The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed.All-wheel-drive vehicles need a differential between each set of drive wheels, and they need one between the front and the back wheels as well, because the front wheels travel a different distance through a turn than the rear wheels.

Viscous Coupling

The viscous coupling is often found in all-wheel-drive vehicles. It is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.The viscous coupling has two sets of plates inside a sealed housing that is filled with a thick fluid. One set of plates is connected to each output shaft. Under normal conditions, both sets of plates and the viscous fluid spin at the same speed. When one set of wheels tries to spin faster, perhaps because it is slipping, the set of plates corresponding to those wheels spins faster than the other. The viscous fluid, stuck between the plates, tries to catch up with the faster disks, dragging the slower disks along. This transfers more torque to the slower moving wheels — the wheels that are not slipping.

When a car is turning, the difference in speed between the wheels is not as large as when one wheel is slipping. The faster the plates are spinning relative to each other, the more torque the viscous coupling transfers. The coupling does not interfere with turns because the amount of torque transferred during a turn is so small. However, this also highlights a disadvantage of the viscous coupling: No torque transfer will occur until a wheel actually starts slipping.

A simple experiment with an egg will help explain the behavior of the viscous coupling. If you set an egg on the kitchen table, the shell and the yolk are both stationary. If you suddenly spin the egg, the shell will be moving at a faster speed than the yolk for a second, but the yolk will quickly catch up. To prove that the yolk is spinning, once you have the egg spinning quickly stop it and then let go — the egg will start to spin again. In this experiment, we used the friction between the shell and the yolk to apply force to the yolk, speeding it up. When we stopped the shell, that friction — between the still-moving yolk and the shell — applied force to the shell, causing it to speed up. In a viscous coupling, the force is applied between the fluid and the sets of plates in the same way as between the yolk and the shell.

Differentials and Traction

There are two factors that determine how much torque can be applied to the wheels: equipment and traction. In dry conditions, when there is plenty of traction, the amount of torque applied to the wheels is limited by the engine and gearing; in a low traction situation, such as when driving on ice, the amount of torque is limited to the greatest amount that will not cause a wheel to slip under those conditions. So, even though a car may be able to produce more torque, there needs to be enough traction to transmit that torque to the ground. If you give the car more gas after the wheels start to slip, the wheels will just spin faster.

On Thin Ice
If you’ve ever driven on ice, you may know of a trick that makes acceleration easier: If you start out in second gear, or even third gear, instead of first, because of the gearing in the transmission you will have less torque available to the wheels. This will make it easier to accelerate without spinning the wheels.

Now what happens if one of the drive wheels has good traction, and the other one is on ice? This is where the problem with open differentials comes in.

Open differentials always apply the same torque to both wheels, and the maximum amount of torque is limited to the greatest amount that will not make the wheels slip. It doesn’t take much torque to make a tire slip on ice. And when the wheel with good traction is only getting the very small amount of torque that can be applied to the wheel with less traction, your car isn’t going to move very much.

Off Road
Another time open differentials might get you into trouble is when you are driving off-road. If you have a four-wheel drive truck, or an SUV, with an open differential on both the front and the back, you could get stuck. Open differentials always apply the same torque to both wheels. If one of the front tires and one of the back tires comes off the ground, they will just spin helplessly in the air, and you won’t be able to move at all.

The solution to these problems is the limited slip differential (LSD), sometimes called positraction. Limited slip differentials use various mechanisms to allow normal differential action when going around turns. When a wheel slips, they allow more torque to be transferred to the non-slipping wheel.

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

• Superior safety
• Superior handling
• Superior comfort
• Low centre of gravity
• Better torque distribution
• Lower fuel consumption
• Permanent AWD
• 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.

Boxer engines got their name because each pair of pistons moves simultaneously in and out rather than alternately, like boxers showing they’re ready by clashing their gloved fists against each other before a fight.

A boxer, or flat engine, is an internal combustion engine with pistons that are all relatively horizontal. A straight engine canted 90 degrees from straight up is a flat engine, as is one in which the cylinders are arranged in two banks on either side of a single crankshaft. In both configurations, the motion of all the pistons is in the horizontal plane.
Usually, each pair of corresponding pistons from each bank of cylinders share one crank pin on the crankshaft, either by master/slave rods or by two ordinary rods side by side. Some authorities divide flat engines into boxer engines which do not share crank pins in this way, and 180° engines which do.
The boxer engine (also known as a horizontally opposed cylinder engine) in which the corresponding pistons reach top dead centeropposed piston engines, which use a completely different concept. simultaneously.

Flat engines are shorter than in-line engines, and have a lower center of gravity than any other common configuration, giving better stability and control. These engines, however, are also wider than more traditional configurations and are more expensive to build. The extra width may cause problems fitting the engine into the engine bay of a front-engined car owing to the interference with the steering wheels.

One benefit of using a boxer engine versus a V-engine is that the design provides good balance because each piston’s momentum is counterbalanced by the corresponding piston movement of the opposite side.

These engines can run very smoothly and free of unbalanced forces with a four-stroke cycle and do not require a balance shaft or counterweights on the crankshaft to balance the weight of the reciprocating parts, which are required in other engine configurations. Note that this is generally true of boxer engines regardless of the number of cylinders, but not true for all V and straight, or inline engines. However, in the case of boxer engines with fewer than six cylinders, unbalanced moments are unavoidable due to the “opposite” cylinders being not exactly opposite but offset slightly.

Boxer engines tend to produce more noise than inline and V-engines because valve clatter is not so well dampened due to lack of covering by air-filters and other components, and produce a larger torsional vibration than a V-engine, and so tend to require a larger flywheel. They have a characteristic smoothness throughout the rev range and, combined with the mounting position immediately ahead of the rear axle, offer a low center of gravity and more neutral handling.