The length of crank throw is periodically discussed, and there is a disposition to jump to the conclusion that excessively high gear ratios may be made easy by increasing throw to 7 or 8 or even to 8½ inches. We do not think it worth while to go into this discussion at present, but will state five propositions: 1. The customary crank throw, like the size of wheel and some other factors, has not been obtained arbitrarily, but as a compromise between opposing considerations. 2. The labor of high gears is not thus easily disposed of, because the increased leverage involves a longer circle of travel, a change in the position of seat relative to pedal, and different angles in the muscular action. 3. The throw is closely related to the length of argument set up by some that proper crank upper and lower leg and the length of foot is fanciful rather than sound. 4. The question of crank throw, like that of vertical or forward thrust, must be counted among individual matters and is not to be disposed of by the dictum of any one person set up against the rest of mankind. 5. A long crank is, however, positively wrong for use by women, because it increases the high rise of the knee which, for them, is so ungraceful and is both mechanically and hygienically wrong.
“CLOCK” DIAGRAM—ORDINARY PEDAL.
GEAR RATIO.
This is a proper place to explain gear ratio or “gear,” which is a phrase not generally well understood, although in constant use; for instance, women have been known to ask dealers for a wheel with low gear, because they liked to be seated near the ground. The term gear, which is an adaptation from the old high wheel, expresses the ratio of forward travel of the bicycle for each pedal revolution, and yet this has nothing to do with either the height of the rider or the length of his leg, or the length of the crank. It depends—with a given size of wheel—solely on the relative size of the two sprockets, as measured by the number of their teeth. For example, if the front sprocket has 20 teeth and the rear has 8, it is plain that each tooth of the former will pull a tooth of the latter; so when the former has made one turn it has pulled 20 teeth on the latter, thus causing the rear sprocket and wheel to make two and a half revolutions; as two and a half times 28 are 70, we say that a bicycle with such sprockets has a 70 gear, meaning that one revolution of the pedal drives it as far as one pedal revolution would drive a wheel actually 70 inches in diameter.
“CLOCK” DIAGRAM—RAMSEY PEDAL.
Computation of this ratio is by the rule of three. Thus as the number of teeth in the small sprocket is to the number in the large one, so is the actual to the equivalent or running diameter of the wheel. Multiply the wheel diameter in inches by the number of teeth in the large sprocket, and divide the product by the number in the small one. Or, for each size of rear sprocket, multiply the number of teeth in the front one by a certain number (which is a constant factor) and the result is the gear. Thus, if the rear sprocket has 7 teeth, multiply by four; if it has 8 multiply by three and a half; if it has 9 multiply by three and one-ninth; if it has 10, multiply by two and four-fifths; if it has 11, multiply by two and six-elevenths; if it has 12, multiply by two and one-third. This is for a twenty-eight-inch wheel; other sizes require slightly different factors.