firehjulsdrift

Firehjulsdrift, literally "four-wheel drive" in Swedish, is a drivetrain configuration in which engine torque can be sent to all four wheels. The main purpose is to improve traction and mobility on surfaces with low grip, such as snow, ice, mud, loose gravel, or uneven terrain. Firehjulsdrift systems are common in trucks, SUVs, and off-road vehicles and can be either part-time or permanent (full-time).

Part-time 4WD uses a transfer case to provide selectable high-range and low-range modes. When engaged, power is sent to both front and rear axles, but there is no center differential, which can cause drivetrain binding on high-traction surfaces. Therefore, part-time 4WD is usually recommended for slippery conditions or off-road use only.

Permanent or full-time 4WD employs a center differential, a viscous coupling, or an electronically controlled coupling to allow front and rear axles to rotate at different speeds during normal driving. Some systems offer a locked center differential or a low-range gear for difficult terrain. In many modern vehicles, torque distribution is automatic, but some models provide selectable modes such as 2WD, 4WD High, and 4WD Low.

Advantages include improved traction, acceleration, braking, and steering control in low-traction conditions. Disadvantages include higher fuel consumption, greater weight and complexity, and, for part-time systems, potential front–rear drivetrain binding when used on dry pavement.

Firehjulsdrift originated in military and off-road applications and later became common in passenger cars and light trucks. The concept remains widely used in SUVs and pickup trucks, with ongoing refinements in center differentials, locking mechanisms, and electronic controls.