Equipment Specs
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Four-wheel Drive

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(Redirected from four-wheel drive)
Mechanical Features and Designs
Four-wheel Drive Equipment is Identifiable by its Protruding Hubs
Four-wheel drive
(4WD) is a feature on a vehicle that allows all four of the wheels to operate. This feature is different from all wheel drive (AWD) in that 4WD is only in operation on a part-time basis when torque (the force required to move the vehicle) needs to be applied to all of the wheels to supply the right amount of traction in conditions where two wheels will not suffice. First appearing in the early 20th century, 4WD is built differently depending on the manufacturer and has a number of pros and cons associated with it, including its ability to manage different surface conditions and its inability to provide sufficient torque in circumstances when one wheel leaves the ground.


[edit] History

[edit] Early Inventions

There is some debate about the inventor of the four-wheel drive. Some sources attribute the invention to Ferdinand Porsche, the same man who founded the Porsche vehicles. Porsche allegedly invented the feature for an Austrian truck manufacturer called Jacob Lohner in 1900. This vehicle was said to consist of electric hub motors positioned for each wheel and an engine that was supplied with power via generator. Porsche, who was 25 years old at the time of his invention, also led the way for NASA to consider using electric wheel hub motors for their lunar vehicles.[1]

Another possibility is that the first instance of four-wheel drive occurred earlier, in 1898, in France by Latil. They were supposedly even manufactured in the same year by a company called Tatra. The same source also credits Porsche with his version of the 4x4 called La Toujours Contente. Porsche also patented an invention called the Mixte transmission. The transmission, which cost too much to produce and become a success, consisted of a petrol engine dynamo and electric motors for the wheels.[2]

Yet another source credits the first four-wheel drive truck to Brookford, an American company that built a 4WD in 1905, although production numbers were very low.[3]

The first mechanical version of the 4WD was produced by a Dutch company called Jacobus Spyker in 1902. Many consider this to be the first invention of the 4WD because Lohner’s invention consisted of many unusual components. Spyker’s automobile was such a success that it was exhibited at a show in Paris within a year of its creation. Spyker’s creation came about when two brothers attempted to produce a vehicle that could successfully climb steeper hills.

Another manufacturer of automobiles was Daimler Benz, who began using 4WD in his line of vehicles, as well as Mercedes in 1903. Some of their versions even included all wheel steering.

The first American version of the 4WD appeared in 1911 by a company called FWD by Zachow and Besserdich. FWD the company was famous for its contribution of 4x4 trucks in World War I. [4] The production of 4WDs in America led to a slew of manufacturers worldwide within the next several years. These included Jeffrey Quad’s trucks (1913), Big Lizzie’s road trains (1915), Citroen (1922), AEC (1929), Miller’s racing cars (1932), and Dodge (1934).

[edit] World War II 4WD Production

By the arrival of the Second World War, 4WD was in full tilt. Ford and Willys-Overland produced as many as 700,000 general purpose army vehicles with 4WD for the war.[5]

Perhaps one of the most important 4WD contributions to the war should be credited to Jeep – who produced the Dana Spicer 18—a four-wheel drive vehicle that played an important part in transportation during that tumultuous time. The Dana Spicer was a cast-iron offset drive transfer case, complete with dual mode and dual range features. In the central rear was a power take-off (PTO) feature, as well as many other features.

FWD and Marmon-Herrington also produced 4WD vehicles. In fact, the event of war is considered the main reason why many manufacturers began developing 4WD vehicles. By the time the war had ceased, GM, Dodge, and Ford had fully recognized the benefits of the 4WD feature.

[edit] 1960s Onward

Manufacturers set out to improve the prototypes of 4WDs in the 1960s and 1970s. Jensen FF, an early 4WD type vehicle, featured a Ferguson Formula 4WD and a Dunlop anti-skid braking system, along with cooling louvers positioned in the front fenders. It was a costly project that was discontinued when the Interceptor was revised. Although some sources consider it the first 4WD supercar, many parts used to make it were rare and it was extremely pricey to repair and improve. Production ceased for the most part in 1971.[6]

Harvey Lamm, the founder of Subaru (America) was one of the first factory-produced 4WD vehicles in 1975. Its simplistic design led to Roy Lunn of Jeep to produce the Quadra-Trac 4WD in 1973. This consisted of a Borg-Warner transfer case that would permit it to tackle all types of road surfaces. Roy Lunn’s version provided the inspiration for AMC’s Eagle. The Eagle was very successful, achieving 200,000 in production sales from 1980 to 1988. The Eagle operated with Lunn’s drivetrain and was considered revolutionary for its transfer case because it consisted of an automatic clutch that allowed the cars to operate with a four-wheel drive on any surface at all given times.[7]

The Eagle was unusual in appearance because of its Concord body and elongated wheel base that stretched from one inch (2.5 cm) to 109 (277 cm). With 110 horsepower and the option of two- or four- door sedans, it generated considerable success. The bumpers were closer in proximity to the body of the vehicle as compared to standard versions and it had 15-inch (38-cm) tires as opposed to the 14-inch (36-cm) ones previously used. According to AMC, the Eagle was meant for light duty only and could tackle very little off-road surfaces. Reviews for the Eagle were mixed, some coining it as the next generational vehicle and others were frustrated wit its inability to operate on more challenging surfaces. One source described the Eagle as looking “like a Hornet standing on its tiptoes.” The Eagle’s success began to decline dramatically during the 1980s and ceased production by 1988.[8]

[edit] Features/How it Works

Four-wheel drive (also known as 4WD or part-time drive) was designed to provide power to all wheels to function on a variety of surfaces. Vehicles with 4WD can be adjusted to use any of the four wheels in a way to combat surfaces that require higher traction such as unpaved roads, snow, or ice and can be switched off when they are no longer needed.

Traction for the four wheels is achieved with the help of torque. Torque is the twisting action that occurs when the engine is operating. The gears of the vehicle, which put the four wheels into motion, are used to divide the force of the torque into each of the wheels. It can be adjusted to provide more torque for particular wheels, depending on the conditions.

The amount of torque required is determined by the traction, not the engine. Traction is defined as the force of the tire as applied to the ground surface. Traction can be determined by many different factors. If a tire has a lot of weight, it will also have more traction and can handle sharp turns and slippery conditions more effectively.

Another factor to consider is the contact that the tire makes with the surface—whether it is static or dynamic. Static is described as a situation where the road and the tire do not slip in a relative manner. Dynamic contact occurs when they do.

When slippage occurs, tires apply force in two ways: through longitude and laterally. Longitude force is supplied by the torque to the tire through the engine or brakes. It can both accelerate and decelerate the speed. Lateral force is applied during turns and occurs when cars change directions.

Longitude and lateral force can be applied to effectively manage driving conditions in a 4WD that otherwise would not be possible. Because a vehicle is able to apply force and traction to all four tires, as opposed to two, longitudinal force can be doubled. This is especially useful on snow-laden roads. Typically, a two wheel drive car couldn’t travel for more than a few inches, but the 4WD can apply the right amount of traction to all four tires to accomplish this task.

On dirt roads and slippery hills, 4WD best provides for these conditions by utilizing traction in all four tires. 4WD can’t be relied upon on its own. Stopping on slippery surfaces also relies heavily on anti-lock braking systems (ABS) and brakes.

[edit] Differentials

Cars have differentials positioned in the rear and in the front of the wheels. Differentials allow the torque to travel through the driveshaft or transmission and provide force for the wheels. They permit the wheels to spin at different speeds depending on the direction they are traveling. For example, when a car drives around a turn, the inner wheels are spinning in a different direction than the outer wheels and at different speeds.

[edit] Transfer Case

A transfer case is an apparatus that allows the power to be divided between the axles in the front and rear of a 4WD. Transfer case locks the front axle to the rear axle and the result is that the wheels spin at the same speed. This makes the tires slip and therefore is only used in low-traction conditions where slippage occurs frequently. Some systems of 4WD also contain a transfer case that has extra gears that provide the car with a low range, or extra torque to drive up steep hills.

[edit] Locking Hubs

The wheels of a car are attached to a locking hub. Located on the front wheels, they allow the hub to disconnect the front wheels from the front differentials and the driveshaft. This occurs to conserve fuel and energy when only the two-wheel drive is being utilized. It can occur manually, as is often seen in older versions, or by automatic locking hubs that adjust while in motion, as seen in newer vehicles.

[edit] Advanced

Manufacturers have implemented more advance systems for the 4WD, introducing ABS systems, a feature that allows brakes to be applied to wheels during the process of slipping. Other types include an electronically-controlled clutch that manipulates the torque applied to the wheels.

[edit] Basic

Basic systems usually consist of rear-wheel drives where the transfer case is directly connected to the transmission. Both the front and rear axles are each turned by a driveshaft. The front and rear driveshafts are locked together to produce a 4WD, resulting in the division of force in each axle while the hubs interlock.

When the transfer case is locked, the vehicle cannot be used on-road because of the open differential of the front and rear axle, which provides them with considerable traction. It also poses a problem in that open differentials tend to split torque force equally, but if one of the two wheels with divided torque force leaves the ground, the torque force of that wheel transforms to nothing. Because the wheels are divided equally, the second wheel will follow suit with zero torque force. Alternatively, if the torque system used by the manufacturer doesn’t divide equally, the tires cannot avoid slippage as effectively.

Improvements to this system have been made. Among them is replacing the open differential with a limited-slip rear differential. The limited-slip applies at least some torque to both of the rear wheels. A second option is the locking differential, which hooks both wheels together so that the both will receive force from the torque onto the axle. This is helpful because if one of the wheels leaves the surface, traction can still be controlled.[9]

[edit] 4WD Hummer

The Hummer (AM General) contains some of the most advanced technology in what some consider to be the most effective 4WD system. It consists of a transfer case that is hooked up to the transmission, as seen in basic systems. But this technology is advanced in that the transfer case does not automatically lock with the front and back axles. Instead, it utilizes gears that are open differential (controlled by the driver). The open differentials are called Torsen differentials. When it is unlocked, both axles operate at different speeds, permitting accessible driving on smooth, dry surfaces. The other option is to lock the differential, providing torque force to whatever wheels require it, making it easier on undesirable surfaces. Torsen gives the differentials a different gear set which allows the torque for one wheel to decrease when a tire is about to slip. While beneficial, it still poses the problem of providing zero torque force to both tires when one leaves the ground.

In an attempt to combat this, the Hummer was revised with a brake traction control system that applies brakes to a wheel when it begins to slip. It also makes use of the traction but attempting to prevent slippage and applies more torque to the other wheel.[10]

[edit] Pros and Cons

One of the biggest problems posed by 4WD vehicles are the different rpms required by the front and rear. The transfer case typically power the both with the same rpms, even though the front axles usually require more. Only all wheel drive (AWD) can manipulate the torque force that is needed by front and back wheels.

The front driveshaft can also cause wide turns imposed by delivering the same rpm to the rear as the front. Another problem is caused by the connection of the front wheels to the transfer case, which means the front wheels cannot be pulled as much as they should be. The speed of the front wheels is decreased with what is called axle binding or driveline binding – a process that allows the mechanics to bind, preventing steering that is difficult. 4WD also make it difficult to drive on surfaces that have high traction, such as asphalt or concrete. When this occurs, a high build-up of pressure is added to the tires producing different axle speeds. On low traction surfaces, such as sand, mud and snow, the understeer of 4WD can be challenging and cause a reduction of traction through slippage of the front wheels.[11]

[edit] 4WD Today

Due to the criticism and different needs of consumers, most manufacturers are producing all-wheel drive vehicles. SUVs primarily use 4WD because of their off-road features while most other commercial vehicles and cars operate under AWD. The cost and complexity of the 4WD makes it a less attractive feature, especially when considering all the wheel-drive options that are available for vehicles today. Systems are created by various manufacturers and are best suited for different conditions.[12]

[edit] References

  1. 4WD. 4x4abc.2008-09-29.
  2. 4WD. 2008-09-29.
  3. Model 18. Novak-Adapt. 2008-09-29.
  4. 4WD 101. 4x4abc. 2008-09-29.
  5. 4WD. 2008-09-29.
  6. Model 18. Novak-Adapt. 2008-09-29.
  7. All-Wheel Drive Revisited. BNet. 2008-09-29.
  8. Jensen FF. 2008-09-29.
  9. Four Wheel Drive. 2008-09-29.
  10. Four Wheel Drive. 2008-09-29.
  11. Turnapart. 4x4abc. 2008-09-29.
  12. Features. Edmunds. 2008-09-29.