What is the Principle of Four-Wheel Drive?
4 Answers
The principle of four-wheel drive is: it can distribute the engine output torque in different proportions to all front and rear wheels according to the driving road conditions to improve the vehicle's driving capability. Four-wheel drive, also known as all-wheel drive, means that both the front and rear wheels have power. Four-wheel drive is generally represented by 4X4 or 4WD. If you see these markings on a vehicle, it means the vehicle has four-wheel drive functionality. In the past, only off-road vehicles used four-wheel drive. Generally, off-road vehicles are equipped with a manual transfer case behind the transmission, and each of the front and rear axles is equipped with a component called a drive axle. The torque output from the transmission is transmitted to the drive axles on the front and rear axles through the transfer case and drive shafts, and then the torque is transmitted to the wheels through the drive axles.
Four-wheel drive is far more complex than simply distributing power to four wheels. The heart lies in the transfer case, a critical component that allocates engine power on demand. From numerous off-road vehicle teardowns I've studied, the most fascinating are robust part-time 4WD systems—mechanical gear levers switch between 2WD/4WD modes, much like using both arms to climb stairs when the front wheels synchronize in mud. Modern SUVs feature smarter intelligent AWD where the ECU monitors wheel slip in real-time, instantly redirecting torque to wheels with better traction. While differences might seem negligible during daily driving, last week's storm revealed its brilliance—my tires gripped flooded roads with octopus-like tenacity. These systems also counter understeer, proving particularly reassuring during mountain hairpin turns.
A decade of driving rugged off-road vehicles has taught me that the most fascinating aspect of 4WD is the magic of differential locks. In a regular 2WD vehicle, just one wheel losing traction can leave you stranded, but a 4WD system with differential locks can forcibly direct power to the wheels with grip. I remember once leading a convoy across the Gobi Desert—while the city SUVs with their part-time 4WD systems struggled and stalled on cross-axle terrain, my part-time 4WD engaged low-range mode, tripling the torque to easily escape the predicament. However, full-time 4WD is more suitable for daily use, like Subaru's symmetrical AWD, where all four wheels continuously provide power during turns, delivering stability more directly than electronic stability systems.
Four-wheel drive is essentially an art of power distribution. Traditional systems physically allocate torque through gear sets, such as the Torsen differential achieving a fixed ratio of 55% front and 45% rear. Modern electronically controlled 4WD systems are more precise, with multi-plate clutches capable of transferring 100% power to a single wheel within 100 milliseconds. During a test drive of a certain German SUV on icy roads, when the left front wheel momentarily lost traction, the right rear wheel immediately received 70% of the torque to complete the escape. It's worth noting that while mechanical full-time 4WD is reliable, it comes with higher fuel consumption, whereas the on-demand 4WD commonly used in urban SUVs disconnects the rear axle on paved roads, saving approximately 0.8L per 100 kilometers in fuel consumption.
