Automotive Dictionary: Overdrive

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Automotive Dictionary: Overdrive


Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.
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The term "overdrive" simply means a device which enables the transmission to turn faster than the engine. With a conventional four-speed gearbox, top gear is normally direct drive, that is a 1:1 ratio, which means that the transmission is being driven at the same speed as the engine. But in all of the other gears the transmission is being driven at a slower speed than the engine. Therefore, the indirect gears are under-drive ratios.

An overdrive provides a step-up gear ratio which, for a given road speed, will allow the engine to rotate slower than it normally would in top gear. This overdrive gear ratio can be used at cruising speeds and will reduce fuel consumption as well as reducing engine wear and vehicle noise. Overdrive can be accommodated in an extension to the standard gearbox housing. Alternatively, an overdrive ratio can be provided in the main gearbox in the form of an additional fifth gear. Most five-speed gearboxes presently being fitted to cars are designed to give overdrive operation of this type. These should not be confused with true five-speed gearboxes where fifth gear is a direct drive ratio.

Overdrive switch from a Triumph Dolomite Sprint
The top image shows an epicyclic gear train. Here the planet carrier is locked and the sun gear is driven - this results in the annulus turning slowly in an anti-clockwise direction. Below shows how overdrive speed is obtained by locking the sun gear, driving the planet carrier and taking the output from the annulus. This method was used in the best known of the overdrive units, the Laycock overdrive.

The Advantages of Overdrive



The main advantage of overdrive is that, at cruising speed, it can make the transfer of the engine's power to the road wheels more efficient. The gearbox of a car is normally designed to provide the maximum possible top speed on a level road when the car is fully-laden, allowing a sufficient margin of engine power for acceleration and hill climbing. This choice of gearing does not, therefore, take into account the fact that for much of the time most cars are not operated at maximum load, speed and acceleration.

Many motorists will cruise along in their cars at 80 to 97 km h (50 to 60mph) for long periods without using the maximum power of the engine. Under these conditions the engine is quite capable of maintaining the car's speed at a lower rate of revs and the use of overdrive can achieve this. When the overdrive ratio is engaged the engine revs fall but, because the propeller shaft is turning faster than the engine, the car's road speed remains the same.

Mechanical efficiency apart, the most immediate advantage of overdrive is that it lowers the car's fuel consumption. Tests have shown that the use of overdrive on a medium-sized family saloon can improve fuel consumption by as much as 20 per cent. This saving does, of course, depend upon the car being driven at cruising speed with overdrive engaged for reasonably long periods of time. The other general advantages of overdrive are the reduction of both noise and mechanical wear. If the engine is turning at a lower speed it will obviously produce less noise and create a less tiring environment for the driver on a long run as well as result in less wear and tear in both the engine and the gearbox.

These factors apply to both forms of overdrive � the "bolt-on" type and the designs that use a fifth ratio in the gearbox itself. But the relative merits of these different ways of obtaining overdrive resulted in some debate and accounts for the number of variations. The commonest type of overdrive is the unit that was bolted to the rear of the existing gearbox, such as that made by Laycock Engineering Ltd., (formerly Laycock de Normanville). This had two main strengths; it could be electrically engaged and disengaged at the flick of a switch and there is no need to use the clutch pedal at any stage of the procedure.

This makes it very useful when extra power is required for acceleration. Furthermore, it can also be used in conjunction with other gear ratios in the main gearbox. This type of overdrive can normally be used on both third and fourth gears; the third gear overdrive giving a ratio between direct top and third gears which is useful for hill climbing when direct third can be too low and direct top too high.

An overdrive unit fitted to a four-speed gearbox and operating on both third and top gears will, therefore, give the equivalent of a six-speed gearbox. Apart from giving a better fuel consumption this can also endow the car with greater flexibility. The Laycock overdrive was rarely used on gears lower than third because the torque transmitted in first or second gear could easily burn out the cone clutch bearings. There have, however, been exceptions to this general rule, notably the Triumph TR range of sports cars. The main disadvantage of this type of overdrive, however, was the high initial cost of the unit. It used epicyclic gears which were expensive to manufacture and the sophisticated control mechanisms add still further to the price. The fifth gear design, however, had the significant advantage of being relatively cheap to manufacture, as it operates in precisely the same way as all the other gears. On the debit side, though, is the fact that it has to be operated by depressing the clutch and moving the main gear lever. Moreover, this type of design can only produce a fifth speed because of the limited space inside the gearbox casing whereas the other mechanism, as explained above, can provide at least six ratios.

Bolt On Overdrive



The "bolt-on" overdrive uses a single train of epicyclic or "sun and planet" gears to provide the step-up in speed between the power input from the gearbox and the power output to the propeller shaft. An epicyclic gear train consists of three elements, the sun gear, the planet gears mounted on a planet carrier and the annulus gear. Various gear ratios can be obtained from such a gear train by driving one element, holding another element and taking the output from the third. To take the simplest case, if the planet carrier is held still and the sun gear is driven clockwise, then the planet gears will rotate anti-clockwise on their own axes and drive the annulus gear in an anti-clockwise direction.

In this case, because the annulus has more teeth on it than the sun gear, it will rotate at a slower speed and in the opposite direction. Therefore, by using the sun gear as the input and the annulus as the output a geared reduction or "step-down" in the reverse direction is achieved. This particular arrangement is obviously of no use as an overdrive gear. However, if the sun gear is then held stationary and the planet carrier is driven round it in a clockwise direction it is clear that the planet gears will also rotate clockwise, driving the annulus in the same direction and at a speed faster than the planet carrier. This arrangement will give a step-up in speed between the input - the planet carrier - and the output - the annulus - and this is how an overdrive ratio is achieved.

The Variomatic automatic transmission used by DAF did, in fact, produce an overdrive ratio towards the top of its speed range. However, the overdrive ratio obtained is so slight that it couldn't, for practical purposes, be considered as a true overdrive. This transmission had been adopted by Volvo for use in the 343 model and is known now as the Continuously Variable Transmission (CVT).

Also see: How It Works - The Overdrive | The Borg-Warner Overdrive | The Laycock Overdrive
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