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4WD and AWD systems explained

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Comments

  • wwestwwest Posts: 10,706
    You'd be amazing at how quickly 4WD transfer case gearing will fail with even the slightest use on roadbeds that are intermittently slippery and tractive. My advice would be to NEVER make continuous use of the 4WD system except on SOLIDLY slippery surfaces and ONLY at relatively low road speeds.
  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,919
    Missed that one a couple of years ago eh?

    I thought about you yesterday. Did a U-Turn on a stretch of dead end pavement here in the UP of Michigan where we live now and got the Subaru stuck. :D

    Went a bit too far on the shoulder that's used for a snowmobile trail and the snow was soft and a bit wet. Had to shovel a yard away from behind each wheel (shod with "performance" all season tires btw).

    If I'd been in the safer FWD minivan, I would have done a 3 point turn and not had any problems. ;)
  • rshollandrsholland Posts: 19,657
    Steve, you're now a "Yooper?" My mother's side of the family is from Ironwood. Is that near you?

    Bob
  • Stever@EdmundsStever@Edmunds YooperlandPosts: 38,919
    No kidding? That's fun, you betcha.

    Nothing is near us, but we have made it over to Ironwood.
  • I got a 2k bravada from my brother in law, the awd seemed to work great, but i got stock yesterday, and the front wheels didnt turn at all. instead of messing with a bunch of stuff to try and get it working, if there a way I can put a lever on it and make it manually controlled? Is this possible and has anyone done it before? Its my wifes car, and I want her to be safe. Let me know, thanks!
  • volkovvolkov Posts: 1,302
    Put a lever on what? Modern awd systems are electronically controlled. If it was stuck but the front wheels wouldn't turn, it is might be the traction control being too sensitive - it will brake the wheels that are slipping, and can take it to the point of not moving at all. Most vehicles have a button to turn the traction control/stability control off, and it can be useful in that situation. Look on the dash for a button with a picture of a car and S shaped tire tracks underneath. Not sure that the 2k had that yet though.
    Do the front wheels show power ever? If the rear wheels were slipping and the front weren't turning in AWD mode, there is a significant problem - abs module or centre diffy is toast would be most likely.
  • wwestwwest Posts: 10,706
    The facts of life are that other than the SH-AWD system(***) there really is NO such thing as a fully functional F/awd system in the marketplace today.

    As has already been said, many of these wannabee F/awd systems allow for TC to be turned off, leaving you with a simple ONEWHEEL drive system. That at least allows you to use wheelspin, if that works, to get unstuck or initially in motion.

    Your brother-in-law shows good sense, not risking his life in one of these. What did he get to replace..? I'm betting on a R/awd vehicle.

    *** Not anywhere near the equal of ANY R/awd system, such as a 4runner, but undoubtedly the best of the best insofar as vehicles with sideways mounted engines are concerned.
  • ateixeiraateixeira Posts: 72,587
    I like the ramp one:

    http://www.subaru.com/engineering/all-wheel-drive.html

    http://www.subaru.com/engineering/safety.html

    Ford Escape does terribly in that avoidance manuever.

    Results certified by USAC, too.
  • wwestwwest Posts: 10,706
    That particular ramp test more indicates an adequate rear LSD, maybe even a virtual one. A good driver could use slight, judicious, e-brake application in all cases to pull that ramp.

    Is it possible that's what the Subbie driver did?

    A rear LSD in a F/awd vehicle, SUBBIE...?? Rear LSDs are pretty rare, VERY rare, in F/awd vehicles.

    Oh, I wonder...

    A virtual LSD via use of TC differential braking at the REAR could be a possibility. Anyone know for sure..??

    So the Subaru doesn't have unequal front drive halfshafts thus "symmetrical" AWD...interesting
  • wwestwwest Posts: 10,706
    edited May 2011
    Driver trick, SIMPLE driver trick, trickery...??

    http://www.subaru.com/engineering/safety.html

    Look at the "set" of the Subbie's suspension at 1:48.

    Now notice the missing frames, video "glip" at about 1:45-7.

    During the frames you do not see the driver has quickly and very briefly oversteared to cause the rear wheels to "skid" out into the "line" he wishes to enter that upcoming hard left turn.

    Just as the Subbie driver is clearing the first set of cones look at the "line" of the Subbie and then at the angle, lack thereof, of the front wheels. The driver has cranked in enough overstear just ahead of the cones that the rear has come about, SWUNG OUT, and now he has counter-steared to correct for the overstearing condition.
  • ateixeiraateixeira Posts: 72,587
    That model has traction control managing the rear axle, so virtual LSD, yes.

    I think the difference is that the others have AWD or traction control since they are usually front axle only. That means the rear axle doesn't really have the virtual LSD, since they provide forward propulsion only part of the time anyway.

    USAC certifies the results, so presumably they didn't cheat with the handbrake...
  • ateixeiraateixeira Posts: 72,587
    edited May 2011
    Are they missing frames, or is it just a low refresh rate/resolution? Seems like the latter to me.

    Oversteer is consistent with what I used to get with my 98 Forester, in the days before stability/traction control (I had open front/rear diffs). It would wag the tail, swing out the rear, then AWD would shift power to the front axle and it would pull me out of the skid.

    Looks like the tail does get loose first, but the stability control catches it quickly, so it completes the turn with pretty good control.

    Doesn't seem like a trick to me - in fact that's more or less what I expect it to do. Watch 1:48 to 1:52, the rear view is closer and smoother, so you see it better.
  • wwestwwest Posts: 10,706
    It appears to me that only the run at ~1:40 was done at the higher speeds equal to the competition.

    Most stability controls, more modern ones anyway, will not interfere if the driver seems to be intentionally invoking overstear. And note that as soon as the rear wheels are aligned in the direction desirable for entering that first turn the front wheels are straightened, turned straight ahead.

    "..Oversteer is consistent..."

    Really...?

    You don't, typically, encounter overstear, tail "wag", in a FWD or F/awd vehicle unless it is invoked intentionally. And once you get a measure, the measure you wish, of that intentional overstear you had better be well ready to crank in the correct level of counter-steer, "early" stability control.

    No obvious deception required, just put highly experienced RWD/R/awd drivers behind the wheel of the competition.

    Or use just one driver who has been "coached".
  • ateixeiraateixeira Posts: 72,587
    My 98 was not FWD-based. It defaulted to 50/50 split and had a viscous coupling between.

    It would predictably break traction on the rear axle first. Weight distribution was 55/45 so the lighter side first makes sense to me.
  • wwestwwest Posts: 10,706
    edited May 2011
    Sorry, marketing wins yet another one.

    Your '98 Subbie either has, 1, a SOLID coupling between the transaxle output and the front drive diff'l input, with a VC to serve as a "soft", rubber bandish, coupling to the rear. Or, 2, has a standard "open" center diff'l with a VC mounted, connected across/between the two diff'l outputs. the

    Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.

    In either case the mostly "flaccid" VC MUST allow a great deal of "give" between the front driveline rotation rate and the rear driveline rotation rate. That's to avoid driveline windup and/or tire scrubbing while driving on reasonably tractive surfaces, especially when turning.

    VC fluid formulation, rate of fluid expansion with heat, determine the "attack" rate of "tightening" of the torque coupling to the rear. Most VC's also have a controlled level of inert gas inside the VC container to delay the onset of the VC's torque coupling. No torque is coupled until the gas is compressed to ZILCH..!

    Most marques manufacturering F/awd vehicles, including Subaru, have now abandoned the use of VCs for use on F/awd vehicles. Instant TC activation upon detection of wheelspin/slip precludes having any substantive level of disparate F/R wheelspin occur thereby making a VC useless.
  • rshollandrsholland Posts: 19,657
    All manual tranny Subarus, except the STI, have a VC.

    Bob
  • wwestwwest Posts: 10,706
    How, where...?

    And I thought I had read the VC was discontinued in the latest models.
  • rshollandrsholland Posts: 19,657
    Nope. As I stated all manual transmission-equipped Subarus, except the STI, still use a viscous coupling on the AWD. Here's a SOA press announcement on the 2011 WRX, and in it is mentioned the viscous coupling:

    http://media.subaru.com/index.php?s=43&item=154

    Bob
  • ateixeiraateixeira Posts: 72,587
    edited May 2011
    Or, 2, has a standard "open" center diff'l with a VC mounted, connected across/between the two diff'l outputs.

    Only in the latter case is it possible to have a "contant" 50/50 F/R "high" torque split, and that only if/when all four tires have roughly equal traction.


    It's #2, except the VC itself acts in place of the center diff, and yes it's fluid-filled like you described. Rotational differences between the front and rear axle thicken the fluid and tighten the coupling temporarily.

    By the way, the VC only applies to manual transmission models, though it's used in several Subaru models. They call it "Continuous AWD" in the brochures.

    Models with an automatic use a different system - no VC. Subaru markets those as "Active AWD", which uses an Electronically Controlled Variable Transfer Clutch. On higher-end models, "Variable Torque Distribution" or VTD is used. Finally, the STI uses DCCD.

    VDC is their marketing term for their stability control system, though in the past it referred to an Outback model that used the VTD AWD system, further confusing matters.

    So really there are 4 separate systems. Symmetrical AWD is a generic term that refers to all 4 of those, though I guess they do have equal length half-shafts in common across the whole lineup, so I guess that's what they're referring to.
  • ateixeiraateixeira Posts: 72,587
    Bob - remember something - ALL WRX models come in a manual, that's why all WRXs minus the STI use a VC.

    But there is no automatic WRX...

    Auto-equipped Foresters, Imprezas, and Outback Sports all still use the variable transfer clutch.
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