Four-wheel drive (4x4) explained
Four-wheel drive, All-wheel drive, AWD, 4WD, or 4x4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously. While many people associate the term with off-road vehicles and Sport utility vehicles, powering all four wheels provides better control than normal road cars on many surfaces, and is an important part in the sport of rallying.
In abbreviations such as 4×4, the first figure is normally the total number of wheels, and the second the number of powered wheels. (The numbers actually refer to axle-ends, which may have more than one wheel.) 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive.By this system, a six wheeled military transport truck would be a "6x6", while the typical American semi-truck tractor unit having two drive axles and a single unpowered steering axle would be a "6x4".
4WD versus AWD
Four wheel drive refers to vehicles that have a transfer case (some of which include a differential that may or may not be lockable) between the front and rear axles, meaning that the front and rear drive shafts will not rotate at different speeds. This provides maximum torque transfer to the axle with the most traction, but can cause binding in high traction, tight turning situations. They are also either full-time or part-time 4WD selectable.
All wheel drive refers to a drive train system that includes a differential between the front and rear drive shafts. This is usually coupled with some sort of anti-slip technology that will allow all wheels to spin at different speeds, but still maintain the ability to transfer torque from one wheel in case of loss of traction at that wheel. All wheels are engaged to the drive full-time.
4WD versus IWD
The term Individual-wheel drive is 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 power to the wheels. The IWD drive is not limited to 4 wheels as there is generally a motor that drives each wheel that can number upwards of 4, but could also identify a single wheeled vehicle.
When powering two wheels simultaneously the wheels must be allowed to rotate at different speeds as the vehicle goes around curves. This is accomplished with a differential. A differential allows one input shaft (e.g. - the driveshaft of car or truck) to drive two output shafts (e.g. - axles shafts that go from the differential to the wheel) independently 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 differential ring gear. Each powered axle requires a differential to distribute power between the left and the right sides. When all four wheels are driven, a third or 'center' differential can be used to distribute power between the front and the 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 wheelslip.
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 isn't 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 tractive 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.
Even though in the general context, the term "four-wheel drive" usually refers to an ability that a vehicle may have, it is also used to designate the entire vehicle itself. In Australia, vehicles without significant off-road abilities are often referred to as All-Wheel Drives (AWD) or SUVs, while those with off-road abilities 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 mistakenly applied to two-wheel-drive variants of these vehicles.
The term 4×4 (read: 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 total number of wheels on a vehicle and the second indicating the number of driven wheels. Today, the term 4×4 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, 4×2 would be appropriate for most two-wheel-drive vehicles, and is often used to describe them as a two-wheel drive. In Australia the term is often used to refer to a ute that sits very high on its suspension. This is to avoid the confusion that the vehicle might be a 4×4 because it appears otherwise suited to off-road applications.
Large American trucks with dual tires on the rear axles and two driven axles are officially designated as 4×4s, despite having six driven wheels, because the "dual" wheels behave as a single wheel for traction and classification purposes, and are not individually powered. True six-wheel-drive vehicles with three powered axles such as the famous M35 2½ ton cargo truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6×6, 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 CB slang, truckers refer to any two-axled vehicle as a "four-wheeler", sometimes in a derogatory context, as distinguished from an "eighteen-wheeler" or tractor/trailer.)
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 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, GB 1113068, a 4WD system 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 E4WD. 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.