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Gears are wheels with teeth that mesh together. This can happen
either directly (the wheels touch each other) or indirectly (a chain
might be used to make one gear turn the other). Sometimes a screw
(worm gear) or a shaft with teeth (a rack) acts in place of one of
the wheels. In your car, a gear is a combination of gear wheels that
are designed to produce certain speeds.
The gear ratio is dependent on the size of the two gear wheels
working together. The larger wheel will always rotate more slowly
than the smaller wheel (the smaller wheel has to turn faster to keep
up with the larger wheel). This means that the greater the size
difference between the two gear wheels, the greater the variance in
speed and force between the two wheels. For example, if the smaller
gear turns three times for every one turn of the larger wheel, the
gear ratio would be 3 to 1 (3:1).
This fundamental principle is at work in the transmission in our
autos. The mechanical elements and complexity of manual transmissions
and the even more complex automatic transmissions are just a more
sophisticated version of a bicycle's derailleur gears. The purpose
of all three types of transmissions is to use the relatively stable
power output from our engines, or bodies, to the varying power needed
to begin and sustain motion.
If you have ever tried to pedal a multi-speed bike uphill in the
"wrong" gear, you have felt this principle at work. Our legs tire
quickly because too much is demanded from them. We can either stop
and walk, or stop and fall over. A better answer is to shift to an
easier (lower) gear. "Downshifting" the gear allows the power from
our bodies to be leveraged, or multiplied; the pedals will turn
faster with less effort. What becomes confusing at this point is that
shifting downward INCREASES the gear ratio. It's easier to go uphill
at a 10 to 1 (10:1) ratio than it is with a five to one (5:1) ratio.
With a 2 to 1 (2:1) gear ratio, the engine turns twice to turn the
wheels once. Gear ratios are determined by the number of teeth on
them. A gear with 20 teeth meshing with a gear with 60 teeth would
yield a 3:1 ratio.
By moving the shift lever, we have increased the number of pedal
revolutions needed to turn the rear wheel, but decreased the amount
of effort required to turn the wheel.
Both the amount of power and the RPM's (revolutions per minute)
needed to turn our bicycle or car wheels are controlled by gears or
sprockets of varying sizes.
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