In the writer’s experiments on the rear wheel, he had an attendant throw a stick between the spokes while in motion; it was not tried at very high speed, however, for the reason, perhaps, that any failure in the theory above provided might end rather disastrously to the experimenter and thereby cause an act of ingratitude to be perpetrated upon the prospective patron of this book, through an inability to ever finish it.
If some ambitious cyclist will kindly complete these experiments, the writer will gladly incorporate an account of them in future editions of this work, together with an appropriate obituary notice in large type.
CHAPTER XIII.
GEARING UP AND DOWN.
This familiar phrase means simply that the number of revolutions made by the drive-wheel in proportion to the number made by the cranks is greater or less. Broadly, it varies the relative amount of motion of the pedals, and consequently of the feet of the rider in travelling over a given distance. In the simple crank device no change can be made in this respect except in the length of the cranks, but in all of the sprocket-chain devices it is also possible to change the amount of motion in the pedals by altering the size of one or the other of the sprocket-wheels. In lever machines and in those which have the sun and planet connection, either with an oscillating lever or full revolving crank, it is generally possible, by some alteration, to produce the same effect as that of changing a sprocket-wheel as mentioned. The variation in the length of crank produces an effect comparable to the change of gearing in so far as the distance through which the feet travel in covering a certain distance is concerned, but the difference lies in this, that altering the crank means a given number of revolutions in a circle of varying radius, while altering the gearing means a variable number of revolutions in a circle of given radius, in order to cover a given length of road.
In popular language, if a rider wishes more power, he must lengthen the crank or decrease the size of the sprocket-wheel on the crank-axle; vice versa, if he desires greater speed and less power, he must shorten the crank or enlarge the sprocket-wheel connected therewith. It is needless to say that enlarging the gear-wheel on the crank-axle produces the same effect as decreasing the size of that on the drive-wheel.
In a sprocket-crank machine the real question of gearing is whether to change the length of crank or proportion of the sizes of the gear-wheels; but you can’t by any combination get power and speed both with the same amount of work done by the rider.
Simple as all this matter of gearing is, it is probable that there is no feature in cycles so indefinitely understood, or, we might say, so persistently distorted. The only trouble is that riders will not stop to apply a most fundamental law of nature. If we gain speed, we lose power; if we gain power, we must lose speed. To apply this particularly to cycles, if you gear up for speed, you must push harder; if we gear down, we need not push so hard, but must kick faster or go more slowly, provided in each case the length of crank is the same. We cannot go fast and push easy unless we increase the strength of the man. To go over the same distance of given road, the same amount of work is required, no matter how the machine is arranged through which it is done.
This subject was better understood when no element but the length of the crank was to be considered; but now, since bicycles have appeared that are capable of being changed to a high or low gear, some riders persist in treating it as an entirely new problem. It has in one respect a new feature in that greater or less speed can be had without decreasing or increasing the length of the crank; that is, since the comparative speed of the pedal and rim of the wheel in space can be varied either by the length of the crank or the number of revolutions of the same, we can make one turn of a six-inch crank do the same work at the same foot-pressure as two turns of a three-inch crank at the same pressure. Now, this is a valuable feature, because it allows us to increase the vertical amplitude through which to transmit power without change in the velocity of the pedal through space.
A convenient standard has been adopted in gearing cycles by comparing the speed of the driver to that of a wheel and crank connected and revolving together, as in the Ordinary; that is to say, a thirty-inch wheel geared to sixty means that one turn of the crank will drive the thirty-inch wheel twice around, as it must do in order to cover the same distance as one turn of a sixty-inch wheel. To find how high the machine is geared, divide the number of teeth in the sprocket-wheel on the crank by the number in the sprocket-wheel on the driver, then multiply the result by the diameter in inches of the drive-wheel. In short, the speed indicated by the size of the drive-wheel of the geared machine is to the real speed as the number of teeth in the gear upon the wheel is to the number of teeth in the gear on the crank-axle.