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Positraction?

Discussion in 'Model S: Driving Dynamics' started by mknox, Nov 12, 2012.

  1. mknox

    mknox Well-Known Member

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    Anyone know if the Model S transfer case has the equivalent of Positraction? (transfer of power from a slipping wheel to one with grip). I know it has traction control, but am unsure about this.
     
  2. ElSupreme

    ElSupreme Model S 03182

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    I don't think the Model S has a limited slip differential (Positraction is a GM brand name). It isn't listed anywhere on the spec sheets, and it isn't something that would be left out.

    And thinking about the gearbox/linkage to the drive shaft I really don't see where they would put a differential.

    I am just speculating, I do not know for sure.
     
  3. TEG

    TEG TMC Moderator

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    I think a lot of modern vehicles forego the mechanical limited slip differentials, and instead use electronic traction/stability controls that can use the ABS system to apply braking to any drive wheel that starts to spin too much faster than the other.
     
  4. dhrivnak

    dhrivnak Active Member

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    #4 dhrivnak, Nov 13, 2012
    Last edited: Nov 13, 2012
    If it is like the Roadster there is no positraction but I say the traction control is better. My Avalanche, and several other cars have had positraction and I have been impressed with what Tesla has done. So no need to worry you will not be stuck.
     
  5. mknox

    mknox Well-Known Member

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    My current Cadillac CTS has torque vectoring AWD, Positraction rear end, Traction Control and Multi-Mode Stability Control. Just words, I guess, but I have had some hairy winter experiences that the car dealt with quite nicely.

    - - - Updated - - -

    There would have to be some sort of differential mechanism to deal with the differing rotational speeds of the wheels as you go around corners...
     
  6. ggr

    ggr Roadster R80 537, SigS P85 29

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    Yes, Teslas use an open differential, with the equivalent of limited slip being provided by braking on the slipping wheel, which automatically transfers power to the other wheel.
     
  7. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Posi-traction is somewhat analogous to traction control. Both limit wheel slip but they use different mechanisms to do so. Stability control is different and shouldn't really be in the same sentence. It's about keeping the front end and back end from swapping places. It also keeps the back aligned with the front when the front starts to slide out. From the manual:

     
  8. William13

    William13 Member

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    Before adding air to one rear tire, my Tesla's tail wiggled when floored. It did not have any spinning/slipping on the road surface. After filling the tires properly this decreased. This result works well for this inexperienced driver going very quickly to 60 mph.
     
  9. TEG

    TEG TMC Moderator

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    Yes, but an "open diff" (which unchecked will send more power to the wheel slipping the most) is smaller/lighter/cheaper than limited slip types.
    Some diagrams of differential innards can be found here:
    http://members.rennlist.com/951_racerx/ps84gleason.html
     
  10. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I agree. The easiest way to think of an "open diff" is that it allows the two wheels to go a different speed, but applies the same torque to each wheel. The problem is that if one wheel is slipping (zero torque) then the other wheel also gets zero torque, and you go nowhere. Good traction control applies some brake action to the slipping wheel, creating some back torque and that torque is reflected to the wheel with traction. The big reason that this is not used more is that by applying the brake to the slipping wheel, you heat up that brake. Good traction control vehicles often need larger heat dissipation in the the brakes to work well.

    The ultimate in traction are the ARB air lockers that I have in my Jeep Cherokee. In 4 wheel low, the center transfer case is locked up and front and back diffs are fed the same speed input. With pneumatic control, I can change the front and rear open diffs to locked diffs and go into bulldoser mode, all wheels going the same speed no mater what. This is not good for tire ware on pavement, but in loose conditions, if one wheel has traction, the Jeep moves. :biggrin:
     

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