All wheel drive, also known as AWD, refers to a vehicle propulsion system that has power delivered to all road wheels in the vehicle, either simultaneously or individually, in a way that provides improved traction to the wheels when compared to two-wheel drive vehicles. The system allows power from the engine to be routed to all road wheels through either a transfer case or a more sophisticated control unit that monitors driving conditions and automatically adjusts power delivery.

Mechanical Components of an AWD System

All wheel drive systems use a combination of mechanical and electronic components to deliver power to all four wheels. At the core is a transfer case or central differential that connects the engine's power take-off unit to both the front and rear axles. This allows power to be split between the front and rear wheels. It also features a center differential that allows the wheels with more traction to receive more torque. Additional components include front and rear differentials that ensure wheels on each axle turn at different speeds during cornering. Some modern AWD variants have electronically controlled locking differentials to further enhance traction.

When Traction is Lost the AWD Kicks In

In most normal driving conditions when good traction is available, an All Wheel Drive vehicle functions like a front or rear-wheel drive car with power being primarily sent to the front or rear axles. However, as soon as one of the driven wheels loses traction, the AWD system automatically transfers power to the axle with the better traction by means of the transfer case and differentials. This allows power delivery to be balanced out across all four wheels to maximize available traction. Compared to two-wheel drive vehicles, AWD provides consistently better acceleration, handling, and control on low traction surfaces.

AWD Driving Modes for Different Terrains

More sophisticated AWD systems feature different driving modes tailored for varying road conditions. For example, some vehicles have a two-wheel drive mode for better fuel efficiency on dry surfaces and an all-wheel drive mode engaged via a dashboard button for inclement weather. Others include settings like snow, mud or sand that optimize power delivery for better traction on slippery surfaces. Some high-end AWD cars have modes like sport that allow for a more rear-biased power split for improved performance and handling characteristics on dry pavement. These settings help drivers get the most out of the all-wheel drive system across different terrains and weather.

Traction Benefits in Adverse Conditions

The most evident benefit of all-wheel drive traction comes into play when driving on low traction surfaces like wet or slippery roads, gravel, snow and ice. With power going to all four wheels, AWD vehicles are much less prone to losing traction and spinning wheels compared to two-wheel drive cars. This makes them inherently safer and more sure-footed on slippery surfaces. The balanced power delivery also ensures better acceleration from a stop without excessive wheelspin, even under poor traction conditions. As a result, AWD is widely recommended or mandated for drivers living in regions with heavy wintry conditions.

Improved Stability and Handling

While the traction advantages dominate under low grip scenarios, all-wheel drive also subtly improves handling dynamics even under normal dry road conditions. By having drive delivered to all four wheels, an AWD vehicle has greater front-rear as well as side-to-side weight transfer under heavy cornering, braking or acceleration forces. This allows the sophisticated computer-controlled differentials to help rotate the car into corners more predictably for razor-sharp turn-in response and balance through bends. It can also enhance steering feel and feedback. Overall, the neutral cornering behavior means better on-road stability at both high and low speeds compared to rear or front-wheel drive cars.

AWD Usage in Performance Cars

While traction was initially the primary motivation, all-wheel drive is now a standard fixture even in performance-oriented sports cars and luxury vehicles. By providing balanced power delivery and optimal grip from all four wheels, AWD improves acceleration off the line and allows cars to put down maximum power smoothly coming out of turns. This translates to deceptively quick straight line speed. Additionally, the neutral cornering balance means AWD sports cars can carry higher cornering speeds through bends without understeer or oversteer. As a result, today even high-end exotic brands like Lamborghini, Ferrari and Porsche offer all-wheel drive variants that take advantage of the technology for scintillating acceleration both on and off the track.

Maintenance Requirements

The complex transfer case and additional differentials do make all-wheel drive vehicles marginally more expensive to maintain compared to two-wheel drive cars. Owners need to change AWD-specific fluids and components like the front and rear differentials, transfer case and associated bearings at the intervals recommended in the owner's manual. It's also advisable to have the all-wheel drive coupling inspected if unusually harsh vibration or noises emerge from the drivetrain. However, with scheduled servicing the maintenance costs are quite reasonable considering the substantial traction and handling benefits all-wheel drive provides year-round and in varied terrain and weather conditions.