How Four-Wheel-Drive Transmissions Work.
Every time we hear of four-wheel-drive transmissions, we think of off-road vehicles dancing in the mud. However, unlike in the past when this transmission was synonymous with SUVs, modern-day manufacturers have it on small cars too. Four-wheel drive is also known as 4WD, or 4×4. It refers to vehicles with two axles whose drive-train can simultaneously provide torque to all wheels. It may either be full time or four-wheel drive on demand.
Full-time Four-Wheel Drive.
These vehicles have all their wheels driven by the engine to provide maximum traction. The system is commonly used in performance vehicles to improve their handling traction. When used in such applications, the drive train has viscous coupling that provides the front and rear wheels with different amounts of torque. It becomes critical especially in tight corners where the rear and front wheels are spinning at different speeds.
A disadvantage of this system is that when one wheel loses traction, the differential transfers all power to that wheel, resulting into a no-drive situation. To overcome this setback, manufacturers equip the system with a center differential lock. The differential lock transmits power to the front and rear axles on a 50/50 basis.
Part-Time Four-Wheel Drive.
In this transmission, the vehicle is driven in two-wheel drive mode during regular applications. When extra traction is needed, the driver switches into four-wheel drive mode. It is preferred because it reduces friction on the drive train, resulting in reduced tire wear and improved fuel economy for your car.
Part-time four-wheel-drive transmissions have locking hubs on their front wheels. The role of these hubs is to disconnect the front wheels from the half shaft, front differential, and driveshaft. In such a way the front differential, half shaft, and driveshaft do not spin when the two-wheel drive mode is engaged. A sliding collar is used in both automatic and manual systems to lock the front half-shafts to the hub. The transfer case in these vehicles is used to split torque between the front and rear axles. Depending on the manufacturer specifications, the vehicle might need to slow down to shift from two-wheel to four-wheel mode. Some vehicles can shift regardless of the speed they are going at the time.
Four-wheel-drive transmissions are essential in rough driving conditions such as in mud, snow, slippery roads, or when towing. Depending on your preference, you can choose to get either a full-time or part-time four-wheel-drive vehicle.