4WD and AWD..

Discussion in 'OT Driven' started by newsroom_can, Jan 24, 2002.

  1. newsroom_can

    newsroom_can Canada eh?

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    This may sounds like dumb question but I am going to ask...

    What is the difference between 4WD and AWD? Are they basically same? How come some car companies are claiming their cars to be AWD while others say 4WD?
     
  2. Astroboy

    Astroboy AcuraZine.com

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    awd generally implies that all the wheels drive all the time...where as a 4x4 can usually be shfted in and out of 4 wheel drive...
     
  3. SycoPhant

    SycoPhant Get out my way

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    AWD is a power transfer system. If your front tires slip, it engages the transfer case and sends part of the power to the rear tires. 4wd has constant power to all 4 tires at all times, unless it is disengaged. Vehicles with full time 4wd usually have it ratioed, for example my dad's Grand Cherokee is 60 to the rear / 40 to the front.
     
  4. Guest

    Guest Guest

    4WD, when engaged, turns only one of the front wheels and one of the rear wheels at any given time UNLESS you lock all three differentials. Locking all three differentials is not possible to do with all vehicles, but with most 4wds, you can lock the rear differential which allows 3 tires to spin while in 4WD (2 rear, one front).

    One vehicle that comes to mind where all three differentials can be locked is the Mercedes G-Wagen.
     
  5. SycoPhant

    SycoPhant Get out my way

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    You sure that's true with all vehicles? I've had all 4 spinning on ice at times.
     
  6. flynfrog

    flynfrog Cool isnt Cheap

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    it will turn all for if they all have equal traction
     
  7. 4wd- all 4 wheels will spin at the same speed all the time.(thats why in a truck if you make a U-turn in 4wd on a dry road the tires will skip).

    AWD-usually controlled by electronic viscious couplings or limited slip differentials (not all 4 wheels will spin at the same time)
     
  8. Guest

    Guest Guest

    these days it comes down to semantics.

    for example, my 1990 toyota 4runner is a 4wd. i.e. transfer case that allows me to go into 4wd.

    my 2002 subaru wrx is an AWD. no transfer case. all 4 wheels have power from the transmission all the time.

    the toyota becomes an AWD when i'm in 4WD...

    but when it comes down to a jeopardy FINAL JEOPARDY round question, the diff is that the AWD has power to all 4 wheels all the time, where as the 4wd has a way of becoming a 2WD via a transfer case...
     
  9. SycoPhant

    SycoPhant Get out my way

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    I'm sorry, how much did you wager? Most AWD systems engage when the drive tires slip.
     
  10. LowkeyG

    LowkeyG OT Supporter

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    well...
    the talon, eclipse, laser, wrx, lancer evo, stealth and 300gt AWD's were 4 wheels all the time
    the thing you are describing sounds alot more like part time 4wd where when the wheels slip the 4wd activates. i believe that tahoes have that
    true 4wd is in vehicles like the jeep grand cherokee
    which is 4wd all the time 60 to the rear 40 to the front

    forgot the most important part, 4wd will blindly turn all wheels regardless of any spinning that may be happening
    AWD will compensate for this (not all of them though)
     
  11. Also most 4WD have a low setting
     
  12. Wolvrin704

    Wolvrin704 Guest

    This might help sum it up.......AWD is better on the street, 4WD is better off-road.
     
  13. Nemesis_152

    Nemesis_152 I'm a delicate desert flower from Arizona.

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    i assume you mean percentages, and you are very wrong. with a direct, non-viscious coupling, you cannot get a torque split, it'll be 50/50.
     
  14. Wolvrin704

    Wolvrin704 Guest

    I've had 2 4WD trucks now and I can't say that I've ever had a tire spinning when I had it engaged. Especially in 4Low, theres just too much torque there.

    I also believe the ratio is 50/50 for 4WD. A true 4WD will always have a Low and High gearing as well, plus be able to run in 2WD.
     
  15. ParTyBoy

    ParTyBoy New Member

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    GMC Syclones & Typhoon's AWD systems work like this... transfercase distributes power.. 35% to thee front .. 65% to the rear..
    As far as the transfercase is concerned read this....

    Taken from an old SyTy post:

    After spending a day with my nose in the factory service manual staring at
    the blow-apart drawings of the transfercase and reading the whole chapter
    on rebuilding one (in a stroke of devine inspiration) I finaly understand
    how the transfer case works. Here is the low down as it relates to the
    removal of the prop shaft. The transfer case alone (with no viscous
    clutch) functions like an open differential. With no VC, if you were to
    lift the rear tires off the ground, all of the torque would go to them and
    the fronts would not turn. Ditto for the front, actually with the open
    FRONT diff on our trucks, just one tire off the ground would stop the rest
    from turning. So, without a VC in the transfer case, torque goes to the
    tires with the least grip. So, if you were to remove the front prop shaft
    (with no VC) and you try to drive, the front output shaft would spin like
    crazy and you would go nowhere. The VC is what transfers all of the
    torque to the rear. The viscous clutch functions like this (this is
    straight from the service manual) "The viscous clutch used in the Borg
    Warner 4472 (transfer case) is a torque distribution device and is
    nonserviceable. The internal construction of the viscous clutch consists
    of alternating plates that are connected to the front and rear outputs of
    the transfer case. The viscous clutch is filled with a high viscosity
    fluid which flows through slots in the plates. The resistance to sheer
    causes the plates to transmit torque." Like I said in my original post, I
    was interested in finding out what type of dammage might be caused by the
    removal of the front propshaft. The book basicly refers to the FLUID
    transfering torque. How can fluid transfer all of the torque to one output
    (wont the fluid slip a little)?? What happens after a while?? Does it get
    worn out kinda like a shock?? What is the service life of the viscous
    fluid?? The book doesnt really go into detail on the VC so I took it upon
    my self to find out. Well, after two days and 14 phone calls to 4
    different states, I finaly got ahold of John Barlage
    (senior production engineer) of GKN Viscodrive. This is the manufacturer
    of the VC that goes in our trucks. So here is the beef as was related to
    me. Basicly the VC progressivly locks the front and rear outputs together
    when there is a speed difference between the two. The bigger the speed
    differnece the faster the VC locks the outputs together. The VC is
    compleatly transparent untill there is a speed difference between the two
    outputs, at this time the viscous fluid tries to speed up the slower plates
    transmitting torque to the slower output. As this is happening the fluid
    is heating up. The fluid has two special qualities, 1 the resistance to
    sheer and 2 a high rate of heat expansion. The VC is purposly not filled
    compleatly with fluid. A certain amount of air is left in it to give the
    fluid room to expand. Once the fluid expands to fill the VC the internal
    pressure starts to climb rapidly, this actually forces the plates together
    (can you say metal to metal contact??) This condition is called self
    induced torque multiplication or HUMP (kinda cool huh) It takes 110 to 140
    degrees C to get to full HUMP (takes me less, hee hee) For those in the
    states thats 230 to 284 degrees F. At this point, the VC wont heat up any
    more (no more slipping = no more heat) but, as it cools it will start to
    slip again. I asked if the heat would thin the fluid and John said that
    actually the opposite happens. The metal particles from the metal to metal
    contact create a condition called jellification. I bet you can guess what
    that means... If you cant... The viscous fluid ends up solidifying. This
    equals no speed differentiating between the front and back which equals
    worn tires or broken transfer case.
     

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