all wheel drive

[peter (archive)]

I'm sure this topis was discussed plenty of times here but since I'm new to this could any one bother explaining this one to me one more time? I drive Subaru Forester X 2004 Automatic. Subaru's website says that "...All-Wheel Drive system (featured on models equipped with 4EAT) can anticipate wheel slippage and tranfers power to the front or rear with the best traction even before slippage occurs". What is the split under normal traction conditions - is it 50/50 like with 5-speed manual transmition models?

Thank You

 

[Yousuf Khan]

I drive Subaru Forester X 2004 Automatic. Subaru's website says that "...All-Wheel Drive system (featured on models equipped with 4EAT) can anticipate wheel slippage and tranfers power to the front or rear with the best traction even before slippage occurs". What is the split under normal traction conditions - is it 50/50 like with 5-speed manual transmition models?

90% towards the front, 10% towards the rear.

Yousuf Khan

 

[Edward Hayes]

According to Subaru of Australia and Subaru of America the default ratio for the 4EAT in the Forester is 60/40 and for the same year Impresa GT it is 40/60.

 

[Carl 1 Lucky Texan]

I have read that selecting 1st or Rev. in the 4EAT switches it to 50/50.

Carl
1 Lucky Texan

 

[John M. (archive)]

I'm sure this topis was discussed plenty of times here but since I'm new to this could any one bother explaining this one to me one more time? I drive Subaru Forester X 2004 Automatic. Subaru's website says that "...All-Wheel Drive system (featured on models equipped with 4EAT) can anticipate wheel slippage and tranfers power to the front or rear with the best traction even before slippage occurs". What is the split under normal traction conditions - is it 50/50 like with 5-speed manual transmition models?

Thank You

From the technical manuals:
For the 4EAT Phase 2 automatic transmission system using VTD (includes the H-6 VDC)-
1) "Assuming the friction of the front and rear tires is the same (normal traction condition), the power is split 54.55% to the rear and 45.45% to the front"
2) "The TCM adjusts the duty ratio of the MPT clutch to maintain the optimum transfer of power. A large speed difference in the rear to the front wheels results in the MPT clutch locking the Read Drive Shaft to the carrier. Power is then split 50% to the front and 50% to the rear."

John

 

[Hal Whelply]

From reading various places, my understanding is 90 front/10 rear usually, going down/up to 60/40 if conditions warrant. I don't know about selected first or reverse gear being 50/50.

HW

 

[Paul Pedersen]

:

From the technical manuals:
For the 4EAT Phase 2 automatic transmission system using VTD (includes the H-6 VDC)-

Except that the Forester doesn't have a VTD transmission, which has a center differential. The Forester only has a transfer clutch which is only used to send power to the rear under certain circumstances (which have never been defined by anyone publicly).

The engine on the Forester is directly linked to the front wheels, while the rear wheels freewheel under normal conditions.

 

[Paul Pedersen]

Carl 1 Lucky Texan wites :

I have read that selecting 1st or Rev. in the 4EAT switches it to 50/50.

If it did this you wouldn't be able to turn the thing.
50/50 means a locked transfer clutch.

 

[Carl 1 Lucky Texan]

?
I'm not sure I ubderstand this statement.
It doesn't mean the differentials aren't working.

Carl
1 Lucky Texan

 

[Carl 1 Lucky Texan]

Correct.
The 4EAT (Active AWD) is usually listed as 90/10 under steady-state cruising. When hard accelerating or rear wheel slippage occurs it can go up to 60/40 (maybe 50/50 in lower gears?). The 'duty solenoid 'c' gets it's duty cycle shifted by the TCU to engage the MPT clutch less or more as is required.(so I've read-something like 100hz). The default(interrupte signal,broken wire) condition is full activation of the MPT clutch. Installing the FWD fuse supplies 12V constantly. Forcing full DISengagement of the MPT clutch. The VTD 4EAT is approx. 55/45 I don't know if this yet different again for the VDC.

Carl
1 Lucky Texan

 

[Dominic Richens]

Carl 1 Lucky Texan wites :

If it did this you wouldn't be able to turn the thing.
50/50 means a locked transfer clutch.

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

 

[Yousuf Khan]

:

Except that the Forester doesn't have a VTD transmission, which has a center differential. The Forester only has a transfer clutch which is only used to send power to the rear under certain circumstances (which have never been defined by anyone publicly).

The engine on the Forester is directly linked to the front wheels, while the rear wheels freewheel under normal conditions.

Actually, no they don't completely freewheel. There is a non-zero minimum friction in the front-to-rear clutchpack, so there is always some minimal amount of power going to the rears. Depending on how tight the clutchpack is set to, you can set the default AWD split at anything from 90:10 to 40:60 depending on Scoobie model.

Yousuf Khan

 

[BlueSTi]

Actually, no they don't completely freewheel. There is a non-zero minimum friction in the front-to-rear clutchpack, so there is always some minimal amount of power going to the rears. Depending on how tight the clutchpack is set to, you can set the default AWD split at anything from 90:10 to 40:60 depending on Scoobie model.

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.

BlueSTi
"Scary-Fast"

 

[Yousuf Khan]

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

In a 5MT, you have a differential between the front and rear wheels. So during turning and stuff, the front and rear wheels take slightly different paths around the corners (the rears actually take a shorter distance). So during turning, you don't actually remain at the default 50/50 torque split, it adjusts to the appropriate split to let the wheels take the right path around. In a 4AT, instead of a differential you have a clutchpack. So in a clutchpack situation, when you're talking about a 50/50 split -- you're talking about a locked 50/50 split. The Scoobie clutchpack actually pulsates on and off like an anti-lock brake, to allow the front/rear wheels to follow their own paths around corners.

Yousuf Khan

 

[Yousuf Khan]

Correct.
The 4EAT (Active AWD) is usually listed as 90/10 under steady-state cruising. When hard accelerating or rear wheel slippage occurs it can go up to 60/40 (maybe 50/50 in lower gears?). The 'duty solenoid 'c' gets it's duty cycle shifted by the TCU to engage the MPT clutch less or more as is required.(so I've read-something like 100hz). The default(interrupte signal,broken wire) condition is full activation of the MPT clutch. Installing the FWD fuse supplies 12V constantly. Forcing full DISengagement of the MPT clutch. The VTD 4EAT is approx. 55/45 I don't know if this yet different again for the VDC.

The VDC 4EAT system actually uses a centre differential like the 5MT uses. But then it combines the differential with the clutchpack of the standarrd 4EAT. The clutchpack in the VDC takes the place of the fluid-coupled limited-slip system providing slip limiting through a clutchpack rather than a fluid-coupler. This clutchpack slip-limiter is electronically controlled so it should be able to provide more "intelligent" active slip-limiting than the completely reactive hydro-mechanical fluid-coupler.

The difference between the standard 4EAT and the VDC 4EAT is that in the standard one, the clutch pack is the distribution mechanism from the front to rear; but in the VDC the front/rear distribution mechanism is the differential, with the clutch pack acting as helper.

Yousuf Khan

 

[Paul Pedersen]

Yousuf Khan writes :

Actually, no they don't completely freewheel. There is a non-zero minimum friction in the front-to-rear clutchpack, so there is always some minimal amount of power going to the rears. Depending on how tight the clutchpack is set to, you can set the default AWD split at anything from 90:10 to 40:60 depending on Scoobie model.

I realize Subaru would like you to think this, but I don't see how it's possible. Unless it is locked, the transfer clutch will be slipping, however slightly, which means that the rear end would be turning less quickly than the front (everything else being cast aside for the moment). But, since the front wheels are connected directly to the engine what happens is that the front wheels pull the car along and the :chassis: will cause the rear wheels to turn, not the transfer clutch, since it's output shaft is turning less quickly than the front (or at least wants to).

So, unless the transfer clutch is locked (which you want it to be sometimes) nothing at all is transfered to the rear.

 

[Edward Hayes]

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. God this thread is getting old. SOA of USA and SOAu says my 2000 Forester ratio is 60/40 when no wheel torque difference is detected.

 

[Carl 1 Lucky Texan]

I can see where there may be a little 'drag' in the clutch pack even under steady state level dry hard surface conditions. Tiny sidewall deflections and very slight slippage on the front would be reported to the TCU as a 'difference' in wheel speed (compared - as you say - to the the rear wheels just being pulled along by the chassis). The even gretaer deflection and slippage under hard acceleration would xfer even more 'power' to the rear wheels. I think the system may be that sensitive. Do searches at www.usmb.net www.nasioc.com and www.i-club.com on 4EAT clutch pack failures. Justa few miles driving with an undersized spare or tire can destroy the clutchpack and/or duty solenoid. Perhaps I misunderstand the exact nature of all the nuances of the 3-4 types of center differential under all conditions in each model (the number of similar threads on various forums indicates I'm not alone) BUT I think they work really well.

Carl
1 Lucky Texan

 

[Patrick Fisher]

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.

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.

 

[John M. (archive)]

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. God this thread is getting old. SOA of USA and SOAu says my 2000 Forester ratio is 60/40 when no wheel torque difference is detected.

It is getting old. It's almost as if folks don't want to hear it just because they can't understand it. Sheesh. Even the quotes directly out of the tech manuals go unheeded.
John