Geared Hub rotor standalone as a Mid drive

Hello Forum,

I want to build a mid drive style project but keeping it as simple, light and efficient as possible (who doesn't right?) So, what I want to try is to use the rotor and stator of a 500-1000 watts hub motor (MAC style), by removing it's freewheel planetary gears and side sideplates, just leaving what's on the image.


replacing that sun gear with an adequate sprocket, attach the axle to the frame and put a chain between the rear wheel and the motor (or what's left of it). Yes, straight to the motor, I don't want to put pedals there right now, I plan to build front pedal recumbent, so that's for later.

I don't have any experience working or even looking at geared hub motors, what I've seen is just what's online, so I was wondering if this setup would be able to withstand pulling the rotor just by one side, you know the chain will push to just one side of the sprocket (the one I'll replace the sungear with), hence maybe wearing down the rotor's roller bearing at an unjustified rate or even missaligning the rotor making the magnets touch the stator. Any thoughts?

Why would I do that? Well, to keep it light, to keep it looseless ( I mean if I'm going to have loosess on the chain I don't want extra loosess on the planetary gears, I don't want to deal with wear of planetary gears, to avoid the unsprung weight scenario and to have regen. Of course I'm considering the appropiate gearing to keep the motor spinning at a confortable rpm.

I like the form factor of geared hubs stators/rotors, and I don't see any direct drive hub motor with a similar size, weight and power that works in the 1000-2000 RPM optimally (I'm planning a 1:5 gearing and a 20 inch wheel).

Do you know about any other type of BLDC motor similar in size, power and rpm? it doesn't matter if it's sensored of not.

If what I suggested above with the geared hub motor/stator is not feasible and there's no other motor type as light powerfull and well suited for my application maybe I will have someone to machine a rotor, which supports itself on both sides of the stator but being lighter than direct drive's cause my sturdiness requirements are not that high.

MexicanElmo said:

So, what I want to try is to use the rotor and stator of a 500-1000 watts hub motor (MAC style), by removing it's freewheel planetary gears and side sideplates, just leaving what's on the image.

Click to expand...

What's the RPM of the motor as it is now? (the sun gear) (probalby several hundred RPM)

What's the planetary reduction ratio? (probably 4:1 or 5:1, or similar)

Is the wheel diameter you have now the same as the wheel diameter the motor came from? If so, is the speed the motor wheel ran the bike at before the same as the speed you want to run it at now?

If so, then you'll need to gear down the motor from the sun gear at the same ratio to the wheel as the planetary did, to keep the motor in it's original (presumably most efficient) RPM range for the pack voltage you have.

I was wondering if this setup would be able to withstand pulling the rotor just by one side,

Click to expand...

Probably the bearing will be ok, if the sprocket is in line with / in place of the sun gear it should put the same forces on the bell / bearings that the gear put on it.

If it's bolted to the side of the bell housing it might also do the same, but you'd need a larger sprocket to clear the housing, making it harder to get the right reduction ratio in a single chain (you might require a jackshaft to do two reductions).


I don't know how far he got with it, but at one point John in CR was going to do something like this with a Fusin geared hubmotor, so you could search his posts for "fusin" and see what's in the thread about it.

Get a used hoverboard wheel they have a low enough kv for what you want, bolt the smallest ring from a mtb cassette on there spaced out a few mill to allow the chain room and have the 21speed etc drivetrain on the right hand side of the bike then add a single speed crank on the left hand side with the large sprocket so you get an aditional 5-1 or so gearing on top of the low kv depending on sprocket choice, then fit the wheel/motor under the bottom bracket remove its tyre, optionally add some small fins to the magnet rings outer and sort electricals, You Can pick a 36v controller up for peanuts doesn't even need reverse a used hoverboard battery or some cheap lipo and it's done, the whole build will only be a few dollar's.

EDIT
I would try get a pedal sensor working rather than a throttle so it feels more of an assistant rather than pushing your cadence upto 350rpm etc for that point I'd fit as small as cog as possible to the motor maybe a 9 tooth rear from a mtb cassette and then a 48 tooth front giving a reduction of 5.3-1 lower than the motors kv at 36v that would give a decent cadence, if its assisting rather than leading the way it could be a very sorted cheap build.

I like this idea, after reading Crossbreak's thread on hub motor conversions to jackshaft middrive usage

https://endless-sphere.com/forums/viewtopic.php?t=45245

this contains somewhere within the many pages a description of a system such as you propose but ist only a small mention and yet to be developed and by thinking about it you have come nearly as far as in the thread

IMO a BPM or cst or SWX series bafang hub would suit your needs they all have the same low (16) pole count and come in several different speed windings

The BPM is the more capable of the three having a slightly wider stator than and more copper massthan the CST and a larger diameter than the swx series , the fastest winding a 12x8t has a peak power around 1500rpm on 48v and a rated speed (hub 336rpm@36v) 5:1 gearbox means this is 1680 motor rpm meaning the motor has a KV of 46 the others 11x9t ,10x10t KV42 , 9x11t KV38 , 8x12t KV 36, any of these could be suitable to either unshell the motor removing the side plates and keep the shaft rotor and stator only, this could be very compact width wise without the need for shaft bearings and the outer hub shell. Only the fixed shaft stator and outrunner rotor of the motor need be retained. The shaft can be cut shorter and supported at both ends, the sun gear removed and replaced with a large tooth (8m htd would be ideal) that is hollow and affixed with 6 screws to the face of the rotor, the stator would benefit from a very large heat sink bonded to the stator centre like Crossbreak's heat bridge ( read above thread has lots of info on cooling and heatloads and ATF cooling). or you could retain the motor hubshell and cut a rectangular hole in the circumference between the spoke flanges for the belt to get inside and use the crossbrake style heat bridge to allow the hub shell to be used as a large protective heatsink. This mod could be done with any of the bafang geared hubs fairly easily and the shell less design with large custom heatsink radiator/ mounting plate would only require a rotor, stator and shaft these parts may be aquired as spares form possible more than one source .

The higher kv one has the highest power producing potential having the lowest resistance windings meaning it will sink more amps and be able to turn more rpms than the slower winds and make more power but this may intern require more reduction to see the rear wheel speed. the higher KV would be likely to have a better efficiency than the lower kv ones unless it needs two stage reduction. The lower pole number according to others here at ES would mean it can have a higher efficient speed than a comparable mack with over twice the pole number because the lower switching frequency needed on the BPM (16 poles) keeps the frequency below the inefficiency point of rising eddy current losses in the thick 0.5mm stator laminates.

the CST could potentially be over volted to even higher rpm due to the thinner 0.35mm laminates in the stator , 3000 rpm at 72 volts would be possible only 400 hz switching freq still in minimal loss for stator eddy currents probably would not trust a large outrunner to more than 3krpm anyway hope its well balanced. I think crossbreak measured saturation effects above 40 amps.