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Derailleur
gears
The
Derailleur gear system moves the bicycle chain between
different gears on both the crank and rear sprocket gearsets. This is achieved
by displacing the chain sideways so that it 'jumps' from one gear and engages
another in a controlled way. The gear ratio for any selection is:
Gear Ratio = (NC / NS)
where NC
and NS are the tooth counts on the chain and sprocket gears
respectively. The ratio gives the number of sprocket revolutions per chainwheel
revolution.
The road advance of the bike per crank revolution
is determined by this gear ratio and the diameter of the rear wheel. A 700 mm rear wheel moves the bike forward by 2.2 m in a single revolution. For this
wheel, the distance traveled by the bike for a single crank revolution is:
Road Advance per Crank Revolution = 2.2 (NC / NS) m.
The
picture shows the rear sprocket mechanism (without the chain). The derailleur
mechanism is to the right of the gears. The two guide wheels of this
mechanism move in a plane normal to the rear wheel axis and transfer the
chain from one sprocket to the next. |
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