Overdrive is a term used to describe the operation of an automobile cruising at sustained speed with reduced engine revolutions per minute (RPM), leading to better fuel consumption, 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 over-geared, and cannot reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning at higher RPM. 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 lower than the absolute maximum RPM to which the engine is limited, the "redline" RPM. A car's speed is limited by the power required to drive it against air resistance, which increases with speed. At the maximum possible speed, the engine is running at its point of maximum power, or power peak, and the car is traveling at the speed where air resistance equals that maximum power. There is therefore one specific gear ratio at which the car can achieve its maximum speed: the one that matches that engine speed with that travel speed. At travel speeds below this maximum, there is a range of gear ratios that can match engine power to air resistance, and the most fuel efficient is the one that results in the lowest engine speed. Therefore, a car needs one gearing to reach maximum speed but another to reach maximum fuel efficiency at a lower 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 engine. 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 optional 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 (1:1) fourth gear 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.
2 Usage 3 How an overdrive unit works 4 In Europe 5 In North America 6 Fuel economy and drivetrain wear 7 References 8 External links
The power needed to propel a car at any given set of conditions and
speed is straightforward to calculate, based primarily on the total
weight and the vehicle's speed. These produce two primary forces
slowing the car: rolling resistance and air drag. The former varies
roughly with the speed of the vehicle, while the latter varies with
the square of the speed. Calculating these from first principles is
generally difficult due to a variety of real-world factors, so this is
often measured directly in wind tunnels and similar systems.
The power produced by an engine increases with the engine's RPM to a
maximum, then falls away. This is known as the point of maximum power.
Given a curve describing the overall drag on the vehicle, it is simple
to find the speed at which the total drag forces are the same as the
maximum power of the engine. This defines the maximum speed the
vehicle is able to reach. The rotational speed of the wheels for that
given forward speed is simple to calculate, it is simply the tyre
circumference multiplied by the RPM.[N 1] As the tire RPM at maximum
speed is not the same as the engine RPM at that power, a transmission
is used with a gear ratio to convert one to the other.[N 2]
At even slightly lower speeds than maximum, the total drag on the
vehicle is considerably less, and the engine needs to deliver this
greatly reduced amount of power. In this case the RPM of the engine
has changed significantly while the RPM of the wheels has changed very
little. Clearly this condition calls for a different gear ratio. If
one is not supplied, the engine is forced to run at a higher RPM than
optimal. As the engine requires more power to overcome internal
friction at higher RPM, this means more fuel is used simply to keep
the engine running at this speed. Every cycle of the engine leads to
wear, so keeping the engine at higher RPM is also unfavorable for
engine life. Additionally, the sound of an engine is strongly related
to the RPM, so running at lower RPM is generally quieter.
If one runs the same RPM transmission exercise outlined above for
maximum speed, but instead sets the "maximum speed" to that of highway
cruising, the output is a higher gear ratio that provides ideal fuel
mileage. In an era when cars were not able to travel very fast, the
maximum power point might be near enough to the desired speed that
additional gears were not needed. But as more powerful cars appeared,
especially during the 1960s, this disparity between the maximum power
point and desired speed grew considerably. This meant that cars were
often operating far from their most efficient point. As the desire for
better fuel economy grew, especially after the 1973 oil crisis, the
need for a "cruising gear" became more pressing.
Dashboard indicator for overdrive (automatic vehicle, manufactured 2000)
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 next highest 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 having an 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
Overdrive button on the gear stick of an automatic vehicle manufactured in 2000.
An 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. 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.
The vast majority of overdrives in European cars were invented and
developed by a man called de Normanville and manufactured by an
English company called
Laycock Engineering (later GKN Laycock), at its
Little London Road site in Sheffield. The system was devised by
Captain Edgar J de Normanville (1882–1968), and made by Laycock
through a chance meeting with a Laycock Products Engineer. De
Normanville overdrives were found in vehicles manufactured by
Standard-Triumph, who were first, followed by Ford, BMC and British
^ For instance, a 15-inch wheel with 215/65 tyres has a diameter of about 26 inches, or a circumference of about 82 inches. At 100 mph, or 1760 inches per second, the wheel will be turning 21.5 times per second, or just under 1,300 RPM. ^ Using the example above, at 100 mph the engine might need to be turning 5,000 RPM to generate the required power to turn the tyres at 1,300 RPM. A transmission with a gear ratio of 4:1 would be appropriate in this case. ^ 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".
^ a b c d e f g h i j k Setright, L. J. K. (1976). "Overdrive". In Ian Ward. Anatomy of the Motor Car. Orbis. pp. 93–95. ISBN 0 85613 230 6. ^ "Obituary: E,J.de Normanville". Motor. 27 January 1968. p. 112. ^ "gearvendors.com". gearvendors.com. Retrieved 2011-09-17.
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"How Automatic Transmissions Work – Overdrive" (with a Flash interactiv