When the first human-powered vehicle -- a four-wheeled contraption driven by rope and gears -- was introduced in 1418, it was little more than a curiosity. Today, bicycles are big business, serving casual weekend riders and elite athletes alike. Single-speed bicycles, or those with just one front and one rear gear, were the popular choice for riders until 20th century. Modern bicycles make use of multiple gears, giving riders a variety of transmission factors, or gear ratios, to choose from.
Definitions
The transmission factor is the ratio of crank wheel, or the sprocket turned directly by the pedals, to the free wheel, usually driven by a chain attached to the crank wheel. For example, if the crank wheel has 30 teeth and the free wheel has 10 teeth, the ratio is 3:1 for a transmission factor of three. That means that for every complete turn of the crank wheel, the free wheel turns three times.
Development
In 1903 Sturmey-Archer, a bicycle component manufacturer still in business as of 2011, developed the three-speed, internal hub gear system. Before the innovation, bicycles had evolved to become lighter and more agile, but they were still powered by just one available transmission factor. The hub design dominated until competitor Simplex developed the cable-driven derailleur with which today's riders are familiar. The new design allowed for more gears at the rear hub increasing the number of transmission factors, or speeds, a rider could call upon for different riding conditions.
Sizes
The actual size of the gears varies dramatically from bike type to bike type. A typical selection of front sprockets for a racing bike, for example, is 39, 42, 52 and 53 teeth, but mountain bikes and hybrid-style commuter bikes make use of various front and rear combinations.
Practical Use
Everyday cyclists interested in the math behind the motion typically refer to bicycle gears by the assigned number manufacturers use on the shift levers. The notation is simple. It begins at 1, just like a car, and progresses up to the maximum number, usually 18, 21 or 24. The lower gears are for getting started or climbing, while the higher gears are for cruising. Basically, when you pair a front sprocket with a similarly sized rear sprocket, pedaling is easier. When the front sprocket is far larger than the rear sprocket, pedaling is more difficult, but the work output is greater due to the transmission factor.
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