viscous coupling

[Rob Munach]

If this is the case then you'd have a FWD car until you had slip this is not
the case on modern manual Subarus.

It is (or was) the case, however, on the non -WRX automatic Subarus.

The rear wheels would be rolling stock.

 

[Rob Munach]

and open


no purpose



Actually, there is a VC in both the automatic and manual versions of
Subies. However, they use the VC in completely different ways in each
case. In the case of manual tranny Subies, the VC acts as a
slip-limiter for the centre diff. In the case of auto Subies, the VC
activates a progressive clutch pack that locks the front and rears
together: there is no centre diff in an auto Subie.

In the auto, it is not a VC (viscous coupling). It is a computer
controlled wet clutch.

 

[YKhan]

In the auto, it is not a VC (viscous coupling). It is a computer
controlled wet clutch.

That's probably a better way to describe it, yes.

Yousuf Khan

 

[l.lichtman]

So, on my auto Subie Outback, if I insert fuse (for running the small
spare) does that simply tell the computer to disengage the wet clutch?

 

[YKhan]

Yes, that's exactly what it means.

Yousuf Khan

 

[transfixed]

There is a center differential. The engine
drives it and one output goes to the front
and the other to the rear. The VC is inside
this center diff.

That is incorrect. The Imprezas (and Forester) do not have a center
diff. The front axle is directly driven by the engine. The manual
transmisson has a viscous coupling that connects the drive shaft to the
rear axle. When there is slippage between the two, the fluid thickens
and some torque will be transmitted to the back. If the front wheels
have very little traction but the back wheels have traction, almost all
of the torque will be to the back (because a slipping tire does not
transfer torque).

Only more expensive Subarus have a center differential.

 

[transfixed]

You are wrong. Simple as that. The center dif
input is hard-coupled to the engine.

No, he is right. You are wrong. There is no center differential in
Imprezas (or Foresters). Just go to the Subaru web site, it is all laid
out there for the different models.

 

[Paul]

That is incorrect. The Imprezas (and Forester) do not have a center
diff. The front axle is directly driven by the engine. The manual
transmisson has a viscous coupling that connects the drive shaft to the
rear axle. When there is slippage between the two, the fluid thickens
and some torque will be transmitted to the back. If the front wheels
have very little traction but the back wheels have traction, almost all
of the torque will be to the back (because a slipping tire does not
transfer torque).

Only more expensive Subarus have a center differential.

All manual transmission Subarus have a center differential. Only the
cheap Impreza, Legacy and Forester automatics don't have one. These
are front wheel drive until the system decides to transfer some torque
to the rear via the clutch pack.

The more expensive automatics with the VTD transmission also have
a center differential.

 

[CompUser]

That is incorrect. The Imprezas (and Forester) do not have a center
diff.
Only more expensive Subarus have a center differential.

Hey---it's that foofnicky guy...the one that
got fired from the Subaru dealership!!!

 

[CompUser]

pack.

The more expensive automatics with the VTD transmission also have
a center differential.

Wow..foonick uses the same terminology under two
different posting nicks...how courageous of him.

 

[Jim Stewart]

No, he is right. You are wrong. There is no center differential in
Imprezas (or Foresters). Just go to the Subaru web site, it is all laid
out there for the different models.

Got a link? Better yet, cut'n paste some text.

 

[Guy Macon]

All manual transmission Subarus have a center differential.

Wrong.

My 1985 Subaru GL 2WD wagon has a 5-speed manual transmission
and no center differential. In fact, I rather suspect that
*no* 2WD Subaru has a center differential... <grin>

My 1985 Subaru GL 4WD wagon has a 10-speed manual transmission
(hi/lo range: 10 forward and 2 reverse) and no center differential.

Took me 20 years, but they are just starting to get broken in...

 

[Larry Van Wormer]

I expect that all AWD Subarus do have a centre differential, can't see
how they would work otherwise. (My MY02 Impreza certainly does...) No, a
viscous coupling doesn't take over that function, it only comes into
action when there is a difference between the front and rear wheel speeds.

For those who like specific quotes, just look at the service manual of
any AWD manual transmission Subaru, it will match the above (tho' likely
better stated!)

Larry Van Wormer

 

[Rob Munach]

That is incorrect. The Imprezas (and Forester) do not have a center
diff. The front axle is directly driven by the engine. The manual
transmisson has a viscous coupling that connects the drive shaft to the
rear axle. When there is slippage between the two, the fluid thickens
and some torque will be transmitted to the back. If the front wheels
have very little traction but the back wheels have traction, almost all
of the torque will be to the back (because a slipping tire does not
transfer torque).

Only more expensive Subarus have a center differential.

I know for a fact my Forester has a center diff. Where are you getting
this info?

 

[Edward Hayes]

Manual transmission models have a viscous center differential. The
automatic transmission models have a continuously variable clutch pack
located in the tail end of the transmission.

 

[YKhan]

No, he is right. You are wrong. There is no center differential in
Imprezas (or Foresters). Just go to the Subaru web site, it is all laid
out there for the different models.

Hunh? Were you looking at just the automatic transmission models? If
you looked at a manual transmission model, you would've seen it.

Yousuf Khan

 

[YKhan]

You know, maybe it is time for an Subaru AWD FAQ? There are so many
different systems that Subaru uses that this question comes up over and
over again.

 

[Guy Macon]

I expect that all AWD Subarus do have a centre differential,
can't see how they would work otherwise.

I can tell you from experience how they work. My 1985 Subaru
GL 4WD wagon has a 10-speed manual transmission (hi/lo range:
10 forward and 2 reverse) with differentials at the front and
rear but no center differential.

Unlike modern AWD Subarus that are designed to drive all four
wheels on dry pavement, the '85 system was designed for off-road
and rain-slick conditions only. If you put it in 4WD on dry
pavement and try to make a tight low-speed turn, the front and
rear wheels try to take different paths and thus roll at different
rates, but the 4WD forces them to rotate at the same rate, which
means that the rear wheels are being dragged forward relative to
what they would normally do and the front wheels are being dragged
backwards relative to what they would normally do. The little 1.8
liter engine isn't powerful enough to make them skid, so this does
an excellent imitation of having the brakes on. At that point
you can't get out of 4WD; the lever will not move. You have to
back up a bit and then go into 2WD. And that, my friends, is why
all modern AWD Subarus have some way of allowing the front wheels
and the rear wheels to move at different speeds.

I tested it in a big parking lot in a heavy rain, and my stopping
distance (maxinmum brakes before skidding) was 20-30 percent
shorter. It also seemed to be more stable while cornering, but
that's subjective. It is also noticably less squirrely at freeway
speeds during the rain in 4WD.

 

[Larry Van Wormer]

Larry Van Wormer wrote:




I can tell you from experience how they work. My 1985 Subaru
GL 4WD wagon has a 10-speed manual transmission (hi/lo range:
10 forward and 2 reverse) with differentials at the front and
rear but no center differential.

Yep, and that's why I specified AWD. That does need a centre diff. The
viscous coupling acts to make the mechanical centre differential a
limited-slip unit.

As you describe well, 4WD is a different beast. May or may not have a
centre differential.

Larry Van Wormer

 

[transfixed]

I know for a fact my Forester has a center diff. Where are you getting
this info?

Straight from Subaru. You do have a differential, but you don't (it's
semantics). The less expensive Subarus drive the front directly. They
do not have a traditional, geared center differential. Subaru still
refers to the mechanism as a center differential, because it partially
takes over the tasks of a center differential. The main difference,
though, is that it does not distribute fore/aft the way a geared system
does (whether planetary or TorSen: one in, two out). Here, the front is
always directly driven, and the rear gets hooked up to the drive shaft
by either the viscous coupling (MT) or by the electronically controlled
clutch pack (AT). The Audi/VW quattro/4Motion Haldex system in the TT,
A3, Golf, and 2006 Passat is similar in that again the front is always
driven, and the clutch pack is connecting the back. The beauty of this
system is that when the front slips, you can still get almost all of the
torque to the back.


If you don't believe me, go to the Subaru web site and look up the
information or ask a question from within the site. You'll get a reply
like:

Forester models do not have a geared center differential.

Continuous AWD: Models equipped with 5-speed manual transmission utilize
a viscous-type locking center differential with torque distribution
configured at a 50/50-split front-to-rear. 2.5 XS and 2.5 XT models also
feature a viscous-type limited-slip rear differential. Active AWD:
Models equipped with 4-speed automatic transmission uses an
electronically controlled variable multi-plate transfer clutch to
distribute power to where traction is needed. Sensors monitor parameters
such as wheel slippage, throttle position and braking to help determine
torque distribution to the wheels with optimum traction. 2.5 XS and 2.5
XT models also include a viscous-type limited-slip rear differential.