 An overdrive is sometimes a separate unit that fits into the back of a gearbox, as with this Fairey unit. A plate warns to only engage the unit in third and fourth gears. Overdrive can refer to two different things. An overdrive is a device which was commonly used on rear-wheel drive automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. Image File history File links No higher resolution available. ...
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The Fairey Aviation Company, Ltd was a British aircraft manufacturer of the first half of the 20th century based in Heyes (Middlesex) and Heaton Chapel/Ringway in Greater Manchester. ...
Car redirects here. ...
Gears on a piece of farm equipment, gear ratio 1:1. ...
Fuel efficiency sometimes means the same as thermal efficiency, that is, the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. ...
Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque (or often called a moment) can informally be thought of as rotational force or angular force which causes a change in rotational motion. ...
It also refers to a combination of gearing inside a transmission or transaxle which results in the output speed being greater than the input speed. In the latter case, "overdrive" does not refer to a physically separate identifiable part/assembly. In mechanics, a transmission or gearbox is the system of gears and/or the hydraulic system (called variously hydrodynamic, fluid or automatic transmission) that transmits mechanical power from a prime mover—such as an engine or electric motor—to a typically rotary output device at a lower angular momentum but...
A transaxle, in the automotive field, is a component that combines the functionality of the transmission, the differential and the drive axle into one integrated assembly. ...
History
Early manual automotive transmissions were limited to three or sometimes four speeds, with only the most sophisticated being five speeds. This left an unfulfilled need for a higher gear ratio for highway cruising, which was filled by the addition of separate overdrive units. However, automotive manual transmissions manufactured since the 1980s tend to include a larger selection of gear ratios than before, the highest of which is usually greater than 1:1. This trend has rendered overdrives a complex and obsolete solution to economy gearing in automobiles, and very few cars are fitted with them today, although they still appear on large trucks, where more gear ratios are always in demand. In mechanics, a transmission or gearbox is the system of gears and/or the hydraulic system (called variously hydrodynamic, fluid or automatic transmission) that transmits mechanical power from a prime mover—such as an engine or electric motor—to a typically rotary output device at a lower angular momentum but...
The 1980s refers to the years of and between 1980 and 1989. ...
Spur gears found on a piece of farm equipment A gear wheel is a wheel with teeth around its circumference, the purpose of the teeth being to mesh with similar teeth on another mechanical device -- possibly another gear wheel -- so that force can be transmitted between the two devices in...
A ratio is a quantity that denotes the proportional amount or magnitude of one quantity relative to another. ...
Usage Generally speaking, overdrive (OD) is the highest gear in the transmission. Most automatic transmissions have three speeds and overdrive (fourth speed). Overdrive allows the engine to operate at a lower rpm for a given road speed. This allows the vehicle to realize better fuel efficiency, and often quieter operation on the highway. When you switch it on, you allow the transmission to shift into overdrive mode after the certain speed is reached (usually 70+ km/h ~ 45+ mph) depending on the load). When it is off, the transmission shifting is limited to the lower gears. For normal driving conditions, operation of the overdrive should be enabled only if the average speed is above 70 km/h. An automatic transmission is an automobile gearbox that can change gear ratios automatically as the vehicle moves, thus freeing the driver from having to shift gears manually. ...
Miles per hour is a unit of speed, expressing the number of international miles covered per hour. ...
It may be necessary to switch it off if the vehicle is being operated in a mountainous area.
The automatic transmission automatically shifts from OD to third gear when more load is present. When less load is present, it shifts back to OD. Under certain conditions, e.g.: driving uphill or towing a trailer, the transmission may "hunt" between OD and the third gear, shifting back and forth. In this case, switching it off can help the transmission to 'decide'. It may also be advantageous to switch it off if engine braking is desired, for example, driving downhill. The vehicle's owner's manual has information and procedures regarding such situations.. It has been suggested that Exhaust brake be merged into this article or section. ...
How an overdrive unit works The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit. It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. A number of such transmissions and transaxles are manufactured by Aisin, for use in vehicles produced by many different manufacturers. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load. Epicyclic gearing is used here to increase output speed. ...
In mechanics, a transmission or gearbox is the system of gears and/or the hydraulic system (called variously hydrodynamic, fluid or automatic transmission) that transmits mechanical power from a prime mover—such as an engine or electric motor—to a typically rotary output device at a lower angular momentum but...
Cardan driveshaft with universal joints A driveshaft or driving shaft or Cardan shaft is a mechanical device for transferring power from the engine or motor to the point where useful work is applied. ...
A propeller shaft connects a propeller to an engine. ...
For other uses, see Clutch (disambiguation). ...
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Overdrive in Europe The vast majority of overdrives in European cars were manufactured by an English company called Laycock de Normanville (later GKN Laycock), which is now defunct, though an uprated hp version of this product is produced in the U.S. by Gear Vendors, and remanufactured in the UK by an ex Laycock de Normanville employee trading as Overdrive Repair Services. De Normanville overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar, Rootes and Volvo to name but a few. Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production in America today. This article is 150 kilobytes or more in size. ...
Motto 2(French) God and my right Anthem God Save the Queen 3 United Kingdom() – on the European continent() – in the European Union() [] Capital London Largest conurbation (population) Greater London Urban Area Official languages English4 Government and Parliamentary democracy - Monarch Elizabeth II - Prime Minister Tony Blair Formation - Acts of Union...
Ford Motor Company is an American multinational corporation and the worlds third largest automaker after Toyota and General Motors, based on worldwide vehicle sales. ...
The British Leyland Motor Corporation (often abbreviated to simply BL), was a Britain in 1968. ...
Jaguar Cars Limited is a British luxury car manufacturer, with headquarters in Browns Lane, Coventry, England. ...
The Rootes Group is a now-defunct British automobile manufacturer. ...
Volvo Cars, or Volvo Personvagnar, is a swedish automobile maker founded in 1927 in the city of Gothenburg in Sweden. ...
The Fairey Aviation Company, Ltd was a British aircraft manufacturer of the first half of the 20th century, notable for a number of important planes, including the Fairey III family and the Fairey Swordfish. ...
Land Rover was the name of one of the first British civilian all-terrain utility vehicles, first produced by Rover in 1947. ...
Fuel economy and drivetrain wear Using overdrive gearing, the car's engine RPM goes down. This reduces engine wear and normally saves fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. However, all engines have a range of peak efficiency; it is possible for the use of overdrive to keep the engine out of this range, thus cutting into any fuel savings from the lower engine speed.
There is some debate on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most will agree that within the transmission, this effect is minimal. The other difficulty can be in the drive shaft rotation speed.
Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is high ratio and an overdrive is used to compensate. This may create unpleasant vibrations at high speeds and possible destruction of the driveshaft due to the centrifugal forces or uneven balance.
The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and contains no internal bracing.
The higher speeds on the driveshaft and related parts can cause heat and wear problems if an overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction. This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing.
The impetus is then to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that automobiles have been coming with more gears in their transmissions. Also, it is why more than one overdrive gear is seldom seen in a vehicle except in special circumstances.
See also Freewheel mechanism In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. ...
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