Four-wheel drive in a vehicle is a system by which power from the engine, applied as torque to the wheels, reaches all four wheels. This is in contrast to other methods, usually a two-wheel drive system, in which power only transfers to one axle and only two of the wheels receive torque. The four-wheel system is advantageous in situations in which poor road conditions are present, such as snow, ice, water, or mud, as the wheels are able to maintain greater torque independently of each other. This is in contrast to all-wheel drive, which allows the system in a vehicle to apply torque as needed to each wheel independently.
Also called a part-time system, four-wheel drive typically refers to a system in a vehicle in which torque is applied to all tires evenly. This type of system is usually used only on vehicles with four tires, as opposed to motorcycles or larger vehicles with more wheels. Simpler drive systems for four-wheeled vehicles provide power only to a single axle, which is the bar connecting two wheels together at the front or back of the vehicle.
In a two-wheel drive system, power, in the form of a force called torque, is provided only to two tires to propel a vehicle forward. If one of the two tires slips or otherwise does not have the traction necessary to move the vehicle, then the vehicle can get stuck. This is often seen in vehicles with rear-wheel drive, in which the back tires spin in mud or snow, while the front wheels do not move and the vehicle is unable to move.
Four-wheel drive, however, provides power and torque to all the tires simultaneously. This means if the back tires slip and cannot provide traction and the force to push the vehicle forward, the front tires can still spin and pull the vehicle instead. Such a system is useful for these types of situations because the wheels are able to provide the force necessary to keep a vehicle moving in poor terrain. It does not work well on dry road conditions, however, so the driver of a vehicle with this type of drive system typically has to manually activate the system.
This type of part-time system is usually what is referred to by the term “four-wheel drive,” while “all-wheel drive” typically indicates a system also referred to as full-time four-wheel drive. The difference is that the full-time system always uses four-wheel drive but allows the tires to function independently enough to still work on dry pavement. A part-time system makes all tires function the same way, which is advantageous in slippery conditions, but makes turning on dry surfaces hard on the vehicle and jarring for the driver. Full-time systems allow the tires to move at different speeds, as is necessary for turning on dry surfaces.