Four-wheel drive

A 4x4 Jeep Wrangler operating off-road

A Four-wheel drive (also called 4WD and 4x4) is a vehicle on which all four wheels get power from the engine to move forward. This is as opposed to a two-wheel drive vehicle. 4WDs are often used off road. Part-time and full-time four wheel drive vehicles uses a transfer case as part of the drivetrain to direct power to one or both axles.^[1] By having a transfer case the driver can shift the vehicle into either "two-wheel-drive" or "four-wheel-drive" mode (part-time systems). Many transfer cases also have a low power range that multiplies the engine power when in four-wheel drive (called four-wheel low gear).

Types

Four Wheel Drive
"A" points at the engine, "B" points at a driving wheel, "C" points at the transfer case or center differential depending on system

There are basically four types of four-wheel drive.

Part-time 4-wheel drive

Part-time systems are the most basic type. They are also still the most popular type for using off-road. The can be shifted (mechanically or electrically) from two-wheel drive to four-wheel drive. This gives them the versatility to operate both on and off-road. When the four-wheel drive system is engaged, the two axles turn at the same speed. There usually is no center differential. For that reason part-time four-wheel drive should not be used on dry pavement.^[2] If operated for any distance on dry pavement tire damage and drivetrain damage is very possible.^[2] Because it operates off-road, the vehicle typically has a higher Ride height. This is to go over obstacles on uneven ground without damage. Even when operated in two-wheel drive on-road, this system will not get the fuel economy of a comparable two-wheel drive vehicle. Locking hubs are available on some models.^[2] They help to get better fuel mileage by disengaging the front axle differential.

Full-time 4-wheel drive

This is the original four-wheel drive. It was and still is designed for serious off-roading.^[3] Common on full-time four-wheel drives are locking differentials. These prevent one wheel slipping on the same axle if the other has traction. These systems do not get good gas mileage due to their typical weight.^[3]

2009 Ford Edge with selectable AWD

All-Wheel Drive

All-wheel drive (AWD) uses a center differential to direct a percentage of the power to both axles, depending on conditions. All wheel drive is not designed specifically to go off-road but is for traction in rain, snow or ice on the road. Body-on frame SUVs are typically designed for off-road use. Unibody crossovers are usually only equipped with the lighter-duty all-wheel drive. However the unibody built Grand Cherokee and newly designed Durango are available with four-wheel drive.

Part-Time All-Wheel Drive

Part-time All-wheel drive (AWD) operate as a front-wheel drive under normal conditions.^[3] When traction is lost for any reason, the system automatically sends a portion of the engine's power to the rear wheels.^[3] As soon as the system detects good traction, it disengages the rear wheels and shifts back to front drive. This is a less expensive system that adds less weight to the vehicle (usually a car). This makes it popular on compact vehicles. It typically gets better fuel economy than all-wheel drive vehicles. They do not have a low range or a transfer case.^[3]

Four-wheel Drive Media

The Jeep Wrangler (pictured is a TJ Wrangler) is a 4WD vehicle with a transfer case to select low-range or high-range four-wheel drive.
Center transfer case sending power from the transmission to the rear axle (right) and front axle (left)
The Lamborghini Murciélago is an AWD that powers the front via a viscous coupling unit if the rear slips
The HMMWV is a 4WD/AWD that powers all wheels evenly (continuously) via a manually lockable center differential, with Torsen differentials for both front and rear
To prevent slippage, some vehicles have controls for independently locking center, front, and rear differentials
Selection lever: 2H for two-wheel drive, 4H for high-range 4WD, 4L for low-range 4WD, and N for neutral
Selection lever: All-time 4WD, neutral, and part-time low-range 4WD
The 1893 Diplock Steam Locomotive was the world's first 4WD land vehicle.
The Lohner–Porsche Mixte Hybrid was both the world's first hybrid vehicle, and the first four-wheel drive without a steam engine.
The 1903 Spyker 60-HP was the world's first 4WD that was directly powered by an internal combustion engine, and the first 4WD race-car.

References

↑ Ronald Montoya (9 July 2013). "Do You Need an All-Wheel-Drive or Four-Wheel-Drive Car?". Edmunds.com, Inc. Retrieved 30 August 2015.
↑ ^2.0 ^2.1 ^2.2 Ann Job (April 2008). "Four-Wheel-Drive Systems Demystified". Autotrader, Inc. Retrieved 30 August 2015.
↑ ^3.0 ^3.1 ^3.2 ^3.3 ^3.4 Jeff Youngs (24 February 2012). "Todays Four-wheel-drive Systems-Which One Do I Need?". J.D. Power and Associates, McGraw Hill Financial. Archived from the original on 27 August 2015. Retrieved 30 August 2015.

[1] Ronald Montoya (9 July 2013). "Do You Need an All-Wheel-Drive or Four-Wheel-Drive Car?". Edmunds.com, Inc. Retrieved 30 August 2015.

[JobAT-2] 2.0 ^2.1 ^2.2 Ann Job (April 2008). "Four-Wheel-Drive Systems Demystified". Autotrader, Inc. Retrieved 30 August 2015.

[YoungsJDPA-3] 3.0 ^3.1 ^3.2 ^3.3 ^3.4 Jeff Youngs (24 February 2012). "Todays Four-wheel-drive Systems-Which One Do I Need?". J.D. Power and Associates, McGraw Hill Financial. Archived from the original on 27 August 2015. Retrieved 30 August 2015.

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