Most 4WD systems on
trucks and utility vehicles use front-wheel drive hubs that can be disengaged
from the front axle when the vehicle is operating in the 2WD mode. When unlocked
in 2WD, the front wheel still turn, but the entire front drivetrain, including
the front axles, the front differential, the front drive shaft, and certain
internal transfer case components, stop turning. This helps reduce wear to these
The front hubs must
be locked during 4WD operation. Some front hub designs lock automatically.
Others require the driver to get out and turn a lever or knob at the center of
each front wheel. By locking the hubs, the wheels turn with the axle for the
most traction. When unlocked, the wheels are free to spin at different
rotational speeds, such as when cornering. The tires scrub on the pavement if
the hubs are locked, causing them to wear faster, so they should only be locked
when venturing off-road or under other low-traction situations, such as heavy
rain, snow, or ice.
The disadvantage of
the self-locking hub is that power cannot be applied to the wheels in reverse.
this can be a big disadvantage if the vehicle gets stuck and backing up is the
only way out. To unlock the automatic type, stop the car, disengage the four
wheel drive, and back up about 3 feet. Often these hubs unlock immediately when
the four-wheel drive is disengaged, without the need to back up.
development in hubs is the fully automatic type. Once four-wheel drive is
engaged, they lock automatically when power is applied, either forward or
reverse. That is, power can be applied in either forward or reverse direction
Some 4WD vehicles are
equipped with ordinary front wheel hubs, but have a vacuum-operated axle
disconnect shift mechanism. The vacuum motor moves a splined collar to connect
or disconnect one of the front axle drive shafts from the front differential.
The axle shafts and differential continue to turn. However, the ring gear,
pinion, and front drive axle remain stationary. This reduce wear to these major
components. This system is used on both trucks and passenger cars.