Overdrive can be also called as the highest gear in the transmission of a motor vehicle that is very much needed at high speeds and also for cruising purposes, which maximizes
fuel economy and reduces the stress caused to the engine. It is an amazing mechanism in transmission that prevents deterioration of engine due to prolonged stress and high speeds while cruising for a long time on the highways.
Generally an automobile, whether its a car or motorcycle has a transmission unit or gear box to transmit power to the wheels from the engine. An ordinary engine on its own directly will not have enough capability to run a vehicle without a transmission system. The minimum idling speed of a motorcycle engine may be around 1000 crankshaft revolutions per minute or RPM. It's visible if you see a tachometer. So what happens to all these revolutions ?
The engine running at 1000 RPM can transmit power to the gear box and makes a vehicle to move around say 10 kilometers per hour. From this, we can conclude that gear box converts all these revolutions to torque required to rotate a wheel according to different load and speed conditions as needed.
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| Overdrive option in car gear shifter of an automatic transmission |
Generally the size of the gears will be of descending order, 1st gear large and next small and so on. We call these combinations as gear ratios. For example, if the top gear is similar to the layshaft gear driven by crankshaft (1:1 ratio) then a overdrive gear will be much more smaller than that of the top gear. ie ratio will be <1.
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| Crankshaft with gears |
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| 1:1 ratio same output. 2:1 reduced output but high torque. |
Under this condition the drive shaft turns more faster than the engine crankshaft. The wheels rotate more freely when compared to engine speed. Therefore it reduces engine stress and improves fuel economy. Imagine if you are cruising around 100 with a stress free engine. Surely this feature is the answer.
Usually the overdrive gear is placed above the top gear in a car or a motorcycle. The only bike in India to have this feature is the Suzuki GS150R as an overdrive 6th gear which unfortunately many of our people don't know.