Two-wheel drive is a type of automobile layout in which two of the four car wheels are used to propel the vehicle forward. The other two wheels spin freely but do not actively drive the vehicle forward. A two-wheel drive vehicle can be front-wheel drive or rear-wheel drive; on a front-wheel drive car, the wheels at the front of the vehicle are used to propel the car forward, while on a rear-wheel drive car, the rear wheels are used to move the car forward. Each design has distinct advantages and disadvantages depending on how the vehicle is intended to be used.
A drivetrain is the collection of components in a vehicle that supply power to the wheels to make them turn. This turning moves the vehicle forward or backward. The engine supplies the power, which is then is transferred to a transmission. A driveshaft is connected to the transmission, and as this driveshaft rotates, a gear turns and spins the axle components. When the axle components turn, so do the wheels. A two-wheel drive system uses only one drive axle instead of two; if two are used, then the vehicle is four-wheel drive or all-wheel drive.
The driveshaft meets the axle of the two-wheel drive system at a component known as the differential. The driveshaft is situated perpendicular to the axle on rear-wheel drive systems, so the torque from the driveshaft must somehow be transferred to the axles. This is done with a series of gears mounted within the differential housing. As the gear on the driveshaft spins, it contacts gears mounted on axle shafts, which then turn as well. The wheels will be attached to these axle shafts, thereby allowing torque to be effectively transferred from the driveshaft to the wheels of the vehicle. The system can differ slightly on front-wheel drive systems.
Two-wheel drive vehicles with the drive wheels in the front tend to have good traction because the weight of the engine keeps the drive wheels in place during slick conditions, but steering can be affected by this weight. Rear-wheel drive cars may not have such good traction in slick conditions, but these two-wheel drive vehicles will tend to have better handling in most driving conditions. Some four-wheel drive vehicles spend most time in a two-wheel drive configuration; the front wheels only drive the vehicle when it is switched into a four-wheel drive configuration. This allows the driver to cut down on fuel consumption when four-wheel drive is not needed.