Forester: viscous coupling locking center differential ??????

[ed]

An earlier post mentioned that the 5 speed Forester had no center diff, only
the viscous coupling giving drive to the rear wheels.
On the Subdriven web site,an article on the 06 Forester
(http://www.subdriven.com/news/publish/Subaru_News/article_369.shtml), has
the following:

In all Forester models equipped with the 5-speed manual transmission, a
viscous coupling locking center differential built into the transmission
case divides engine power 50 percent front / 50 percent rear. Wheel slippage
at the front causes more power to shift to the rear, and slippage at the
rear transfers power to the front, ensuring that the wheels with the best
traction receive more power.

Anyone know if the Forester now has an actual center diff ? A viscous
coupling does act like a differential, but to call it a locking center
differential seems like marketing fluff.

From the wording. seems like it doesn't, since it would say it has a viscous
COUPLED center diff, if there was an actual diff in addition to the VC.

 

[Orienteer]

Probably confusion with the auto versions, which do not have a centre diff.
Instead it is solid front wheel drive, with an electronically controlled
'viscous' clutch to the rear wheels.

The distribution front to rear varies according to circumstances.

 

[Edward Hayes]

Manual transmission models have a locking center differential with a
default ratio of 50/50 until wheel slip is detected then power is
shifted. The automatic has a continuously variable clutch pack that
has a default ratio in my Forester of 60/40 and power is transferred
when a difference in wheel torque is present. The automatic is NOT a
front driver at any time. Subaru of America and Subaru of Australia
have both stated this and for more information with pictures refer to
the Factory service manual for drivelines. Ed

 

[Danny Russell]

An earlier post mentioned that the 5 speed Forester had no center diff, only
the viscous coupling giving drive to the rear wheels.
On the Subdriven web site,an article on the 06 Forester
(http://www.subdriven.com/news/publish/Subaru_News/article_369.shtml), has
the following:

In all Forester models equipped with the 5-speed manual transmission, a
viscous coupling locking center differential built into the transmission
case divides engine power 50 percent front / 50 percent rear. Wheel slippage
at the front causes more power to shift to the rear, and slippage at the
rear transfers power to the front, ensuring that the wheels with the best
traction receive more power.

Anyone know if the Forester now has an actual center diff ? A viscous
coupling does act like a differential, but to call it a locking center
differential seems like marketing fluff.

From the wording. seems like it doesn't, since it would say it has a viscous
COUPLED center diff, if there was an actual diff in addition to the VC.

Ed, here's a link to a scan of the innards:

http://www.main.experiencetherave.com:8080/subaru_manual_scans/FSM_Scans/AWDMT_description.jpg

Study it and see if you can figure it out.

If you can, then please explain it to me, cause I can't make heads or tails of the drawing (there's
inner and outer shafts). There IS an actual third (center) diff aside from the front and rear.

 

[Edward Hayes]

Danny: This drawing is somewhat like my factory manual and shows the
innards of the MT model. This may be a drawing of the latest setup. In
this drawing it shows that the front wheels are direct drive. The
transmission output shaft goes to a transfer gear box which has a set
of gears that sets the 50/50 default ratio. The output shaft from the
main transmission case drives the viscous coupling/center
differential. It is certainly more oblivious in my 2000 Forester
manual. All of this "stuff" in within the transmission case which has
really three compartments. One compartment is for the front
differential, one for the main gearing and one for the transfer

 

[Danny Russell]

Danny: This drawing is somewhat like my factory manual and shows the
innards of the MT model. This may be a drawing of the latest setup. In
this drawing it shows that the front wheels are direct drive.

That's what I thought when I first studied it, but looking closer It looks like the output of the
trans powers the input of the center diff via the outside housing structure, then the two outputs of
the center diff power the respective front & rear shafts. The drawing is a little bit ambiguous.

The transmission output shaft goes to a transfer gear box which has a set
of gears that sets the 50/50 default ratio. The output shaft from the
main transmission case drives the viscous coupling/center
differential.

But look closely, you'll notice that the output from the trans is actually a hollow tube (labeled
"Driven Shaft") and it surrounds the front drive shaft (labeled "Drive Pinion Shaft").

 

[Paul]

The center differential and the viscous coupling are two completely
independent things. The center differential distributes the power
between front and rear (just like the rear differential on a rear-drive
car). There are two more differentials, one at the front, one at the
rear, for left-right distribution.

The visous coupling is a friction device between the front and rear
shafts that attempts to dampen any large difference in speeds
between the two.

The center differential and the viscous coupling are in parallel.

 

[Dominic Richens]

From the wording. seems like it doesn't, since it would say it has a
viscous COUPLED center diff, if there was an actual diff in addition
to the VC.

Subaru MT use three differentials;
- one between the front wheels to balance torque so that the wheels can turn
at different speeds
- one between the back wheels to balance torque so that the wheels can turn
at different speeds
- and one between the front axle and rear axle (driving the above
differentials), so that the front and rear axles can turn at different
speeds.

Now because differentials balance torque, if one wheel should get spin
freely on ice (zero torque) all the differentials will see that all the
wheels get zero torque and you won't go anyway.

So between the two "driven" gears of the centre differential, they stick a
VC unit which allows the two shafts to turn at different speeds, but
provides resistance, and that resistance increases as difference in shaft
speed increases.

Which is why after 5 hours on the highway on a hot hot day, I can bearly
turn my Subaru into a parking spot!