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How Does it Work /
4 Four Wheel Drive
How Does All Wheel (4 Wheel Drive) Drive 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.
All Wheel Drive Example (appearance may vary)
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 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:
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.
Manual Axle Hub
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.
How Does it Work
/ 4 Four Wheel Drive
Check Manufacturer Specific 4 Wheel
Drive Questions and Answers
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