Four wheel drive allows a vehicle to supply power to all four of its wheels. Drivers depend on four wheel drive systems to give them better traction on snow, ice, and off-road terrain. Vehicles with 4WD can be full-time or part-time. A part time 4WD system can be engaged and disengaged as desired by the driver and is designed to be used under conditions where poor traction is an issue. Standard full-time 4WD systems consistently deliver power to all four wheels, and can be operated while driving on any type of surface. The goal of a 4WD system is to send torque to the wheels to prevent slippage. A four wheel drive system is composed of various parts. These include the differentials, hubs, and the transfer case. A transfer case is responsible for delivering power to both the front and rear axles, while hubs help secure the wheels. To keep a 4WD vehicle performing as intended, drivers should watch over the different parts of the system and have them routinely inspected during a 4WD service.