The Audi Quattro four-wheel-drive system sends power to all four tires, giving it significant off-road and road-gripping capabilities. A four-wheel-drive system typically only sends power to both car axles when a toggle on the driver's dashboard is engaged, sending power to just the rear axle the rest of the time. This is because with standard open and locking differentials, it is difficult for a vehicle to turn on flat road when four-wheel drive is engaged. The Audi Quattro has found a way around this turning problem and developed a system that sends power to all four wheels all of the time. Technically, such a vehicle is labeled an all-wheel drive.
The structure of the Quattro four-wheel drive begins with the transmission. Ordinarily, four-wheel-drive cars feature a drive-transfer case with two drive shafts leading to differentials sitting between both the rear and front axle. Audi's version, however, does not have a drive-transfer case. In its place is a Torsen differential, from which extend two drive shafts. Each of those connects to a Torsen differential on each of the vehicle's axles.
Power passes through all three Torsen differentials to the wheels equally when the Audi Quattro is driving on a straight, smooth surface. The power is divided when there is more resistance on one wheel than another. For example, when the front passenger-side wheel edges off the road onto stiff gravel it puts more resistance against the wheel than the front driver-side wheel. This causes a shift in the power being sent to both wheels by the front-axle Torsen differential, sending more power to the driver-side wheel to compensate. Likewise, if both front wheels were to go off-road, the Torsen differential would send more power to the rear axle than the front.