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Encyclopedia > Four wheel drive
The Jeep Wrangler is a 4WD vehicle with a transfer case to select low range or high range 4WD.
The Jeep Wrangler is a 4WD vehicle with a transfer case to select low range or high range 4WD.
The Lamborghini Murciélago is a 4WD/AWD that powers the front via a VCU if the rear slips.
The AWD VW Golf is a 4WD/AWD that powers the rear via a multi-plate clutch if the front slips.
The AWD VW Golf is a 4WD/AWD that powers the rear via a multi-plate clutch if the front slips.
The Mercedes-Benz M-Class is a 4WD/AWD that powers all wheels evenly (continuously) via a plain differential and uses traction control to recover from wheel spin.
The Mercedes-Benz M-Class is a 4WD/AWD that powers all wheels evenly (continuously) via a plain differential and uses traction control to recover from wheel spin.
The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a plain (but manually lockable) center differential, oddly with Torsen differentials for both front and rear.
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The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a plain (but manually lockable) center differential, oddly with Torsen differentials for both front and rear.
A Subaru Impreza rally car uses 4WD for traction on loose dirt.
A Subaru Impreza rally car uses 4WD for traction on loose dirt.

Four wheel drive, 4WD, 4x4 ("four by four"), all wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people think exclusively of off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads. Jeep Wrangler TJ File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Jeep Wrangler TJ File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Wikimedia Commons has media related to: Jeep Wrangler The Wrangler (also known as the YJ and TJ, as explained below) is a small ORV from Jeep. ... Image File history File links Download high resolution version (2592x1944, 2115 KB) Please see the file description page for further information. ... Image File history File links Download high resolution version (2592x1944, 2115 KB) Please see the file description page for further information. ... Lamborghini Murciélago Roadster Lamborghini Murciélago The Lamborghini Murciélago is a GT and supercar automobile made by Lamborghini and designed by Luc Donckerwolke. ... 2003 US model Volkswagen 20th Anniversary edition of the MKIV GTI File links The following pages link to this file: VW Golf Categories: GFDL images ... 2003 US model Volkswagen 20th Anniversary edition of the MKIV GTI File links The following pages link to this file: VW Golf Categories: GFDL images ... VW Golf Mk. ... Image File history File links Download high resolution version (946x546, 50 KB) This is a copyrighted image that has been released by a company or organization to promote their work in the media. ... Image File history File links Download high resolution version (946x546, 50 KB) This is a copyrighted image that has been released by a company or organization to promote their work in the media. ... 2002 Mercedes-Benz ML55 AMG Mercedes-Benz M-Class is a 5-door SUV with light off-road capability, first offered in 1997, and built in Tuscaloosa County, Alabama. ... In this differential, input torque is applied to the ring gear (blue). ... Traction control and Vehicle Stability Control systems, on current production vehicles, are typically (but not necessarily) electro-hydraulic systems designed to prevent loss of control when excessive throttle or steering is applied by the driver. ... Download high resolution version (928x512, 104 KB) Wikipedia does not have an article with this exact name. ... Download high resolution version (928x512, 104 KB) Wikipedia does not have an article with this exact name. ... This article refers to the Military HMMWV, not the civilian Hummer sold by General Motors General Characteristics (Humvee) Manufacturer: AM General Length: 4. ... The Torsen limited slip differential was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. ... Image of a subaru WRX at the Rally of Victoria, October 2003. ... Image of a subaru WRX at the Rally of Victoria, October 2003. ... 1997 Subaru Impreza Outback Sport 2005 Subaru Impreza Sport Wagon 2005 Subaru Impreza GX Sport UK in WR Blue For the high-performance versions of the Impreza, see Subaru Impreza WRX and Subaru Impreza WRX STi The Subaru Impreza is a compact car that was first introduced by Subaru in... Rallying (international) or rally racing (US) is a form of automobile racing that takes place on normal roads with modified production or specially built road cars. ... A driving wheel on a steam locomotive. ... An engine is something that produces some effect from a given input. ... The term off-road refers to a driving surface which is not conventionally paved. ... Rallying (international) or rally racing (US) is a form of automobile racing that takes place on public roads with modified production or specially built road cars. ...


Four wheel drive (4WD or 4x4 for short) was the original term, often used to describe truck-like vehicles that required the driver to manually switch between a two wheel drive mode for streets and a four wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all wheel drive" term (AWD for short) was invented to distinguish vehicles that are capable of driving all four wheels on normal roads without causing poor control and excessive tire and drivetrain wear. The AWD term is now being used to market vehicles which do not continuously drive all four wheels, but instead switch from two wheel drive to four wheel drive automatically as needed. The terms are thus quite vague in modern usage, with AWD being used to describe vehicles with a wide variety of very different drivetrains. The driver of this DAF tractor with an auto-transport semi-trailer prepares to offload Skoda Octavia cars in Cardiff, Wales For further uses of the word truck, see Truck (disambiguation). ...


The buyer must be wary. It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, both Quattro and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential. Quattro is a permanent all wheel drive system used on Audi brand automobiles. ... Quattro is a permanent all wheel drive system used on Audi brand automobiles. ... The Torsen limited slip differential was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. ...

Contents


Design

When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.


A differential allows one input shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requies a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles. In this differential, input torque is applied to the ring gear (blue). ... In physics, torque can be thought of informally as rotational force. Torque is measured in units of newton metres, and its symbol is Ï„. The concept of Torque, also called moment or couple, originated with the work of Archimedes on levers. ... Angular velocity describes the speed of rotation. ...


Such a design would handle very well. It distributes power evenly and smoothly, making it unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be... making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simplistic design works acceptably well for a 2WD vehicle. Since a 4WD is twice as likely to have a driven wheel on an icy patch, the simplistic design is usually considered unacceptable.


Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake can be automatically applied to that wheel. The torque will then be matched, causing power to be divided between the pavement (for the non-slipping wheel) and the brake. This is effective, though it does cause brake wear and a sudden jolt that can make handling less predictable. By extending traction control to act on all four wheels, the simple 4WD vehicle design based on three differentials can now recover from wheel spin. One nice feature of this design, is that it is traction control, and thus will not work against traction control. This design is commonly seen on luxury crossover SUVs. Traction control and Vehicle Stability Control systems, on current production vehicles, are typically (but not necessarily) electro-hydraulic systems designed to prevent loss of control when excessive throttle or steering is applied by the driver. ... A brake is a device for slowing or stopping the motion of a machine, and to keep it from starting to move. ... A crossover SUV or XUV is an automobile with a sport utility vehicle appearance but is built upon a more economical and fuel-efficient car-based platform. ...


Another way to solve the problem is to temporarily lock together the differential's output shafts, usually just for the center differential that distributes power between front and rear. Recall that a drivetrain without differentials will fight the driver, causing tire wear and handling problems. This is of little concern when the wheels are already slipping. One very common design joins the output shafts together via a multi-plate clutch under computer control. This design causes a small jolt when it activates, which can disturb the driver or cause more wheels to lose traction. Another common design uses a viscous coupling unit. A dilatant fluid inside the viscous coupling unit acts like a solid when under shear stress caused by high shaft speed differences, causing the two shafts to become connected. This design suffers from fluid degradation with age and exponential locking (joining) behavior. It can also waste fuel, because it requires that there be a slight shaft speed difference under normal driving conditions (via gearing) to prepare to fluid for operation. Older designs used manually operated locking devices. A clutch is a subcomponent of an engines transmission designed to allow engagement or disengagement of the engine to the gearbox or whatever apparatus is being driven. ... Viscous coupling is an application used in all-wheel drive systems. ... A dilatant material is one in which viscosity increases with the rate of shear (also termed Shear thickening). ... The term exponential may refer to any of several topics in mathematics: Exponential distribution Exponential function Exponential growth, exponential decay Exponential time Matrix exponential Exponential map (in differential geometry) All relate in some fashion to exponents. ...


Yet another way to solve the problem is via a Torsen differential. When a normal differential is replaced with a Torsen differential, it is possible to drive the output shafts with different amounts of torque. While this is useless in a zero-torque situation, it will help greatly when the slippage is not so extreme. As the slipping side begins to spin out of control, more power is delivered to the other side. A typical Torsen differential can deliver up to twice as much power to the non-slipping side as it delivers to the slipping side. Most Audi Quattro cars, notably excluding the A3 and TT, use a center Torsen differential. For a time, the Volkswagen Passat 4motion shared this design. The HMMWV uses front and rear Torsen differentials, but only has a normal differential in the center. Torsen differentials generally work very well, though they are expensive and heavy. The Torsen limited slip differential was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. ... Audi is an automobile maker in Germany, and a wholly-owned subsidiary of the Volkswagen Group. ... Quattro is a permanent all wheel drive system used on Audi brand automobiles. ... The A3 is a compact luxury car built by Audi since 1996. ... Audi TT coupe. ... Volkswagen, [literally: peoples car] (also known as VW) is an automobile manufacturer based in Wolfsburg, Germany. ... The passat is a type of Wind. ... Quattro is a permanent all wheel drive system used on Audi brand automobiles. ... This article refers to the Military HMMWV, not the civilian Hummer sold by General Motors General Characteristics (Humvee) Manufacturer: AM General Length: 4. ...


Many lower-cost vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and real axles. Such systems distribute power unevenly under normal conditions, and thus do not help prevent loss of traction; they only enable recovery once traction has been lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95%/5% or 90%/10% power split. Light trucks and SUVs tend to use multi-plate clutches under computer control, often with 100% of the power going to the rear axle under normal conditions. Sports cars using this type of system always drive only the rear under normal conditions. For example, Lamborghini uses a viscous coupling unit to drive the front, and the Nissan Skyline GT-R uses a clutch. The Audi TT normally powers the front, and has a multi-plate clutch to power the rear. Lamborghini logo 2004 Lamborghini Gallardo 2004 Lamborghini Murciélago Roadster Automobili Lamborghini S.p. ... Nissan Motor Co. ... Nismo (Nissan) Skyline GT-R 400R The Nissan Skyline GT-R is an iconic Japanese sports coupe in the Nissan Skyline range. ... Audi TT coupe. ...


History

The first ever four wheel drive car (as well as hill-climb racer), the so-called Spyker 60 HP, was built in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. Designs for four wheel drive in the US, came from the Twyford company of Brookville, PA in 1905. The first US four wheel drive vehicle was built in 1911 by the Four Wheel Drive auto company (FWD) of Wisconsin. FWD would later produce over 20,000 of its four wheel drive Model B trucks for the British and American armies during World War I. Bugatti created a total of three four wheel drive racers, the Type 53, in 1932, but the cars were legendary for having poor handling. It was not until "go-anywhere" vehicles were needed for the military that four wheel drive found its place. The Jeep, originally developed by American Bantam but mass-produced by Willys and Ford, became the best-known four wheel drive vehicle in the world during World War II. Willys (since 1950 owner of the Jeep name) introduced the CJ-2A in 1945 as the first full-production four wheel drive passenger vehicle. 1903 (MCMIII) was a common year starting on Thursday (see link for calendar). ... Amsterdam Location Country The Netherlands Province North Holland Population 742,951(1 January 2005) Coordinates 52°22′N 4°54′E Website www. ... 1911 (MCMXI) was a common year starting on Sunday (click on link for calendar). ... State nickname: Badger State Official languages None Capital Madison Largest city Milwaukee Governor Jim Doyle (D) Senators Herb Kohl (D) Russ Feingold (D) Area  - Total  - % water Ranked 23rd 169,790 km² 17 Population  - Total (2000)  - Density Ranked 18th 5,453,896 38. ... Clockwise from top: Trenches in frontline, a British Mark I Tank crossing a trench, the Royal Navy battleship HMS Irresistible sinking after striking a mine at the battle of the Dardanelles, a Vickers machine gun crew with gas masks and a Sopwith Camel biplane. ... Bugatti is one of the most celebrated marques of automobile and the one of the most exclusive Italian car producers of all time. ... The 5. ... 1932 (MCMXXXII) is a leap year starting on Friday. ... Jeep is an automobile marque (and registered trademark) of DaimlerChrysler. ... American Bantam was the name given cars produced in Butler, Pennsylvania following the failure of the American Austin Car Company. ... Combatants Allied Powers Axis Powers Commanders {{{commander1}}} {{{commander2}}} Strength {{{strength1}}} {{{strength2}}} Casualties 17 million military deaths 7 million military deaths World War II, also known as the Second World War, was a mid-20th century conflict that engulfed much of the globe and is accepted as the largest and deadliest... 1920 Willys-Knight advertisement Willys (pronounced WILL-iss) was the brand name used by the United States automobile company Willys-Overland Motors, best known for its production of military and civilian Jeeps, during the last century. ... The Jeep CJ (or Civilian Jeep) was a commercial version of the famous Military Jeep from World War II. The first CJ (the CJ-2) was introduced in 1944 by Willys, and the same basic vehicle stayed in production through 7 variants and 3 corporate parents until 1986. ... 1945 (MCMXLV) was a common year starting on Monday (link will take you to calendar). ...


However, it was not until Jensen applied the Formula Ferguson four wheel drive system to their 1966 Jensen FF that the system was used in a production sports car, but with a total of 320 build units this did not sell in appreciable numbers. The first manufacturer to develop four-wheel drive for road-going cars was Subaru, who introduced the mass-produced 4WD Leone in 1972. This model eventually became the best-selling 4WD car in the world. Audi introduced the first permanently all-wheel driven high volume road-going car, the Audi quattro, in 1980. Audi's chassis engineer, Jörg Bensinger, had noticed in winter tests in Scandinavia that a vehicle used by the German Army, the Volkswagen Iltis, could beat any high performance Audi. He proposed developing a four-wheel drive car, soon used for rallying to improve Audi's conservative image. This feature was also extended to Audi's production cars and is still available nowadays. 1959 Jensen 541R This page is about car manufacturer, for other uses see Jensen (disambiguation) Jensen Motors was a British manufacturer of sports cars based in West Bromwich (in the West Midlands west of Birmingham). ... 1966 (MCMLXVI) was a common year starting on Saturday (link goes to calendar) // Events January January 1 - In a coup, Colonel Jean-Bédel Bokassa ousts president David Dacko and takes over the Central African Republic. ... The Jensen FF (Ferguson Formula) was an all wheel drive GT car produced by the British manufacturer Jensen between 1966 and 1971. ... Subaru (In katakana: スバル), a Japanese car company, is a subsidiary of Fuji Heavy Industries. ... Audi is an automobile maker in Germany, and a wholly-owned subsidiary of the Volkswagen Group. ... Volkswagen, [literally: peoples car] (also known as VW) is an automobile manufacturer based in Wolfsburg, Germany. ... The Iltis is a military vehicle made by Volkswagen, and then Bombardier. ...


Some of the earliest mid-engined four wheel drive cars were the various road-legal rally cars made for Group B homologation, such as the Ford RS200 made from 1984-1986. In 1989 niche maker Panther Westwinds created a mid-engined four wheel drive, the Panther Solo 2. Today, sophisticated all wheel drive systems are found in many passenger vehicles and most exotic sports cars and supercars. In Automobile design, an MR or Mid-engine, Rear wheel drive layout drives the rear wheels with an engine placed just in front of them, behind the passenger compartment. ... Group B Ford RS200 In relation to motorsport governed by the FIA, Group A and Group B referred to two sets of regulations for competition vehicles in touring car and rally racing. ... An RS200 seen in California in 2004. ... 1984 (MCMLXXXIV) is a leap year starting on Sunday of the Gregorian calendar. ... 1986 (MCMLXXXVI) is a common year starting on Wednesday of the Gregorian calendar. ... 1989 (MCMLXXXIX) is a common year starting on Sunday of the Gregorian calendar. ... Panther Westwinds (commonly known as Panther) was a specialty automobile manufacturer in Surrey, United Kingdom. ... In Automobile design, an MR or Mid-engine, Rear wheel drive layout drives the rear wheels with an engine placed just in front of them, behind the passenger compartment. ... The Solo was a mid-engined sports car from the British Panther Westwinds company. ... Honda NSX sports car A sports car is a type of automobile designed for sporting performance. ... A supercar is a term used for a sports car, typically an exotic or rare one, whose performance is highly superlative to its contemporary sports cars. ...


Terminology

Although in the strictest sense, the term "four wheel drive" refers to a capability that a vehicle may have, it is also used to denote the entire vehicle itself. In Australia, vehicles with offroad capabilities are referred to as "four wheel drives". This term is sometimes also used in North America, somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles. World map showing North America A satellite composite image of North America North America is a continent in the northern hemisphere bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the south by the Caribbean Sea, and on the west by the... A sport utility vehicle (SUV) or off-roader is a vehicle that combines the load-hauling and passenger-carrying capacity of a large station wagon or minivan with features designed for off-road driving. ...


The term 4x4 (read either four by four or full times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive. The term 4x4 is common in North America and is generally used when marketing a new or used vehicle, and is sometimes applied as badging on a vehicle equipped with four wheel drive. Similarly, a 4x2 would be appropriate for most two-wheel-drive vehicles, although this is rarely used in practice, as vehicles are assumed to be two-wheel-drive unless stated otherwise. A 2×4, however, is unambiguously a piece of lumber. Dimensional lumber is a term used in North America for lumber that is cut to standardized width and depth specified in inches. ... Lumber is the name used, generally in North America, for wood that has been cut into boards or other shapes for the purpose of woodworking or construction. ...


Large American trucks with dual tires on the rear axles (also called duallys or duallies) and two driven axles are officially badged as 4x4s, despite having six driven wheels because the 'dual' wheels behave as a single wheel for traction purposes and are not individually powered. True 6x6 vehicles with three powered axles such as the famous "Deuce and a Half" truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6x6. Six wheel drive, 6WD, and 6x6 are terms used to describe a six-wheeled vehicle with a drivetrain that allows all six wheels to receive power from the engine simultaneously. ... The Army is the branch of the United States armed forces which has primary responsibility for land-based military operations. ...


Another related term is 4-wheeler (or four wheeler). This generally refers to all-terrain vehicles with four wheels and does not indicate the number of driven wheels; a "four wheeler" may have two or four wheel drive. A group of “quad bike” all terrain vehicles The term all-terrain vehicle is used to describe a number of small open motorised buggies and tricycles designed for off-road use. ...


Four wheel drives in Australia

There are two main players in the Australian market: Toyota and Nissan. The typically more massive American four wheel drive trucks and SUVs are generally not as popular among Australian consumers because they are not well suited to the Australian outback. They are often not rugged enough for the harsh conditions, and with their typically larger size they are too wide to fit on the existing wheel tracks created by previous cars (so the driver ends up attempting to carve out his or her own track). As in other countries, four wheel drives have become popular with city-dwelling people, who by and large will never actually drive "off road". This is commonly referred to as driving a Toorak Tractor. Toyota redirects here. ... Nissan Motor Co. ... A sport utility vehicle (SUV) or off-roader is a vehicle that combines the load-hauling and passenger-carrying capacity of a large station wagon or minivan with features designed for off-road driving. ... For the restaurant chain, see Outback Steakhouse; for the station wagon, see Subaru Outback. ... In Australian slang (particularly in Victoria), a Toorak tractor is a pejorative name for a SUV. The reference to the affluent Melbourne suburb of Toorak alludes to the ownership and use of such vehicles as status symbols, rather than for their designed off-road abilities. ...


Unusual Four wheel drive systems

Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, the French motor manufacturer Citroën developed the 2CV Sahara. Unlike other 4x4 vehicles which use a conventional transfer case to drive two axles, the Sahara had a modified front axle, engine and gearbox fitted to the rear of the vehicle, facing backwards. The throttles, clutches and gearchange mechanisms could be linked, so both 12bhp engines could run together, or they could be split and the car driven on either engine. Combined with twin fuel tanks and twin batteries (which could be set up to run either or both engines), the redundancy of two separate drive trains meant that they could make it back to civilization even after major mechanical failures. Only around 700 of these cars were built, and there are no clear records of how many still exist. Enthusiasts have built their own "new" Saharas, by rebuilding a 2CV and fitting the modified engine, gearbox and axle onto a new, strengthened chassis. Citroën is a French automobile manufacturer, started in 1919 by André Citroën, today part of PSA Peugeot Citroën. ... (Redirected from 2CV) 1960 2CV. Notice suicide doors The 2CV (deux chevaux - French, literally two horses, from the tax power rating) was a popular French car made by Citroën. ...


BMC experimented with a twin-engined Mini Moke in the mid-1960s, but never put it into production. The British Motor Corporation (BMC) was a car company, formed by the merger of the Austin and Morris companies in 1952. ... Mini Mokes packed up for export. ...


References

See also


  Results from FactBites:
 
cars - Four wheel drive (1535 words)
Four wheel drive or 4x4, is a type of four wheeled vehicle drivetrain configuration that enables all four wheels to receive power from the engine simultaneously in order to provide maximum traction.
When a part-time four wheel drive system is enabled, the transfer case connects both the front and rear drive shafts with the transmission.
The sum of the output angular velocity of the two output shafts (connected to the wheels) is linearly related to the angular velocity of the input shaft (connected to the transfer case).
Four Wheel Drive - Wikicars (2820 words)
Four-wheel drive, 4WD, 4x4 ("four by four"), all wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously.
Four-wheel drive (4WD or 4x4 for short) was the original term, often used to describe truck-like vehicles that required the driver to manually switch between a two wheel drive mode for streets and a four-wheel drive mode for low traction conditions such as ice, mud, or loose gravel.
The term 4x4 (read either four by four or four times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive.
  More results at FactBites »

 
 

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