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Gear inches
Gear inches is one of several relative measures of bicycle gearing, giving an indication of the mechanical advantage of different gears. Values for 'gear inches' typically range from 20 (very low gearing) via 70 (medium gearing) to 125 (very high gearing); as in a car, low gearing is for going up hills and high gearing is for going fast.
'Gear inches' is actually the diameter in inches of the drive wheel of a penny-farthing bicycle with equivalent gearing.
==Origin of the term==
When the high wheeler or penny-farthing was the "ordinary" bicycle form, the comparative diameter in inches of the driven wheel was an indication of relative speed and effort. A 60 inch wheel propelled a bicycle faster than a 50 inch wheel when both were cranked at the same cadence. The technology of the high wheeler imposed a natural limit—a 60 inch wheel was about the maximum size that could be straddled by ordinary sized legs. When "safeties" replaced "ordinaries," chains and sprockets allowed small wheels to be turned faster than the pedal cranks. As result, a 28 inch wheel could be made to move a bicycle at the same speed as a 60 inch wheel. Such a bicycle was then said to be geared at 60 gear inches and pedalled similar to an ordinary with a 60 inch wheel. Thus on a bicycle geared at 72 gear inches one revolution of the pedals advances the bicycle the distance that a 72 inch wheel would in one revolution.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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