all wheel drive

[peter]

thx!

 

[Yousuf Khan]

So where does the STi fit in? Does it use the same AWD system as all
the other subies? I know it has a DCCD. And 60R/40F is the standard
power distribution when running with it in Automatic mode.

Actually, I think the STI is using the same differential system that the
4EAT-VDC's are using, except it's using it with an MT.

Yousuf Khan

 

[Paul Pedersen]

Dominic Richens writes :

my 5MT is 50/50 all the time and I don't have trouble parking.

You're right, but you have a center differential, as do
the high-end automatics (VTD), but not the Imprezas, Foresters
and regular Legacy automatics.

And I really meant that it wouldn't be easy to turn the thing,
with one wheel or two dragging. Now if it pulsates (and interesting
notion, which as far as I know isn't mentioned in any service manual)
that's a bit different, you'll only be dragging wheels a part of
the time.

 

[Paul Pedersen]

Edward Hayes writes :

NO!!!! there is a gearset behind the transmission but inside
the same housing. The gears set ratio is 60/40 which is the
default for the Std 4EAT transmission and that is why the
front to rear default ratio is 60/40.

If you would read the service manual, Volume 1, "General Information",
Chapter "Specifications", Section "D. Transmission" you would
see that it is stated for the 4AT :

"Reduction gear (Front drive)

1st reduction 1.000
Final reduction 4.111

Reduction gear (Rear drive)

Transfer reduction "--" (N/A)
Final reduction 4.111 "

To me, this means that there is no difference between
front and rear shaft speeds.

Now, if you look at a 2 litre turbo (WRX) 5MT manual
transmission you'll find :

"Reduction gear (Front drive)

1st reduction "--"
Final reduction 3.900

Reduction gear (Rear drive)

Transfer reduction 1.100
Final reduction 3.545 "

This may very well give you the 40/60 split you and your
sources mention.

If you read the Subaru documentation carefully, it is quite
clear that whenever they sing the praise of their system,
they are talking either about their manual transmissions,
or their VTD auto transmissions. They deal with the MPT
transmissions mainly by omission.

 

[Paul Pedersen]

The STi only comes with a manual transmission. This thread
was discussing the automatic transmission of a Forester.

The service manual states for the STi 6MT :

"Reduction gear (Front drive)

1st reduction "--"
Final reduction 3.900

Reduction gear (Rear drive)

Transfer reduction 1.000
Final reduction 3.900 "

So the STi is split 50/50.

 

[Patrick Fisher]

The service manual states for the STi 6MT :

"Reduction gear (Front drive)

1st reduction "--"
Final reduction 3.900

Reduction gear (Rear drive)

Transfer reduction 1.000
Final reduction 3.900 "

So the STi is split 50/50.

This is not correct. The STi has the same final drive ratio for front and
rear diffs, but this is because they have to be the same. The only reason
the EAT is different is because it's a viscous coupling. In order to allow a
more progressive lockup, rather than an abrupt transfer of torque when the
fluid is suddenly heated up, they use slightly different final drive ratios,
to cause a constant amount of "slip" in the center diff, allowing some
marginal transfer of power to the rear wheels before wheel slip actually
occurs.

The torque split in a mechanical diff is determined by the diff's own
internal gears, but by the final drive ratio of the two axles.

 

[Edward Hayes]

Paul !!!! It may be that the 60/40 front to rear split (gear set) that is in
the 4EAT depends on a certain amount of torque difference favoring the
lighter weight rear of the car. This would mean that the default ratio 60/40
may be very close to 50/50 in normal driving which would result in very low
slippage on the continuously variable clutch pack Sound reasonable Paul?
Just a guess on my part. I did read that the transfer gearset is reversed to
40/60 for the Legacy GT (2000 model) to "give the driver a more sporting
feel" I do have the factory service manual for my 2000 Forester (section
5)(Transmission & Differential). Unfortunately the factory service manual
does states that for my 2000 Forester with the 4EAT the front differential
ratio as 4.444:1 and no real ratio is given unless it's elsewhere in the
section. Ed Hayes

 

[Dominic Richens]

Dominic Richens writes :


You're right, but you have a center differential, as do
the high-end automatics (VTD), but not the Imprezas, Foresters
and regular Legacy automatics.

Actually I was being a bit facitious - especially since my VCD in my 95
Brighton is toast and tends to lock up after a bit of hwy driving. I tried
to back into a parking spot the other day and stalled it twice because of
binding!

 

[John M.]

the high-end automatics (VTD), but not the Imprezas, Foresters

and regular Legacy automatics.

And I really meant that it wouldn't be easy to turn the thing,
with one wheel or two dragging. Now if it pulsates (and interesting
notion, which as far as I know isn't mentioned in any service manual)
that's a bit different, you'll only be dragging wheels a part of
the time.

Actually, the tech manual does mention the 'pulsating' in a sense... it's
referred to a 'duty cycles':
"Transfer Clutch Assembly (AWD). The transfer unit consists of a hydraulic
multi-plate clutch and a hydraulic control system incorporating a duty
solenoid.... Duty solenoid "C" regulates the MPT clutch. It is controlled by
the TCU, which determines the degree of AWD by altering the duty ration. As
the duty ration increases the amount of AWD decreases. The clutch itself
features friction discs that are designed to slip. This eliminates torque
binding during tight cornering. In order to get power to the front wheels,
the reduction gear powers the reduction driven gear, which is attached to
the drive pinion shaft. For the rear wheels, power goes from the reduction
drive shaft to the MPT clutch hub..... Beginning in the 1990 model year a
new transfer piston was added. This improved torque split control,
preventing the MPT clutch from further applying during high speed driving."

So much for some of the other 'theories' out there....
John

 

[John M.]

thx!

Thx!??? That's *it*?? All of that arguing you've caused and all you say is
Thx???!!! For shame!
<smile>
John

 

[Edward Hayes]

At least he could have spelled it out so it would sound sincere!!!!!! Ed

 

[John Emdall]

People tend to confuse torque distribution with limited slip lockup. The
STi (as with just about every AWD car) has a fixed torque distribution,
based on the gears in the center diff. This distribution never changes*.
For the STi, it's 64/36 rear/front. The DCCD uses an electromagnetic system
to vary the amount of differential lockup continuously. In Auto mode, it
uses various sensors to decide how much slip-limiting action to apply. In
manual mode, you dial in an infinitely variable setting that fixes the
amount of lockup. The torque distribution remains constant, but the center
diff allows torque to be applied to one end even if the other is slipping.
So it's just an electronically controlled limited slip with lockup
capability - no variable torque transfer like the Porsche 996 and Nissan
Skyline (ATESSA-II system) have.

*In full lock mode, the torque distribution is still "64/36", but since it's
locked, it's effectively proportional to the traction available at each
end - on dry ground, this means basically 50/50.

you have just gone in a circle with your conclusion. probably just
a minor oversight. by your own reasoning, the torque is not a function
of the ground or anything other than the f/r ratio in the center diff.

there are two modes, when there is no wheelspin, and when
there is wheelspin at one end and/or the other. when there is no
wheelspin the distribution remains 64/36, regardless of the
DCCD setting, all it does is increase drag (friction) in the system as
it approches lock. when there is wheelspin,
if the diff is in "open" the torque will be sent to the wheels with the
least traction, and when the diff is in "lock" the torque will again be
64/36. 50/50 cannot be achieved under any circumstances.

comments please?

 

[Jim Stewart]

John Emdall wrote:
when there is wheelspin,

if the diff is in "open" the torque will be sent to the wheels with the
least traction, and when the diff is in "lock" the torque will again be
64/36. 50/50 cannot be achieved under any circumstances.

A minor nit...

When there is wheelspin and the diff is locked,
for all practical purposes the torque split is
determined by which axle has the traction and
which doesn't, not the the diff or anything
else in the drivetrain.

Imagine the car sitting with it's front wheels
on dry pavement and the rears on glare ice.
Just for fun, the car is hitched to a big
trailer frozen to the ice. You put it in
gear and press on the gas. The rears will
spin until the diff locks. Now the axles
both turn together. Since the rear continues
to spin, there is zero torque (and I'm using
the word torque in the physics sense) coupled
to the ground. On the other hand, the front
axle, as long as the front tires don't break
traction, couple 100% of the engine's torque
to the ground.

 

[Jim Stewart]

John Emdall wrote:
when there is wheelspin,



A minor nit...

When there is wheelspin and the diff is locked,
for all practical purposes the torque split is
determined by which axle has the traction and
which doesn't, not the the diff or anything
else in the drivetrain.

Imagine the car sitting with it's front wheels
on dry pavement and the rears on glare ice.
Just for fun, the car is hitched to a big
trailer frozen to the ice. You put it in
gear and press on the gas. The rears will
spin until the diff locks. Now the axles
both turn together.

A minor correction to my minor nit...

The car moves forward. The front axle
is supplying 100% of the torque because
it has 100% of the traction. The rear
axle, even though it is not spinning,
contributes 0% of the torque because
it has 0% of the traction, since it's
on the ice.


********* disregard *********************

Since the rear continues

 

[Paul Pedersen]

John Emdall writes :

there are two modes, when there is no wheelspin, and when
there is wheelspin at one end and/or the other. when there is no
wheelspin the distribution remains 64/36, regardless of the
DCCD setting, all it does is increase drag (friction) in the system as
it approches lock. when there is wheelspin,
if the diff is in "open" the torque will be sent to the wheels with the
least traction, and when the diff is in "lock" the torque will again be
64/36. 50/50 cannot be achieved under any circumstances.

comments please?

I'd be interested in knowing where you get the 64/36
figure front. The service manual states that the gearing
is identical front and back, so I don't see how it can
be anything other that 50/50 (nominally).

 

[DH]

There's a planetary center differential, which divides the torque unequally
from front to rear. The transmission output shaft connects to the planet
gear carrier, the front axle connects to the ring gear, and the rear axle to
the sun gear. The ratio between the diameter of the ring gear and the sun
gear is the ratio of torque split from front to rear. This works because the
planet gears divide the force on the gear teeth equally between the sun and
ring gear (because otherwise the planet gears would rotate), and the torque
on the output shafts depends on the force on the teeth multiplied by the
radius of the gear. You can get any torque split you want by changing the
dimensions of the center differential.

 

[BlueSTi]

John Emdall writes :


I'd be interested in knowing where you get the 64/36
figure front. The service manual states that the gearing
is identical front and back, so I don't see how it can
be anything other that 50/50 (nominally).

Back to my original question: Does anyone know if the STi AWD system
is the same as the other AWD Subaru systems? Other than of course it
being a 6-Speed and having the DCCD.

BlueSTi
"Scary-Fast"

 

[Carl 1 Lucky Texan]

The most detail would probably be found at www.i-club.com or
www.nasioc.com

It is my understanding thet the STi uses an electromagnetic center diff.

Carl
1 Lucky Texan

 

[bill-robinson]

Well the RSK B4 5 spped MT that I have only has a viscous centre diff
there is no planetary gear set or spider gearset at all. Just a
viscous diff.

Bill

 

[Patrick Fisher]

Back to my original question: Does anyone know if the STi AWD system
is the same as the other AWD Subaru systems? Other than of course it
being a 6-Speed and having the DCCD.

No, it's different. It uses a planetary transfer case providing a fixed
64/36 torque split, with an computer-controlled electromagnetic limited slip
coupling. The WRX uses a viscous center limited slip diff that isn't nearly
as good at putting the power where it's needed.