fourwheeldrive

Four-wheel drive (4WD) is a drivetrain that delivers torque to all four wheels to improve traction on low-friction surfaces. In traditional 4WD, a transfer case splits power between the front and rear axles and allows mode selection. Some 4WD systems are part-time and manual; others are full-time or automatic.

Core components include the engine, transmission, transfer case, front and rear differentials, and drive shafts. A center differential or clutch within the transfer case permits wheel-speed differences to reduce drivetrain binding on paved roads. Low-range gearing provides extra torque at low speed for difficult terrain.

Variants include part-time 4WD, which often has no center differential and is ill-suited for dry pavement; full-time or permanent 4WD with a center differential or coupling for on-road use; and related all-wheel drive (AWD), which automatically powers all wheels. Some systems also include electronic controls or torque-vectoring to optimize power distribution.

Applications and considerations: 4WD improves traction for off-road, snow, mud, and steep grades, but adds weight, complexity, and reduces fuel economy. It is unnecessary on dry highways and may cause drivetrain wear if misused. Maintenance includes regular oil changes in the transfer case and differentials and inspecting seals.

History: 4WD appeared in tractors and military vehicles in the early 20th century, with civilian use growing after World War II. Vehicles such as the Jeep and Land Rover popularized 4WD in the mid-20th century; today, many light trucks and SUVs offer 4WD, while many cars use AWD.