4WD

Four-wheel drive means the engine's torque is transmitted to all four wheels simultaneously, not just the front or rear pair. The system uses a transfer case, a gearbox mounted between the transmission and axles, to split power equally or variably between front and rear differentials. This gives traction on loose, soft, or slippery surfaces where two-wheel drive would lose grip and bog down.

4WD systems come in two main types: part-time and full-time. Part-time 4WD uses a manual or electronic clutch to engage the front axle only when needed; the driver shifts into 4WD for off-road use and must disengage it before driving on hard pavement, or the drivetrain binds and tire scrub causes wear and steering strain. Full-time 4WD, found in many SUVs and crossovers, runs continuously and uses a center differential to allow front and rear wheels to turn at different speeds on tarmac, eliminating binding.

Variants and torque distribution

All-wheel drive (AWD) is often confused with 4WD but works differently. AWD uses viscous couplings, clutches, or electronic differentials to vary torque delivery moment by moment, sending more power to whichever axle has grip. 4WD divides power statically or in fixed ratios. Vehicles with locking differentials can lock the front and rear axles together, forcing both wheels on an axle to turn at the same speed; this gives maximum grip in extreme terrain but damages tires on pavement.

4WD adds weight, complexity, and fuel consumption compared to rear-wheel or front-wheel drive. The transfer case, front differential, and heavier axle shafts increase the vehicle's mass by 200 to 400 kg. Maintenance is more demanding: 4WD vehicles need separate differential fluid checks, transfer case oil changes, and regular inspections of U-joints and driveshafts. Tire wear accelerates if drivers engage 4WD on dry roads or fail to disengage low-range gearing.

4WD is standard on trucks, off-road SUVs, and utility vehicles used in agriculture, mining, forestry, and remote construction. Commercial operators use 4WD for poor-condition access roads, mud, snow, and steep slopes. The term originated in the 1930s when manufacturers began engineering vehicles with powered front axles; it has remained unchanged in vocabulary even as the technology evolved from mechanical to electronic engagement and torque control.