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All-wheel Drive

From RitchieWiki

Mechanical Features and Designs

All-wheel drive (AWD) is a feature on a vehicle that allows all four of the wheels to operate at all times, normally without the possibility of being disengaged. This feature is different from four-wheel drive (4WD), which only allows use of all four wheels on a part-time basis. All-wheel drive employs torque (the force required to move the vehicle) to allow it to control traction and direction on hard and slippery surfaces. All-wheel drive is normally only featured on sports vehicles, luxury trucks and “special purpose” vehicles.[1]

It is not known exactly when all-wheel drive made its first appearance, although it arrived after the 4WD was introduced in the early 1900s.

Contents

[edit] Features/How it Works

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.[2]

[edit] Differentials

Cars have differentials positioned in the rear and in 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] Pros and Cons

Only all-wheel drive can manipulate the amount of torque force that is needed by front and back wheels. In a 4WD, different revolutions per minute (rpm) are required for the front and rear and the transfer case is only capable of providing them with the same amount of rpms.[3]

AWD is designed to function with all four wheels in all driving situations, but when a 4WD is used on hard, smooth surfaces, it causes damage to a number of the vehicle’s core components, including the tires, transfer cases and drive axles. AWD also allows the tires to rotate at different speeds, which enable it to control traction easily.[4]

AWD is best for sporting vehicles that want to achieve a quick and rapid start in a short amount of time. The Porsche 911 Turbo, for example, is an AWD vehicle capable of going from zero-to-sixty in just 3.4 seconds. The downside to AWD is that, to compensate for the amount of traction and speed it can acquire, exorbitant costs are associated with it. It is also a more complex vehicle to produce. There is also strain on the drivetrain which affects its ability to make the most of gasoline.[5]

[edit] Today

Today, all-wheel drive systems consist of front wheel drivetrains that come with an auxiliary rear drive axle. One example would be Jaguar’s X-Type sedan, which is able to manipulate the balance of the car with its drivetrain. It provides additional traction and stability. Not only does it consist of aviscous coupling differential but it also has a planetary gear, which can set the amount of torque that is provided for each axle.

Different manufacturers utilize the AWD in different ways. Buick has produced the Rendezvous AWD, a system designed to use the front wheel drive and consisting of an auxiliary drive on the transaxle. The transaxle spins the driveshaft and connects to the rear axle. With the rear axle and the front-wheel drive, an AWD is created. Acura’s MDX does the same.[6]

[edit] References

  1. Winter. Canadian Driver. 2008-09-29.
  2. Four-Wheel Drive. Howstuffworks.com. 2008-09-29.
  3. Turnpart. 4x4abc. 2008-09-29.
  4. Winter. Canadian Driver. 2008-09-29.
  5. Features. Edmunds. 2008-09-29.
  6. Winter. Canadian Driver. 2008-09-29.