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
* 1.1 4×4 * 1.2 4WD * 1.3 AWD * 1.4 IWD * 1.5 SAE Recommended Practices
* 2 Design
* 2.1 Differentials * 2.2 Limiting slippage * 2.3 Operating Modes
* 3 History
* 3.1 Early 1900s * 3.2 1930s–1944 * 3.3 1945-1960s * 3.4 1970s-1990s * 3.5 2000-present
* 4 Uses
* 4.1 Road racing * 4.2 In heavy trucks * 4.3 In construction equipment
* 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
Main article: Differential The
Lamborghini Murciélago is an
AWD that powers the front via a viscous coupling unit if the rear
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. To prevent slippage from happening some vehicles have controls for independently locking center, front, and rear differentials
Main article: Limited-slip differential (LSD)
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).
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
The 1903 Spyker 60 H.P. 4WD Center transfer case sending power from the transmission to the rear axle (right) and front axle (left) Selection lever: 2H for two-wheel drive, 4H for high-range 4WD, 4L for low-range 4WD, and N for Neutral
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
Ferdinand Porsche designed and built a four-wheel-driven electric
vehicle for the k. u. k. Hofwagenfabrik Ludwig Lohner 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
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
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
The first Russian produced four-wheel-drive vehicle, also in part for
civilian use, was the
GAZ-61 , developed in the
It was not until "go-anywhere" vehicles were needed for the military
that four-wheel drive found its place. The
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.
The similarly boxy, also inline-4 powered
Land Rover appeared at the
With the acquisition of the "Jeep" name in 1950
Jensen applied the Formula Ferguson (FF) full-time all-wheel-drive
system to 318 units of their
American Motors (AMC) acquired Kaiser's
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
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
Some of the earliest mid-engined four-wheel-drive cars were the
various road-legal rally cars made for
In the United States, as of late 2013, AWD vehicles comprised 32 per
cent of new light vehicle sales, up five per cent since 2008. This is
in large part due to the popularity of the crossover . Most
crossovers offer the popular technology, in spite of it increasing
vehicle price and fuel consumption.
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
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
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
GM 4×4 medium-duty trucks *
Heavy duty International Workstar *
IN CONSTRUCTION EQUIPMENT
A Case backhoe loader with 4WD
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
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
Some hybrid vehicles such as the
The 4RM system used in the Ferrari FF in 2011 is unique in that it has a rear transaxle with secondary front transaxle connected directly to the engine. The car operates primarily as a rear wheel drive vehicle. Clutches in the front transaxle engage when the rear wheels slip. 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 front transaxle has three gears, two forward, and reverse. The two forward gears of the front transmission match the lower four forward gears of the rear transmission. It is not used in higher 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.
INTRODUCTION TO OFF-ROADERS
YEAR MANUFACTURER MODEL/SERIES MASS MILITARY
1941 Volkswagen Typ 128 Schwimmwagen
1948 Rover Land Rover Series x
1979 Mercedes-Benz G-Class x
INTRODUCTION TO PASSENGER CARS
YEAR MANUFACTURER MODEL/SERIES MASS SMALL SERIES
x (4677 units)
x (320 units)
x (200 units)
x (220 units)
x (292 units)
x (ca. 300 units)
2001 Jaguar Jaguar X-Type x
2008 Saab Saab 9-3 Turbo X x
2010 Dacia Dacia Duster x
x (ca. 800 units)
SYSTEMS BY DESIGN TYPE
CENTER DIFFERENTIAL WITH MECHANICAL LOCK
TORSEN CENTER DIFFERENTIAL
* 156 Crosswagon and Sportwagon * 159 * Brera , Spider
* Audis with quattro - various iterations of Torsen, the T-3 starting from the 2007 B7 RS4
* 80, 90 & Coupé (Typ 89) * 100 & 200 * A4 , S4 , RS4 * A5 & S5 * A6 , S6 , RS6 * A8 , S8 * Q5, Q7
Bentley Continental GT
NON-LOCKING CENTER DIFFERENTIAL
The above systems ending with "#" function by selectively using the traction control system (via ABS) to brake a slipping wheel.
Note: the above all function like 2WD when multi-plate clutch
coupling is not engaged (with exception of
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.
* ^ A B Mohan, Sankar (12 June 2000). "All - Wheel Drive / Four -
Wheel Drive Systems and Strategies" (PDF). Seoul 2000 FISITA World
* ^ 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.
* ^ A B "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.
* ^ A B 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
* ^ A B Sheppard, Tom (1 September 2005). "