Mechanics

Transfer Case

The term four-wheel drive is used interchangeably with all-wheel drive and describes the ability of a vehicle to transfer the engine's power to all four wheels. The majority of vehicles on the road do not offer this feature, as either the front or rear wheels are driven by the engine's power. However, a four-wheel drive system offers a distinct advantage when traction is limited in slippery conditions such as on snow, mud, loose gravel or sand. In the past 4 wheel drive meant trucks with big tires and big engines. Although trucks still enjoy the many benefits of the all wheel drive feature most manufacturers have developed a new technologically all wheel drive system that can accelerate and handle corners more efficiently. 

 There are many different all wheel drive systems offered on the automotive market today; this can be confusing to the average consumer. Each manufacturer will use a unique term for their specific four-wheel drive system - whether it is the Audi Quattro all-wheel drive, the Honda real-time four-wheel drive, Volkswagen's 4Motion or Mercedes-Benz's 4Matic. However, most of the four-wheel drive systems offered today can be broken down into two main categories:

Part Time Four Wheel Drive

Part-time four wheel drive: As its name implies, this form of four-wheel drive powers all four wheels only when the 4WD mechanism is engaged. Typically, these systems power the rear wheels during ideal weather conditions to reduce the wear on the drive train and improve fuel economy. However, when four wheel drive is engaged, engine power is transferred to the front wheels as well as the rear. In a part time four wheel drive vehicle the engine's power is transferred into a transfer case that is mounted to the rear of the transmission. The transfer case then divides the torque evenly between a front and rear driveshaft 50% to the front, 50% to the rear. The drive shafts are connected to both differentials (front and rear), which divides power to each wheel.

On some older systems the part time four wheel drive system can be disengaged from powering the front axle by unlocking the front hubs (front hubs are used to attach the driven wheels to the axle). The front hubs are either disengaged manually by the driver, or automatically when the driver presses a switch on the dashboard. When the front hubs are disengaged the wheels are allowed to spin freely, power from the engine is transferred solely to the rear wheels. To return to four-wheel drive at a later time, the hubs must once again be locked to the front wheels.

All Wheel Drive

This system is gaining popularity in the newer cars and trucks, some manufacturers such as Subaru market their vehicles by making their entire model line all-wheel drive. In a typical all-wheel drive system all four wheels are powered at all times. However, unlike a true four-wheel drive vehicle, the power split between the front and rear axles are not set at a fixed value (typically 50% front, 50% rear) but can be varied depending on available traction. All wheel drive systems typically work by having an active center differential (located on the rear of the transmission) that under normal driving conditions splits power evenly between the front and rear axles. However, when driving conditions change and wheel slip is detected at one axle, the center differential responds by transferring more torque to the axle with the most traction. 

This change in torque split maximizes the traction available at each axle and in extreme conditions it is possible for 100% of power to be transferred to just one axle. One other kind of all-wheel drive system that is becoming relatively common can be best described as part time all wheel drive. In this system, either the front or rear axle receives all of the engine's power during normal driving, but when slip is detected, power is transferred to the other axle in just a fraction of a second. Four-wheel drive (4WD) vehicles have gained mainstream acceptance over the last two decades due to the popularity of the Sport Utility Vehicles in the United States. Car manufacturers have also marketed four-wheel drive vehicles because of their added traction on slippery roads and rugged terrain.

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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-20)