Subaru: Subaru Automatic AWD isn't really, in most models

I thought the normal automatics we 60:40 front:rear and could adjust to
50:50 max rear
The can also adjust to 95:5 front:rear.

I found this info at http://subaru.com.au/awd/ in the shockwave animation

I do agree thought that it is not as good as the manual. But it is alot
better than the CRV



"Paul Pedersen" <com> wrote in message
news:com... 

Paul Pedersen wrote: 
Great description, Paul. However, the auto is more proactive than you
are describing (so I have been told - anyhow). It is my understanding
that it is tied into the engine computer so that it can proactively send
torque to the rear wheels when you get accelerate - to reduce wheel spin
and in other sitiations. I agreewith you , however, that I would rather
have the manual transmission.

Subaru of America (tech dept) and Subaru of Australia claim that my 2000
Forester AUTOMATIC has a default ratio of 60 front/40 rear until wheel
Torque difference is detected. The AT is almost always in a state of flux
and is always activated unlike the locking center differential on the MT
models. The 60/40 default ratio is fixed by the transfer gear set as shown
in the Subaru Factory Manual. Power is transferred by a "continuously
variable clutch pack" behind the transfer gear set in the transmission. The
same gear set is reversed to obtain a default ratio of 40/60 for the Impresa
GT ? To quote Subaru literature "to give a more sporting feel in spirited
driving" I think the "old" Subarus that were front or all wheel drive
depending on whether the system was activated may be where the 90/10 or
100/0 idea came from. The AT system even in the base Subaru is a far more
advanced AWD system. In fact Subaru called it the thinking mans transmission
for its almost instantaneous reaction to wheel torque compared to the
locking differential which is locked until wheels slip and then the silicone
oil heats and then power is transferred. Subaru factory racers have a switch
to deactivate the locked center differential so as to bypass the much
delayed power transfer. Let us get our facts in line before typing.

"Subie25l" <com> wrote in message
news:3f95255b$0$23615$iinet.net.au... 

Paul Pedersen wrote: 

Yes, you are basically correct here (and this has been discussed several
times in this news group). But as Rob says, even Subaru's inexpensive
AT AWD is a bit more proactive, e.g., when you start driving, or under
strong acceleration. However, it does not have the full set of sensors
that the VTD/VCD has, and does not give you the benefits a set-up with
~50-50 default has.

As I have said before here, for technical reasons and given the lack of
a center diff, the clutch pack can basically only work in an on-off
mode. That is, when it is off, the distribution by definition must be
almost 100:0. When it is on, it can be anything because with an
(almost) locked differential, the torque goes to where the traction is.
That is, the torque distribution then is variable not due to variable
clutch pressure, but because traction to the ends varies due to surface
conditions.

This is a trick that is used by several manufacturers to reduce fuel
consumption. So, Subaru can claim that the AT rquires as little
gasoline as the MT. (Wether this is true under practical circumstances,
I don't want to get into).

- D.

Paul Pedersen <com> wrote in message news:<com>... 

The Suburu Auto AWD is not the same as the Honda and should not be
compared. The Subuaru system is a proactive system. I see little or no
disadvantage with the Auto AWD when compared to the manual AWD. It is
a proactive system. I have tested this out, and it is very difficult
if not impossible to generate wheel spin.

Case #1: Raining Day. Flooring gas pedal from a traffic light. The AWD
system automatically shift power to rear wheels to compensate for the
weight transfer. In fact in this case, I think it is better than the
50/50 in the manual system, since the manual system waits to slip
before transfer power!!!!

Case #2: Ice Storm. Again same as above flooring from a traffic light.
No slippage what so ever. AWD system already compensates for weight
transfer to rear wheels!!!

Etienne

Glad to hear from you Transfixed. Ed
"TransFixed" <jp> wrote in message
news:S%qlb.80555$.. 

Paul Pedersen <com> wrote in message news:<com>... 

For what i have read, all AT subaru are now full-time AWD. Some models
used to be part time AWD, but not anymore. In fact, the AT AWD is more
advanced than the MT AWD. The automatic transmission pump pressure
allows the design of this more advance AWD. It is the same system as
the Audi A8. In 1st and 2nd gear the power distribution is 60/40. To
reduce gas consumption, the distribution goes to 80/20 or 90/10 in 3rd
and 4th gear.

For the MT AWD, when there is slippage, the center differential which
is normally open, locks to 50/50.
For the AT AWD, when there is slippage torque is sent to the axel with
less slippage, hence torque on each axel is adjusted dynamically,
which make the AT AWD better. Of course, it doesn't mean the the MT
AWD doesn't do the job well. In fact it is a good system also, it is
just less advanced and adaptative.

quote from "http://home.comcast.net/~eliot_www/awd.html"
"Subaru deserves mention here because in the automatic version of the
Legacy and Impreza (including the Outback variants), it uses a
computer controlled system much like those found in the Mercedes
4Matic, automatic Audi A8/V8 and the earlier Porsches. Subaru has been
offering this sophisticated system for a long time in a relatively
inexpensive car."

http://home.comcast.net/~eliot_www/awd.html is a very interesting
document on AWD.

Good going Subaru !

bilgab

Edward Hayes writes :
 

Funny, I was talking to a Subaru salesman and he claimed
50-50 for the non-VTD automatic. Due to my disagreement
we went to the mechanic and he said 80-20.

It would be so nice if Subaru cleared all this up for those
of us who like to understand things. Here are the different
numbers I've come across, all from Subaru :


Off the Subaru Canada website
-----------------------------

http://www.subaru.ca/interface/Subaru02/WebPage.asp?WebPageID=4768&WebSiteID=282

-> "Click to view our AWD demo"

"The different Subaru AWD systems are matched to each
vehicle's character. A 60% front 40% rear torque
transfer on our basic system prioritizes safety while
a 36% front 64% rear torque transfer on our STI helps
maximize performance where sporty driving is the
priority."

[--> no mention made of which transmission this might apply to]


Out of the Outback 2003/4 brochure
----------------------------------

"Manual transmission models split engine power
50/50 between front and rear wheels. Automatic
transmission models use Active All-Wheel Drive,
an electronically controlled multiplate transfer
clutch. VDC model uses a Variable Torque
Distribution System."

[--> no problem, except it violates the previous ]


Off the Subaru Global website
-----------------------------

http://www.subaru-global.com/about/awd/2321.html

[manual transmissions]

"Normally, the centre differential distributes the torque at a
ratio of 50:50 to the front and rear wheels for extremely
stable driving and maximum traction. Where the balance of
traction is lost when front or rear wheels slip, viscous
LSD automatically redistributes torque to maximise grip,
so full traction and driveability are always available."


[automatic transmissions]

[--> no values for the MPT trnasmission]

http://www.subaru-global.com/about/awd/2323.html

[--> ie WRX ]

"V-T-D AWD is an AWD system for automatic transmission vehicles
that provides positive, sporty driving by making improvements
in turning-in while maintaining the basic driving safety
performance of Subaru AWD. The system uses complex planetary
gear type centre differentials that distribute the basic torque
at a ratio of 35 for the front wheels and 65 for the rear.
Distributing more torque to the rear wheels reduces the
tendency to understeer when accelerating while cornering to
provide smoother, more confident handling."

[--> here are the gear reductions for the VTD WRX :

front : 4.111 rear : 4.111 ]


"Torque distribution is also optimally controlled to suit road
conditions. This system provides both sporty driving and
stability under any road conditions by automatically
equalizing the front and rear wheel torque distribution to a
maximum ratio of 50:50."


Out of the Impreza 2004 Service Manual (Mechanism and function)
---------------------------------------------------------------

Automatic Transmission : Transmission Control Module (TCM)

AWD transfer clutch control - Ordinary transfer control

Input signals :

Throttle position sensor
Rear vehicle speed sensor
Front vehicle speed sensor
Inhibitor switch
ATF temperature sensor
FWD switch

[--> there aren't any 'torque sensors']


Automatic Transmission : AWD Transfer System

A: MPT MODELS

"This all-wheel-drive (AWD) transfer system uses an
electronically controlled multi-plate type transfer
clutch. The clutch is controlled by the TCM through
the transfer hydraulic pressure control unit which
consists of a duty-cycle-controlled solenoid valve
and is located at the rear of the automatic
transmission section together with the vehicle speed
sensor.

The TCM has in its memory a set of duty ratio data,
each defining at what ratio the transfer clutch should
transmit the torque for a particular driving condition.
Based on the driving condition information it receives
from the corresponding sensors (vehicle speed, throttle
opening, gear range, slip of wheels, etc.), the TCM
selects an appropriate duty ratio from the memory and
uses it to control the solenoid valve. The solenoid
valve then regulates the pilot pressure of the transfer
control valve which creates the pressure to the clutch
from the line pressure. The clutch is engaged to a
degree determined by the transfer clutch pressure
thus created. Through this process, the torque from the
engine is distributed to the rear wheels optimally
according to driving conditions.

B: VTD MODELS [ie WRX]

The center differential performs the differential functions
of absorbing the speed difference between the front and rear
wheels and also distributes drive forces to the front and
rear wheels at a predetermined ratio. In normal conditions
(when there is almost no difference in the speed between the
front and rear wheels), the drive force distribution ratio
is 45.5 % to the front wheels and 54.5 % to the rear wheels.
The hydraulic multi-plate clutch connected in parallel with
the center differential between the carrier and 2nd sun gear
functions as a differential action limiting device (LSD) and
also as a device that controls torque distribution according
to driving conditions."


From Subaru's Drive magazine
----------------------------

http://www.drivesubaru.com/SubaruTakesSafe.html

"The VDC system first uses the transmission’s VTD AWD to help
maintain control of the vehicle by transferring power between
the front and rear wheels. Under normal driving conditions
the VTD AWD system directs 45% of the engine power to the
front wheels and 55% to the rear wheels. This slight rear-wheel
bias produces a more neutral balance by reducing understeer
and providing more of a performance driving feel."


For another non-Subaru description see :

http://www.autoworld.com/news/Subaru/Subaru_All-Wheel.htm

(this article mentions 90%-10%)


 

At least for the 2004 non-turbo Impreza (and Forester ?)
the gear reductions are the same front and back 4.111
so any torque difference must come from the viscous
coupling in the transfer clutch, which under normal
conditions is mostly disengaged. The 90-10 figures I've
seen look sensible for this condition.

With the transfer clutch fully engaged (locked) the
distribution will be 50-50. An interesting thing here
is that I don't beleive it's possible to spin a rear
wheel without a front wheel also being in the spinning
state.

Another thing, I don't see a transfer clutch as a
replacement for a center differential. There's no
balancing function. When the clutch is disengaged
there is little going to the rear, then as the
clutch closes, there is more and more going to the
rear - but - there is never anything to handle the
difference in wheel speeds front and back and
vehicle handling must go out the window. (Just try
driving a car with a locked center differential).

My personal vote is for 90-10 (if not 100-0) unless
conditions dictate a shift to the rear, which can
go to a max of 50-50.

It would seem that Subaru shies away from numbers
when they talk about the MPT transmission...