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Gear
Systems
In
bicycles, gears are required to couple the cyclist's output to
the motion of the rear wheel. A selection of gear ratios is desirable to maintain
a good impedance match between the cyclist and the bicycle. The tooth
count on the chainwheel and the rear cog determines the turn ratio between the
pedals and the rear wheel. Crank diameter and wheel size must also be taken
into account in matching the human power source to the machine. In boat and
airplane applications of the gear and chain system it is usually adequate to
select a single gear ratio to drive a propeller in either air or water.
Several
different types of gearbox are used for bicycles. The three-speed
epicyclic-gear hub and the multi-chainwheel Derailleur gears are the most common.
The latter can provide a wider range of gear ratios and is commonly used
with mountain and touring bikes.
The number of teeth in the chainwheel
for a 70 cm wheel touring bike may be selected to be between 24 and 54,
and the corresponding rear sprocket may have between 12 and 34 teeth. For a
mountain bike (66 cm wheels) a similar tooth range is available, but the distance
travelled per crank revolution with the same gears will be different.
Wheel size influences the overall gear ratio.
With the Derailleur gears, the chain length determines
the set of ratios that can be used. Not all rear sprockets can be used with
all chainwheels in the set. A gearset is selected that provides the performance
desired by the rider, perhaps with a very low gear for hill climbing
and high gear for high speed flat road performance. |
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