Page:The Cycle Industry (1921).djvu/72

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One of the devices on a bicycle which has done more to popularize cycle touring than any other, with the exception of the pneumatic tyre, is the change speed gear.

It was shown in Chapter I that the diameter of the velocipede or high bicycle driving wheel defined the method of calculating gear ratio, i.e. the ratio or number of turns of the pedal crank shaft and the rear wheel of a safety bicycle. If the road wheel is 28 ins. diameter, and the gearing multiplies the revolutions of the wheel by two, that is called a 56-in. gear because the wheel circumference travels a distance equal to one turn of a 56-in. diameter wheel; if the ratio was one to three, the gear would be called 84 ins., and so on.

A change speed gear is a device that allows the gear ratio to be altered to suit the conditions of the road or the riding conditions prevailing at the time. A low gear ratio, one to two, for example, means that the rider is enabled to climb hills better but not faster; the power of the rider does not vary but the power strokes in pedalling are divided into a greater number of efforts in a given time. The higher gears are useful for normal conditions or for riding with the wind blowing on the rider's back or down slopes when the gradient is not steep enough to let the machine run down quickly by its own weight. Under the latter conditions, viz., with the higher gears in use, the rider's feet revolve the crank axle slower, but the ratio of gearing being about one to two and a half turns of the road wheel