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  1. #1
    PENGUINBLASTER!!!'s Avatar
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    Why aren't outrunners used in surface vehicles?

    Outrunners have great torque and powerband, it seems like they would be ideal for many electric buggies and trucks.

    Anybody know why they aren't used?

  2. #2
    HandyRacing's Avatar
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    RE: Why aren't outrunners used in surface vehicles?

    A fellow somewhere here on RCU did this:

    Promoting the Great Lakes Region racing scene via www.YouTube.com/HandyRacing

  3. #3
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    RE: Why aren't outrunners used in surface vehicles?

    You could use one, but they don't react well to large quantities of dirt.
    *Compact battery box is waterproof against dust!
    Rustler, Slash, TA03F, 4Tec, Evader, X-Crawler, Cx3 heli, more?...

  4. #4
    HandyRacing's Avatar
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    RE: Why aren't outrunners used in surface vehicles?


    ORIGINAL: FairTax4me

    You could use one, but they don't react well to large quantities of dirt.
    Yeah, the guys who run the older NEUs that have cooling holes in the can use a wrap type filter to keep out the dirt.
    Promoting the Great Lakes Region racing scene via www.YouTube.com/HandyRacing

  5. #5

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    RE: Why aren't outrunners used in surface vehicles?

    I use them in my crawlers and minis, I haven't tried on in any of my 1/10 vehicles yet.

  6. #6
    Metallover's Avatar
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    RE: Why aren't outrunners used in surface vehicles?

    I've got an outrunner in my crt .5.








    I think the reason they aren't used as much is because a brushless inrunner is a good replacement a nitro engine or standard brushed motor. An inrunner turns at high enough rpms. An outrunner turns much slower. You need to usually put them in a direct drive setup like I did to use the power. This has it advantages like less weight, but the only way you can change the gearing is with bigger or smaller tires.

    I have to add outrunners are SICK. They have unholy torque. I picked my outrunner to run on 4s lipo. When I run on 3s lipo I get about 35mph, lots of wheelies, and ice cold electronics. They pull very low amps compared to inrunners.
    Dimebag Darrel 1966-2004 RIP
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  7. #7

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    RE: Why aren't outrunners used in surface vehicles?

    Outrunners are great and I wish we'd see more direct-drive outrunner setups too. You could have an outrunner for each wheel and then control the steering like that, or something similar. But overall outrunners have an efficiency problem, it's easier to build a very efficient inrunner (or a multi-pole inrunner if you need the torque) than a very efficient outrunner. This is why most of the high-end brushless companies like Neu won't do outrunners. I saw some of their huge stuff and an industry trade show and it was all multi-pole inrunners.

  8. #8

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    RE: Why aren't outrunners used in surface vehicles?

    Can you fill us in a little how you got the driveshafts on both ends of the motor? Do outrunner rotor shafts extend on both sides? This setup seems very interesting!

  9. #9
    Metallover's Avatar
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    RE: Why aren't outrunners used in surface vehicles?


    ORIGINAL: Access

    Outrunners are great and I wish we'd see more direct-drive outrunner setups too. You could have an outrunner for each wheel and then control the steering like that, or something similar. But overall outrunners have an efficiency problem, it's easier to build a very efficient inrunner (or a multi-pole inrunner if you need the torque) than a very efficient outrunner. This is why most of the high-end brushless companies like Neu won't do outrunners. I saw some of their huge stuff and an industry trade show and it was all multi-pole inrunners.
    Woah, there. The newer outrunner are coming close. A Scorpion outrunner will be at about 76% effiency and Axi will have a 91%. A Medusa motor is at 81% and Neu is about 85%. They are pretty close in that thought.

    The thing that puts outrunner ahead is that inrunners generally need a reduction. An inrunner will lose probably5% efficency with the standard spur/pinion reduction used in RC vehicles. It will probably use another 5% if there is some kind of complex transmission used.

    An outrunner will be more efficent in an RC setup. My car doesn't even get the m warm on 3s lipo. The bec is even still running. A 2wd car will be lighter and if used with a MM and say 4900kv motor to get the same speeds I'm getting, the MM will get at least warm. It will probably pull 65-75a. My outrunner might be pushed 55a. Outrunners FTW!
    Dimebag Darrel 1966-2004 RIP
    FS - E-Jato

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    RE: Why aren't outrunners used in surface vehicles?

    ORIGINAL: SidewaysLS4

    Can you fill us in a little how you got the driveshafts on both ends of the motor? Do outrunner rotor shafts extend on both sides? This setup seems very interesting!
    On all the outrunners I've dealt with in my planes the shaft can be changed to stick out either side, depending on how you need to mount the motor (inside the firewall, or outside). All you would have to do is split the difference and it would basically stick out half way on each end.

    If you needed more length you could probably just get a longer shaft.

  11. #11
    Metallover's Avatar
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    RE: Why aren't outrunners used in surface vehicles?


    ORIGINAL: SidewaysLS4

    Can you fill us in a little how you got the driveshafts on both ends of the motor? Do outrunner rotor shafts extend on both sides? This setup seems very interesting!
    You need to slide the shaft though. Imagine an outrunner as two parts. One part is the stator which stays there and has the windings on it. Imagine the rotor, which has the bell that goes around the rotor and the shaft is attached to that with a grub screw. You just need to take out the grub screw. The shaft will slide out with a little force. You can put it back in wherever you want. In a direct drive setup, that will be hanging out on both ends.
    Dimebag Darrel 1966-2004 RIP
    FS - E-Jato

  12. #12

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    RE: Why aren't outrunners used in surface vehicles?

    ORIGINAL: Metallover
    The thing that puts outrunner ahead is that inrunners generally need a reduction. An inrunner will lose probably5% efficency with the standard spur/pinion reduction used in RC vehicles. It will probably use another 5% if there is some kind of complex transmission used.

    An outrunner will be more efficent in an RC setup. My car doesn't even get the m warm on 3s lipo. The bec is even still running. A 2wd car will be lighter
    The standard inrunner's loss of efficiency to gearing does not generate heat at the motor. Some of the larger inrunners I saw at the AUVSI show had just as many poles as an outrunner. When you're looking at motor heat, there is a huge difference between a 92.5% efficient motor and a 85% efficient motor, the latter one will generate twice the amount of waste heat. Even if the 85% efficiency motor loses nothing in gearing, and the 92.5% efficiency motor loses 10% in gearing, the 85% will still get twice as hot. The 10% loss in gearing only means more efficient motor must produce 10% more energy, increasing the relative heat generation ~1% in this case.

    I'm guessing the poor efficiency has to do with the poles being in the center, this means the poles are close together and you get flux creep from one pole to the adjacent poles. With an inrunner the poles are in the can, which means it's easier to sink the heat they generate, and they're farther apart so less flux creep and such. Just a guess.

    Direct-drive is still something I want to see, it seems like a good idea in a truck without a lot of downsides.

  13. #13
    Metallover's Avatar
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    RE: Why aren't outrunners used in surface vehicles?

    I would have to disagree slightly. The friction created by the transmission gears will cause the motor to heat up more. It's not because the motor is less or more efficient, but becase the motor is under more load. When I say efficiency, I mean efficeincy at the wheels after all the transmissions and diffs. That being said, IMO the outrunner will be a little more efficient because of the motor having no transmission.

    I think that evidence shows an outrunner set up for the same speed on the same voltage will draw less amps then an inrunner with a gearing reduction.
    Dimebag Darrel 1966-2004 RIP
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  14. #14

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    RE: Why aren't outrunners used in surface vehicles?

    ORIGINAL: Metallover

    I would have to disagree slightly. The friction created by the transmission gears will cause the motor to heat up more. It's not because the motor is less or more efficient, but becase the motor is under more load. When I say efficiency, I mean efficeincy at the wheels after all the transmissions and diffs. That being said, IMO the outrunner will be a little more efficient because of the motor having no transmission.

    I think that evidence shows an outrunner set up for the same speed on the same voltage will draw less amps then an inrunner with a gearing reduction.
    Only a little more, here's the math. It's a 'differing bases' type problem that can fool a lot of people.

    Say a vehicle requires 100W power with direct drive. With a geared transmission (10% inefficiency) it requires 110W to get the same performance.

    Say the motor driving the geared transmission is 90% efficient, the one driving the direct drive is 85% efficient. This means the geared motor generates 10% waste heat and the direct drive one generates 15% waste heat.

    For the geared vehicle, 110W @ 90% efficiency requires 122W, roughly ~12W waste heat (at the motor).
    For the non-geared vehicle, 100W @ 85% efficiency requires 118W, roughly ~18W waste heat (at the motor).

    If the geared vehicle had a 'perfect', no-loss gearing system, it would only save 1W in waste heat at the motor. The motor would drop to 100W @ 90% requiring 111W and generating 11W waste heat. Therefore in a heat-limited system, the efficiency of the motor is far more important than things like gearing losses, etc. We're willing to accept some gearing loss even if it makes the overall system less efficient, for the sake of keeping the motor cooler.

    You are right that the outrunner setup may well draw less overall power, though. That's perfectly believeable. But, the only way that helps you is a little more runtime, and a little easier on your batteries. If a setup isn't heat limited at the motor, the outrunner direct drive could be an improvement.


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