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Torque Sensing Differential

Discussion in 'Model S: Driving Dynamics' started by Olle, Oct 17, 2014.

  1. Olle

    Olle Member

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    Anybody know why the Model S doesn't have a torsen diff? Now with its open differential, MS depends on breaking the spinning wheel in order to bring the torque over to the wheel with traction. Breaking wheels while driving feels counter intuitive on an EV where you want to conserve every Wh possible to optimize range.

    Are there drawbacks with Torsen diffs?

    Thoughts?
     
  2. Htuork

    Htuork Member

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    If you are spinning a wheel you are not really driving for efficiency, so it is being in-efficient for a very small amount of time at a moment that it doesn't really matter. My guess would be that a Locking Differential was not used for reasons of overall efficiency.
     
  3. Olle

    Olle Member

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    When I drove upstate NY last winter for miles on end through 1/2 foot of snow, the breaks were constantly working the rear wheels to the tune of funny sounds. I tried to drive for efficiency.

    Limited slip diffs are very inefficient so I totally understand why TM doesn't use them.

    But is the TorSen diff less efficient than an open diff, that's what I am trying to understand.
     
  4. Saghost

    Saghost Active Member

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    In straight line operation, it should have the same efficiency.

    However, one you start to turn the car, the Torsen uses the side gears to bias torque to the slower wheel - and the side gears and their bushings eat some power to do that. Not much, but a little, and power an open differential wouldn't take.

    When the car is braking the spinning wheel, the power it is dissipating is probably similar to the losses in the Torsen, or possibly a little higher. In other turns it uses less power.

    The Torsen means more parts to break and more cost and weight, but doesn't really change the results much when you have an EDL/TC system as sophisticated as Tesla has - and that's with conventional brakes. The new EM brake cars are probably even better...
    Walter
     
  5. Kbsilver

    Kbsilver Member

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    Biggest drawback is cost and special oils needed for a limited slip (aka more maintenance). Very few manufacturers use limited slip diffs these days except on models that are likely to end up on the track, such as BMW M line. We all know the MS is not a car that belongs on the track.
     
  6. Olle

    Olle Member

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    I thought that what is known as a limited slip diff is an oil viscosity based design that per definition turns kinetic energy into heat, whereas a torsen diff's worm gears only use mechanical leverage to stop the slip and therefore doesn't involve energy loss, no?
     
  7. vgrinshpun

    vgrinshpun Active Member

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    #7 vgrinshpun, Oct 17, 2014
    Last edited: Oct 17, 2014
    I think that the main drawback of torsen differential is actually functional.

    While torsen differential allows to lock and split torque at the set design ratio based on its mechanical design (for example 75%-25%), applying low latency highly modulated brake inputs in conjunction with an open differential allows infinite control of the torque split between left and right wheel. I think that this was the reason for introduction of electromagnetic brake system according to Elon's presentation - it allows for low latency and precise modulation that probably can't be achieved with conventional hydraulic system.
     
  8. Saghost

    Saghost Active Member

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    With an open differential, the torque on the two output shafts is always the same - and adding the brake based control system doesn't change that.

    The reason the control system is braking the spinning wheel is to raise the torque across the whole system, thereby delivering some power to the wheel with grip.
    Walter
     
  9. Olle

    Olle Member

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    With today's hydraulic brake system my Model S slows down significantly when a wheel starts to slip. It is especially obvious when you take a turn while accelerating on dry pavement, the inner wheel lifts, the break applies with a slight delay and you feel an abrupt power cut, just a tad too late. Don't others recognize the feeling? In this situation a Torsen diff would have continued to shove you forward with no brake application.

    Perhaps Torsen is a thing of the past when the electric brake system gets implemented. But how does the new brake system get away from the fact that the brakes will subtract energy, especially with constant slippage in for example snow?
     
  10. LittoDevil

    LittoDevil Member

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    Porsche offers their PTV as an option on their vehicles, it's Porsche Torque Vectoring system, but even less expensive cars can benefit by using individual brakes to do the same job.. when you apply brakes to one side the torque is transferred to the other wheel. It's basically now a brake torque vectoring system. How does it get away from subtracting energy? I have no idea.. If you want my opinion, if a car is consistently slipping in snow, in a normal car you have your electronic stability control kicking in to reduce power. If using the brakes to transfer torque to the wheel with the most traction means I get out of the snow then so be it.. even if it means losing some energy.

    So the conclusion after typing all of that out is.. if your "driving" in the snow, you wont be constantly slipping.. and if you are constantly slipping right on the edge, the car will reduce power.. traction control will kick in. So I'm not sure if it will get to the point where the brake torque vectoring will come into play anyways since your not going fast enough. Or you do.. and it does kick in..

    Either way I don't believe the calipers are redesigned to become electronic brakes, I believe the electronic servo will be used to actuate the master brake cylinder.

    Larry
     
  11. Saghost

    Saghost Active Member

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    The brake based systems can't avoid the fact that they are subtracting energy - but so is the torsen differential:

    Torsen - Wikipedia, the free encyclopedia

    Note the key phrase:


    The torsen differential is generating the change in torque by braking the faster spinning axle internally. I don't think there's a huge difference in energy efficiency between the two strategies - and a sufficiently well designed EDL based on EM brakes might be equally quick and responsive, too.

    The only way out of the energy loss is separate motors for each wheel, which I think we will be seeing become standard in a decade or two for this reason among others.
    Walter
     
  12. trils0n

    trils0n 2013 P85

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    I'm pretty sure even high performance track cars are moving away from Torsen diffs and the like. I think the McClaren P1 has an open diff and uses the brakes to perform that function. Brake based torque vectoring is probably more accurate and efficient, especially with the new electromechanical braking system Tesla is deploying.
     
  13. EarlyAdopter

    EarlyAdopter Active Member

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    #13 EarlyAdopter, Oct 18, 2014
    Last edited by a moderator: Mar 13, 2016
    Audi offers an active rear differential that can vector torque left/right with minimal loss. They call it their "sports diff." I had it on my S4 at it was _amazing_.

    It will simultaneously push with the outside rear wheel while retrograding the inside wheel, creating a moment of inertia rotating in the direction of the turn.

    Felt like the hand of god reaching down and helping push the car around corners. Awesome on the track, and daily driving. You could punch it while exiting a turn earlier than normal and the car would sort it out without having to cut engine power or fire the brakes. Really defied physics.

    So, torque vectoring left/right is possible with only one engine. Downsides are it was larger, heavier, more expensive, and more maintenance prone than other types of diff.

    Here's a nice video explaining the difference between torque vectoring front/back with an electromechanical differential lock (EDL) I.e. brake based diff lock on an open differential vs. their active "sports diff."

     
  14. glhs272

    glhs272 Unnamed plug faced villian

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    My prior car was a Honda Civic SI with a limited slip differential. I loved it, it pulled that car around corners when you would floor it through the corner. I never had to do anything with it as far as maintenance, but I only kept the car to 88K miles. I think the Model S RWD cars could see some handling improvement with them. But if it came at the expense of range I wouldn't do it.
     
  15. Saghost

    Saghost Active Member

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    Well, that's new. :)

    Somewhat complicated and presumably fairly heavy, but it certainly can push torque to one wheel without paying the full frictional losses. I've never seen anyone take that approach to a differential before, with each axle shaft tied both through the spiders and at a separate gear ratio to the ring directly by a clutch.

    If I understood it right, when they close one clutch, it's going to force the spiders to spin the other way at a rate proportional to the gear ratio (and bias the torque distribution by the gear ratio towards the side with the clutch closed.) It will be mechanically impossible to turn any of the shafts with both clutches closed. Since it's a biasing ratio, in true zero grip situations (on ice or a wheel in the air,) you'll still need EDL braking to move the car, but for more normal driving it'll be a more efficient solution (worth the weight/cost/complexity? I dunno.)
    Walter
     
  16. jcaspar

    jcaspar Member

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    I noticed this when I tried a smoking burnout yesterday. Felt like only the right wheel spun. Made it really easy to break a tire loose with all torque only going to one wheel.
     
  17. LittoDevil

    LittoDevil Member

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    Oh nice! I saw a picture of a MS doing a burn out but.. the picture was taken from the passenger side.. haha I wonder if most burn outs are done with the passenger rear tire lol...

    Larry
     
  18. vgrinshpun

    vgrinshpun Active Member

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    Electronic Differential Lock (EDL) is a limited slip differential imitation - this is an electonic system that detects wheelspin via ABS sensors and applies brakes to spinning wheels. This results in the torque being transferred via open differentials to another wheel that has more traction.

    Electronic Differential Lock (Edl) | awd cars, 4x4 vehicles, 4wd trucks, 4motion, quattro, xDrive, SH-AWD, Haldex, Torsen, wiki - How it works
     
  19. vgrinshpun

    vgrinshpun Active Member

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    #19 vgrinshpun, Oct 18, 2014
    Last edited by a moderator: Mar 13, 2016
    What this video does not mention is the response time of the system. It is on the order of 0.1 second - seemingly good result, until you grab calculator and see that during 0.1 second interval, at 60mph car travels more than 8 feet. It all sounds good on paper/advertising video, but not very practical from the point of view of vehicle stability.

    What I think Tesla has in store with their "D" is the system that will have response time on the order of a millisecond, i.e. will be able to respond to changing road conditions within time that vehicle moves 1 inch (!) at 60 mph. As far as on-road stability is concerned, this system will blow any other system currently on the road out of the water (well, except $0.5 million Mercedes SLS Electric Drive - Mercedes SLS Electric Drive. Can Volts Ever Match Pistons? - /CHRIS HARRIS ON CARS - YouTube ). I can't wait until car magazines and Consumer Reports will get their hands on P85D -
     
  20. vgrinshpun

    vgrinshpun Active Member

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    The abrupt power cut is the consequence of the slow response of the conventional hydraulic brake system. The best of them have response time of more than .05s. While Elon did not mentioned response time of the new electromagnetic brake system, I think it will be on the order of the response time of the drive unit, which he mention to be on the order of a millisecond on more than one occasion. The result of this would be that brake intervention will be almost imperceptible.

    The longer the brake system takes to decide what to do in response to the changing road conditions, the higher braking force need to be applied to compensate for the lost time...
     

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