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The tooth count on the
chainwheel and the rear cog determines the turn ratio between the pedals and
the rear wheel. Several different types of gearbox are used for this application,
with the three speed epicyclic gear hub and the multi-chainwheel Derailleur
gears being the most common. The latter provides a wider range of gear
ratios and is 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. 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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