Subaru Offers Four Versions of Symmetrical All-Wheel Drive for 2009




Дата канвертавання27.04.2016
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Subaru Offers Four Versions of Symmetrical All-Wheel Drive for 2009
Subaru, an All-Wheel Drive pioneer, is also an All-Wheel Drive technology leader. Having first offered a four-wheel drive passenger car more than 30 years ago and full-time automatic All-Wheel Drive in 1987, Subaru is not following the All-Wheel Drive trend in the car industry – Subaru is leading the trend. In 1997, Subaru began equipping all of its models with All-Wheel drive as standard. All Subaru models come equipped exclusively with Symmetrical All-Wheel Drive, a complete system that includes a lightweight horizontally opposed Subaru Boxer engine and full-time All-Wheel Drive. Rather than take a “one type fits all” approach, Subaru tailors its four different versions of Symmetrical All-Wheel Drive to each Subaru model, according to the engine and transmission.

Continuous

Which models use it

All Subaru models equipped with a 5-speed manual transmission, plus the Legacy 2.5GT spec.B, which features a 6-speed manual transmission.

How it works


A viscous-coupling locking bevel-gear center differential built into the transmission case distributes torque 50:50 front-to-rear. Slippage at the front or rear wheels causes more power to transfer to the opposite set of wheels.




Active

Which models use it

All Subaru models equipped with the 4-speed automatic transmission.

How it works


An electronically managed continuously variable transfer clutch actively manages power distribution in response to driving conditions and acceleration.




Variable Torque Distribution (VTD)

Which models use it

All Subaru models equipped with a 6-cylinder Boxer engine, as well as Legacy and Outback models powered by the turbocharged 4-cylinder Boxer engine teamed with the 5-speed automatic transmission.

How it works


An electronically controlled, continuously variable hydraulic transfer clutch works with a planetary gear-type center differential to control power distribution between the front and rear wheels. Under most conditions, VTD uses a 45:55 torque split, with the rear-wheel bias contributing to improved handling agility. VTD responds to driving conditions to continually optimize power distribution on all road surfaces.




Driver Controlled Center Differential (DCCD)

Which models use it

Exclusive to Impreza WRX STI®

How it works

A limited-slip, planetary gear-type center differential, augmented by an electronically controlled center limited-slip differential, provides a performance-oriented 41:59 torque split. The mechanical limited slip differential has a quicker response and activates just prior to the electronic limited-slip differential. In any of the three automatic modes, the electronically managed continuously variable transfer clutch can vary the distribution ratio through the center differential as needed to suit driving and road-surface conditions. The DCCD AWD System has three automatic modes in the 2009 WRX STI: “Auto” is the same as in the previous-generation model. The new “Auto (-) Active Sport setting shifts the torque bias to the rear and also opens the center limited-slip differential (no locking factor), which improves steering feel. For driving on slippery surfaces, such as gravel or snow, the Auto (+) setting tightens the LSD. In manual mode – which offers six driver-selectable settings -- DCCD allows the driver to vary the front-to-rear torque distribution to optimize All-Wheel Drive performance to suit specific driving conditions.



At-A-Glance: Subaru Symmetrical All-Wheel Drive by Model




Continuous


(5- and 6-speed man. trans.)

Active

(4-speed auto. trans.)



VTD

(5-speed auto trans.)



DCCD

(6-speed manual trans.)






Impreza®













2.5i models

X

X







Outback Sport

X

X







2.5GT




X







WRX®

X













Impreza WRX® STI®










X




Forester®













2.5X models

X

X







2.5XT models

X

X










Legacy®













2.5i models

X

X







2.5i Limited




X







2.5GT Limited

X




X




2.5GT spec.B

X










3.0R models







X







Outback®













2.5i

X

X







2.5i Limited




X







2.5XT Limited

X




X




3.0R Limited







X







Tribeca™







X






All-Wheel Drive Built In, Not Added On


Subaru develops its vehicles around Symmetrical All-Wheel Drive – it does not simply adapt AWD components to a front- or rear-wheel drive vehicle. All versions of Subaru Symmetrical All-Wheel Drive distribute power to all four wheels all the time, reducing the load on each wheel and reducing and even helping to prevent tire slip, especially on slippery or loose surfaces.

In contrast, some AWD systems on the market function passively, transferring power away from


the main drive wheels only when they slip. When there is no slippage, vehicles equipped with such systems essentially operate in two-wheel drive. Although such automatic “part-time” or “on-demand” systems can help a vehicle traverse a snow-covered road, for example, they may not provide the all-road handling benefits of a true full-time All-Wheel Drive system.

Vehicle Dynamics Control Augments All-Wheel Drive Capability


The Vehicle Dynamics Control (VDC) stability and four-wheel traction control system (TCS) is standard on nearly every Subaru model for 2009. The VDC system uses a variety of sensors to compare where the vehicle is heading to where the driver is steering it. If corrective action is needed to help keep the vehicle on course, the VDC system can apply momentary brake pressure at individual wheels and can also reduce torque at the wheels via the electronic throttle control system. Traction control functions as a second line of defense against wheelslip, after All-Wheel Drive.

Boxer Engine: Compact and Light


Subaru introduced its first horizontally opposed (Boxer) engine over 40 years ago and today remains a staunch adherent to this engine configuration. The Subaru Boxer engine is ideal for an All-Wheel Drive application because it is inherently compact. The layout concentrates the engine’s mass in a small area and helps provide a lower center of gravity, which contributes to responsive handling and steering. Aluminum-alloy construction of the engine and transmission case results in a lightweight drivetrain.

Mounting the Boxer engine longitudinally allows the transmission to be located directly behind it and within the vehicle’s wheelbase. Power travels in a straight, near-horizontal line to the rear differential, minimizing frictional loss. This symmetrical, uniform layout also provides excellent left-right balance.



By comparison, in a vehicle with a transverse-mounted engine, an All-Wheel Drive system requires additional transfer gearing to reroute the power from transverse to longitudinal orientation. Such a system can cause more friction and can add extra weight on one side of the vehicle.
In addition, the horizontally opposed layout yields an inherently smooth-running engine as well, because the motion of the pistons from one cylinder bank (and the vibration the motion creates) cancels the vibrations of the opposing bank. Lower vibration contributes to durability, a Subaru hallmark. Some inline and V-type engines require a balancer shaft to help reduce reciprocating vibration, and this feature adds both weight and complexity.

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