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4WD and AWD systems explained

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Comments

  • rshollandrsholland Posts: 19,663
    I know—which is why I stated in an earlier post that "all manual Subarus" (except the STI), use a VC.

    Bob
  • ateixeiraateixeira Posts: 72,587
    Gotcha, I misread your post, sorry.
  • wwestwwest Posts: 10,706
    "..It's #2 except..."

    If there is no ACTUAL center diff'l then the VC use must be as stated in #1.

    That places the VC in "series" with the rear driveline, just as in the Chrysler T&C, and there is evry little torque to the rear unless the VC has reason to "tighten up". The more recent problem with this type of VC F/awd implementation is that TC activation, INSTANT TC activation, upon even the slightest level of front wheelspin/slip, prevents the VC from EVER tightening.

    That's the main reason Chrysler discontinued the F/awd T&C, the adoption of TC (and "VSC") made their VC implementation non-functional, useless.
  • ateixeiraateixeira Posts: 72,587
    Having driven one in snow for years and years, I can guarantee that the rear axle gets power full-time. Hence why the rear axle breaks traction first and you get oversteer.
  • wwestwwest Posts: 10,706
    You're only fooling..... Yourself.

    "...I can guarantee...gets power full-time.."

    Even a beginning automotive design engineer could put the fallacy to that statement. Even the new Subbies with the ECU controlled clutch pak only route engine torque to the rear for times, during acceleration, when excessive drive torque is most likely to result in front wheelspin/slip

    If that were to be so your tire treads would have an a very short life and early, premature driveline component failures would be the NORM.
  • ateixeiraateixeira Posts: 72,587
    edited May 2011
    Manual trans so that '98 had the VC setup, no clutches.

    Rear axle would break traction first when on power in turns. Less weight on that axle, but it would definitely get power - how else would it break traction and spin the rear wheels first?

    I drove it for 9 winters, and that's just how it behaved.

    Wife has an 09 automatic now and that feels different - it'll understeer. Night and day difference in feel.
  • Is the AWD of the 2012 4-cylinder Outback different between the 6-speed manual and the CVT automatic? If so, which is better for normal on-road driving?
  • ateixeiraateixeira Posts: 72,587
    Yes...

    Manuals use a viscous coupling that is fluid-filled. When the axles spin at different rates the fuild shears and thickens to temporarily bind the axles together. The default split is 50/50.

    The CVT uses clutches and defaults to a more FWD-like default, then sends more power to the rear axle as needed.

    I've owned Subaru with both, and I prefer the VC system in the manual. It doesn't feel like FWD in day to day use. Both are effective in the snow, particularly when paired up with good tires.

    To be honest the limiting factor on a new Outback is probably the approach and departure angles, due to longish overhands. It has plenty of ground clearance.
  • wwestwwest Posts: 10,706
    "..When the axles spin at different rates..." TC will INSTANTLY activate, fully dethrottling the engine while simultaneously braking the driven, front, wheels to alleviate the potential for loss of directional control.

    With the advent, adoption, of TC it is not possible for a VC to be functional absent a TC off capability. Additionally, actual VC functionality, assuming TC off, will always compromise ABS and stability controls.

    As a result of the above limitations/restrictions most modern day systems using a VC have had the fluid reformulated such that the VC remains forever flaccid.
  • wwestwwest Posts: 10,706
    "..sends more power to the rear axle as needed.."

    Not technically correct.

    PRE-EMPTIVELY sends more power to the rear axle, "needed" or no, at times when loss of directional control is most likely to otherwise result. Those times fall into only 2 categories, low speed acceleration, and when turning tightly or turning while accelerating.

    Other than those times the system will default into a definite front torque biasing mode. Regardless of the instantaneous mode of operation loss of directional control is such a dire threat for FWD or F/awd vehicles that the INSTANT response to wheelspin/slip, DRIVEN (front) wheelspin/slip, will be activation of TC.
  • wwestwwest Posts: 10,706
    There is also an unstated, unspoken, advantage to the DCCD F/awd system in that it can be disabled INSTANTLY should ABS, TC, or VSC (stability control) activate.
  • ateixeiraateixeira Posts: 72,587
    absent a TC off capability

    There's an off switch.
  • dcm61dcm61 Posts: 1,459
    But, but ... that's not possible. WRX a "fake" F/AWD and RAM is a "real" R/4WD. :P
  • wwestwwest Posts: 10,706
    edited July 2012
    6 wheels driving, the 2 front on the subie + the 4WD RAM, will always provide more traction.....

    And...

    Is that a WRX (no hood scoop) and is it the WRX with the "make-do" VC coupling.
  • ateixeiraateixeira Posts: 72,587
    One has excess traction, the other not enough.

    The Subie happens to have enough excess traction to compensate for the other.
  • larryvlarryv Posts: 9
    I got stuck recently in the Sierras when I backed over a large angular piece of granite that dropped the right rear wheel into a hole. All the other wheels were firmly on the ground and the chassis did not high center. I thought the AWD in the 5-speed would pull me out with no problem. But all it did was spin the right rear wheel and burn up the clutch. I managed to get out by slipping the jack under the chassis in front of the right rear wheel and jacking up the car enough to put some rocks and large branches under the wheel and the floor matte over the rocks/branches and up the rock, and I finally got out. But I was very disappointed in the car. So, what gives? Thanks. LRV
  • wwestwwest Posts: 10,706
    Subaru is not shy regarding marketing "AWD" systems that have VERY serious shortcomings. The current WRX, for instance, uses a completely non-functioning VC, Viscous Clutch, in order to tout "AWD" capability.

    Sounds as if your's is one of the many F/awd systems out there in the market today that are really "ONE-WHEEL" drive systems. Totally open center differential, "AWD" mode only if all four tires have roughly EQUAL traction.

    Most of those have a TC implemented "AWD" system, braking of "slipping" wheels to simulate traction accompanied INSTANTLY by full engine dethrottling...not very helpful in the situation you describe.

    Turn off TC = ONE-WHEEL drive.
  • rshollandrsholland Posts: 19,663
    Did you turn off the traction control? I think that would have helped. Subaru says to turn it off in deep snow and mud. It certainly would have helped your clutch.

    Bob
  • wwestwwest Posts: 10,706
    edited August 2012
    Since turning off TC results in the car reverting to a simple ONE-WHEEL drive he got exactly the results one would expect.

    With TC on the engine dethrottling would have been so severe the engine torque wasn't enough to pull that wheel over the "hump".

    TC off functionality is pretty much restricted to "feathering" the throttling right up to, but not beyond, the point of loss of traction. Wheelspin, rocking the car back and forth, to get unstuck, can also be used constructively.
  • dcm61dcm61 Posts: 1,459
    edited August 2012
    Did you turn off the traction control?

    Wake up before you post, Bob :P ... 2005 Outback 2.5L does not have traction control.
  • rshollandrsholland Posts: 19,663
    What's the little yellow light that shows a car skidding on the instrument cluster? I can turn that off in my car, and you're saying that can't be turned off in the Legacy 2.5?

    Bob
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,460
    VDC wasn't available on the 2.5L models until 2008 as an option, then standard in 2009.

    Even so, the car should have had traction in the front axle unless the center differential had gone out. It's unusual that he would have power to the rear axle under that circumstance, though, as typically it is the rear axle that would stop working as opposed to the front axle. In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that, so under limited traction conditions they are generally 2wd (one on each axle).

    I managed to get my '96 Outback stuck more than a few times in deep snow by taking too much weight off the wheels, and it would always default to spinning the left front and right rear tires. :blush:
  • wwestwwest Posts: 10,706
    "..does not have..."

    That would be strange indeed. Even as far back as 2001 it ws well understood industry wide that TC could prevent a LOT of loss of control accidents rising from the use of to much engine torque applied to the drive wheels.

    Prior to that period TC was found mostly only on "pure" FWD vehicles, non-AWD versions.

    That was one of the aspects facilitated by DBW, the engine could be dethrottled without compromising the catalytic converter.
  • wwestwwest Posts: 10,706
    VDC probably refers to stability control INCLUSIVE of TC, the 2008 timing would be about right for that happenstance.

    Not knowing anything at all about the specifics of this particular "AWD" system in question the description, only the wheel with traction spinning freely, indicates a fully open center diff'l with the TC turned off.

    "..were limited slip..."

    But more likely than otherwise "simulated" limited slip using TC braking.

    Sounds as if your '96 had some method to automatically "lock" the center diff'l or PTO, VC's were commonly used for that back then. Once TC was implemented those methods because useless, non-functional.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,460
    But more likely than otherwise "simulated" limited slip using TC braking.

    No, they weren't, they were viscous LSD. After 2009, however, they did go to open front/rear differentials using the VDC system to compensate. It is a poor compensation, at best.

    The WRX retains a rear LSD, while the STi has both front and rear LSD.
  • dcm61dcm61 Posts: 1,459
    edited August 2012
    In 2007, 08, and 09, the rear axles on the Outback were limited slip, but they were open prior to (and after) that,

    2000 (w/AWP) and 2001 to 2006 2.5L also had rear LSD. Not sure which years/models of H6 had rear LSD.
  • wwestwwest Posts: 10,706
    I know of a few high end marques that use VC(s) to implement a form of rear diff'l LSD but I suspect in Subbies case you mean center diff'l or PTO drive "locking" is done using a VC.

    "..The WRX retains a rear LSD,..."

    That, an actual mechanical LSD, would be a bit of foolishness since the current WRX model has no means for driving the rear diff'l except under TC braking, along with full engine dethrottling, of the front wheels. Probably not even then unless you use an extended period of TC activation.

    The above assumes the WRX's VC center "locking" is at least minimaly functional, not something I have any faith in at all.

    "..STi...has both..."

    I'm willing to bet you good money that if the Sti has a rear LSD it is of a "virtual" nature using "differential" rear wheel braking.

    And front LSD...??!!

    NOT.

    Those are found ONLY on the most robust off-road vehicles (Hummer, etc.)wherein the driver is expected to be fully aware, expectent, experienced, with what a front LSD will do to your fingers, thumbs, etc.

    I think you will find that the Subbies version of front LSD is via TC braking and of BOTH wheels simultaneously even if only one wheel is slipping. To do otherwise would often result in yanking the stearing wheel right out of the hands of an unwary driver not expecting the resulting hard "TUG".
  • wwestwwest Posts: 10,706
    "Virtual" or "hard" LSD...?
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