Page:Motors and motor-driving (1902).djvu/348

316 The engine is placed vertically, and the Simms-Bosch magneto ignition is adopted. A modified surface type of carburetter is employed, and one small lever suffices to effect the whole of the control. In the Derby machine, a chain from the engine propels a friction roller bearing on the tyre, which is thus driven by frictional contact. It is claimed by the advocates of this system that no extra wear on the tyre results. Latterly, chains or cog-wheels have been experimented with, but machines with this kind of driving cannot as yet be said to have been thoroughly tested, and it remains to be seen whether chain or gear driving will obviate the drawbacks to the belt without introducing disadvantages of their own. Chains are employed in the Humber motor bicycle, a friction disc faced with leather being introduced to slip slightly when undue pressure falls on the chain. In this case the motor is an essential part of the frame, and forms the bottom tube.

No final judgment can be given on this question of transmission, for like everything else about motor bicycles, the whole subject is in a state of transition. But the difficulty is especially great when we find experts divided. There are two gentlemen of the name of Craig who have each devoted much attention to this subject. Mr. A. Craig of Putney tells us that he considers 'that in spite of slipping and breaking, belt transmission is the best. It is simplest and cheapest, and under good conditions an almost ideal drive.' He prefers a flat belt, of at least one inch width, and a jockey pulley on ball bearings. The jockey pulley is always abused because its purpose is misunderstood. It is not meant to jam the belt up tight, but to make it hug as much as possible of the circumference of the driving pulley. Twisted leather belts are in his opinion a nuisance. On the other hand, Mr. A. Craig of Coventry says, 'It has been a matter of surprise that the belt should have survived so long as a means of transmission in motor bicycles. Probably, the belt drive will eventually lose favour except for low-powered machines, and some form of chain or gear drive take its place.'