Off-Road Vehicle

An off-road vehicle is considered to be any type of vehicle which is capable of driving off paved or gravel surface. It is generally characterized by having caterpillar tracks or large tires with deep, open treads and a flexible suspension. Other vehicles that do not travel streets or highways are generally termed off-highway vehicles, which would include things like tractors, forklifts, cranes, backhoes, bulldozers and Golf carts.

Off-road vehicles have an enthusiastic following because of their many uses and versatility. Several types of motorsports involve racing off-road vehicles. The three largest "4 wheel vehicle" off-road types of competitions are Rally, Desert Racing, and Rockcrawling. The three largest types of All Terrain Vehicle (ATV) / Motorcycle competitions are Motocross, Enduro, and Desert Racing. These sports are often celebrated in competition events due to public interest. The most common use of these vehicles is for sight seeing in areas distant from pavement. The use of higher clearance and higher traction vehicles enables access on trails and forest roads that have rough and low traction surfaces.

History

One of the first modified off-road vehicles was the Kégresse track, a conversion undertaken first by Adolphe Kégresse, who designed the original while working for Tsar Nicholas II of Russia between 1906 and 1916[1]. The system uses an unusual caterpillar track which uses a flexible belt rather than interlocking metal segments. It can be fitted to a conventional car or truck to turn it into a half-track, suitable for use over rough or soft ground. Conventional front wheels and steering are used.

After the Russian Revolution of 1917 Kégresse returned to his native France where the system was used on Citroën cars between 1921 and 1937 for off-road and military vehicles. The Citroën company sponsored several overland expeditions with their vehicles crossing North Africa and Central Asia.

After World War II a huge surplus of light off-road vehicles like the Jeep and heavier lorries were available on the market. The Jeeps in particular were popular with buyers who used them as utility vehicles. This was also the start of off-roading as a hobby. The wartime Jeeps soon wore out, though, and the Jeep company started to produce civilian derivatives, closely followed by similar vehicles from British Land Rover and Japanese Toyota and Mitsubishi. These were all alike; small, compact four wheel drives with at most a small hardtop to protect the occupants from the elements.

From the 1960s more comfortable vehicles, later called SUVs, were produced. First they were popular for many years with rural buyers due to their off-road and load-lugging capabilities. The US Jeep Wagoneer and the Ford Bronco, the British Range Rover and the station wagon-bodied Japanese Toyota Land Cruiser were early SUV examples, essentially a station wagon body on a light truck frame with four wheel drive drivetrain.

Information Courtesy of Wikipedia, the free encyclopedia

Four-Wheel Drive

Four-wheel drive, 4WD, 4x4 ("four by four"), all-wheel drive, or AWD, is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. In the United States, these cars are included in the broader sport utility vehicle category. While many people associate the term with off-road vehicles, powering all four wheels provides better control on many surfaces, and is an important part of rally racing on mostly-paved roads.

The term four-wheel drive describes truck-like vehicles that require the driver to manually switch between two-wheel drive mode for streets and four-wheel drive mode for low traction conditions such as ice, mud, or loose gravel. All-wheel drive (AWD) 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, such as the Subaru Impreza, whereas 4WD is used to describe vehicles meant primarily for heavy off-road use.

Identical drivetrain systems are commonly marketed under different names for upmarket and downmarket branding, and conversely different drivetrain systems are commonly marketed under the same name for brand uniformity. For example, Audi's quattro, DaimlerChrysler's 4MATIC, and Volkswagen's 4motion can mean either an automatically-engaging system with a Haldex clutch or a continuously-operating system with a Torsen differential.

There is often confusion as to the difference between 4X4s and SUVs. This leads to criticisms of 4X4 vehicles in the media that should actually be directed at SUVs (see SUV Criticism).

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 the right sides. When all four wheels are driven, a third differential can be used to distribute power between the front and the rear axles.

Such a design handles well. It distributes power evenly and smoothly, and makes slippage unlikely. However, once it does slip, recovery is difficult. Suppose that the left front wheel slips on an icy patch of road with a design that drives all four wheels. The slipping wheel will receive all of the power, causing it to 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. A similar problem occurs between the front and rear axles via the center differential. The front left wheel receives all of the power, again doubling its speed while the rear wheels stop moving; again, the average speed is maintained. The available torque goes to zero, and the left front wheel actually turns four times as fast as it should be turning, preventing the other three wheels from 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 2WD vehicles, but since 4WD vehicles are more likely to have a driven wheel on an icy patch, the differential design is less acceptable.

Limiting slippage

Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake is automatically applied to that wheel. By preventing one wheel from spinning freely, power is divided between the pavement for the non-slipping wheel and the brake for the slipping wheel. This is an effective solution, although it causes additional brake wear and may cause a sudden jolt that impacts handling. By extending traction control to act on all four wheels, the simple three-differential 4WD design will see limited wheel spin. This design is commonly seen on luxury crossover SUVs.

Locking differentials temporarily lock 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 locking differentials use either a computer-controlled multi-plate clutch or viscous coupling unit to join the shafts, while older differentials generally used 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, which hastens torque transfer.

A third approach to limiting slippage is the 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, it 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.

Finally, 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 the loss of traction, instead only enabling recovery once traction is 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.

The first ever four-wheel drive car (as well as hill-climb racer) with combustion engine, the Spyker 60 H.P., was presented in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. The two seater sports car, which was also the first ever car equipped with a six-cylinder engine and featured, as another novelty, brakes for all four wheels, is nowadays exhibit of the Louwman Collection (the former Nationaal Automobiel Museum) at Raamsdonksveer in The Netherlands.

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 annually[1]. The second US four-wheel drive vehicle was built in 1908 by (what became) 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.

Daimler-Benz also has a history in four-wheel drive. In 1907 the Daimler Motoren Gesellschaft had build 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 4x4 featured full time four-wheel drive, four-wheel steering, three differential lockers, 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 an exception of fully independent suspension since it hinders suspension articulation. The Unimog is another Mercedes great.

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. The inspiration was from a Willys MB that was frequently offroaded on the farm belonging to chief engineer Maurice Wilks, and was felt that it needed some refinement.

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.

Information Courtesy of Wikipedia, the free encyclopedia