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Multiple Gear Ratio Options

Discussion in 'Future Cars' started by doubeld, Feb 3, 2016.

  1. doubeld

    doubeld Member

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    Couldn't see this posted elsewhere... exactly.

    What are the chances of Tesla providing future models (I'm thinking Model S and *maybe* Model 3) with different gearing options?
    We all know that the acceleration rate is unbeatable in anything even close to its class, so perhaps those customers in high-velocity jurisdictions might prefer a taller gearing at the expense of lower end torque?

    Perhaps they could get the car to 155MPH at far less than 16,000RPM. I'm no EE, but that seems like one option to increase top-end speed if a customer doesn't mind the loss of the low end a bit. It's not like we could use more low-end torque since they already need to dial down the power to prevent wheel slip.
     
  2. omarsultan

    omarsultan Active Member

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    Are you asking for different gearing as a BTO option or a multi-speed gearbox. In either case, I think the odds are low. The former because of supply chain complexity and the latter because of their experiences with the 2-speed transmission in the Roadster--at least until they find a gearbox that can deal with EV torque.

    I think what you might see is them getting more sophisticated in using the front and rear motors on "D" models to deliver some of the benefits of different gear ratios.
     
  3. doubeld

    doubeld Member

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    BTO. Getting rid of the multiple speed gearbox is half the reason an EV performs so well and has reduced complexity.

    Good call. Yes, perhaps having taller gears in one of the motors might make more sense. (The front?)
     
  4. ThosEM

    ThosEM Space Weatherman

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    I thought there were different ratios front and back and that this was the basis for claims of lower ultimate consumption with the D models (which may have been overblown?).
     
  5. EcoHeliGuy

    EcoHeliGuy Member

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    Definitely not over blown, as you can see in one of Bjøn videos.

    The dual motors do have different final gear rations. Forward motor has less torque and rear motor has more. But the design and size of the motor itself has a lot to do with it. Minimizing energy losses within the motor its self.
     
  6. Gerasimental

    Gerasimental Member

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    In development. Good EV-specific options now that didn't exist in Roadster days.
     
  7. Red Sage

    Red Sage The Cybernetic Samurai

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    It's only been posted -- exactly -- in the neighborhood of 8,000,000,000 times. Having multiple gears would work great if:

    1. You want to make EVs a lot less capable.
    2. You want to make EVs a lot less reliable.
    3. You want to make EVs have a lot less performance.
    4. You want to make EVs have a lot less punch.
    5. You want to make EVs with a lot less power.
    6. You want to make EVs with a lot less range.
    7. You want to make EVs using permanent magnet motors.
    8. You want to make EVs using tiny battery packs.
    9. You want to make EVs no one wants to buy.
    10. You want to make EVs no one wants to drive.

    Dude, if you really want to make sure that no one, anywhere, is impressed by EVs in the slightest, go ahead.
     
    • Like x 1
  8. doubeld

    doubeld Member

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    Dude, you need to calm down a bit. Find me one post where someone has suggested or requested a different gear ratio during the build/design process. I'm not talking about a multiple-gear drive train. I'm talking about requesting either a high gear ratio drivetrain, OR a low gear ratio drivetrain.

    The outcomes would be like this:
    Drive train option 1: 0-60 of 3.0 seconds. Top speed: 130MPH. Gear Ratio: 9.73:1
    Drive train option 2: 0-60 of 4.2 seconds (WAG). Top speed: 158MPH. Gear Ratio: 8:1

    1. The EV is just as capable, I'd suggest it is even more capable as your ability to reach higher speeds is greater (Someone from Germany might appreciate this)
    2. Reliability is the same.
    3. 0-60 will take a slight hit, but really, the Model S can already afford to sacrifice some 0-60 for a benefit on the top end.
    4. Marginal
    5. Disagree
    6. Potentially, if driven at higher speeds, but that's obvious.
    7. No.
    8. No. You don't understand the question as I intended
    9. You've lost sight of the topic
    10. Now you're just making me laugh.

    I understand you're missing the TM forums a little bit, but you don't _Need_ to respond to EVERY thread on here ;)

    Be happy. Buy a Kia.
     
  9. bxr140

    bxr140 Member

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    Formula E proves these assertions incorrect.
     
  10. Red Sage

    Red Sage The Cybernetic Samurai

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    There is no need for a higher 'theoretical' top speed -- the aerodynamics of the vehicle will limit its top speed regardless of gear ratio. There is absolutely nothing gained from having different final drive ratios offered on an otherwise identical platform. Tesla Motors has already shown the correct means of differentiating between cars is with battery pack capacity.
     
  11. EcoHeliGuy

    EcoHeliGuy Member

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    Gear ratios are set based on weight of the car, optimum output the motor. Aerodynamic resistance. And size of the motor, to name a few.

    You would need to charge other factors in the equation to warrant a gear ratio change.

    This isn't like an ICE where you have a very narrow optimum torque band, power band, stall speed, efficiency speed, and an upper limit durability speed. That you need to constantly adjust based on output speed, and torque load.
     
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  12. Red Sage

    Red Sage The Cybernetic Samurai

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    Exactly.
     
  13. tga

    tga Active Member

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    Disagree. Aerodynamics coupled with the amount of torque delivered to the pavement, while not overspeeding the motor defines top speed. You can max out top speed because wind resistance offsets the power available, or because you hit max motor RPM. But there are too many variables at play to make a definitive assertion.
     
  14. Red Sage

    Red Sage The Cybernetic Samurai

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    When it comes to gearing, you can have a theoretical top speed of 200+ MPH -- and never, ever reach it. All you'll get is a car that takes much longer to get to whatever its actual top speed may be. A motor of a given horsepower will not exceed the maximum possible aerodynamic top speed no matter what gearing you choose. Once again, the premise of offering a 'choice' of different gearing -- on purpose -- to offer a different experience with either top speed or initial acceleration is a complete fallacy.
     
  15. tga

    tga Active Member

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    Steve Saleen would disagree.

    Top speed at a given gear ratio occurs at (a) motor redline or (b) at the motor/ICE peak power output (if aerodynamically limited).

    A P85's 245/35R21 has a tread diameter of 21" + 2*245mm*35%, or 27.75". Circumference is 87.19".

    At 10mph, the tire is rotating 10miles/hour * 5280 feet/1 mile * 12inches/1 feet * 1 rev/87.19inches * 1 hour/60 min = 121.1 rpm. At 100 mph, 1211rpm. At 155 mph, 1877rpm

    With the 9.73:1 gear ratio, the motor is turning 121.1rpm*9.73 = 1178rpm at 10mph. At 100mph, 11780. At 155, 18263.

    So at the top speed of 155mph, a P85's motor is turning ~18000 rpm. Is top speed limited by (a) drag, (b) motor RPM redline, or (c) an arbitrary electronic limiter? We don't really know, but 155mph is a magic number in Europe (the 250km/h "gentleman's agreement"), so it's probably c.

    Now change the final drive to a shorter ratio (let's use Saleen's 11.39:1 for example).

    At 18263 motor RPM, the tire rpm will be 1603rpm, and the car will be going 132mph. If that's the motor's redline, that's the new top speed. If not, the car may go faster, depending on the motor's torque curve/electronic limiter.

    With shorter gears in the final drive, the car will accelerate faster, but top speed will be the same or reduced, depending on why the speed was limited in the original case. I'm not going to try to guess what that does to range.

    Tesla probably picked the final drive that gave the best trade off for range, acceleration, top speed, and drive unit wear. Changing to shorter gears will improve 0-60, but likely reduce the others.
     
  16. EcoHeliGuy

    EcoHeliGuy Member

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    Slowing down a cars top speed is very different then trying to push passed aerodynamic forces. There is a crazy amount of engineering to let something turn a 18000rpm let alone any faster then that. So this comes back to that gear ratio is picked based on all perimeters and not the limiting factor
     
  17. jaguar36

    jaguar36 Member

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    You're missing the fact that the Tesla's top speed right now is not limited by its available power. It's limited by the RPM limit of the motors. With the Tesla's abundance of power and very low cd it would probably have a very high top speed with different gearing.

    Course its all moot really, range at the current top speed is very very low and you start overheating the battery after a few minutes anyway. Would be a fun exercise to claim some top speed records, but beyond that its not a commercially useful idea.
     
  18. Zeus

    Zeus New Member

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    Steve Saleen tried this out with the Saleen Foursixteen (a tuned Tesla with multiple gears and a body kit). He and others at Saleen claimed it was an improvement over the original Tesla Performance (416 HP version) but specific improvements were never discussed. Furthermore, I have my suspicions that Tesla limited the Model S's speed to 155 because of heat related issues. If you don't believe me, take a look at the Nurburgring. The P85 wasn't able to complete even 1 lap without overheating and going into limp mode. So, to reach speeds beyond 155, Tesla will first have to deal with the heat issues. Although, I am aware that they already do a fantastic job at it.
     
  19. doubeld

    doubeld Member

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    Would the majority of the heat issues be due to high motor RPM or battery pack heating?

    Again, this is more of a technical exercise to determine if it would be viable to offer an option for a smaller gear ratio for higher speed environments. Yes, you're pushing more air, but if Tesla can hit the 155MPH (electronically limited) at around 18263RPM as tga says, it stands to reason they could manage the cooling better and get a higher top speed.

    Thanks for the tech info folks. I wonder if Saleen would be interested in testing changing the ratios in the other direction.
     
  20. jaguar36

    jaguar36 Member

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    I believe its primarily battery heating. The motor heating probably doesn't help though. There were certainly be an opportunity for improved cooling, would be more useful if you wanted to make a track version rather than for pure top speed. Above 130mph the range gets to be so small that top speed is really irrelevant for traveling purposes.
     

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