4WD

Four-wheel drive (4WD, 4x4) refers to a drivetrain configuration that sends torque to all four wheels. It is designed to improve traction in low-friction conditions such as snow, mud, or uneven terrain. 4WD is distinct from all-wheel drive (AWD), which is usually permanently engaged and designed primarily for on-road use, with variable torque distribution managed by differentials and electronics.

Drive systems come in part-time and full-time varieties. Part-time 4WD requires manually selecting 4WD mode and typically lacks a center differential; it provides high and low range in the transfer case, which means it is intended for off-road use and can cause driveline binding on dry pavement if driven in 4WD high.

Full-time or permanent 4WD uses a center differential or an electronically controlled coupling to allow all-wheel drive on any surface, with torque distributed variably. Some systems add a locked mode for extreme traction, but most rely on limited-slip differentials or traction-control systems.

Key components include the transfer case, front and rear differentials, driveshafts, and locking mechanisms; control may be mechanical, hydraulic, or electronic. Vehicles with 4WD often feature a low-range gear ratio for crawling and rock-stomping, enhancing torque at low speeds.

Historically, 4WD originated with military and civilian utility vehicles in the mid-20th century, notably Jeep and Land Rover models after World War II. Today, 4WD is common in trucks and sport utility vehicles and remains a preferred option for off-road driving and difficult terrain.