Ultimate lightweight wheel-motor concept

[johnrobholmes]

I would design the "frame" with an inner adapter that would work with ISO disc mounts or cassettes. The freewheel could not be used alone to support the rotor inside the stator, there would be another set of bearings left and right of the rotor to keep the airgap consistent. Freewheels have way too much axial and tangent play to rely on it for accurate alignment. Cassettes have a lot of play as well, it can not be relied on to keep the motor aligned radially more than +/- 0.005".


I'm not made of money and would not foot the materials cost on someone's word to purchase a motor. Materials cost up front, payment for the machining and labor before shipment. I'm fine if some people just want to purchase just the stator and magnets and do their own thing for the rest. Or maybe they just want a housing and raw materials. Even if I flaked on making housings, the materials could still be used by another and the initial materials investment would not be lost by the buyer.


If you aren't aware, I make R/C motors for a living. Most are brushed motors, we manufacture about 3,000 a year in house. We also make small sensored brushless motors for a UAV "drone" company. The only parts I do not design are bearings and stators (if at all possible I try to use off the shelf designs). We have other factories make the magnets and shafts to our specs, the rest is in house manufacturing for as much volume as we can stand. With Miles giving this concept motor a kick start with the rotor and stator designs it is simple work to finish the rest and build them, IMHO.

 

[madin88]

The freewheel could not be used alone to support the rotor inside the stator, there would be another set of bearings left and right of the rotor to keep the airgap consistent.

thats exactly what i thought. i did a bit of description on the pic. sorry for the bad yellow text color.
mill turn a flange on the freewheel for the rotor bearings. Mount the rotor on the sprocket. The freewheel bearings only will twist when pedalling. these are good sealed bearings (Flux and Fly brand is no cheap crap).
The question is: will an adapter for cassette mount fit inside the freewheel (M35 thread i guess)?

I'm fine if some people just want to purchase just the stator and magnets and do their own thing for the rest.

this would be a good option, but first lets see what the final construct will look like and cost.

With Miles giving this concept motor a kick start with the rotor and stator designs it is simple work to finish the rest and build them, IMHO.

Yes, without Miles there would be no rotor and stator concept. A very big THX to him for the time he spent doing this. Its great.

btw. is this design yet optimal or could it be improved?

 

[johnrobholmes]

I see no reason to use the freewheel body for bearing mounts. If that freewheel fails you are stuck with a motor that needs custom machine work to get running again, and the freewheel will be the most likely part to fail. Better to use slightly larger bearings with the "axle" dipping down to meet the iso or freewheel adapter. Then at least the freewheel can be off the shelf parts.


We have estimated in this thread between $2000 to $2500 for a completed motor. I think it is an accurate cost for one-off motors of this size. Until one is built, there is not an exact price that can be made. I would attribute about half assembly labor and half materials cost.

 

[Miles]

I have done some work on the structural aspect. One problem was making it work for all wheel sizes. In order for it to work with 20" wheels, it needs to be offset inwards of the cassette driver. I used 100mm (OD) 80mm (ID) bearings to get the clearance needed for this.

There's no electro-magnetic advantage in laminating the rotor yoke, in this case, it could be machined from solid.

 

[oger]

Hi Guys,
I read through most of the pages covering this Motor idea.
Quite interesting, as I have also thought about constructing a large diameter Motor for torque and efficiency reasons.
I am interested in what the design looks like. I did not find any drawings except for housing and a DXF File I could not open.
Can this design compete with the Bionx D? That Motor has a very small airgap (about 1mm) I saw on Pictures.

 

[Miles]

I am interested in what the design looks like. I did not find any drawings except for housing and a DXF File I could not open. Can this design compete with the Bionx D? That Motor has a very small airgap (about 1mm) I saw on Pictures.

Welcome oger,

That aspect is still being developed. Feel free to make any suggestions.

I used a 1mm airgap thickness for this design. To be honest, I've no idea how that compares to the thicknesses used in the different hub motors.

 

[speedmd]

We have some advantages to a standard hub motor with regards to air gap as the wheel loads will not be a factor on this setup. Thinking, All we have to really worry about is getting the stator hot too quickly(heating faster than the rotor on cold start up), and bearing or lam/ magnet run out causing rubbing. We may be able to push the envelope on gap if that makes any improvement in performance without other negative trade offs. Brake side mount would be excellent with a locking sprag setup that could be released for pedal only use and locked up for ebrake use. Has to be something simple we can adapt. Still looking.