Author Topic: Gearing


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« on: February 17, 2008 »
Cyclopedia gets its cherry popped

Gearing is a system employed in the drive trains of many machines, including cycles. It allows the drive wheel (chainwheel on a cycle) to rotate at a different speed from the driven wheel (sprocket). This enables the drive (cyclist's legs) to rotate at a mechanically-acceptable rate, while producing a wide range of rotational speeds in the driven wheel. In the case of cyclists, 60 to 120 rpm is widely reckoned to equate to a comfortable rate of pedalling, although there are debates over both what is optimal and (especially among the fixed-wheel community) what is the maximum achievable.

It follows that almost all modern cycles have gearing, and it is not accurate to describe a single-speed machine as having no gears. Rather, it has a single gear, as opposed to variable gearing such as hub or derailleur types.

In the traditional British system, gears are measured in inches. The measurement represents the equivalent diameter of a direct-drive wheel on an Ordinary cycle (high-wheeler or "penny-farthing"). This is because the early safety cycles were sold in competition with the then-dominant Ordinaries, for which wheel diameter directly limited potential speed, but was in turn limited by the height of the rider. Therefore, the gear measurement of a safety cycle gave a means of comparing it with that of an Ordinary.

The measurement is calculated by multiplying the ratio of:

chainwheel size * wheel size in inches
                   sprocket size

and the sizes of the chainwheel and sprocket are most conveniently represented by counting the number of teeth, hence:

chainwheel teeth * wheel size in inches
                    sprocket teeth

Although modern 700C wheels are actually slightly smaller, it is usual to assume the old 27" wheel size. On significantly-smaller wheels, the true diameter must be used.

Gears in the 60-70" range typically suit ordinary riders on the flat, unless they are moving particularly fast. However, rider preference and fitness play as large a part as terrain and speed. Touring cycles may have gears as low as 30" and racing cycles as high as 120", and some will go beyond these.

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