Difference between revisions of "Four-wheel drive"
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[[Image:Jeep Wrangler TJ.jpg|thumb|right|224px|The [[Jeep Wrangler]] is a 4WD vehicle with a transfer case to select low range or high range 4WD.]] | [[Image:Jeep Wrangler TJ.jpg|thumb|right|224px|The [[Jeep Wrangler]] is a 4WD vehicle with a transfer case to select low range or high range 4WD.]] | ||
[[Image:80EagleDriveTrain.jpg|thumb|right|224px|1980 [[AMC Eagle]] 4x4 drivetrain and suspension components. The third differential is in the center. This setup is used on most AWD vehicles to this day.]] | [[Image:80EagleDriveTrain.jpg|thumb|right|224px|1980 [[AMC Eagle]] 4x4 drivetrain and suspension components. The third differential is in the center. This setup is used on most AWD vehicles to this day.]] | ||
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[[Image:Hmmwv-036.jpg|thumb|right|224px|The [[HMMWV]] is a 4WD/AWD that powers all wheels evenly (continuously) via a plain (but manually lockable) centre differential, oddly with [[Torsen]] differentials for both front and rear.]] | [[Image:Hmmwv-036.jpg|thumb|right|224px|The [[HMMWV]] is a 4WD/AWD that powers all wheels evenly (continuously) via a plain (but manually lockable) centre differential, oddly with [[Torsen]] differentials for both front and rear.]] | ||
[[Image:Car rally.jpg|thumb|right|224px|A [[Subaru Impreza]] [[rally car]] uses 4WD for traction on loose dirt.]] | [[Image:Car rally.jpg|thumb|right|224px|A [[Subaru Impreza]] [[rally car]] uses 4WD for traction on loose dirt.]] | ||
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'''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 [[wheel]]s to receive power from the [[engine]] simultaneously. While many people associate the term with [[off-road vehicle]]s, powering all four wheels provides better control on slick ice and is an important part of [[rallying|rally racing]] on mostly-paved roads. | '''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 [[wheel]]s to receive power from the [[engine]] simultaneously. While many people associate the term with [[off-road vehicle]]s, powering all four wheels provides better control on slick ice and is an important part of [[rallying|rally racing]] on mostly-paved roads. | ||
Latest revision as of 13:39, 24 September 2009
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 associate the term with 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.
Four-wheel drive (4WD or 4x4 for short) is the original term and is often used to describe truck-like vehicles that require 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" (AWD) term is a marketing term used to sell primarily on-road 4WD vehicles. However, in Australia, AWD is generally used for passenger vehicles that drive all four wheels all the time (e.g., a Subaru Impreza), whereas 4WD is used for vehicles designed primarily for heavy off-road use, normally with a low range transfer case (e.g. a Toyota Land Cruiser). The terms are thus quite vague in modern usage.
It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and it is also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, quattro, 4MATIC and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential.
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 requires 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.
Such a design would handle very well. It distributes power evenly and smoothly, making it very 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 just as likely to have a driven wheel on an icy patch, the simplistic design is usually considered marginally acceptable.
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 prevent wheel spin up. One nice feature of this design is that it does not work against traction control - it is traction control. This design is commonly seen on luxury crossover SUVs.
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 one possible optimization reduces latency via a small rotational difference (via gearing) to hasten torque transfer. Older designs used manually operated locking devices.
Yet another way to prevent this 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 traction difference is not so extreme. 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 tractive 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 lockable differential in the center. Torsen differentials generally work very well, though they can be expensive and heavy.
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 rear 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 usually 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.
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, six were made there around 1906; one still exists and is displayed annuallyTemplate:Citationneeded. The second 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.
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. Possibly beaten by the 1941 GAZ-61.
It was in 1948 that the vehicle whose name is synonymous with Four Wheel Drive in many countries was introduced. The Land Rover appeared at the Amsterdam Motor Show, originally conceived as a stop-gap product for the struggling Rover car company, and despite chronic under-investment succeeded far better than the passenger cars. Land Rover also had a luxury 4WD with the Range Rover in the 1970s, which unlike most subsequent offerings from other manufacturers, was capable of serious off-road use.
Kaiser Jeep introduced a 4WD wagon called the Wagoneer in 1963. It was revolutionary at the time, not only because of its technical innovations such as an independent front suspension and the first automatic transmission with 4WD, but also because it was equipped and finished as a regular passenger automobile. The Super Wagoneer (1966 to 1969) was powered by Rambler or Buick V8s. Its high level of equipment made it the first "luxury" SUV. American Motors (AMC) acquired Kaiser's Jeep Division in 1970 and quickly upgraded and expanded the entire line of serious off-road built 4WD vehicles. The top range full-size Wagoneer Limited continued to compete with traditional luxury cars. It was relatively unchanged during its production, even after Chrysler's buyout of AMC, all the way through 1991.
Jensen applied the Formula Ferguson four-wheel drive system to their 1966 Jensen FF marking the first time 4WD was used in a production sports car. However, 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. Subaru's success in marketing AWD vehicles has led to an AWD-only lineup in almost all of its markets outside of Japan. By 1998, Subaru discontinued all two-wheel drive vehicles in North America, where it remains the only brand to be exclusively AWD.
American Motors introduced the innovative Eagle for the 1980 model year. This was the world's first complete line (sedan, coupe, and station wagon) of permanent automatic all-wheel drive passenger models. The new Eagles combined Jeep technology with an existing and proven AMC passenger car 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 central 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 year after Chrysler Corporation 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, soon used for rallying to improve Audi's conservative image, the resulting rally bred Audi Quattro was a famous and historically significant Rally car. This feature was also extended to Audi's production cars and is still available.
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.
4WD in road racing
Bugatti created a total of three four-wheel drive racers, the Type 53, in 1932, but the cars were legendary for having poor handling. Ferguson Research Ltd. built the front-engined P99 Formula One car that actually won a non-WC race with Stirling Moss in 1961. In 1969, Team Lotus raced cars in F1 and the Indy 500 that had both turbine-engines and 4WD, as well as the 4WD-Lotus 63 that had the standard Cosworth engine. Matra also raced a similar MS84, while Team McLaren tested its design only. All these F1 cars were considered inferior to their RWD counterparts and the idea was discontinued, even though Lotus tried repeatedly.
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 without significant offroad capabilities are often referred to as All-Wheel Drives (AWD) or SUVs, while those 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.
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. 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 the USA in practice. In Australia the term is often used to describe a pickup truck that sits very high on its suspension. This is to avoid the confusion that the vehicle might be a 4x4 because it appears to be otherwise suited to off-road applications. A 2×4, however, is unambiguously a piece of lumber.
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, as is the Pinzgauer, which is popular with defense forces around the globe.
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.
In the UK, the derogatory nickname "Chelsea tractor"[1] is sometimes used to describe large privately owned four-wheel drive vehicles that rarely are used off-road. The term originally applies mostly to Range Rovers but may also be applied to any similar large four-wheel-drive vehicle or SUV. Many people often give this name to small, fuel-efficient and environmentally clean cars like the Fiat Nuova Panda 4x4 and the Suzuki SX4.
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 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 gearchange mechanisms could be linked, so both 12 bhp 425 cc 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.Template:Fact Enthusiasts have built their own "new" Saharas, by rebuilding a 2CV and fitting the modified engine, gearbox and axle onto a new, strengthened chassis.
BMC experimented with a twin-engined Mini Moke in the mid-1960s, but never put it into production.
Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Though actual numbers were never released, this twin-engined vehicle is believed to weigh less than 2000 pounds and produce nearly 1000bhp. Each engine is a twin-turbo charged 2.0L V6 mated to a sequential 6-speed manual transmission.
Nissan Motors has developed a system called E4WD wherein the rear wheels in a car that is normally front-wheel drive are driven by electric motors. This system was introduced in some variants of the Nissan Cube and Tiida.
Most recently, DaimlerChrysler's Jeep Division debuted the twin engine, 670 hp 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 in 360 degrees in place. It also has dual Hemi V8s.
See also
- Category:All wheel drive vehicles
- Limited slip differential
- Off-road vehicle
- Sport utility vehicle
- Front-wheel drive
- Rear-wheel drive
- FR layout
- MR layout
- RR layout
- Dune bashing
- Rock crawling