What Is Four-Wheel Drive?
This is the more traditional system that comes to mind when most people think of drivetrains that power all four of a vehicle's wheels. It isn't surprising since the principle goes back almost to the beginning of motorized transportation. The stereotypical picture of a 4WD vehicle is of a truck with high ground clearance, a shielded underbody, tow hooks and big, knobby tires. And it's true that this system is still found primarily in large trucks and SUVs.
But through the years, 4WD engineering has become increasingly sophisticated, and although it generally remains capable of more serious off-road use, it can now be found on a wider variety of comfortable, even luxurious, models. 4WD systems deliver torque through a series of front, rear and center differentials, transfer cases and couplings, which allow the vehicle to operate at maximum traction under a variety of conditions.
How Does Four-Wheel Drive Work?
Like AWD systems, 4WD is designed to send torque to all four of a vehicle's wheels to increase traction when needed. But 4WD systems tend to be more robust than AWD ones and can generally handle more rugged terrain. And they, too, come in two types: full-time and part-time.
Many 4WD systems also have low and high ranges that can be selected by the driver, either with an electronic switch or a floor-mounted mechanical lever. The low setting provides maximum traction in an off-road environment, while the high setting is the default configuration, useful for slippery on-road conditions, such as packed snow, ice, loose sand or gravel.
Full-Time Four-Wheel Drive
Full-time 4WD operates as a full-time AWD system does, with all four wheels receiving power on a continuous basis. In some designs, the driver may have the option of controlling how power is apportioned to the front and rear axles through selectable modes.
Part-Time Four-Wheel Drive
This type is the real traditionalist of four-wheel propulsion and can most often be found in trucks and SUVs that are designed to work and play in more extreme conditions. In this case, the vehicle is typically driven by two wheels, most often in the rear. The driver needs to make the decision to engage 4WD when needed and either push a button or shift a lever. Some systems also allow the driver to lock the vehicle's differentials for extra traction in extreme off-road conditions.
Four-Wheel-Drive Pros and Cons
4WD vehicles are generally best at handling adverse conditions, both on road and off. Even though these systems are now available in well-appointed luxury trucks and SUVs, at heart they still tend to be designed for ruggedness and maximum pulling power, and they are well-suited for work and play in difficult terrain.
These days, 4WD design has become increasingly refined, as has the design of the vehicles that can be ordered with it. But, depending on the make and model, 4WD still often delivers a stiffer ride than 2WD. These systems also have a detrimental effect on fuel economy and increase the initial cost of the vehicle.