Subaru All Wheel Drive
The symmetrical All-Wheel Drive (AWD) characteristic, unique to Subaru vehicles, maximises road traction, thus improving control. The transfer of power from the wheels that slip to the wheels that grip provides fundamental driving superiority, particularly in active safety, when compared to two-wheel or 4x4 vehicles.
The AWD in conjunction with the compact, Horizontally Opposed, lightweight Boxer™ engine, which has a low centre of gravity, operates through a perfectly symmetrical drivetrain - a combination that ensures superb balance and handling capabilities.
Many people believe that 4x4 is essentially the preserve of large 'jeep' shaped vehicles, even giving them the generic title of 'four wheel drives'. This is not so with Subaru providing a full-time drive to all wheels in passenger vehicles providing a level of inherent safety, economy, and reassurance not experienced in the larger truck-like 4x4s.
Full-time symmetrical All-Wheel Drive is a standard feature on all Subaru vehicles, and not just an added extra as is the case with some manufacturers. It is important to realise that All-Wheel Drive is not a 'fad' or a clever marketing initiative designed to sell cars in greater numbers. It provides a genuine contribution to road safety and will continue to be an integral part of a Subaru vehicle's make-up. AWD, pioneered by Subaru in 1972, has also been adopted by some luxury vehicle manufacturers such as Volvo, Jaguar and BMW as an add on, and has even been incorporated into the technology of the Formula One racing cars.
The key benefit of All-Wheel Drive is greater traction.
Traction is the force that keeps tyres in stable contact with the road surface during take-off, acceleration, hill climbing, turning and braking. The better a car's traction, the safer it is. Safe running depends on constant stable traction and traction stability is largely determined by the car's drive method.
Distributing power to each of the four wheels, All-Wheel Drive achieves much better traction than either of the two-wheel drive methods (Front or Rear Wheel Drive). To understand the reasons for this you have to realise that car tyres lose traction and slip when the maximum value of the frictional force between the tyres and road surface is exceeded.
An AWD vehicle distributes motive power to all four wheels equally, which means the traction limit is approximately twice that of a two-wheel drive. In the case of a 100kW-engine output to a two-wheel drive, 50kWs would be delivered to each wheel. But if each wheel only has 30kW traction, 20kWs on each wheel is wasted and normally results in spinning.
With AWD the 100kW will be distributed to four wheels, assuming the weight distribution is symmetrical, resulting in 25kW per wheel … 5kWs below the tyre’s threshold of 30kW. The result is that the vehicle utilises its maximum power and moves away faster without wheel spin while in full control.
All-Wheel Drive benefits
The benefit of AWD is easy to understand in that it provides up to twice the traction of two-wheel drive, which breaks surface contact resulting in skidding because the tyres simply cannot cope with the sudden power output of the engine.
What is generally not understood is the difference between AWD and 4x4 vehicles.
Subarus have permanent All-Wheel Drive, providing superior handling by distributing torque to wheels that are gripping while taking power away from those slipping in wet weather, icy conditions or on gravel roads. They also have an off-road capability.
4x4s are generally cumbersome, heavy, truck-like vehicles with high fuel consumption and a high centre of gravity that can affect handling and safety. They have longer stopping distances due to sheer weight. Smaller 4x4s offer few creature comforts and compromise ride quality on normal roads.
Competing manufacturers incorporating AWD are at a disadvantage when compared to the advanced technology developed by pioneers Fuji Heavy Industries of Japan. The Horizontally-Opposed Boxer™ engines in Subarus allow a low centre of gravity and a perfectly symmetrical drivetrain that delivers better handling and a smooth, comfortable ride.
Most engines adapted to AWD result in off-centre configurations and imbalances that cause vibrations. Additional mechanisms to compensate add to vehicle and running costs.