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Let me be clear: I am speaking about the rear wheel drive model.

After I've asked https://mechanics.stackexchange.com/questions/15342/number-of-motors-rwd-tesla , it seems Tesla's rear axle has only one motor.

Searching the Internet, it could not be clear for me if there is a mechanical differential at the center or if it the distribution of different speeds per wheel is done by software, as the info I find on it, seems all based on suppositions.

Now, if instead of using one motor, it used two smaller ones, one for left wheel and another for the right wheel. With the motor outputs not spinning on the axle center, but in the left an right extremes. And the differential made by electronic control.

I see some disadvantages on this approach: need for two separate gearboxes/clutches and need electronics to control two motors separately.

But are there advantages?

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    I'm wondering if the Model S has a multi speed gearbox and clutch at all? Compared to fuel engines, electrical motors are able to produce power/torque over a much wider range, which should eliminate the need for a gearbox (other than single speed).
    – Roland Mieslinger
    Commented Feb 8, 2015 at 14:49
  • It is a basic requirement of a modern car to distribute power to the drive wheels separately somehow. The existing mechanical differentials are reliable, and can be locked or unlocked, or have posi-traction, etc.. I think having separate motors for each wheel would open a lot of possibilities, especially for off-road vehicles, but it is a more dramatic departure from the norm. The motors would have to be moved to the sides of the vehicle which could be awkward.
    – mkeith
    Commented Feb 8, 2015 at 17:16
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    The Tesla doesn't have a multispeed gearbox, it only has a single, fixed gear ratio. I'm not sure how it distributes power though. The Electric Mercedes SLS AMG has 4 separate motors directly driving each wheel, which has some benefits for track driving (and I assume off road but it's not an off road car), but it's a lot of extra effort when you don't need it.
    – michaelyoyo
    Commented Feb 8, 2015 at 17:28
  • I believe the tesla model S uses an open differential. It probably uses some form of traction control strategy to manage traction by individually braking the slipping wheel to lock the differential and controlling the overall output of the motor. Individual motors - torque vectoring? However even torque vectoring can be achieved through the current setup (similar to tcs implementation) , though i don't believe it's currently implemented.
    – chilljeet
    Commented Feb 10, 2015 at 7:42
  • In youtube.com/watch?v=M9rndjtiE6E it seems the Rimac uses four different motors, one for each wheel, and the differential logic seems to be managed by electronic torque vectoring.
    – sergiol
    Commented Sep 2, 2016 at 23:00

2 Answers 2

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I can think of several advantages of having two independent motors, one for each wheel:

  • no need for a diff/gearbox

    Today's technology allows electric motors to be attached directly to the wheel, so a two-motor configuration would do away with the need for a mechanical differential or gearbox.

    Less components = less manufacturing cost, so it is a compelling configuration for Tesla to consider, which they would likely trade-off against the additional control complexity and cost of the additional motor.

  • packaging - more floor space is freed up

    The two axles can become much smaller in the absence of a diff/gearbox, giving back more floor space back to the cabin interior and boot/trunk.

  • independent control - torque-vectoring

    Sure the control systems would be more complex, but the mechanical limitations of a diff would no longer be present, meaning that the right amount of torque can be delivered to each wheel independent of the other. This can make the vehicle handle fast corners much better than a RWD with a mechanical diff.

  • more efficient transmission of power to the wheels

    There's no gearbox or diff in the middle, which means there are no parasitic mechanical losses. This equates to more power at the wheels.

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    I agree with all your statements except one ... I don't see as how two motors in and of themselves would free up floor space. You now have two motors, two power converters, two of everything to make it run (except extra batteries). This would be offset somewhat by no differential, but I think overall you are going to be using more space than you'd be saving. That could be a matter of interpretation and opinion, though.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 19, 2015 at 2:54
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    @Paulster2 turns out there's a pretty nice wiki page.
    – Zaid
    Commented Dec 19, 2015 at 3:10
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    @Paulster2 I'd also think that the two motors would each be smaller diameter than a single motor that needed enough torque for both wheels, so would fit more easily under the floor.
    – Nick C ♦
    Commented Dec 21, 2015 at 17:18
  • @NickC - As proposed on the page which Zaid produced the link for, most vehicles with a motor/wheel setup is going to have the motors located in what we'd consider the "hub", so Zaid's premise is a lot more spot on than I'd first imagined. I'm not sure, but the Chevy Volt might be this way (IIRC).
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Dec 21, 2015 at 18:10
  • This is interesting. Tesla released a two motor car but it's a motor per axle as opposed to a motor per side. teslamotors.com/blog/dual-motor-model-s-and-autopilot
    – DucatiKiller
    Commented Dec 22, 2015 at 23:03
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+50

There are no clear advantages on the production vehicle mentioned. The whole process of producing the vehicle is not taken into account. Imagine having to build a vehicle with the two motors instead of one. You'd have twice the amount of motors (and supporting equipment) to build, service, diagnose, and replace. Then comes the issue of driving dynamics. Without a differential you would have to not only split the torque but also control how much of it reaches the wheel.

Did you also think about the effect of the torque twisting the chassis? The car would need to have the motors running in opposite directions to avoid issues with the chassis twisting itself into a pretzel. Running the motors in opposite directions means you'd have to make it absolutely clear in the software and hardware. Imagine a mechanic plugging in the motors wrong or a bug in the software that caused the car to suddenly spin both motors in the same direction. This increases complexity and cost for no apparent improvement in performance.

There is no clear performance advantage versus one or two (or more) motors. Its not like adding more motors increases the cars performance linearly with each additional one. There was a lot of experimentation done with multiple engines back in the 60's drag racing scene. Look into it. You will see that the only reason they ran multiple engines was to have more power. Which in the Tesla mentioned is not really a big deal. The car already has a potent enough motor.

To answer your question: No, there are no advantage. There are multiple disadvantages that affect not only the cars configuration, but the whole logistics chain.

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  • I think there is clear advantage in the removal of a torque converter. If you didn't have a diff there is less power loss and the motors can be controlled independently. The independent control is reduced complexity in that aspect. I don't necessarily think there is a right answer to this questions as there are possible multiple viewpoints to it. The chassis twisting component is simply shoring up certain points where force may twist it but at the end of the day, apply all the force you want, it won't exceed the friction coefficient of traction between the road and the tires.
    – DucatiKiller
    Commented Dec 18, 2015 at 16:37
  • I think that the point I brought up about the logistic chain is being missed. The question is about a production vehicle. Not a prototype. I'm taking into account the benefits of efficiency and the impact on the supply chain and cost. If the question were for a prototype vehicle that does not need to meet deadlines and regulations then we could expand on efficiency. Maybe the OP could make the question without that explicit detail?
    – race fever
    Commented Dec 18, 2015 at 17:01
  • I'm not sure I can see the logic that simply because it has two motors or these additional parts that there would be a failure in the supply chain. It's too implicit. I suppose it puts more stress on the supply chain but there is no guaranteed failure. as well, I see the question as a pro's/con's for performance, reliability, etc. rather than a manufacturing question.
    – DucatiKiller
    Commented Dec 18, 2015 at 17:06
  • The question does leave room for interpretation. You mention reliability. Which I didn't take into account. I'd say it would be less reliable due to having more points of failure. Could we close this question? Its driving me insane. :)
    – race fever
    Commented Dec 18, 2015 at 17:33

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