Outrunners... what's the catch?

Discussion in 'Electric Bicycles' started by abudabit, Sep 15, 2008.

  1. abudabit

    abudabit New Member

    Outrunner motors look great on paper... superb power to weight, great cost, good efficiency... what's the catch? How come they are only used for model aircraft and not ev's or light industrial equipment?

  2. terrence

    terrence Member

    Forgive my ignorance. Im not familiar with these. Do you have a link or
    a place to educate myself on these. thanks, terrence
  3. abudabit

    abudabit New Member

    Outrunners are brushless dc's, except the magnet is the core and the wiring is the perimeter.

    Here is a source of outrunners

    Here is one, for example:
    It says 2kw, although that is probably when it is being air cooled by flying around. Probably more like 1kw - 1.5kw. Still, that's a lot of power for a $79 motor that weighs only 1 lb. Also it is very small, and supposedly they are very efficient even by electric motor standards. They have lower rpms but higher torque than their inrunner (standard) brushless dc cousins.

    I can't help but think there must be something wrong with them though.
  4. Drunkskunk

    Drunkskunk Member

    they are used. For an Ebike, its used as a hub motor. lace the spokes to the outside of the motor, and you have a direct drive.

    The are being tested by a few companies for cars as well, but the efficancy band is smaller than a conventional geared drive. thats fine for a bike with a narrow speed range, or power tools, pump motors, forklifts, ect... but for a car, there are other, more efficent ways to make them work
  5. skyl4rk

    skyl4rk Guest

  6. abudabit

    abudabit New Member

    That's the theoretical unloaded top turning speed, where RPM = kv * volts. So 12 volts into a 3500Kv (K is uppercase and v is lowercase to avoid confusion with kilovolts) motor would be 42000 RPM.

    Their peak efficiency point supposedly, and perhaps someone can correct me on this because I haven't seen many performance charts for those hobby outrunners, is at 80% of Kv. So if you were putting 12 volts into that motor, 33600 RPM would be optimal efficiency.

    A 3500Kv motor is probably designed for low voltage use. For 36-48v you will probably want something a lot lower. Here is an example of a beast with low Kv:


    At 36 volts you would have an optimal RPM of 3744. If 2400 rpm is 60 mph on a 26 inch wheel, you reduce the motor rotation in half and get 1872 or 46 mph optimal running speed which it would easily take. Sounds dangerous on a bike though, but that's just an example of how one would be used.
    Last edited: Sep 19, 2008
  7. Drunkskunk

    Drunkskunk Member

    thats an oddball motor, 3600kv is inrunner teritory. Its also listed as 6-10 nicad cells, which means 12 volts max.

    inrunners are better at higher RPMs, as the narrower stator design means they accelerate faster. they also run more efficently at high RPMs.
    Out runners are best around 1500Kv or less. For a bike, a 150Kv motor would be great for a gear drive, and an 8 Kv would be ideal for a hub motor (Like my Clyte motor is)
  8. Bigwheel

    Bigwheel Member

    This is all very interesting. I really like the size of these motors and their corresponding weight which coupled with LiPo should yield a nice light package? So forgive my ignorance on the electronics end of things but the way I am reading it one of these motors set up with a jackshaft running to gears would work? Similar to a setup like this:http://www.ecospeed.net/emddet.html

    Thanks for any feedback in advance.
  9. skyl4rk

    skyl4rk Guest

    A outrunner coupled with a internally geared hub (perhaps with jackshaft to reduce rpms) might be the way to go. If it is "safe" to propose a system with no pedals.
  10. Bigwheel

    Bigwheel Member

    But there would be pedals when using the jackshaft, that is what the jackshaft accomplishes is the ability to utilize the front cranks, a motor and a gear system both internal and derailleur/cassette style. So independent use of both could be used or in sync. Like the Sickshifter that is becoming so popular with the HT crowd, kind of sort of without the freewheeling front crank?
  11. abudabit

    abudabit New Member

    After doing research, it looks like a 2 speed helps a lot for these. Either that or a current limiter. Current limiters should help with efficiency a lot and extend the life of all your parts, because stalling / near stalling on outrunners is very bad.

    For a pedaless tradpole I'm building I'm going with a current limiter to limit my system to 3600 kw max, probably will do 1 speed since I'll be using a belt. It won't be a true current limiter though, I think I'll have it reduce throttle when the current limit is exceeded for longer than 1/10th of a second. The tadpole will be higher and wider than normal tadpoles so it can handle the higher speeds on the streets safer.
  12. abudabit

    abudabit New Member

    Long time since I posted the previous post, just as an update I ruled these motors out. They are sensorless, which means they are extremely inefficient at lower speeds. So any application where low end torque is needed (road vehicles) I would be cautious.

    On a pedal bike I wouldn't engage throttle on one of these until you are doing several mph.
  13. Peter

    Peter New Member

  14. Drunkskunk

    Drunkskunk Member

    Not sure where you got your info, but its not right.

    Brushless outrunners are far more powerfull and efficent at low rpm, and at there worst, are usualy better than a brushed motor at its best.

    As for sensorless, that has nothing to do with efficancy. a sensorless controller uses the phase wire's back emf pulse as the motor turns for a sensor, instead of an external sensor
  15. recumpence

    recumpence Member

    Hey Guys,

    I am the owner/builder of the E-cumbent posted earlier.

    Brushless, sensorless outrunners are very good for bike use. There really isn't a down side (or a catch) other than they are not designed for bike use and, therefore, more difficult to mount.

    I am getting 13wh per mile at 15mph and 17wh per mile at 20mph.

    Also, I pull 5,000 watts easily without the motor running hot. 0 to 35mph in 7 seconds without trying hard.

    Startup and low speed tractability is fantastic. No pedalling needed to start the bike. The motor just starts and runs smoothly. Most of that is the controller programming.

    My entire system (motor, lipo pack, reduction unit and related equipment) is 12 pounds.

    I hope that helps answer some the questions!