Four-wheel drive
A four-wheel drive, also called 4×4 ("four by four") or 4WD, is a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive shaft and, in many instances, additional gear ranges.
Several terms redirect here. For other uses, see Four-wheel drive (disambiguation), 4×4 (disambiguation), and Four by four (disambiguation).A four-wheel drive vehicle with torque supplied to both axles is described as "all-wheel drive" (AWD). However, "four-wheel drive" typically refers to a set of specific components and functions, and intended off-road application, which generally complies with modern use of the terminology.
Uses[edit]
Road racing[edit]
Spyker is credited with building and racing the first four-wheel-drive racing car, the Spyker 60 HP in 1903.[53][17]
Bugatti created a total of three four-wheel-drive racers, the Type 53, in 1932, but the cars were notorious for having poor handling.
Miller produced the first 4WD car to qualify for the Indianapolis 500, the 1938 Miller Gulf Special.
Ferguson Research Ltd. built the front-engined 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 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, 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 controversial 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 midseason 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.[54] One example is the Audi R18 e-tron quattro (winner of 2012 race, the first hybrid/4WD to win Le Mans), using an electric motor in the front axle with the gasoline engine in the rear.[55]
In heavy trucks[edit]
Medium-duty and heavy-duty trucks have recently adopted 4×4 drivetrains; 4×4 medium-duty trucks became common after Ford began selling Ford Super Duty trucks. These 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. 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.
Terminology[edit]
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,[57] 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, are classified as 6×6s regardless of how many wheels they have. Examples of these with two rear, one front axle are the six-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.
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.
Unusual systems[edit]
Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, 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 axles, 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.[58]
BMC experimented with a twin-engined 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 the 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 six-speed manual transmission.
Nissan Motors has developed a system called E-4WD, 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.[59])
Chrysler's Jeep Division debuted the twin-engined, 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 the 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 from 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 and Purosangue is unique in that it has a rear transaxle with a 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 that 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.[60]