and reliabil ity trials have done much to improve the motorcycle, and the modern machine has a low centre of gravity, a smooth running and relatively well-silenced engine, large pneumatic tyres, a three speed gear-box with a handle-bar controlled clutch, a kick-starter and all-chain drive, efficient internal expanding brakes, a well sprung saddle and front forks, and mudguards which give a mod erate degree of protection. Electric lighting sets came into gen eral use in 1927, and almost any machine can be obtained with one at a small extra charge.
Motorcycles are made in all sizes from about 1.5 to 12 h.p., and a machine of 3.5 h.p. or more is capable of taking a side-car. The side-car is attached to the left side of the machine in Great Britain, and to the right side in the United States, according to the rule of the road of the country in which the outfit is being used. In most countries, however, there is no law prohibiting the fitment of the side-car on what is considered locally to be the wrong side. Horse-power is reckoned on the capacity of the engine; i.e., the volume swept by the piston or pistons, in cubic centimetres, 100 c.c. being considered equal to one horse-power. Thus an engine with a capacity of 35o c.c. is stated to have a horse-power of 3.5, and so on. Some manufacturers, however, cling to horse-power denominations based on an old and inaccu rate rating. In America the most favoured type of motorcycle is the large twin- or four-cylinder machine of 10-12 h.p., for either solo or side-car use, but in Great Britain and most Euro pean countries solo machines of above 5 h.p. are not widely used, the larger sizes generally being employed with side-cars. Ma chines of from 1.5 to 2.5 h.p. are very popular in Europe or wherever the roads are moderately smooth, since they are light and easily handled, but in districts where roads are poorly con structed the larger sizes are usually chosen.
At the beginning of 1928, the majority of racing successes stood to Britain's credit. Every inter national tourist trophy race except one (the 1911 5 h.p. class, in which an American "Indian" was successful) had been won by a British motorcycle, and in most continental grand prix races when British motorcycles were present they had proved themselves to be infinitely superior to their competitors. As an indication of the increase of speeds in recent years it is interesting to note that whereas the first tourist trophy race (1907) was won at 38 miles per hour, the 1927 race, on a larger and more difficult course, was won at 68 miles per hour. Most of the world's records for high speeds are held by British machines.
The following is a list of record speeds over a distance of one kilometre, up to the end of 1927.
Solo motor cycles: 2.5 h.p. 89.96 m.p.h.; 3.5 h•p• 202.99 m.p.h.; 5 h.p. 114•02 m.p.h.; '0 h.p. 121.41 m.p.h.
Side-car machines: 6 h.p. 99•07 m.p.h.; I() h.p., 104.12 m.p.h.