Auto Questions: How Four-Wheel Drive Works

Four wheel drive systems have enhanced and helped our abilty to drive in adverse driving conditions. This article will help you to understand how these systems work.

Four-wheel drive (4WD) vehicles have gained mainstream acceptance over the last two decades due to the popularity of the Sport Utility Vehicle in the United States. Car manufacturers also market four-wheel drive vehicles heavily due to their added traction on slippery roads, and during the wintry months dealership sales are normally very strong. So how does the four-wheel drive system work?

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 - due to four driven wheels offering more traction than two.

There are many different four-wheel drive systems offered on the automotive market today and 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 Audi's quattro all-wheel drive, Honda's 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:

1) Part-time four-wheel drive

2) All-wheel drive

Part-time four wheel drive: Like 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, power is transferred to the front wheels as well.

In a part-time four-wheel drive vehicle, the engine's power is transferred to a transfer case inside the transmission that then splits the torque evenly between a front and rear driveshaft (50% to the front, 50% to the rear). The driveshafts are connected to two axle differentials (front and rear), which split power to the wheels at each corner.

During ideal driving conditions, the part-time four-wheel drive system can be disengaged from powering the front axle by unlocking the front hubs (hubs are used on vehicles to attach the driven wheels to the axle). The front hubs are either disengaged manually by the driver, or hydraulically when the driver presses a switch on the dashboard. When the front hubs are disengaged and 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 onto the front wheels.

All-wheel drive: This system is gaining popularity and some manufacturers such as Subaru market their vehicles by making their entire model line all-wheel drive. In the 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 is not set at 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 in 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. However, the normal torque split returns when the vehicle is on a grippy surface again.

One other kind of all-wheel drive system that's 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. Some part-time all-wheel drive systems are so advanced and lightning quick, that the wheels which normally receive 100% of the engine's power only need to slip a sixth of a revolution before power is transferred to the other axle! However, once traction is regained, the vehicle returns to being two-wheel drive once more.

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