SUBARU'S ALL WHEEL DRIVING SYSTEM

Shouldn't the question really be why not? Subaru believes every driver should have the benefit of all-wheel drive. And Subaru is the only manufacturer to build it into every car they make. It's all part of the Subaru commitment to active safety. It's simple, really -- a safe vehicle is one that helps you avoid an accident, not just survive one. And the Subaru All-Wheel Driving System actively works to help you do just that, every time you take the wheel.

The Subaru All-Wheel Driving System is a powerful combination of full-time, All-Wheel Drive, a horizontally opposed boxer engine and an advanced, long travel four-wheel independent system. All of which work together for superior traction, handling and driving confidence. Otherwise known as The Beauty of All-Wheel Drive.®

FULL TIME ALL WHEEL DRIVE

Consider this. If all-wheel drive is designed to help you avoid accidents, why should you have to stop to think about it? Not to mention the time it takes to engage four-wheel drive in most cars. By the time it's all said and done, you've most likely gotten yourself into a messy situation.

"Subaru All-Wheel Drive is always on -- watching out for your personal safety, you might say. Maybe that's why Subaru is so popular with police and fire professionals and educators.

"Subaru full-time All-Wheel Drive provides outstanding traction on almost any wet or dry surface, in any weather. It constantly monitors road conditions, sensing any loss of traction and automatically transfers power from the wheels that slip to the wheels that grip. And wheels that grip are especially nice if you're into skiing or any other sport that demands a lot from a vehicle. Like rally racing, where Subaru is a consistent champion, proving itself year after year on some of the world's most challenging courses.

"So whether you're headed for the slopes, the Safari Rally in Kenya or the supermarket, you can count on Subaru All-Wheel Drive to be working for you, full time."

While the Subaru All-Wheel Driving System is all about different components working together to keep you safely on the road, there are four parts that constantly demonstrate their independence: the wheels.

You see, the Subaru long travel four-wheel independent suspension handles rough roads, unexpected dips and bumps beautifully. Because each wheel is on its own, which means the suspension absorbs road irregularities to keep each tire firmly planted on the road. It's no wonder so many mountain bikers and paddlesports enthusiasts count on Subaru to get them to the most remote trailheads and streams.

"Sure-footed stability. That's what you get when you let four wheels think for themselves."

Anti-lock disc brakes come on almost every Subaru, and unlike other Subaru safety features, they're noted for what they don't do. They're designed to not lock up on you. Step hard on the brake pedal and you'll feel the Anti-Lock Braking System (ABS) automatically help balance braking pressure for better control and safer, more even stopping.

"So you'll be ready for whatever comes your way -- a child who forgot to look both ways, the guy in front of you who suddenly stops for no apparent reason, and so on. Just remember, for your ABS system to work properly, you should never pump your pedal.

"Safer stopping power. Keeping you safely on the road, not fishtailing off it. That's what the Subaru Anti-Lock Braking System is all about."

"After all this talk about active safety, let's talk passive safety features. These are the safety features built into every Subaru that we hope you'll never have to use. But everyone knows accidents can happen.

"First, there's the rigid unibody construction, which reduces flex and makes your Subaru one strong machine. Then there are the front and rear energy-absorbing crumple zones, which absorb a lot of the impact in a collision so you don't have to. Same for the side impact door beams. And, of course, dual front air bags* come standard on every Subaru model.

Passive safety. You might not always think about it, but it's sure nice to know it's there if you need it."

*The Supplemental Restraint System (air bag) affords additional protection in a frontal collision. This system provides supplemental protection only, and seat belts must be worn in order to avoid injuries to out-of-position occupants upon bag deployment and to provide the best combined protection in a serious accident. Children should always be properly restrained in the back seat.

Drivetrain

All Subaru models sold in the U.S. feature the Subaru All-Wheel Driving System. This system consists of several subsystems working in harmony to ensure maximum traction availability without any driver input. Unlike many four-wheel drive or all-wheel drive systems, the Subaru all-wheel drive system consists of power transfer mechanisms that are small enough to fit inside the transmission case. These small components not only hold down weight and power loss, but also ownership costs of Subaru vehicles, as no extra maintenance is required for the system.

Subaru all-wheel drive works differently in manual and automatic transmissions. Read on for more technical information.

All-Wheel Drive (Automatic Transmission)

Active all-wheel drive is a term coined by Subaru to differentiate the all-wheel drive system in the automatic transmission from other "reactive" all-wheel drive systems on the market today. What makes this all-wheel drive system so special is its ability to anticipate traction needs and act before a wheel slips.

The mechanism that transfers torque fore and aft is contained within the transmission’s tailshaft. To the casual observer it looks just like a typical hydraulic clutch found in any automatic. The key difference in this clutch pack is its operation. It’s designed to slip according to how much all-wheel drive is needed. When an automatic’s clutch slips, it is due to a malfunction and will eventually burn up. But the multi-plate transfer (MPT) clutch uses a special friction material that easily withstands the friction loads generated during torque transfer.

The MPT’s operation is controlled by the Transmission Control Unit (or TCU) and constantly changes dependent on how the vehicle is being driven. To get more all-wheel drive, the TCU increases the hydraulic pressure to the clutch for less slippage. Less all-wheel drive calls for more slip and the TCU reduces the hydraulic pressure to the clutch.

Under normal, dry pavement operation torque split is about 90% front and 10% rear. This distribution helps to compensate for the car’s weight distribution and resultant smaller effective rolling diameter of the front tires. As weight transfers to the rear of the vehicle, (i.e., under acceleration), the TCU shifts the torque split more toward the rear wheels. Under hard braking, torque is directed forward. Torque distribution is changed based upon how the vehicle is being driven. Throttle position, gearshift lever position, current gear and other factors combine to influence the TCU and it, in turn, selects a software map that determines how aggressively torque split will be adjusted.

Two speed sensors are used by the TCU to detect wheel slippage. One sensor monitors the front axle set, the other the rear axle set. Pre-programmed variables help the TCU differentiate between slipping wheels and normal wheel speed differentials as what occurs when cornering. A speed differential (front-to-rear) of up to 20% signals the TCU that the vehicle is cornering and torque is distributed to the front wheels to help increase traction during the turn. Anything above 20%, however, indicates to the TCU that wheel slippage is occurring and torque is then distributed to the rear wheels.

Another feature of the all-wheel drive system is its interaction with the anti-lock brake system. When ABS is engaged, the transmission selects third gear, reducing the unpredictability of engine braking and, thus, reducing the possibility of wheel lock-up. But all four wheels are still connected to the engine through the AWD system and are brought back up to overall vehicle speed quicker and can, therefore, be controlled again sooner. In a two-wheel drive system if the locking wheel isn’t a drive wheel, it can only be brought back up to overall wheel speed by whatever traction exists between it and the road. The quicker a wheel is controlled the better the stopping performance

All-Wheel Drive (Manual Transmission)

The 5-speed manual transmission’s all-wheel drive is referred to as a continuous all-wheel drive system. It uses a center differential located inside the transmission case that is controlled by a viscous coupling device. In effect, the center differential is a limited-slip differential.

In normal operation, power is distributed equally to the front and rear wheels. Plates are alternately attached to the front and rear output shafts inside the viscous coupling. When a rotational difference occurs between the front and back wheels, the plates inside the viscous housing shear inside the contained fluid (a type of silicone) heating it and causing the fluid to thicken. The thickened fluid causes the plates to transfer torque from those that rotate faster (the slipping wheels) to the plates that rotate slower (the wheels with the best traction).

This no-maintenance system is simple, compact and virtually invisible in its operation. The system can distribute torque from a 50:50 torque split for maximum traction to mostly front or rear wheel drive.