Understanding The Different Drive Systems
There are three basic automobile drivetrain configurations, and each configuration has its own drive characteristics that set it apart from the others.
FRONT-ENGINE/REAR WHEEL DRIVE: The engine is in the front of the vehicle and drives the rear wheels. Rear-wheel drive vehicles tend to oversteer when the rear wheels lose traction – this means the back end of the vehicle may break free and skid under certain conditions, which may cause a spin. Vehicles with this drivetrain configuration don’t have the advantage of having the engine weight over the drive wheels to improve traction.
FRONT-ENGINE/FRONT WHEEL DRIVE: The engine is in the front of the vehicle and drives the front wheels. Front-wheel drive vehicles tend to understeer. Under certain conditions, the front wheels may lose traction, forcing the vehicle to want to go straight – or “plow” – to the outside of a curve during hard cornering. In general, front-wheel drive vehicles offer better traction than rear-wheel drive vehicles because the weight of the engine and transmission is directly over the drive wheels.
FRONT-ENGINE/ALL-WHEEL DRIVE: The engine is in the front and drives all four wheels. All-wheel drive helps provide more neutral handling, virtually eliminating unwanted oversteer and understeer. This is the configuration in place in the Subaru model line.
Four-Wheel Drive Defined
Four-wheel drive is not the same as all-wheel drive. There are two basic types of four-wheel systems:
“PART-DRIVE” four-wheel drive systems typically route power to the rear wheels. When the driver goes off-road or encounters slippery conditions, the front wheels have to be manually engaged. Only then is the vehicle powered at all four wheels. These systems can only be utilized properly when driving off-road or in slippery conditions and depend on the driver to engage the system.
“FULL-TIME” four-wheel drive powers all four wheels all of the time. The amount of power is evenly divided among all four wheels.
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 4×4 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 4×4 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 4×4s.
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
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Superior safety
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Superior handling
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Superior comfort
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Low centre of gravity
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Better torque distribution
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Lower fuel consumption
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Permanent AWD
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Lower insurance premiums
4×4s 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 4×4s 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.
