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Torque Sensing Differential

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

  1. Saghost

    Saghost Well-Known Member

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    That description matches my understanding, and also my description above. Since it was delivered without comment, I'm not sure if you thought it contradicted my description or were merely adding details.

    The brake on the spinning wheel creates resistance to its rotation, thus requiring torque to keep it turning.

    The spider gears keep the torque balanced on both sides, so the wheel with grip has torque applied to it, as well. Make sense?
    Walter
     
  2. vgrinshpun

    vgrinshpun Supporting Member

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    My original post stating that 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 was immediately followed by your post, without the comment, stating that 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. I am not sure now what was the reason for your post, but it appeared that you disagreed with what I posted.

    The reason for the quote is to emphasize that I was talking about transferring torque to the wheel that has grip, and it was the correct statement.
     
  3. Saghost

    Saghost Well-Known Member

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    Hmm. It's a subtle distinction, but I'll agree that what you wrote is correct.

    The left and right axle shafts of an open differential always have the same torque on them, though how much that is varies greatly from moment to moment. However, the axle torque is the combination of the wheel and brake torques, so the torque on one wheel can be different from the torque on the other with differential braking.
    Walter
     
  4. vgrinshpun

    vgrinshpun Supporting Member

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    Exactly. We are on the same page now. :smile:
     
  5. gene

    gene Supporting Member

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    A limited slip or locking differential in ice conditions can be very dangerous on paved roads. Reason being, imagine going around a curvy mountain road and hitting ice at the moment you happen to accelerate, even a tiny bit. With an open diff, the wheel with the better traction is holding you on the road so you don't slide off the cliff. With a limited slip, the one wheel that has the better traction and is preventing you from sliding off the cliff as you drive around the curve, now, imagine sending torque over to it and breaking it loose, even a little bit, there you go, slide right off the cliff.

    Using an open diff with brake pulsation is much better by reasons of safety in snowy and icy conditions on pavement.

    I have a lot of off-road driving experience from the arctic to South America, to Africa and all places in between. This is where limited and locking diffs are valuable. We are talking slow going over rocky and stair step terrain, a locking diff is wonderful. In soft sand limited slip will sink you more easily. On rocky side slanting mountain roads where the roads tend to slant towards a cliff, the limited slip diff will attempt to move you over towards the edge of the cliff so you must correct for the movement. This is safe as it is all at slow speed.

    This video shows a step-up in a canyon from a washout where an open diff Toyota Tacoma and a limited diff Jeep have difficulty. You'll see my Dodge Cummins with locking diff goes over with ease. Manual trans and very low transfer case gives the Dodge a 100:1 final drive ratio which means it crawls at full torque at 2 mph.

    Also at minute 1:40 you'll notice the value of flexible suspension. This is stock suspension, hopefully if Tesla ever builds a 4wd truck, they will be able to have a good flexible suspension like the Dodge. The air suspension on the X will be useless in this sort of terrain.


    Saline Fiddlers Visit Butte Valley - YouTube
     
  6. LittoDevil

    LittoDevil Member

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    Sorry I couldn't help but noticed you said electromagnetic.. those are found in suspension dampeners not brakes.. hehe.. the new brake system is still hydraulic but has an electric mechanical servo somewhere in the system that's why the brake pedal is almost like a drive by wire system. This eliminates the need for a vacuum pump since a electric mechanical braking system wouldn't need the help of a vacuum pump to create the pressure required at the master brake cylinder. The ABS system will still respond in the same quick millisec fashion though.

    Oh.. and what Olle mentioned about "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."

    I believe he's talking about the traction control system kicking in to reduce power to the tires which causes the abrupt power cut however the brakes doesn't actually engage. Some of my other vehicles have very well defined traction control system or.. programmable ones where you can dial in the "forgiveness" of it so you can still kick out the rear end without the ECU interfering. The Model S has a very basic traction control system which it mainly just reduces power. It definitely can use some refinement since the abrupt cut in power messes with the chassis dynamic and weight transfer of the vehicle when driving and cornering at it's outer limits.

    I'm curious if the Model S has a Yaw sensor.. never looked into it.. I'm almost certain it has it for stability control purposes.

    Larry
     
  7. vgrinshpun

    vgrinshpun Supporting Member

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    #27 vgrinshpun, Oct 20, 2014
    Last edited: Oct 20, 2014
    During the presentation Elon mentioned electro-mechanical brake system. According to several internet searches another name for this system is electro-magnetic brake system, and it does eliminate hydraulics. So I was not talking about suspension dampeners (resisting my urge to insert hehe here :smile:).

    While I agree that there is some room for uncertainty here, I am far from sure that the system "is still hydraulic". Can you share any evidence of this, or it is just your conjecture?
     
  8. Olle

    Olle Member

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    The calipers looked hydraulic on a new Model S (yes it was the new auto pilot car) I just saw in the showroom.
     
  9. Saghost

    Saghost Well-Known Member

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    Well, I don't know anything for certain, and there's a lot I'd still like to know. However, two quotes from recent Road and Track articles would seem to support Larry here:

    Tesla Model S P85D - Dual Motors, AWD, 691 hp, 3.2 seconds to 60 - Road & Track




    Porsche 918 Spyder - Road Tests - Road & Track



    It sounds like Road and Track thinks both systems are using a hydraulic pump and accumulator to provide the power, and electronic valves to control the operation.
    Walter
     
  10. vgrinshpun

    vgrinshpun Supporting Member

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    I did see the Road and Track article, but it is not conclusive enough for me. I guess I really want these brakes to be fully electromagnetic :smile:. I have posted couple of questions to TDial, a TMC member, asking him to look at the Manual, which should settle all the questions on this issue: P85D - Electric Mechanical Braking System - Page 7
     
  11. LittoDevil

    LittoDevil Member

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    Well the truth is, of course I can't be 100% sure cuz the car isn't out publicly haha so ur free to laugh at everything I say :).. The only thing I can share is that I went to look at the most recently built vehicles and they have the same calipers from Brembo on the car.. and there's no new part number neither. But.. who knows.. I could be wrong.. i did watch the live event and recall him saying that.. but I heard it differently I guess. The only thing I couldn't understand was.. someone.. said.. or maybe it was Elon himself.. said it's like a power steering rack on it's side.. which to me personally that sounds like electric actuators like on the electronic e-brakes on many newer vehicles.

    If it means anything my sources goes all the way to the factory floor.. but.. as I said even I can't be certain enough to say for sure at the moment. :) Would hate to be the laughing stalk when it comes out and it really is some crazy unique braking system.. LOL! I did manage to get the current P85+ brakes to smoke on my own car after some hard driving... like.. literally .. smoke... haha I just hope the new braking system will be able to apply enough force to lock the wheels up as well as.. gentle enough for day to day driving just like Elon said..

    Larry
     
  12. vgrinshpun

    vgrinshpun Supporting Member

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    Well, my hopes for a fully electromagnetic braking system in Model S were not realized. As several people mentioned, it is a hydraulic system, with electromechanical booster replacing the vacuum boost. The electromechanical booster is Bosch iBooster unit.
    P85D - Electric Mechanical Braking System - Page 8
     
  13. Theshadows

    Theshadows Active Member

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    Isn't the new braking system on the D only? Which means we won't see it until they start rolling off the production line and/or they start offering test drives.
     
  14. ecarfan

    ecarfan Well-Known Member

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    My understanding is the the new braking system is installed on all cars, just like the new sensors are installed on all cars. Both are required for the coming Auto Pilot software option.
     
  15. vgrinshpun

    vgrinshpun Supporting Member

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    The new system is installed on all cars, similar to the autopilot hardware. At least three people posting in the Electric Mechanical Braking System thread had the new system installed on their recently delivered cars.
     
  16. Theshadows

    Theshadows Active Member

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    Thanks for clarifying. So the new system still uses hydraulic fluid?
     
  17. vgrinshpun

    vgrinshpun Supporting Member

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    Yes, unfortunately. I was really hoping for a fully electromagnetic system...
     

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