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 items.

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.

The latest 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 once locked.

Axle Disconnects

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.