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’09 Forester AWD, F/R Torque Split, VDC & TC, and “Virtual” Rear LSD

14415 Views 7 Replies 6 Participants Last post by  The Rookie
Interesting 2009 Forester technical descriptions from a March 2008 Subaru Canada document:

AWD, Transmission, and F/R Torque Split

Forester uses two different versions of Symmetrical AWD, both with Subaru's standard Vehicle Dynamics Control electronic stability system.

Models equipped with the 5-speed manual transmission use a locking viscous-coupling locking centre differential that distributes power 50/50, for well-balanced handling at all speeds.

In all Forester models with the automatic transmission, an electronically-controlled multi-plate transfer clutch system actively manages power distribution based on acceleration, deceleration, and available traction. This system distributes power 60/40 front/rear under normal conditions, but balances power 50/50 when slippage is detected for maximum grip. Overall AWD performance is enhanced in the 2009 Forester via improved communication over the computerized system. Control logic in this AWD system enhances stability by monitoring individual wheel speeds and reacting more quickly to wheel slippage. The control logic also helps to improve tight corner braking, which contributes to more neutral handling response (reduced understeer and oversteer).

Compared to the previous-generation Forester, overall AWD performance is enhanced via improved communication over the local area network (LAN). The LAN constantly monitors speed information from each wheel and engine output to accurately calculate changes in road conditions and steering in real time. The system uses this information to distribute torque to the front and rear wheels to suit the driving conditions and driving style.
Vehicle Dynamics Control (VDC) and Traction Control System (TC or TCS)

Vehicle Dynamics Control

Vehicle Dynamics Control becomes standard on all Forester models for the 2009 model year. Vehicle Dynamics Control is a highly sophisticated electronic stability control system that monitors input from the ABS brake system as well as taking input from steering wheel angle, and yaw and lateral g-force sensors. The system adjusts individual wheel braking as needed, helping to maintain vehicle control under a variety of driving conditions. A Vehicle Dynamics Control 'off' switch is useful for driving out of slushy roads, deep snow or gravel. The 'off' position deactivates the system's torque-reduction control, while the ABS and traction control remain active. (Blue font emphasis from Jim / crewzer)

In Forester models with an automatic transmission, Vehicle Dynamics Control also actively controls the centre differential's power distribution by means of the continuously variable hydraulic transfer clutch. Ultra-high-speed communication between the engine and transmission systems over the onboard LAN makes this possible.

The Vehicle Dynamics Control system in the 2009 Forester features several enhancements over that used in the turbocharged Forester models for 2008. Compact, lightweight sensors are used for improved control accuracy and the new system uses a magnetic wheel rotation measurement device within the oil seal, rather than a sensor mounted on the axle hub.
Electronic “Virtual” Rear Limited Slip Differential (LSD)

Electronic Limited Slip Differential Function

In all 2009 Foresters, Vehicle Dynamics Control also provides traction control and a 'virtual' limited slip rear differential function. Using the virtual rear LSD in place of the viscous-type unit used on previous Forester models provides the traction benefits of an LSD but without the weight or the potentially negative impact on steering response, turning ability or stability.
Hope this is useful!
Jim / crewzer
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I'll be honest. I don't like the 4EAT AWD system. There has to be a better set-up. Is there any reason the 4EAT can't also get a viscous center diff? Why not use a front and rear Torsen diffs and a rear-bias planetary center diff. Throw some VDC in there and you'd have a superior AWD system, no?

The 4EAT has a fixed coupling to the front wheels and the variable MPT coupling to the rear wheels. The way the manual has the centre diff (including viscous coupling) at the rear of the gearbox which then connects to the front differential via a shaft that goes through the hollow gearbox lower shaft is an arrangement that I'd wager is not possible with the automatic, hence the autos use MPT.

The good news is that some have converted the 4EAT to use VTD rather than MPT, and VTD is probably superior again to the viscous LSD centre in the manual cars. See thread
1 - 1 of 8 Posts