[s-cars] Re: Subie WRX AWD (NAC)

James Murray (LMC) James.Murray at ericsson.ca
Wed Jan 15 10:00:45 EST 2003

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Ya know what looks darn interesting... the VW RS32 ! and it's coming to Nor=
th America !!!

Check it out: http://www.vwvortex.com/news/01_03/NAIAS/R32/index.shtml


> -----Original Message-----
> From:	Keith Maddock [SMTP:Keith.Maddock at trw.com]
> Sent:	January 15, 2003 9:30 AM
> To:	skippertgore at hotmail.com; t44tq at mindspring.com
> Cc:	s-car-list at audifans.com
> Subject:	RE: [s-cars] Re: Subie WRX AWD  (NAC)
> Taka schooled Keith:
> > Actually Keith-
> > I'm pretty sure that the automatic WRX comes with a standard 50/50
> > torque split until slippage occurs. The torque available to each axle is
> > significantly greater with the computer-controlled diff, hence the
> > difference in SCCA SoloII classification (at least initially) for the
> > WRX manual and auto and also the reason why some Subaru
> > tuners offer a conversion to the computer-controlled
> > diff for the cars that didn't come with it from the factory.
> Taka,
> I stand humbly corrected, however I will slightly recorrect your correcti=
> Thanks for pointing this out, I now have a new thing to learn more about.
> According to Subie, Automatic WRX with VTD has a 45/55 F/R bias under "no=
rmal driving conditions".
> From mentions in press releases, it sounds like all the USA 2.5L STi mode=
ls will have this VTD system, right?
> Taka, do you have any good links for more info on VTD?
> Good descriptions from Subie USA Website explaining all their AWD systems=
 (at least ones available in the USA)
> "Impreza WRX models with the optional four-speed electronic automatic tra=
nsmission (4EAT) also feature Variable Torque distribution (VTD) All-Wheel =
Drive. "
> "Variable Torque Distribution (VTD) All-Wheel Drive uses an electronicall=
y controlled hydraulic transfer clutch that works with a planetary gear typ=
e center differential to control power distribution between the front and r=
ear wheels. Under normal driving conditions, the VTD system splits power 45=
% front and 55% rear to deliver more of a "performance"driving feel. The VT=
D All-Wheel Drive system responds to wheel slippage by sending the power to=
 the wheel or wheels with the best traction. "
> "VTD All-Wheel Drive
> Impreza WRX models with the optional four-speed electronic automatic tran=
smission (4EAT) features Variable Torque distribution (VTD) All-Wheel Drive=
. The new VTD system uses an electronically controlled hydraulic transfer c=
lutch that works with a planetary gear-type center differential to control =
power distribution between the front and rear wheels. VTD All-Wheel Drive s=
plits the power 45 percent front/55 percent rear, with the slight rear-whee=
l bias enhancing the performance driving feel. VTD All-wheel Drive constant=
ly monitors throttle input to account for weight transfer and responds to d=
riving conditions to continually optimize power distribution on all road su=
rfaces. "
> And RE their manual transmission models:
> "In vehicles with the 5-speed manual transmission, the All-Wheel Drive us=
es a viscous coupling in a center differential inside the transaxle case. T=
he viscous coupling contains a series of opposing discs attached to the fro=
nt and rear output shafts, surrounded by a silicone fluid. In normal operat=
ion, power is distributed equally between the front and rear wheels (50/50 =
power split). Slippage at the front or rear wheels causes a rotational diff=
erence between the front and rear discs in the viscous unit, which then she=
ars the fluid.
> The shearing action heats the fluid, causing it to thicken. As the fluid =
thickens, power transfers from the slipping wheels to the wheels with the b=
est traction. When the slippage ceases, all the discs turn at the same spee=
d, restoring the 50/50 power split. The process is quick and imperceptible =
to the driver and passengers.
> The continuous All-Wheel Driving System is simple, compact, and virtually=
 invisible during operation. Its traction adds a significant margin of safe=
ty on slippery or unpaved roads, and enhances dry-road handling. ">
> And RE their older/non-WRX auto-trans system:
> "
> Subaru models equipped with the 4-speed electronic automatic transmission=
 (4EAT) employ Active All-Wheel Drive. Active All-Wheel Drive optimizes pow=
er distribution before slippage occurs.
> Instead of a viscous coupling center differential, 4EAT-equipped Subaru v=
ehicles feature an electronically managed variable transfer clutch in the t=
ransaxle tailshaft. Power transfer is governed by slippage in the clutch pl=
ates, which use a special friction material that easily handles the loads g=
enerated during power transfer.
> The electronic Transmission Control Module (TCM) controls the All-Wheel D=
rive multi-plate clutch. Active AWD can adjust the power split in an instan=
t, depending on many input factors. If the front wheels begin to slip, the =
TCM increases hydraulic pressure on the clutch, sending power to the rear w=
heels. As the front wheels regain traction, the TCM reduces pressure on the=
 clutch, increasing slippage of the plates and transferring power to the fr=
ont wheels.
> Intelligent Control
> With Active All-Wheel Drive, the TCM monitors input from speed sensors on=
 the front and rear output shafts and also takes input from the throttle po=
sition and transmission. All of these factors cause the TCM to choose how a=
ggressively it adjusts the power split.
> Subaru Active All-Wheel Drive varies the power split according to how you=
 drive the vehicle. Under acceleration, weight transfers to the rear, and t=
he system responds by transferring more power to the rear wheels (in a fron=
t-wheel drive vehicle, this weight transfer on acceleration can cause wheel=
 spin, compromising traction). When braking, weight transfers to the front =
of the vehicle, and the All-Wheel Driving system transfers power to the fro=
nt wheels to help reduce stopping distance.
> Compared with standard front- or rear-wheel drive, All-Wheel Drive also e=
nhances cornering performance. When you enter a turn, lifting off the gas a=
nd applying the brake transfers power to the front wheels for greater steer=
ing control. As you exit the turn and accelerate, power transfers to the re=
ar wheels for added traction under acceleration. Power transfer occurs quic=
kly and imperceptibly, just as it does with the continuous AWD in a manual-=
> ****************************************************************
> Keith Maddock, TRW Automotive,  Koblenz, Germany
> Slip Control Systems, Systems Design, Traction Control
> +49 (0)261/ 895 2474     -    -    keith.maddock at trw.com
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