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Car Forum / Subaru Cars / May 2004all wheel drive
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:) > 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 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. > > I drive Subaru Forester X 2004 Automatic. Subaru's website says that > > "...All-Wheel Drive system (featured on models equipped with 4EAT) can [quoted text clipped - 6 lines] > > Yousuf Khan I have read that selecting 1st or Rev. in the 4EAT switches it to 50/50. Carl 1 Lucky Texan > 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 [quoted text clipped - 10 lines] >> >> Yousuf Khan  Signatureto reply, change ( .not) to ( .net) 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. ? 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 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. Signatureto reply, change ( .not) to ( .net) > Carl 1 Lucky Texan wites : > [quoted text clipped - 3 lines] > 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. -- Dominic Richens | dominic@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!" >> Carl 1 Lucky Texan wites : >> [quoted text clipped - 5 lines] > > 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 Dominic Richens writes : > > Carl 1 Lucky Texan wites : > > [quoted text clipped - 5 lines] > > 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. > Dominic Richens writes : > [quoted text clipped - 11 lines] > 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! -- Dominic Richens | dominic@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!" > the high-end automatics (VTD), but not the Imprezas, Foresters > and regular Legacy automatics. [quoted text clipped - 4 lines] > 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 > 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?:) [quoted text clipped - 6 lines] > > 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 "John M." writes : > 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. 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 > "John M." writes : > [quoted text clipped - 10 lines] > The engine on the Forester is directly linked to the front > wheels, while the rear wheels freewheel under normal conditions. Signatureto reply, change ( .not) to ( .net) > Correct. > The 4EAT (Active AWD) is usually listed as 90/10 under steady-state [quoted text clipped - 7 lines] > 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 > "John M." writes : > [quoted text clipped - 10 lines] > 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 >> "John M." writes : >> [quoted text clipped - 18 lines] > > 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" > 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. > > 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 [quoted text clipped - 16 lines] > 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? 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. > when there is wheelspin, > [quoted text clipped - 17 lines] > 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 > 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. 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 [quoted text clipped - 6 lines] > > 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). 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. > John Emdall writes : > [quoted text clipped - 13 lines] > is identical front and back, so I don't see how it can > be anything other that 50/50 (nominally). 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 >There's a planetary center differential, which divides the torque unequally >from front to rear. The transmission output shaft connects to the planet [quoted text clipped - 24 lines] >> is identical front and back, so I don't see how it can >> be anything other that 50/50 (nominally). > >There's a planetary center differential, which divides the > >torque unequally from front to rear. The transmission output [quoted text clipped - 8 lines] > >radius of the gear. You can get any torque split you want > >by changing the dimensions of the center differential. I understand that you can get any split you want through the center differential. But wouldn't this show up in the gear reductions stated in the service manual ? The WRX's specs show different gearing front and back (balanced by the center differential) but the Sti's specs are equal front and back. > I understand that you can get any split you want through the > center differential. But wouldn't this show up in the gear > reductions stated in the service manual ? The WRX's specs > show different gearing front and back (balanced by the center > differential) but the Sti's specs are equal front and back. No. Why would they? The only reason some cars utilize different ratios is to allow a continuous, small transfer of torque to one axle when the center coupling is viscous. In a viscous coupling, no torque is transferred until there is some slip. If the ratios are the same, then generally there will be no slip, and the torque goes to one axle (the front). As soon as the front wheels started to spin, torque would suddenly be transferred to the rear as the fluid heats up and thickens. The last thing you want is sudden torque transfer to the rear in a low traction situation. So they "pre-tension" the coupling by having different final drive ratios. It "seems" like the rear is always slipping, so there's almost no "play" in the coupling. Torque transfer under slip conditions is smoother. In any other diff setup, different final drive ratios will just cause the diff to melt - you'll be constantly fighting the diff's slip-limiting action by causing perpetual "slip". >> I understand that you can get any split you want through the >> center differential. But wouldn't this show up in the gear [quoted text clipped - 14 lines] > rear is always slipping, so there's almost no "play" in the coupling. > Torque transfer under slip conditions is smoother. I think there is some confusion here. The viscous coupling in Subaru's 5MT centre diff is there just to aid a fully mechanical gear-based differential. The two ends of the diff are usually spinning at exactly the same speed (when travelling in a straight line), and they don't really have a speed differential until the car starts turning. The viscous coupling then puts some resistance on the two ends of the diff to prevent slippage. It only takes effect during turns. Yousuf Khan > I think there is some confusion here. The viscous coupling in > Subaru's 5MT centre diff is there just to aid a fully mechanical [quoted text clipped - 4 lines] > two ends of the diff to prevent slippage. It only takes effect during > turns. I would go futher than that. The center differential is an open differential with the two output shafts connected in the middle by a visous coupling. Even without the viscous coupling, you still have an AWD vehicle; i.e. equal torque to all four wheels, which is fine so long as all four wheels have traction. Where the VCD comes in is when you are stuck in the snow/mud with one wheel with no traction (i.e. zero torque - hence all wheels have zero torque). The VCD heats up and essentially stops the differential from being a differential (i.e. locks up). -- Dominic Richens | dominic@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!" All I can say is the STi handles like a rear wheel drive until you really nail the throttle in a corner. My previous AWD vehicles seemed to handle more like a FWD. The STi feels more like a "push" than a "pull". BlueSTi "Scary-Fast" >John Emdall writes : > [quoted text clipped - 13 lines] >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" 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 >>John Emdall writes : >> [quoted text clipped - 20 lines] > BlueSTi > "Scary-Fast" Signatureto reply, change ( .not) to ( .net) > 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. > 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 > > 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. 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. > The service manual states for the STi 6MT : > [quoted text clipped - 9 lines] > > 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. 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. 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. > Yousuf Khan writes : > [quoted text clipped - 16 lines] > So, unless the transfer clutch is locked (which you want it to > be sometimes) nothing at all is transfered to the rear. > 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 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 !!!! 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 > Edward Hayes writes : > [quoted text clipped - 41 lines] > or their VTD auto transmissions. They deal with the MPT > transmissions mainly by omission. 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 > Yousuf Khan writes : > [quoted text clipped - 16 lines] > So, unless the transfer clutch is locked (which you want it to > be sometimes) nothing at all is transfered to the rear. Signatureto reply, change ( .not) to ( .net) 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 > 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?:) [quoted text clipped - 6 lines] > > Thank You:) thx! > thx! Thx!??? That's *it*?? All of that arguing you've caused and all you say is Thx???!!! For shame! <smile> John At least he could have spelled it out so it would sound sincere!!!!!! Ed > > thx! > > > Thx!??? That's *it*?? All of that arguing you've caused and all you say is > Thx???!!! For shame! > <smile> > John |
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