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  1. #1
    Senior Member DudeMan will become famous soon enough DudeMan's Avatar

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    Default Viscous Coupling = 2WD?

    Was reading some stuff on carbibles.com today when I came across this;


    Finally, all-wheel drive or full-time 4WD. Found mostly on sportier cars, but also on some SUVs, there are two types of AWD, both designed to try to overcome the problems with 4WD. The simplest form has two open differentials - one on each axle - and a viscous coupling between. The engine drives the gearbox which drives two output shafts. One goes to the front open differential and the other goes to the viscous coupling, the output of which is connected to the rear open differential. Under normal conditions, this type of AWD system functions exactly like a 2WD car, driving only the front axle (unlike a 4WD which normally drives the rear axle). Lower end Subarus and some of the Honda trucks use this system. The front wheels turn at a certain rate, and the rear wheels are dragged along for the ride. Both halves of the viscous coupling are spinning at the same speed so no torque is sent to the rear axle. If the front wheels begin to slip and spin, the input to the front of the viscous coupling begins to spin faster than the rear and because of its torque-converter-like design, this causes the rear output to want to speed up. At this point, the drivetrain is now transferring torque to the rear axle and the car starts to function in AWD mode. Actally, AWD is a bit of a misnomer at this point, because unless the car has limited-slip differentials front and rear, it's still only really driving two wheels in this mode - the one on the front and the one on the rear axles that have the most traction.

    Read more: http://www.carbibles.com/transmissio...#ixzz0mXJS5WVM

    The AllTrac/GT4 has this same setup, except some rear differentials are torsen, correct?

    Does this mean our cars are basically 2WD?

    Can someone clear this up for me?

  2. #2
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    Default

    While I can't speak for the Alltrac/GT4, I can say that the Mazdaspeed6 is like that, as well as the other Mazda AWD/fulltime 4wd vehicles. Along with that, the Ford AWD vehicles (suvs/etc).

    The Mazdaspeed6 AWD system is FWD only, and when one tire looses traction, the rear wheels will have power adjusted to make up for the slippage.
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  3. #3
    Senior Member DudeMan will become famous soon enough DudeMan's Avatar

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    Default

    I'm starting to think that the AllTrac doesn't even have a true all wheel drive system.

    Here's a link to a good article on the subject, if anyone's interested.

    I'm kinda disappointed to be honest.

  4. #4

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    From what I understand about our system is that the 50/50 split is under normal driving conditions. If any slip happens in the front or rear the VC goes into effect. The part I am unsure of is if the VC locks and maintains the 50:50 split, or if it does any power transfer from the faster moving (slipping) side.

    The main flaw in our system is it lacks a front lsd, which effects corner exit.
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  5. #5
    Senior Member DudeMan will become famous soon enough DudeMan's Avatar

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    See that's how I always thought it worked but reading these articles made me think otherwise.

    Sounded more like a FWD system with part time AWD.

    I'm pretty sure your right CMS, since the BGB even says 50/50.



    Its funny, I was so disappointed at first but after finding that AllTrac's are in fact full time AWD and that a lot of other supposed 'AWD vehicles' actually aren't, I'm even happier.

  6. #6

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    From what I read about VC, I think the point of it is to maintain the 50/50 split. I wish it moved power to the rear.
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  7. #7

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    I'd like to read some original tech on the E50F2 transaxle. Since the E150F is based on it, might give more info about the power distribution when slip happens.
    Also, the ST185 got a beefier center diff and it makes me wonder if it had something to do with the power transfer for the 50/50 split.
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  8. #8

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    I pretty sure the system awd. Awd lambo uses the same VC centeral diff type as ours.

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