Overdrive (OD) is a term used to describe a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise and lower wear. Use of the term is confused, as it is applied to several different, but related, meanings.
The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly. The purpose of such a gear may not be immediately obvious. The power produced by an engine increases with the engine's RPM to a maximum, then falls away. The point of maximum power is somewhat slower than the absolute maximum RPM to which the engine is limited, the "redline". A car's speed is limited by the power available to drive it against air resistance—so the maximum possible speed is obtained at the engine's point of maximum power, or power peak, and the gear ratio necessary to achieve this will be the single ratio between these two speeds. As drag causes the power needed to increase with the cube of the velocity (v3), most cars will be capable of achieving a fast cruising speed less than their maximum, with far less power being required. This power is available well below the engine's power peak and so the ideal cruising gear is an overdrive gear, a ratio higher than that for absolute top speed.
With the early development of cars and the almost universal rear-wheel drive layout, the final drive (i.e. rear axle) ratio for fast cars was chosen to give the ratio for maximum speed. The gearbox was designed so that, for efficiency, the fastest ratio would be a "direct-drive" or "straight-through" 1:1 ratio, avoiding frictional losses in the gears. Achieving an overdriven ratio for cruising thus required a gearbox ratio even higher than this, i.e. the gearbox output shaft rotating faster than the original engine RPM. The propeller shaft linking gearbox and rear axle is thus overdriven, and a transmission capable of doing this became termed an "overdrive" transmission.
The device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever. These were often an optional extra on some models of the same car.
As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis, the use of 5-speed gearboxes became more common in mass-market cars. These had a direct fourth speed with an overdrive 5th gear, replacing the need for the separate overdrive gearbox.
With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle. There is no longer a propeller shaft and so one meaning of "overdrive" can no longer be applied. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all cars.
The transmission of a car reduces the rotational speed of the drivetrain from that appropriate to the engine to that suiting the wheels. It also allows this ratio to change when shifting gear, so that the best ratio is in force for the road speed, keeping the engine RPM roughly constant. At top speed, this ratio might be around 4:1 with far higher ratios in the lower gears.
In the widely-used rear-wheel drive layout, the transmission consisted of two parts: a "gearbox" that provided the additional reduction in the lower gears and also allowed gear shifting (manual or automatic) and also a "final drive" that provided a 4:1 (approximately) reduction ratio that was always in effect. The gearbox was usually mounted forwards, next to the engine, and the final drive was mounted on the rear axle where it also housed the bevel drive gears to change the direction of rotation and the differential. To allow a lightweight drive shaft to be used to connect these, its rotational speed was kept high, thus reducing the torque it had to carry and thus the strength and weight it required. For this reason, the final drive retained the last step of speed reduction and this was not performed by the gearbox.
The top ratio of a non-overdrive gearbox is 1:1 or "direct drive". This is chosen for efficiency, as it does not require any gears to transmit power and so reduces the power lost by them. This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient. As techniques of gear-cutting improved, the losses of a gear train reduced and it became practical to use an overdrive gearbox. This usually still offered the same direct ratio, but also had an additional ratio even higher, so that the output speed was faster than the input engine speed. Mechanically this had the same effect as installing a higher final drive ratio, but was simpler to provide.
In practice, the use of a separate overdrive gearbox was popular, mounted as an extension to the rear of the main gearbox. This was often an optional extra, or limited to the top models in a range, the standard gearbox being supplied without. This gearbox was also often electro-hydraulically activated by a simple switch on the dashboard. At a time when gear changing was generally slow and heavy, this easy shift for overtaking at high speed was popular just for its convenience.
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 (auxiliary or bolt-on), in order to simulate an extra gear. Today, however, automotive transmissions manufactured since the 1980s tend to include overdrive within the transmission. In the aftermarket, early non-overdrive vehicles and 3/4 ton or heavier diesel trucks (where fuel economy and more gears are always in demand) are the main use of bolt-on auxiliary overdrives.
With the use of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle. There is no longer a drive shaft between them and so the notion of "direct drive" ratios is simply inapplicable. Although "overdrive" is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive.
Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [40-45 mph or more] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer.) With a manual transmission, overdrive should usually be selected when the average speed is above 70 km/h (40-45 mph).
The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example 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 when driving downhill. The vehicle's owner's manual will often contain information and suitable procedures regarding such situations, for each given vehicle.
Virtually all vehicles (cars and trucks) have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. Overdrive was widely used in European automobiles with manual transmission in the 60s and 70s to improve mileage and sport driving as a bolt-on option but it became increasingly more common for later transmissions to have this gear built in. If a vehicle is equipped with a bolt-on overdrive (e.g.: GKN or Gear Vendors) as opposed to a having overdrive built in one will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect "third-and-a-half" and a fifth gear. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.
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.
Overdrive in Europe
The vast majority of overdrives in European cars were manufactured by an English company called Laycock de Normanville (later GKN Laycock), at its Little London Road site in Sheffield, which is now demolished and remanufactured in the UK by an ex-Laycock de Normanville employee trading as Overdrive Spares. The system was devised by a Briton, named Captain Edgar J de Normanville (1884–1968), through a chance meeting with a Laycock Products Engineer. De Normanville overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar, Rootes Group and Volvo to name 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.
The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, this was fitted to many sports cars during the 1950s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC and Armstrong Siddeley. Later the A-type overdrive was to be fitted to the entire range of TR cars.
In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962-1967 MGBs (those with 3-synchro transmissions).
The J-type overdrive was introduced in the early 1970s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit vans.
The P-type overdrive marked the last updates and included both a Gear Vendors U.S. version and a Volvo version. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier. The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case.
Over a period of 40 years, Laycock Engineering manufactured over three and a half million overdrive Units, and over one million of these were fitted to Volvo motorcars.
The system features an oil pressure operated device attached to the back of the standard gearbox operating on the gearbox output shaft. Through a system of oil pressure, solenoids and pistons, the overdrive would drop the revs on whatever gears it was used on by 22%. For instance, the overdrive system applied to a Triumph TR5 operates on 2nd, 3rd and top gear. When engaged, the overdrive would drop the revs by approximately 450 RPM. The advantages this had on fuel consumption was quite marked over long distances.
Overdrive in North America
In the days before automatic transmissions were common, especially in the 1950s, many rear-wheel drive American cars were available with an overdrive option. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedal, the action was much like a semi-automatic. Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic. A knob connected to a bowden cable, similar to some emergency brake applications, was also provided to lock out the unit mechanically. Using overdrive with the main 3-speed transmission in 2nd gear was similar in ratio to 3rd gear, and with the main transmission in 3rd, the overall ratio was fractional (i.e., "true overdrive").
Such add-on overdrive boxes were available from the 1930s to the 1970s for cars and light trucks.
Today, most gas and diesel cars and trucks come with an overdrive transmission because of the benefit to fuel economy. Overdrive is included in both automatic and manual transmissions as an extra gear (or two in some cases).
Since 1979, add-on overdrives are provided by Gear Vendors, Inc., El Cajon CA. for most U.S. made vehicles from the 1920s through to current (2010) cars and light trucks.
Fuel economy and drivetrain wear
When using overdrive gearing, the car's engine speed drops, reducing wear and normally saving fuel. Since 1981 U.S. corporate average fuel economy (CAFE) legislation on virtually all domestic vehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if used at inappropriate speeds, thus cutting into any fuel savings from the lower engine speed.
There is some debate[by whom?] on the overall efficiency of overdrive gearing, as it requires more moving parts than direct 1:1 drive, but most[who?] 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 a 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 centripetal 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 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 modern automobiles tend to have larger numbers of gears in their transmissions. It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances i.e. where high (numerical) diff gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vehicles, often with a small number on the axle gear reduction, but usually only engage at speeds exceeding 100 km/h or about 60 mph.
- ^ a b c d e f g h i Setright, L. J. K. (1976). "Overdrive". In Ian Ward. Anatomy of the Motor Car. Orbis. pp. 93–95. ISBN 0 85613 230 6.
- ^ This ratio varies between cars, from around 3.5:1 to 5:1. American cars with large-slow-revving engines would use higher ratios, European compact cars with small high-revving engines were lower. Often the final drive ratio varied between models within a range, a "sports" model having a lower ratio.
- ^ Small Volkswagens of the 1980s, such as the Polo, were marketed to an environmentally-conscious market with an overdrive top ratio labelled on the gear shift as "E", variously described as "Efficiency", "Economy" or "Environment".
- ^ "ODspares.com". ODspares.com. 2011-09-11. http://www.odspares.com/. Retrieved 2011-09-17.
- ^ "Obituary: E,J.de Normanville". Motor: page 112. date 27 January 1968.
- ^ "gearvendors.com". gearvendors.com. http://gearvendors.com/. Retrieved 2011-09-17.
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