Four-wheel drive, also called 4×4 ("four by four") or 4WD refers to type of a vehicle, specifically one with its drivetrain capable of providing torque to all wheel ends of a two-axled vehicle simultaneously. It may be full-time, or on-demand, and is typically linked via a transfer case which provides an additional output drive-shaft, along with additional gear ranges.
When a four-wheeled vehicle has torque supplied to both axles, this is described as "all-wheel drive" (AWD). However, "four-wheel drive" typically refers to a set of specific components and functions, and/or intended offroad application, which generally complies with modern use of the terminology.
- 1 Definitions
- 2 Design
- 3 History
- 4 Uses
- 5 Terminology
- 6 Unusual systems
- 7 Introduction to off-roaders
- 8 Introduction to passenger cars
- 9 Systems by design type
- 10 See also
- 11 References
- 12 External links
4×4/4WD/AWD systems were developed in many different markets and used in many different vehicle platforms. There is no universally accepted set of terminology to describe the various architectures and functions. The terms used by various manufactures often reflect marketing rather than engineering considerations or significant technical differences between systems. SAE International's standard J1952 recommends only the term All-Wheel-Drive with additional sub classifications which cover all types of AWD/4WD/4x4 systems found on production vehicles.
Four-by-four (4×4) refers to the general class of vehicles. The first figure represents the total wheels (more precisely, axle ends), and the second, the number that are powered. Syntactically, 4×2 means a four-wheel vehicle that transmits engine torque to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive. Alternatively, a 6×4 vehicle has three axles, two of which provide torque to two wheel ends each.
Four wheel drive (4WD) refers to vehicles with two axles providing torque to four wheel ends. In the North American market the term generally refers to a system that is optimized for off-road driving conditions. The term "4WD" is typically designated for vehicles equipped with a transfercase which switches between 2WD and 4WD operating modes, either manually or automatically.
All wheel drive (AWD) historically was synonymous with "four-wheel drive" on four-wheeled vehicles, and six-wheel drive on 6×6s, and so on, being used in that fashion at least as early as the 1920s. Today in North America the term is applied to both heavy vehicles as well as light passenger vehicles. When referring to heavy vehicles the term is increasingly applied to mean "permanent multiple-wheel drive" on 2×2, 4×4, 6×6 or 8×8 drive train systems that include a differential between the front and rear drive shafts. This is often coupled with some sort of anti-slip technology, increasingly hydraulic-based, that allows differentials to spin at different speeds but still be capable of transferring torque from a wheel with poor traction to one with better. Typical AWD systems work well on all surfaces, but are not intended for more extreme off-road use. When used to describe AWD systems in light passenger vehicles, it refers to a system that applies torque to all four wheels (permanently or on demand) and/or is targeted at improving on-road traction and performance (particularly in inclement conditions), rather than for off-road applications.
Some all wheel drive electric vehicles solve this challenge using one motor for each axle, thereby eliminating a mechanical differential between the front and rear axles. An example of this is the dual motor variant of the Tesla Model S, which on a millisecond scale can control the torque distribution electronically between its two motors.
Individual-wheel drive (IWD) was coined to identify those electric vehicles whereby each wheel is driven by its own individual electric motor. This system essentially has inherent characteristics that would be generally attributed to four-wheel drive systems like the distribution of the available torque to the wheels. However, because of the inherent characteristics of electric motors, torque can be negative, as seen in the Rimac Concept One and SLS AMG Electric. This can have drastic effects, as in better handling in tight corners.
The term IWD can refer to a vehicle with any number of wheels. For example, the Mars rovers are 6-wheel IWD.
SAE Recommended Practices
Per the SAE International standard J1952 AWD is the preferred term for all the systems described above. The standard subdivides AWD systems into three categories.
Part-Time AWD systems require driver intervention to couple and decouple the secondary axle from the primarily driven axle and these systems do not have a center differential (or similar device). The definition notes that part-time systems may have a low range.
Full-Time AWD systems drive both front and rear axles at all times via a center (inter-axle) differential. The torque split of that differential may be fix or variable depending on the type of center differential. This system can be used on any surface at any speed. The definition does not address inclusion or exclusion of a low range gear.
On-Demand AWD systems drive the secondary axle via an active or passive coupling device or "by an independently powered drive system". The standard notes that in some cases the secondary drive system may also provide the primary vehicle propulsion. On-demand systems function primarily with only one powered axle until torque is required by the second axle. At that point either a passive or active coupling sends torque to the secondary axle.
In addition to the above primary classifications the J1952 standard notes secondary classifications resulting in a total of eight system designations:
- Part-Time Non Synchro System
- Part-Time Synchro System
- Full-Time Fixed Torque System
- Full-Time Variable Torque Passive System
- Full-Time Variable Torque Active System
- On-Demand Synchro Variable Torque Passive System
- On-Demand Synchro Variable Torque Active System
- On-Demand Independently Powered Variable Torque Active System
Two wheels fixed to the same axle turn at the same speed as a vehicle goes around curves. This either forces one to slip, if possible, to balance the apparent distance covered, or creates uncomfortable and mechanically stressful wheel hop. To prevent this the wheels are allowed to turn at different speeds using a mechanical or hydraulic differential. This allows one driveshaft to independently drive two output shafts, axles that go from the differential to the wheel, at different speeds.
The differential does this by distributing angular force (in the form of torque) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the differential ring gear. When powered each axle requires a differential to distribute power between the left and right sides. When power is distributed to all four wheels a third or 'center' differential can be used to distribute power between the front and rear axles.
The described system handles extremely well, as it is able to accommodate various forces of movement and distribute power evenly and smoothly, making slippage unlikely. Once it does slip, however, recovery is difficult. If the left front wheel of a 4WD vehicle slips on an icy patch of road, for instance, the slipping wheel will spin faster than the other wheels due to the lower traction at that wheel. Since a differential applies equal torque to each half-shaft, power is reduced at the other wheels, even if they have good traction. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a surface with little traction or raised off the ground. The simplistic design works acceptably well for 2WD vehicles. It is much less acceptable for 4WD vehicles, because 4WD vehicles have twice as many wheels with which to lose traction, increasing the likelihood that it may happen. 4WD vehicles may also be more likely to drive on surfaces with reduced traction. However, since torque is divided amongst four wheels rather than two, each wheel receives approximately half the torque of a 2WD vehicle, reducing the potential for wheel slip.
Many differentials have no way of limiting the amount of engine power that gets sent to its attached output shafts. As a result, if a tire loses traction on acceleration, either because of a low-traction situation (e.g., - driving on gravel or ice) or the engine power overcomes available traction, the tire that is not slipping receives little or no power from the engine. In very low traction situations, this can prevent the vehicle from moving at all. To overcome this, there are several designs of differentials that can either limit the amount of slip (these are called 'limited-slip' differentials) or temporarily lock the two output shafts together to ensure that engine power reaches all driven wheels equally.
Locking differentials work by temporarily locking together a differential's output shafts, causing all wheels to turn at the same rate, providing torque in case of slippage. This is generally used for the center differential, which distributes power between the front and the rear axles. While a drivetrain that turns all wheels equally would normally fight the driver and cause handling problems, this is not a concern when wheels are slipping.
The two most common factory-installed locking differentials use either a computer-controlled multi-plate clutch or viscous coupling unit to join the shafts, while other differentials more commonly used on off-road vehicles generally use manually operated locking devices. In the multi-plate clutch the vehicle's computer senses slippage and locks the shafts, causing a small jolt when it activates, which can disturb the driver or cause additional traction loss. In the viscous coupling differentials the shear stress of high shaft speed differences causes a dilatant fluid in the differential to become solid, linking the two shafts. This design suffers from fluid degradation with age and from exponential locking behavior. Some designs use gearing to create a small rotational difference that hastens torque transfer.
A third approach to limiting slippage is taken by a Torsen differential. A Torsen differential allows the output shafts to receive different amounts of torque. This design does not provide for traction when one wheel is spinning freely, where there is no torque, but provides excellent handling in less extreme situations. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower traction side.
A fairly recent innovation in automobiles is electronic traction control. Traction control typically uses a vehicle's braking system to slow a spinning wheel. This forced slowing emulates the function of a limited-slip differential, and, by using the brakes more aggressively to ensure wheels are being driven at the same speed, can also emulate a locking differential. It should be noted that this technique normally requires wheel sensors to detect when a wheel is slipping, and only activates when wheel slip is detected. Therefore, there is typically no mechanism to actively prevent wheel slip (i.e., you can't "lock the differential" in advance of wheel slip), rather the system is designed to expressly permit wheel slip to occur, and then attempt to send torque to the wheels with the best traction. If preventing all-wheel slip is a requirement, this is a limiting design.
The architecture of an AWD/4WD system can be described by describing its possible operating modes. A single vehicle may have the ability to operate in multiple modes depending on driver selection. Mohan describes the modes as follows:
Two Wheel Drive (2WD) Mode - In this mode only one axle (typically the rear axle) is driven. The drive to the other axle is disconnected. The operating torque split ratio is 0:100.
Four Wheel Drive (4WD) Mode - Here, depending on the nature of torque transfer to the axles, we can define three sub-modes (below).
In addition to these basic modes there could be implementations that combine these modes. The system could have a clutch across the center differential, for example, capable of modulating the front axle torque from a Full-time mode with the 30:70 torque split of the center differential to the 0:100 torque split of the 2WD mode
- Part-time Mode - The front and rear axle drives are rigidly coupled in the transfer case. Since the driveline does not permit any speed differentiation between the axles and would cause driveline wind-up, this mode is recommended only for ‘part-time’ use in off-road or loose surface conditions where driveline wind-up is unlikely. Up to full torque could go to either axle depending on the road condition and the weight over the axles.
- Full-time Mode - Both axles are driven at all times but an inter-axle differential permits the axles to turn at different speeds as needed. This allows the vehicle to be driven ‘full-time’ in this mode, regardless of the road surface, without fear of driveline wind-up. With standard bevel gear differentials the torque split is 50:50. Planetary differentials can provide asymmetric torque splits as needed. A system that operates permanently in the full-time mode is sometimes called ‘All-the-Time 4WD’, 'All-Wheel-Drive' or ‘AWD’. If the inter-axle differential is locked out then the mode reverts to a ‘part-time mode’.
- On-Demand Mode - In this mode the transfer case operates primarily in the 2WD mode. Torque is transferred to the secondary axle as needed by modulating the transfer clutch from ‘open’ to a rigidly coupled state while avoiding any driveline wind-up. The torque modulation may be achieved by active electronic/hydraulic control systems, or by passive devices, based on wheel slip or wheel torque, as described in the section on traction control systems.
In 1893, before the establishment of a modern automotive industry in Britain, English engineer Bramah Joseph Diplock patented a four-wheel-drive system for a traction engine, including four-wheel steering and three differentials, which was subsequently built. The development also incorporated Bramah's Pedrail wheel system in what was one of the first four-wheel-drive automobiles to display an intentional ability to travel on challenging road surfaces. It stemmed from Bramagh's previous idea of developing an engine that would reduce the amount of damage to public roads.
Ferdinand Porsche designed and built a four-wheel-driven electric vehicle for the k. u. k. Hofwagenfabrik Ludwig Lohner & Co. at Vienna in 1899, presented to the public during the 1900 World Exhibition at Paris. An electric hub motor at each wheel powered the vehicle. It was clumsily and heavy and to its unusual status the so-called Lohner-Porsche is not widely credited as the first four-wheel-driven automobile.
Another four-wheel-drive car, as well as hill-climb racer, with internal combustion engine, the Spyker 60 H.P., was presented in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. The two-seat sports car, which was also the first ever car equipped with a six-cylinder engine, is now an exhibit in the Louwman Collection (the former Nationaal Automobiel Museum) at the Hague in The Netherlands.
Designs for four-wheel drive in America came from the Twyford Company of Brookville, Pennsylvania in 1905, six were made there around 1906; one still exists and is displayed annually. The second American four-wheel-drive vehicle was built in 1908 by (what became) the Four Wheel Drive Auto Company (FWD) of Wisconsin (not to be confused with the term "FWD" as an acronym for front-wheel drive). FWD would later produce around 15,000 of its four-wheel-drive Model B trucks for the British and American armies during World War I. Approximately 11,500 of the Jeffery or Nash Quad models (1913–1919) were similarly used. The Quad not only came with four-wheel drive and four-wheel brakes, but also featured four-wheel steering.
The Reynolds-Alberta Museum has a four-wheel-drive vehicle, named "Michigan", from 1905 in an unrestored storage. The American Marmon-Herrington Company was founded in 1931 to serve a growing market for moderately priced four-wheel-drive vehicles. Marmon-Herrington specialized in converting Ford trucks to four-wheel drive and got off to a successful start by procuring contracts for military aircraft refueling trucks, 4×4 chassis for towing light weaponry, commercial aircraft refueling trucks, and an order from the Iraqi Pipeline Company for what were the largest trucks ever built at the time.
Daimler-Benz also has a history in four-wheel drive. In 1907 the Daimler Motoren Gesellschaft had built a four-wheel-driven vehicle called Dernburg-Wagen, also equipped with four-wheel steering, that was used by German colonial civil servant, Bernhard Dernburg, in Namibia. Mercedes and BMW, in 1926, introduced a rather sophisticated four-wheel drive, the G1, the G4 and G4 following. The 1937 Mercedes-Benz G5 and BMW 325 4×4 featured full-time four-wheel drive, four-wheel steering, three locking differentials, and fully independent suspension. They were produced because of a government demand for a four-wheel-drive passenger vehicle. The modern G-series/Wolf such as the G500 and G55 AMG still feature some of the attributes, with the exception of fully independent suspension since it hinders suspension articulation. The Unimog is another Mercedes truck.
The first Russian produced four-wheel-drive vehicle, also in part for civilian use, was the GAZ-61, developed in the Soviet Union in 1938. "Civilian use" may be a bit of a misnomer, as most if not all were used by the Soviet government and military (as command cars), but the GAZ-61-73 version is the first four-wheel drive vehicle with a normal closed sedan body. Elements of the chassis were used in military vehicles such as the GAZ-64, GAZ-67, GAZ-69, and also the properly civilian GAZ-M-72 (based on the rear-wheel drive GAZ-20 "Victory" and built from 1955-1958). Soviet civilian life did not allow the proliferation of civilian products such as the Jeep in North America, but through the 1960s the technology of Soviet 4×4 vehicles stayed on par with British, German, and American models, even exceeding it in some aspects, and for military purposes just as actively developed, produced and used.
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.
In 1937, the Japanese company Tokyu Kurogane Kogyo built approximately 4,700 four-wheel-drive roadsters, called the Type 95 used by the Imperial Japanese Army from 1937 until 1944, used during the Second Sino-Japanese War. Three different bodystyles were manufactured; a 2-door roadster, a 2-door pickup truck and a 4-door phaeton, all equipped with a transfer case that engaged the front wheels, powered by a 1.3 litre, 2-cylinder, air-cooled OHV V-twin engine.
Willys introduced the model CJ-2A in 1945, the first full-production four-wheel-drive vehicle for sale in the general marketplace. Thanks to the ubiquitous World War II jeep's success, its rugged utilitarianism set the pattern for many four-wheel drive vehicles to come.
The similarly boxy, also inline-4 powered Land Rover appeared at the Amsterdam Motor Show in 1948. Originally conceived as a stop-gap product for the struggling Rover car company, despite chronic under-investment it succeeded far better than the passenger cars. Inspired by a Willys MB that was frequently run off-road on the farm belonging to chief engineer Maurice Wilks, Land Rover developed the more refined yet still off-road capable luxury 4WD Range Rover in the 1970s.
With the acquisition of the "Jeep" name in 1950 Willys had cornered the brand. Its successor, Kaiser Jeep, introduced a revolutionary 4WD wagon called the Wagoneer in 1963. Not only was it technically innovative, with independent front suspension and the first automatic transmission coupled to 4WD, it was equipped and finished as a regular passenger automobile. In effect, it was the ancestor of the modern SUV. The luxury Rambler or Buick V8s powered Super Wagoneer produced from 1966 to 1969 raised the bar even higher.
Jensen applied the Formula Ferguson (FF) full-time all-wheel-drive system to 318 units of their Jensen FF built from 1966 to 1971, marking the first time 4WD was used in a production GT sports car. While most 4WD systems split torque evenly, the Jensen split torque roughly 40% front, 60% rear by gearing the front and rear at different ratios.
American Motors (AMC) acquired Kaiser's Jeep Division in 1970 and quickly upgraded and expanded the entire line of off-road 4WD vehicles. With its added roadworthiness, the top range full-size Grand Wagoneer continued to compete with traditional luxury cars. Partially hand-built, it was relatively unchanged during its production through 1991, even after Chrysler's buyout of AMC.
Subaru introduced the category-expanding Leone in 1972, an inexpensive compact station wagon with a light-duty, part-time four-wheel drive system that could not be engaged on dry pavement. In September AMC introduced Quadra Trac full-time AWD for the 1973 model year Jeep Cherokee and Wagoneer. Thanks to full-time AWD, which relieved the driver of getting out to lock hubs and having to manually select between 2WD and 4WD modes, it dominated all other makes in FIA rally competition. Gene Henderson and Ken Pogue won the Press-on-Regardless Rally FIA championship with a Quadra Trac equipped Jeep in 1972.
American Motors introduced the innovative Eagle for the 1980 model year. These were the first American mass production cars to use the complete front-engine, four-wheel drive system. The AMC Eagle was offered as a sedan, coupe, and station wagon with permanent automatic all-wheel-drive passenger models. The new Eagles combined Jeep technology with an existing and proven AMC passenger automobile platform. They ushered a whole new product category of "sport-utility" or crossover SUV. AMC's Eagles came with the comfort and high-level appointments expected of regular passenger models and used the off-road technology for an extra margin of safety and traction.
The Eagle's thick viscous fluid center differential provided quiet and smooth transfer of power that was directed proportionally to the axle with the greatest traction. This was a true full-time system operating only in four-wheel drive without undue wear on suspension or driveline components. There was no low range in the transfer case. This became the forerunner of the designs that followed from other manufacturers. The automobile press at the time tested the traction of the Eagles and described it as far superior to the Subaru's and that it could beat many so-called off-road vehicles. Four Wheeler magazine concluded that the AMC Eagle was "The beginning of a new generation of cars."
The Eagles were popular (particularly in the snowbelt), had towing capacity, and came in several equipment levels including sport and luxury trims. Two additional models were added in 1981, the sub-compact SX/4 and Kammback. A manual transmission and a front axle-disconnect feature were also made available for greater fuel economy. During 1981 and 1982 a unique convertible was added to the line. The Eagle's monocoque body was reinforced for the conversion and had a steel targa bar with a removable fiberglass roof section. The Eagle station wagon remained in production for one model year after Chrysler acquired AMC in 1987.
Audi also introduced a permanently all-wheel-driven road-going car, the Audi Quattro, in 1980. Audi's chassis engineer, Jorg 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, that would also be used for rallying to improve Audi's conservative image. The Audi Quattro system became a feature on production cars.
In 1987, Toyota also developed a car built for competition in rally campaigns. A limited number of road-going FIA Homologation Special Vehicle Celica GT-Four (known as Toyota Celica All-Trac Turbo in North America) were produced. The All-Trac system was later available on serial production Toyota Camry, Toyota Corolla, and Toyota Previa models.
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 to 1986. In 1989, niche maker Panther Westwinds created a mid-engined four-wheel-drive, the Panther Solo 2.
In 2008, Nissan introduced the GT-R featuring a rear-mounted transaxle. The AWD system requires two drive shafts, one main shaft from the engine to the transaxle and differential and a second drive shaft from the transaxle to the front wheels.
The Ferrari FF introduced in 2011 features a unique system called 4RM, which does away with the heavy center differential and instead attaches a smaller, second transaxle that draws power from the front of the engine. This allows the car to keep the traditional rear transaxle design without the need for a second drive-shaft for the front wheels. However, after fourth gear the awd system shuts off or disables itself.
Today, sophisticated all-wheel-drive systems are found in many passenger vehicles and some exotic sports cars and supercars. Mainstream luxury and near-luxury vehicles which can absorb the extra cost involved, and for which the fuel economy penalty of some 5% is not an issue, are offered with AWD (or "4×4" or "4WD") as a safety enhancement to meet owners' expectations for a full complement of up-to-date technology.
Miller produced the first 4WD car to qualify for the Indianapolis 500, the 1938 Miller Gulf Special.
Ferguson Research Ltd. built the front-engine P99 Formula One car that actually won a non-World Championship race with Stirling Moss in 1961. In 1968, Team Lotus raced cars in the Indy 500 and three years later in Formula 1 with the Lotus 56, that had both turbine engines and 4WD, as well as the 1969 4WD-Lotus 63 that had the standard 3-litre V8 Ford Cosworth engine. Matra also raced a similar MS84, and McLaren entered their M9A in the British Grand Prix, while engine manufacturers Ford-Cosworth produced their own version which was tested but never raced. All these F1 cars were considered inferior to their RWD counterparts, as the advent of aerodynamic downforce meant that adequate traction could be obtained in a lighter and more mechanically efficient manner, and the idea was discontinued, even though Lotus tried repeatedly.
Nissan and Audi had success with all-wheel drive in road racing with the former's advent of the Nissan Skyline GT-R in 1989. So successful was the car that it dominated the Japanese circuit for the first years of production, going on to bigger and more impressive wins in Australia before weight penalties eventually levied a de facto ban on the car. Most controversially was the win pulled off at the 1990 Macau Grand Prix where the car led from start to finish. Audi's dominance in the Trans-Am Series in 1988 was equally controversial as it led to a weight penalty mid season and to a rule revision banning all-AWD cars, its dominance in Super Touring eventually led to a FIA ban on AWD system in 1998.
New 2011 24 Hours of Le Mans regulations may revive AWD/4WD in road racing, though such systems are only allowed in new hybrid-powered Le Mans Prototypes. One example is the Audi R18 e-tron quattro (winner of 2012 race, the first ever hybrid/4WD to win Le Mans), utilizing an electric motor in the front axle while combining the engine motor in the rear.
In heavy trucks
Medium-duty trucks and Heavy-duty trucks have recently adopted 4×4 drive-trains. 4×4 medium-duty trucks became common after Ford Motor Company began selling Ford Super Duty trucks. The Super Duty trucks shared many parts between the light duty and medium duty, reducing production costs. The Dana 60 front axle is used on both medium- and light-duty Super Duty trucks. Furthermore, the Big Three share/shared parts between the companies reducing costs. The Dana S 110 is currently being used for the rear drive, under Ford and Ram's medium-duty trucks. The Dana 110 was also used on the General Motors 4×4s as well. Ram Trucks began selling medium-duty trucks, 4×4 and 4×2, in 2008. General Motors sold a 4×4 for model years 2005-2009.
In construction equipment
In engineering terms, "four-wheel drive" designates a vehicle with power delivered to four wheel ends spread over at least two axles. The term 4×4 (pronounced four by four) was in use to describe North American military four-wheel-drive vehicles as early as the 1940s, with the first number indicating the number of wheel ends on a vehicle and the second indicating the number of driven wheels.
Trucks with dual tires on the rear axle and two driven axles are designated as 4×4s despite having six wheels because the paired rear wheels behave as a single wheel for traction and classification purposes. True 6×6 vehicles which have three powered axles, and are classified as 6×6s regardless of how many wheels they have. Examples of these with two rear, one front axle are the 6-wheeled Pinzgauer, which is popular with defense forces around the globe, and 10-wheeled GMC CCKW made famous by the U.S. Army in World War II.
4-wheeler (or four-wheeler) is a related term applying to all-terrain vehicles, and not to be confused with four-wheel drive. The "four" in the instance referring to the vehicle having four wheels, not necessarily all driven.
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 in 1958. Unlike other 4×4 vehicles which use a conventional transfer case to drive the front and rear axle, the Sahara had two engines, each independently driving a separate axle, with the rear engine facing backwards. The two throttles, clutches and gear change mechanisms could be linked, so the two 12 hp (9 kW) 425 cc (26 cu in) engines could run together, or they could be split and the car driven solely by 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 only 27 are known to exist today.
BMC experimented with a twin-engine Mini Moke (dubbed the "Twini Moke") in the mid-1960s, but never put it into production. This made advantage of the Mini's 'power pack' layout, with a transverse engine and the gearbox in the engine sump. Simply by fitting a second engine/gearbox unit across the rear, a rudimentary 4×4 system could be produced. Early prototypes had separate gear levers and clutch systems for each engine. Later versions sent for evaluation by the British Army had more user-friendly linked systems.
In 1965, A. J. M. Chadwick patented a 4WD system, GB 1113068, that used hemispherical wheels for an all-terrain vehicle. Twenty years later, B. T. E. Warne, patented, GB 2172558, an improvement on Chadwick's design that did not use differential gear assemblies. By using near-spherical wheels with provision to tilt and turn each wheel co-ordinatively, the driven wheels maintain constant traction. Furthermore, all driven wheels steer and, as pairing of wheels is not necessary, vehicles with an odd number of wheels are possible without affecting the system's integrity. Progressive deceleration is made possible by dynamically changing the front-to-rear effective wheel diameter ratios.
Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Earlier Suzuki versions were twin engined, from 1996 on the engine is a twin-turbocharged 2.0 L V6, mated to a sequential 6-speed manual transmission.
Nissan Motors has developed a system called E-4WD. It is designed for cars that are normally front-wheel drive, however the rear wheels are powered by electric motors. This system was introduced in some variants of the Nissan Cube and Tiida. (This is similar to the system used on the Ford Escape Hybrid AWD.)
Chrysler's Jeep Division debuted the twin engine, 670 hp (500 kW) Jeep Hurricane concept at the 2005 North American International Auto Show in Detroit. This vehicle has a unique "crab crawl" capability, which allows it to rotate 360° in place. This is accomplished by driving the left wheels as a pair and right wheels as a pair, as opposed to driving the front and rear pairs. A central gearbox allows one side to drive in the opposite direction to the other. It also has dual Hemi V8s.
Some hybrid vehicles such as the Lexus RX400h provide power to an AWD system through a pair of electric motors, one to the front wheels and one to the rear. In the case of the AWD model version of the Lexus RX400h (and its Toyota-branded counterpart, the Harrier hybrid), the front wheels can also receive drive power directly from the vehicle's gasoline engine as well as via the electric motors, whereas the rear wheels derive power only from the second electric motor. Transfer of power is managed automatically by internal electronics based on traction conditions and need, making this an all-wheel-drive system.
The 4RM system used in the Ferrari FF is unique in that it uses two gearboxes, in addition to the traditional Ferrari rear transaxle behind the engine, the system also uses a smaller transaxle drawing power from the front of the engine, sending it to the front wheels as needed. To save space and weight the front transaxle only has three forward speeds (and no reverse). Drive to the front wheels is transmitted through two infinitely-variable clutch packs which are allowed to 'slip' to give the required road wheel speeds. The system usually operates in the rear-drive only mode and only engages the clutches for the front transaxle when the rear wheels start losing grip.
Introduction to off-roaders
|1941||Volkswagen||Typ 128 Schwimmwagen||x|
|1948||Rover||Land Rover Series||x|
|1951||Alfa Romeo||Alfa Romeo Matta||x|
|1951||Austin Motor Company||Austin Champ||x|
|1956||Auto Union||DKW Munga||x|
|1963||Kaiser Jeep||Wagoneer (SJ)||x|
|1970||British Leyland||Range Rover||x|
Introduction to passenger cars
Systems by design type
Center differential with mechanical lock
- Alfa Romeo 164 Q4 (central viscous coupling, epicyclic unit and Torsen rear differential)
- Alfa Romeo 155 Q4 (central epicyclic unit, Ferguson viscous coupling and Torsen rear differential)
- AMC Eagle (central viscous coupling)
- Audi - Quattro Coupé, 80, 90, 100 & 200 (locking center and rear differentials) - up to 1987
- Audi Q7 -double pinion 50/50 with lockup clutch pack
- BMW 3 series and 5 series in the 1980s - planetary center differential with a 37-63 (front-back) torque split and viscous lock (also in rear differential but not front differential)
- Chevrolet Rounded-Line K Fleetside, K Stepside, K Blazer, and K Suburban - permanent four-wheel drive (1973-1979) two-speed New Process 203 transfer case, center differential with 50:50 torque split and lock. An Eaton Automatic Differential Lock was optional for the rear hypoid differential.
- Ford - Escort (RS 2000 16v 4×4 models and RS Cosworth), Sierra Cosworth, Sierra and Granada 4×4 models,
- Dodge Power Wagon - permanent four-wheel drive (1974-1979) two-speed New Process 203 transfer case, center differential with 50:50 torque split and lock.
- Ford Expedition (1997–present) and Expedition EL/Max (2007–present) - automatic ControlTrac four-wheel drive with two-speed dual range BorgWarner transfer case and intelligent locking center multi-disc differential
- Ford Explorer (1995–2010) - automatic ControlTrac four-wheel drive with two-speed dual range BorgWarner transfer case and intelligent locking center multi-disc differential
- Ford F-Series - permanent four-wheel drive (1974-1979) two-speed New Process 203 transfer case, center differential with 50:50 torque split and lock.
- GMC Rounded-Line K Wideside, K Fenderside, K Jimmy, and K Suburban - permanent four-wheel drive (1973-1979) two-speed New Process 203 transfer case, center planetary differential with 50:50 torque split and lock. An Eaton Automatic Differential Lock was optional for the rear hypoid differential.
- H1 & Humvee NVG 242HD AMG open center differential, locked center differential, Neutral, low range locked. Also Torsen1 differential at the front and rear axle, The H1 moved to Torsen2 when ABS was added. The H1 Alpha had optional locking differentials in place of torsens
- Hummer H2, H3 40/60 planetary with lock
- Jeep Grand Cherokee, Commander (except models equipped with Quadra-Trac I)
- Jeep Liberty, Jeep Cherokee (XJ), Dodge Durango (Select-Trac) - NV 242 transfer case- rear drive, open center differential, locked center differential, Neutral, low range
- Full size Jeeps with Borg Warner QuadraTrac: limited slip center differential, 50/50 locked center differential. Low range could be used in locked or unlocked mode, allowing for use of low range on pavement.
- Land Rover Defender (and Series III V8 models)
- Land Rover Discovery/LR3
- Land Rover Freelander
- Lada Niva (VAZ-2121) - full-time 4WD using open center differential. Transfer case with high/low range and manual central diff lock. Low range selectable in locked or unlocked mode, allowing use on pavement.
- Lexus RX300 -viscous coupling across the otherwise open center differential.
- Lincoln Navigator (1998–2006) - automatic ControlTrac four-wheel drive with two-speed dual range BorgWarner transfer case and intelligent locking center multi-disc differential
- Navigator and Navigator L (2007–present) use one-speed single range transfer case, no reduction gearing
- Mercedes-Benz Unimog (locking center and rear with up to 10 low range gears).
- Mercedes-Benz G-Class (locking center and lockers on both front- and rear axle)
- Mercedes-Benz GL-Class - 4Matic all-wheel-drive system
- Mitsubishi Pajero (also known as Montero or Shogun)
- Porsche Cayenne - 38/62 planetary with lockup clutch pack
- Range Rover Classic 1970–1995 all full-time 4WD either plate LSD, manual lock or Ferguson viscous centre differential.
- Range Rover 2nd Gen. 1994–2002 full-time 4WD Ferguson viscous centre differential
- Suzuki Grand Vitara/Escudo -full-time 4WD using limited-slip center differential, off-road 4WD with selectable center differential lock and low range transfer case 4 mode (4h, 4h lock, 4l n), traction control and electronic stability control
- Subaru - manual transmissions come with 50/50 viscous-type center differential; performance models include a planetary differential with computer regulated lockup; automatic transmission models have an electronically controlled variable transfer clutch.
- Toyota Land Cruiser
- Toyota Sequoia (Multi-mode)
- Volkswagen Touareg -double pinion 50/50 with lockup clutch pack
Torsen center differential
- Alfa Romeo Q4s - with (Torsen T-3):
- Audis with quattro - various iterations of Torsen, the T-3 starting from the 2007 B7 RS4
- Bentley Continental GT, Bentley Continental Flying Spur (2005) initially Torsen T-2, current have T-3
- Chevrolet Trailblazer SS Torsen T-3
- Lexus GX470, Toyota Land Cruiser Prado 120 Torsen T-3
- Range Rover 3rd Gen. 2002–2009
- Toyota 4Runner (only Limited V8 model & 2010 Limited V6 model) Torsen T-3 with lock
- Toyota FJ Cruiser (only manual models) Torsen T-3 with lock
- Toyota Hilux Surf Torsen T-3 with lock
- Toyota Land Cruiser 200/2008/V8 Torsen T-3 with lock
- Toyota Sequoia (North America) (only 2005-07 Models)
- Volkswagen Passenger Cars with 4motion:
Non-locking center differential
- BMW 3-series and X5 between 2001 and xDrive - planetary center differential with permanent 38-62 (front-back) torque split #
- Cadillac Escalade, STS AWD, SRX AWD (The first two generations had a viscous clutch on the center differential) #
- Chrysler 300C AWD#
- Dodge Ramcharger 1974–1981 - NP203 FullTime 4WD Transfer Case
- Dodge Magnum, Charger AWD #
- GMC Yukon Denali, XL Denali, Sierra Denali #
- Mercedes 4MATIC cars, R class, and ML class (note some MLs had low range) #
- Plymouth Trailduster 1974–1981 - NP203 FullTime 4WD Transfer Case
- Toyota Highlander #
- Toyota Sienna AWD (-2010 only) #
The above systems ending with "#" function by selectively using the traction control system (via ABS) to brake a slipping wheel.
- Acura RL, RDX (SH-AWD) Right and left axle shaft
- Acura MDX SH-AWD & VTM4
- Ford Explorer - Ford's full-time shift-on-the-fly Intelligent 4WD System (I-4WD) on the 2011 Explorer with Terrain Management System and RSC (Roll Stability Control), Curve Control functionality, HDC (Hill Descent Control) and HAA (Hill Ascent Assist).
- Honda Ridgeline
- Honda Pilot
- Infiniti FX (ATTESA E-TS)
- Mercedes-Benz 1st generation 4MATIC (normally rear-drive, automatic clutch in transfer case engages 4WD on demand)
- Mitsubishi GTO MR/3000GT VR-4
- Mitsubishi Lancer Evolution Series S-AWC
- 2010 Mitsubishi Outlander GT S-AWC
- Mitsubishi Outlander (2003–2006) independent front and rear axle coupling, and Active Center Differential.
- Nissan GT-R (ATTESA E-TS)
- Nissan Skyline GT-R (ATTESA E-TS and ATTESA E-TS-PRO) front axle coupling, rear differential locking
- Nissan Skyline GTS4 (ATTESA E-TS)
- Nissan A31 Cefiro SE4 (ATTESA E-TS)
- Porsche 959 PSK front axle coupling, rear differential locking
- Saab 9-3, Saab 9-5, Saab 9-4X (Saab XWD).
Multi-plate clutch coupling
- Audi A3 quattro, Audi S3, Audi TT quattro, Audi R8 (with Haldex Traction)
- BMW xDrive: latest 3 Series, latest 5 series, X3, latest X5 series
- Chevrolet Equinox (GMPCA)
- Chrysler Pacifica (BorgWarner ITM3e) (on 2007 model)
- Dodge Nitro (Quadra-Trac 1)
- Dodge Caliber
- Ford: Escape, Freestyle, Edge, Fusion, Five Hundred (Freestyle, FiveHundred Haldex Traction based)(Escape Control Trac II, based)
- Honda CR-V, HR-V, Element
- Hyundai Santa Fe, Hyundai Tucson Borg-Warner ITM 3e magnetic multi-plate clutch coupling
- Hyundai Veracruz IMJ magnetic multi-plate clutch coupling
- Infiniti: G35x, M35x
- Jeep Compass (Freedom Drive)
- Jeep Grand Cherokee and SRT8 NVG 249, 247
- Land Rover Freelander 2/LR2 (also Haldex Traction)
- Lamborghini: AWD variants VT series (viscous traction)
- Lincoln: MKS, MKZ
- Mazdaspeed6 (a power takeoff unit linked to clutch pack with torque sensitive rear differential)
- Mazda: Tribute, CX-7, CX-9 (tribute Control Trac II, based)
- Mercury: Milan, Montego, Mariner (Montego Haldex Traction-based)
- Mitsubishi Outlander (current generation)
- Nissan Murano automatic with manual lockup switch
- Porsche 911 AWD variants (a version of BorgWarner ITM3e) — excluding the 964-series Porsche 911 Carrera 4 31/69 planetary center differential
- Pontiac Torrent (GMPCA)
- Subaru low powered automatic transmission models
- Subaru Legacy, Outback, Impreza, Forester, Tribeca automatic transmission models: mechanical front drive, clutch coupled rear axle.
- Suzuki: SX4, XL7, Aerio, Swift/Cultus based Subaru Justy. (viscous clutch)
- Toyota RAV4 - from 2005 (third generation only)
- Toyota Sienna AWD (2011 and newer only)
- Volkswagen Golf 4motion, Volkswagen Jetta 4motion, Volkswagen Tiguan 4motion, Volkswagen Passat (B6) 4motion (initially viscous coupling, later with Haldex Traction)
- Volvo: S40, S60, S80, V50, V70, XC70, XC90 (Visco system until 2003; then all Haldex Traction-based)
Note: the above all function like 2WD when multi-plate clutch coupling is not engaged (with exception of Subaru models), and like 4WD highrange in a part-time 4WD system when the clutch is engaged (usually by computer although some allow manual control). Some in this category have varying degrees of control in the torque distribution between front and rear by allowing some of the clutches in a multi-plate clutch coupling to engage and slip varying amounts. An example of a system like this is the BorgWarner i-Trac(TM) system. Note: the Haldex Traction-based car list was created from the list on Haldex Traction corporate web site: Haldex Cars. A version of the BorgWarner ITM3e system is used on 2006 and up Porsche 911TT's. The Borg-Warner ITM 3e is also used in the 2006-now Hyundai Santa Fe and the Hyundai Tucson. In the Hyundais, the ITM 3e acts like a full-time AWD with 95:5 normal torque split. In extreme conditions, the system can be locked in a 50:50 split via the 4WD LOCK button.
- Ferrari FF (4RM) rear transaxle with secondary front transaxle connected directly to the engine.
In Ferrari's 4RM (so far the only system of this type), the engine is set between the traditional Ferrari rear transaxle and a secondary front transaxle (PTU - "power take off unit"). The car itself operates primarily as a rear wheel drive vehicle, clutches in the front transaxle only engage when the rear wheels begin to slip. The front transaxle itself only has 3 gears equivalent to 2nd, 4th and reverse in the rear transaxle (with gear ratios 6% taller than the corresponding ratios in the main gearbox), so the system is only active in 1st to 4th gears. The connection between this gearbox and each front wheel is via independent haldex-type clutches, without a differential. Due to the difference in ratios "the clutches continually slip" and only transmit, at most, 20% of the engine's torque.
These are vehicles that have no center differential. Since there is no center differential to allow for speed differences between the front and rear wheels when turning, a small amount of tire slippage must occur during turns. When used on slick surfaces, this is not a problem, but when turning on dry pavement, the tires grip, then are forced to slip, then grip again, and so on, until the turn is completed. This causes the vehicle to exhibit a 'hopping' sensation. Using an engaged part-time 4WD system on a hard surface is not recommended, as damage to the drive-line eventually occurs.
- Chevrolet Rounded-Line K Fleetside, K Stepside, K Blazer, and K Suburban - conventional four-wheel drive (1973-1987) or shift-on-the-move four-wheel drive (1981-1987) two-speed New Process 205 or 208 transfer case. 0:100 torque split in Two High. 50:50 torque split lock in Four High and Four Low. An Eaton Automatic Differential Lock was optional for the rear hypoid differential. Note Rounded-Line "K" Pickups and Utilities were temporarily renamed to "V" for 1987
- Chevrolet Tahoe, Trailblazer (LT1 and LT3 models only), Tracker, Suburban, Silverado, Avalanche, Colorado, S-10 series, K5 Blazer
- Dodge Power Wagon (a Ram version with front and rear differential locks)
- Dodge Ram, Dakota
- Dodge Nitro (Quadra-Trac 2)
- Ford F series
- Ford Explorer (1991-1994) & Sport Trac
- Ford Ranger
- Geo Tracker
- GMC Rounded-Line K Wideside, K Fenderside, K Jimmy, and K Suburban - conventional four-wheel drive (1973-1987) or shift-on-the-move four-wheel drive (1981-1987) two-speed New Process 205 or 208 transfer case. 0:100 torque split in Two High. 50:50 torque split lock in Four High and Four Low. An Eaton Automatic Differential Lock was optional for the rear hypoid differential. Note Rounded-Line "K" Pickups and Utilities were temporarily renamed to "V" for 1987
- GMC Envoy, Yukon, Sierra, Jimmy, Sonoma
- Infiniti QX56 (All-mode 4WD) Auto-engages 4WD with slip
- Isuzu i-series, Isuzu Wizard
- Jeep Cherokee (Quadra-Trac 2)
- Jeep Cherokee (XJ), Jeep Comanche, Jeep Grand Cherokee (ZJ), Jeep Liberty (Command-Trac)
- Jeep Wrangler (Rubicon model has locking front and rear differentials)
- Kia Sorento (some 2002-2009 models with 2WD/4HI/4LO - mostly LX)
- Land Rover Series I, II & III (except V8 models)
- Lincoln Mark LT
- Mazda B-series
- Mercedes-Benz G-Class
- Mitsubishi Raider
- Nissan Patrol
- Nissan Terrano II
- Nissan Armada, Pathfinder (All-mode 4WD) Auto-engages 4WD with slip
- Nissan Titan, Xterra, Frontier (rear locker an option)
- Subaru Loyale, GL/DL, BRAT Front/4wd/4wd lo, Justy 4WD
- Suzuki Sidekick, Jimny, Vitara
- Toyota Hilux
- Toyota Tacoma (rear locking differential optional)
- Toyota Tundra TRD
- Toyota FJ Cruiser (automatic transmission models) (also locking rear differential)
- Toyota 4Runner (only SR5 and pre 2010 Limited V6 models, 2010 Trail edition V6 models) (also locking rear differential on 2010 V6)
- Category:All-wheel-drive vehicles
- Four-wheel drive in Formula One
- Limited slip differential
- Off-road vehicle
- Sport utility vehicle
- Dune bashing
- Rock crawling
- Transfer case
- Mohan, Sankar (12 June 2000). "All - Wheel Drive / Four - Wheel Drive Systems and Strategies" (PDF). Seoul 2000 FISITA World Automotive Congres.
- Andreev, Alexandr F.; Kabanau, Viachaslau; Vantsevich, Vladimir (2010). Driveline Systems of Ground Vehicles: Theory and Design. CRC Press. p. 34.
- Dykes, Alex. "Alphabet Soup: 4×4 vs 4WD vs AWD Where’s the Differential?". thetruthaboutcars.com. Retrieved 18 December 2015.
- "Surface Vehicle Recommended Practice - J1952 - All-Wheel Drive System Classification". SAE International. October 2013.
- Walczak, Jim. "4WD vs 2WD: The Differences Between 4×4 And 4×2". about com. Retrieved 7 August 2010.
- Collard, Chris. "2WD vs AWD vs 4WD". ConsumerReports.com. Consumer Reports Magazine. Retrieved 15 December 2015.
- Williams, Mark. "4WD vs AWD: What's the Diff". MotorTrend.com. Motor Trend Magazine. Retrieved 15 December 2015.
- Allisons.org Automotive History "1929: AEC started to build AWD trucks in conjunction with FWD (UK)"
- MEYER, DONALD E. THE FIRST CENTURY OF GMC TRUCK HISTORY  "By December , GMC had orders for more than 4,400 all-wheel-drive military trucks."
- Sheppard, Tom (1 September 2005). "Jeep Grand Cherokee 4WD System Breakthrough - 4x... For?". Four Wheeler Network. Retrieved 27 May 2014.
- Davies, Alex (10 October 2014). "The Model D Is Tesla’s Most Powerful Car Ever, Plus Autopilot". Wired.com. Retrieved 11 October 2014.
Musk said the added efficiency is thanks to the electronic system that will shift power between the front and rear motors from one millisecond to the next, so each is always operating at its most efficient point
- Henry, Allan (1975). The 4-Wheel Drives: Racing's Formula for Failure?. Macmillan.
- "1903 Spyker 60 HP". Retrieved 6 May 2011.
- "The Novak Guide to the Dana Spicer Model 18 Transfer Case". Novak Conversions. Retrieved 16 September 2012.
- Eckermann, Erik (2001). World history of the automobile. Society of Automotive Engineers. p. 76. ISBN 978-0-7680-0800-5. Retrieved 21 January 2013.
- Nylund, Jimmy (February 2009). "27 Trailer Towing Tips". Four Wheeler. Retrieved 16 September 2012.
- Editors of Air Force Journal of Logistics (2000). The Logistics of War - a historical perspective. The Air Force Logistics Management Agency. p. 282. ISBN 9781428993785. Retrieved 21 January 2013.
It became a famous vehicle around the world and is mentioned in a majority of the histories.
- Nunney, M J J (2012). Light and Heavy Vehicle Technology (Fourth ed.). Routledge. p. 431. ISBN 9780750680370. Retrieved 21 January 2013.
- Foster, Patrick (January 2008). "1963 Jeep Wagoneer - Landmark Vehicle". 4 Wheel Drive & Sport Utility Magazine. Retrieved 21 January 2013.
- Auto Editors of Consumer Guide (25 July 2007). "1966–1971 Jensen FF". howstuffworks com. Retrieved 7 August 2010.
- Auto Editors of Consumer Guide (29 August 2007). "1978-1979 Jeep Wagoneer Limited". HowStuffWorks.com. Retrieved 5 January 2011.
- Auto Editors of Consumer Guide (29 August 2007). "1967–1977 Jeep Wagoneer". howstuffworks com. Retrieved 7 August 2010.
- Calvin, Jean (1974). Rallying to win: a complete guide to North American rallying. W.W. Norton. p. 150. ISBN 978-0-87880-017-9. Retrieved 5 January 2011.
- Sherman, Don (February 2001). "All-Wheel-Drive Revisited: AMC's 1980 Eagle pioneered the cross-over SUV". Automotive Industries. Retrieved 21 January 2013.
- Rettie, John (August 1987). "Four-Wheeling Into Your Future". Popular Mechanics. 164 (8): 58. Retrieved 5 January 2011.
- Sass, Rob (9 March 2008). "A Breed of 4-by-4 Hatched on the Fly". The New York Times. Retrieved 21 January 2013.
- Padgett, Martin (2004). Hummer. Zenith. p. 26. ISBN 978-0-7603-1863-8. Retrieved 21 January 2013.
- Foster, Patrick R. "1980-88 AMC Eagle 4wd". Autoweek (archived at amxfiles). Archived from the original on 30 April 2008. Retrieved 21 January 2013.
- Consumer's Guide, ed. (2005). History of the American Auto. Publications International. p. 532. ISBN 978-0-7853-9874-5.
- "Celica All Trac and Gt-Four FAQ". Alltrac.net. Retrieved 7 August 2010.
- Loh, Edward (December 2007). "First Look: 2008 Nissan GT-R". Retrieved 5 January 2011.
- Phelan, Mark (27 April 2011). "AWD helps sell luxury; Ford, GM use systems to build top brands". Detroit Free Press. Retrieved 6 May 2011.
- "The 2011 Le Mans 24 Hours Regulations". lemans.org. 6 October 2010. Retrieved 21 January 2013.
- "Hybrid Tech Dominates The 24 Hours Of Le Mans Endurance Race [VIDEO]". Autoblog.com.
- "Volvo BM 646 Backhoe/Loader" (PDF). Volvo AB.
- "Instruction Book: Driving, Maintenance, Repair. Department of National Defence, Canada". bcoy1cpb.pacdat.net. 1940. Retrieved 21 January 2013.
- Melissen, Wouter (1 December 2004). "Citroën 2CV Sahara". Ultimatecarpage.com. Retrieved 21 January 2013.
- "2009 Ford Escape Hybrid Review and Specs". Edmunds.com. Archived from the original on 1 March 2009. Retrieved 1 August 2009.
- Abuelsamid, Sam (26 July 2010). "2011 Ford Explorer puts four-cylinder EcoBoost engine above V6 in pecking order". Autoblog.com. Retrieved 26 September 2010.
- "Ford - Cars, SUVs, Trucks & Crossovers" (in Spanish). FordVehicles.com. Retrieved 26 September 2010.
- according to Car and Driver Vol52No8 Feb 07 page 110
- Horrell, Paul (25 January 2011). "Ferrari FF The inside scoop on Ferrari's new FF". Topgear.com. Retrieved 6 May 2011.