Cycle Repairing and Adjusting
With a Chapter on building a Bicycle from a Set of Parts

VARIABLE GEAR DEVICES 119 wheel ; multiply by the diameter (in inches) of the back wheel ; and divide by the number of teeth in the hub chain ring. Assume a back wheel 26 in. in diameter and the chain wheel and the hub chain ring to contain 50 X 26 50 teeth and 18 teeth respectively. Then -----------— = 72f, say, 72. Or assume a 28-in. back wheel and a 20-tooth hub chain ring ; how many teeth must the chain wheel have to make the gear of the machine equal 70 ? Call the oc X 28 number of teeth in the chain wheel x. Then ——— = 20 70. Therefore, inverting the numbers as follows : — x x = 70, and simplifying this to - x 70 = 50 = the number of the teeth in the chain wheel. “High” and “Low” Gears.—It is obvious that with a low gear the cyclist travels a comparatively short distance, and with a high gear a comparatively long distance, per one revolution of the cranks. Therefore, the power required to make one revolution in the former case is much less than in the latter case, since less work is done. The user of a high gear, therefore, although he travels farther per one revolution of the cranks than the user of a low gear, is obliged to exert a great deal more power in the making of that revolution ; he feels the advantage when the running conditions are easy, such, for example, as when descending a slight hill, or when pedalling with the wind behind hia back.