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When a bicyclist wants to race on a level track, he gears up his drive wheel with a largersprocket, so that one revolution of thecrank takes him further. Yet if he takes his wheel, with this large sprocket on the pedal shaft, out on the road where there arehills, he must get off and walk, orexert an extra lot of power.
The same principle applies to the automobile. For this reason the automobile is provided with four or five changes of gears. These gears are contained in a gearbox usually placed at the back of the clutch.
The principle, upon which all change-speedgears work, is the fact that when twogear-wheels orspur-gearsaremeshed together, the larger wheel turns more slowly than the smaller.
In the gear-boxthere are twoshafts — the upper one coming from the engine through the clutch, and the lower one continuing to theback axle.
Each shaft is fitted with four or five gear wheels of different size.
Those on the upper shaft are fixed to the shaft itself, but those on the lower shaft are able to slide on akeyway, to right and left along the shaft. The lower shaft is square so the sleeve of the gear wheels can slidebackward andforward, but they cannot revolve independently of the lower shaft.
In order to vary the speed of the car, it is only necessary to slide the gear wheels along the lower shaft until the correct two gearscome into mesh to form the gearing required.