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4WD and AWD systems explained



  • paisanpaisan Posts: 21,181
    I know the front is not an LSD.
    I know the center is not a Locking Center Diffy.
    The rear *may* have an LSD but I'm not positive on that.

  • nrossinrossi Posts: 47

    I think I got the lo down on the Seq. It has three open diffs. The modes are 2 wd, 4 Hi, and
    4 Lo. When in 4 hi the Center diff sends power to the front and rear axles. The TRACS system monitors for slippage and makes adjustments. VCS can be engaged.

    When in 4 Lo and low gear, the center diff locks and the TRACS and VCS are automatically disengaged.

    Do you think this system is superior to the center LSD and locking rear diff on the Denali?

  • paisanpaisan Posts: 21,181
    It'll last 2x as long and is priced roughly the same. Also gives you low gears that the denali doesn't.

  • Currently I drive a 96 Explorer 4WD. I hope to acquire a 2002 Mountaineer AWD. I realize the new Mercury will not have the benefit of low gearing present.
    Will the AWD Mountain function on a beach?
    Will a limited slip differential help on the beach?
    Will driving on a beach once in a while put excessive wear and tear on the AWD system.
    Thank You
    please email me at
    [email protected]
  • paisanpaisan Posts: 21,181
    Shouldn't be a problem. A rear LSD would help. I know many people who have taken AWD subarus onto the beach w/o a problem. Make sure to air-down the tires before going on the beach.

  • tshadletshadle Posts: 38
    In regard to Drew's previous post #83 about beach driving:
    "BTW, with the way the Highlander's AWD system is set up, it is possible to get stuck even with the rear LSD."
    Is this just because of the HL's limited slip center diffy?
  • paisanpaisan Posts: 21,181
    Basically there are 2 reasons the highlander gets stuck:

    1) It's 100% FWD until slippage occurs, so basically it's like driving your camary around the beach. Once you are spinning the front wheels the rears kick in and by that time you are probably already dug into a rut.

    2) You can't turn off the traction control on it. What happens is that once you are spinning the wheels, and the rear wheels kick in, it will apply the brakes onto any one or all the wheels that are spinning so that they won't spin, this will lead you to getting stuck as well.

    The Highlander is not really designed as any kind of off-road vehicle, IMHO. I'm not a fan of the Mountaineer either. For my $27.5K, I'd buy a trooper (AWD, 2wd, 4wd Lo, more room than the highlander and mountaineer, 10/120K warranty, etc.etc.)

  • ateixeiraateixeira Posts: 72,587
    AWD is fine for sand. I'm one of the Subie owners Mike was talking about, and even with open front and rear differentials I drove along just fine.

    I'll admit I got a lot of shocked looks from the Suburban owners out there, though!

    I did feel that a low range would have helped, and I hope Subaru brings their low range tranny offered in Australia to the US. Deep sand eats up torque, so it feels like you're driving a Geo Metro with 5 big people in it!

    Never got stuck, though.

  • nrossinrossi Posts: 47
    When slippage occurs, the fluid in a VC gets heated and becomes solid, locking whatever diff it is connected to, which then transfers torque to the wheel/axle with traction. What happens next?
    When the fluid is in a solid state, there is no friction so wont it go become a fluid again, and the whole process repeat?
  • paisanpaisan Posts: 21,181
    Once it cools, it then goes back to it's original state, that is why it's a limited slip diffy instead of a fully locked differential.

  • tonychrystonychrys Posts: 1,310
    I know you are only trying to help Mike, but in your Trooper zeal, you are giving out less than correct info to folks. You said: "Highlander.. It's 100% FWD until slippage occurs, so basically it's like driving your camary around the beach."

    Which is just plain wrong on the AWD version, which is permanent AWD. This is straight from Edmunds: "The Highlander's 4WD operation is similar to the RX 300's and RAV4's, with a 50/50 front-to-rear torque split on a full time basis. Unless there is tire slippage, then the viscous coupling will apply torque as necessary, front to rear, depending on which wheels are slipping. Four-wheel-drive models can be equipped with an optional limited-slip differential to further improve traction in slippery conditions."
  • paisanpaisan Posts: 21,181
    But from another review I read they said it slipped up a steep hill because it was mostly fwd. Sorry for the wrong info.

  • tomshadletomshadle Posts: 11
    Thanks for jumping in.
    I'd like to see if I can get a Highlander to join the increasing number of Outbacks I see on the beach, all of which seem to do just fine. My days of part-time shift on the fly are done.
    Mike's point #2 of there being no VSC-off switch is well taken, and I have ruled out getting the VSC on the HL because of this.
    I'm still trying to decide whether the optional Limited-slip rear would be advantageous in the sand. Drew doesn't think so but apparently it's because of something specific to the HL's AWD system.
  • drew_drew_ Posts: 3,382
    Actually, if you want the HL w/o VSC, definitely do get the rear LSD. Unless you do mostly beach driving (and in deep sand), I'd still go for the VSC since the benefits far outweigh the disadvantages. Stability control can dramatically improve on-road handling and equally dramatically reduce the risk of rollovers. Even without VSC, if you spin your tires a lot in the sand, you'll dig a hole for yourself and sink in.

    What Mike said is true (about the wheels braking). However, if they keep spinning, you'll loose power anyway since with an open differential, it goes to the wheel that has the least resistance (read: the one with no traction).

    I guess the only way you can test this out is to actually take the vehicle down to the beach and try it out. However, I'm not sure if the saleguy will be happy with you airing the tires down to 20psi (which you should on sand to get a wider footprint) :-)
  • Thanks for the input at least i know i got a shot at it at the beach with a new 2002 Mountaineer AWD.

    What is the difference between permanent AWD and full time AWD?
  • tonychrystonychrys Posts: 1,310
    "What is the difference between permanent AWD and full time AWD?"

    Drew does an excellent job of describing this at the beginning of the discussion here drew_ Feb 5, 2001 8:24pm
  • paisanpaisan Posts: 21,181
    Depends which company the car is coming from, those 2 terms IIRC are interchangeable (Toyota LC calls it Full Time AWD, Ford may call it permenant awd...)

  • drew_drew_ Posts: 3,382
    Hmm, that's strange. I thought that I had already answered your questions, but I can't find the response above. Oh well...

    AFAIK, the '02 GMC Envoy/Chevy Trailblazer/Olds Bravada all have the same auto 4WD system (AutoTrac). In the past, the Bravada had an permanent AWD system, which was later substituted for a full-time AWD system. See my post #2 in this topic if you're unsure about the terminology.

    As for the MDX vs. the Bravada/Envoy/Trailblazer, the main difference is that the Acura MDX lacks a low range. The MDX is also FWD based, whereas the GM trucks are primarily RWD.

    IMHO, both of these systems' main disadvantage is that they're not permanent systems. That is to say, not all 4 wheels are powered at the same time. The MDX has the advantage over the GM trucks here because it does transfer power to the rear wheels upon acceleration (depending on the speed) and a few other situations. In contrast, the GM trucks only transfer power forward when rear wheel slippage is detected (read: it is purely a reactive system; the MDX is also reactive when coasting and at higher speeds though - FWD-only till wheel slippage is detected).

    VTM-4 utilises a sophisticated, but pricey rear differential that can transfer power side to side, sort of like the GM's locking rear differential. However, the MDX's rear differential requires maintenance (basically a fluid change) every 7500 miles or so, I believe.

    Hope this helps!
  • drew_drew_ Posts: 3,382
    Yes, the button (in the Tribute) or the rotary dial in the Escape does split the power 50/50, hence making it permanent AWD; front and rear prop shafts are bound together and spin at the same rate. However, this system should not be used on dry roads because it can cause binding especially in turns.

    I can't recall if the Tribute/Escape offers a rear limited slip differential. I don't think it does. If so, the answer to your question is yes, the vehicle will get stuck if there is no traction on one side of the vehicle, and also if 3 wheels do not have traction. The only way to get around this is with locking front/centre/rear differentials, or with 4 wheel traction control.
  • drew_drew_ Posts: 3,382
    Since you do not plan to go off-roading, I'd go for the permanent AWD system since it is pro-active (all 4 wheels are powered and helping to stabilise the vehicle/improve handling at all times). Actually, the power is permamently split 30/60 between front and rear axles.

    The auto 4WD system (AutoTrac) actually does react quite quickly to rear wheel slippage - its RWD based, and transfers power to the front when needed. However, I like the fact that the permanent AWD system is helping out, regardless of slippage or no slippage, dry or snowy roads. There should be little if any fuel consumption difference between the two different systems.

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  • ateixeiraateixeira Posts: 72,587
    More and more I'm beginning to agree with Drew on the value of stability/traction control.

    Check out the new C&D, for the wagon comparo in snow. The Subaru Outback VDC was the only one that did NOT plow into a snow bank, and require a tow truck to yank it out.

    The VDC finished 4th out of 4, but that's probably because they like the smaller, sportier wagons (some with manual trannies). More importantly, they don't own the cars or pay those carsh-into-a-snow-bank repair bills!

  • nrossinrossi Posts: 47
    Traction control braking prevents wheel spin. So, if wheels start spinning it applies brakes to the slipping wheels and simulates traction. If all the wheels are slipping, or there is no or low traction, like on snow, slush, mud or sand, are the brakes on all the wheels applied so that no wheel spin is permitted?

    Sometimes wheel spin can help to get out of a stuck situation by rocking or sliding.
  • bwhbwh Posts: 76
    Most sytems that I am aware of have a defeat switch if you feel the need to spin the tires. Also most allow a small amount of wheel spin, they don't force 0 wheel slip. I agree though, I heard a story of a fancy new Volvo XC wagon getting stuck in the snow because the traction control wouldn't allow enough wheel spin to maintain forward momentum. The car eventually slowed itself to a stop and was stuck.
  • ateixeiraateixeira Posts: 72,587
    That's why there is an off switch on some systems.

    Subaru says it's system isn't that intrusive; that it lets the AWD system act first. A rep from Subaru (who participates in the Subaru Crew forums under Owners Clubs) took a demo VDC to some soft sand (volleyball courts) and couldn't get it stuck no matter what she did.

    Even though C&D felt it was intrusive and reduced their lap times, they also said it was best in the snow (and safest), so this is consistent with her testing.

    I think the sensors work by comparing wheel speed, so brakes are applied if one wheel is spinning faster than the others. If all four wheels spin, the sensors would not not a difference in speed and probably would just let all tires spin, or at least cycle through them one by one.

  • gluizgluiz Posts: 17
    One thing I have noticed is that people like to interchange power with torque, like 50/50 torque/power split. They are not the same thing. The difference is key to many things. For example, an open differential will not always split power 50/50, but it does split torque 50/50. It is usually easier to think of torque first.

    An open differential will always keep torque balanced between the two outputs. When you get one wheel in snow and one on pavement, you don't go anywhere b/c the snow wheel has little traction and does not accept much torque. Hence, the other wheel cannot receive much torque (must be balanced). Traction control works by adding drag to the low traction side, absorbing more torque, and allowing the other side to get more torque.

    A locked diff forces equal rotational speed regardless of torque or power. In fact, it works for us because it does not require a 50/50 torque split. In the above example, locking the diff prevents one wheel from overspeeding by putting more torque to the traction side. However, the forced speed matching prevents locked diffs from utilizing all traction in many situations.

    A limited slip diff usually starts with an open diff. When it sees slip, it tries to couple the two outputs, once again to allow a torque imbalance. The mechanics then approach a locked unit.

    While appearing similar, a Torsen is not a limited slip. Its bevahior is controlled by available traction and not on output shaft speed. It works by allowing a torque imbalance between outputs, regardless of speed, and not by preventing speed differences. It is done in such a way that the side with more traction has more torque. Torsens are characterized by a bias or torque ratio, for example 2:1. An open diff has a 1:1 ratio. Since it is biased, it is limited in cases of extreme torque imbalances (2x of 0 is still 0). However they also maximize the benefit of traction control b/c the require the least amount of drag.

    And finally, there is a difference between a viscous limited slip center differential and a viscous (center) coupling. The former is a limited slip version of an open diff and the latter is used for full-time awd (using this forum's terminology) to redirect torque only with slippage.

    Sorry for the long post.
  • bwhbwh Posts: 76
    I am curious, what type of scenario would you say would cause a locked diff to not take full advantage of the availible traction?
  • gluizgluiz Posts: 17
    It would not utilize all traction in any situation where one side needs to turn more than the other, i.e. a situation where you would scrub.

    Imagine a bump in the road under one side of the vehicle. That tire must roll farther to go up and over as opposed to the other side which is flat. With a locked diff, one tire must loose traction and slip to go over the bump.

    Another example is just plain going around a corner. One side or the other must slip even through both sides may have equal traction.

    These examples may sound trival but they are a huge factor in mobility for everything except rock crawling.
  • 2001rx2001rx Posts: 8
    I have found this forum very interesting and informative, but I still have a question. I have a 2001 Lexus AWD RX300. VSC (vehicle skid control) is standard on both 2WD and AWD models. This is probably the same system which is optional on the Toyota Highlander. I had been wondering what the actual details are about traction and stability control. The Owner's Manual and the 2001 Brochure do not give complete information on how these systems function. A Lexus Owners' newsletter came in the mail recently sheds some light on this subject:

    "... The RX300 gets the Lexus Vehicle Skid Control (VSC) system for the 2001 model year. This computer-controlled system replaces the mechanical limited-slip center differential on four-wheel-drive models. Besides saving more than 50 pounds of the performance-robbing weight of the differential, VSC improves traction and cornering grip by monitoring speed, yaw rate and steering angle, selectively applying one or more brakes, and automatically reducing the throttle to keep the RX300 on its intended course. This sophisticated technology is also included on front-wheel-drive models, ...."

    Edmund's information on the 2001 RX300 seems inconsistent.

    Excerpt from Edmund's Review: ".... the RX 300 can be had with your choice of front-wheel drive or full-time four-wheel drive. Not only does the FWD version perform better and get better fuel economy, but it benefits from standard electronic traction control to improve poor weather performance. Four-wheelers include a viscous center differential that directs torque to the wheels with the most traction whenever slippage occurs. For 2001, Lexus has added Vehicle Skid Control (VSC -- a stability control system) and Brake Assist (which applies full braking pressure in panic situations quicker than your feet can) as standard equipment on all RX 300s."

    I'm not sure the viscous center differential is still present or needed. Could it be the 50 pound differential weight which was eliminated? Does anyone know for certain?

    Incidentally, I recently drove aggressively around an off-vehicle road area in the California desert (near Ridgecrest) and had no problems.
  • bwhbwh Posts: 76
    I'm still a little foggy on how you are actually loosing traction.

    The bump in the road scenario; so say my front locker is engaged and I go up over the bump on the drivers side and the suspension is compressed. The drivers side tire (more traction) climbs up and over while the pass side must "skid" along a bit. Is that what you consider loosing traction? Seems like a moot point if forward progress was not sacrificed.

    The corner scenario; You shouldn't be going around corners on dry pavement with the diff (any) locked. Again I fail to see the logic or application of it.

    My truck has three (front,center,rear) normally open diffs. It drives with normal mobility in this mode. If slip is a possibility the diffs can be locked to raise the number of wheels with maximum torque, increasing availible traction, up to and including true fully locked 4wd. Admittedly steering is difficult in this mode, but the truck is nearly unstoppable as well.

    I have never found locking diffs to do anything but increase traction, the ability to produce progress along a given surface. Just because one tire sacrifices some grip to allow the opposite one to maintain progress does not to me constitute an under utilization of traction. The slipping tires traction was not required, and if the gripping tire begins to slip, with the locker, the other side immediately takes over and progress continues.
  • tshadletshadle Posts: 38
    Excellent example. On my way home from work every day I have a slippery corner to turn where there is a bump under one wheel. My 97 Pathfinder with Limited Slip rear always seems to spin the rear wheels a little and then "jump" as I make that turn even though the bump is not that big. I think it is because the Limited Slip rear is trying to simulate a locked diff. Thanks.
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