Hub Motor Idea

Joined
Oct 18, 2021
Messages
16
I am a hobbyist designer and have developed a concept design for a type of hub motor for electric bikes which I think has not been tried before, and which I think may have some significant advantages over existing types. I have built a working CAD model to illustrate the principle but I don't have the facilities or funds to take the project further- building prototypes, testing and iterating the design and taking it to market.
Any ideas how I can take this idea further? I would like to form a collaboration with someone who has the relevant expertise in motor building to validate the concept and work towards a prototype. So far I have tried emailing a few manufacturers (including Grin) but no response yet. I'm in the UK.

Thanks
 
Everything has been done, friend. Once upon a time, in the time before us, with motors. Every day I think I have a "new idea" but.. nope. Been done.


We are very interested in your design, regardless. Please show us the concept and we can help you here. Many here make DIY motors.

Hire an engineer if you want very good data. I suppose you do not want to publish the detail? MOtor (design) property is very well known.
 
You're best bet is going to build a prototype yourself which is probably not as expensive as you think, this forum has hundreds of examples of people building very complex motor and drive systems with very limited tools and outlay. Will it take a lot of time and effort, yes, but it can be done. If you can produce a prototype the amount you'll learn about the design and how to turn it into a viable product will be invaluable. (that being said you probably should tell us the design now and possibly save yourself a lot of effort having others with much more experience weigh in on it)
 
Well I don't think it's been done. It's a dual rotor axial flux motor with twin offset cycloid gear reduction. The cycloid gears use bearings as gearteeth to transfer the torque with minimal friction. It fits in a 135mm dropout with a 9 speed cassette.
 
Ah interesting, so what are the advantages besides those of an axial flux motor?

I hate to rain on your parade but the problems I see are the standard problems with axial flux motors and cycloidal gearboxes, they are both very expensive to manufacture, which is the reason they aren't more commonly used. The axial flux design partially because it's not very common but also due to issues of how the frame that holds the coils is held in place, cooling said coils, etc, all possible to solve but at added cost which is a bad idea when trying to sell hub motors. The Cyclodial gearboxes require very tight tolerances to be reliable and that makes them much more expensive than molded gears (Nylon or MIM) or no gears and when compared to nylon gears they are often noisy which is also not popular when trying to sell hub motors.
 
I can tell you right now you'll probably have to deal with several kinds of specialists: a motor winding specialist and a gear cutting specialist at least. The gear maker can probably do all your other machining if he's willing to do it and you're willing to pay his rate.
 
The coils are air core and fit into cutouts in the stator ring. I think they will be relatively easy to wind and just pop them in. There is space in the middle for a circular PCB which they can be connected up to.

Single cycloid gearboxes suffer from vibraton but this is counteracted by using two, offset by 180 degrees. I think it's possible to make the cycloid gear itself from nylon since the wear will be minimal due to using bearings as the gearteeth on the outer ring.

I'm not sure about cooling, and I haven't been able to do any meaningful calculations, this is is just a concept really.

I'm trying to compress a gif animation so I can upload it..
 
ebike-motor-cycloidal-gear (1).gif

apologies for the jerky animation, I had to just screen record and crank it by hand as my computer isnt powerful enough to render the full animation
 
A high reduction means you can use a very fast winding in the coils. It's also called a "low turn count", and that means filling the available stator-coil space with fewer turns of thicker wire. Luke Workman has persuaded me that the lower resistance of the thicker wire is worth the trade-offs, so...I think your idea has merit.

I've seen several cycloidal reductions, and other than some noise, they are a great option. Having two of them configured to 180-degrees is what Wankel did with the rotary engine, rather than having one larger rotor. I'd look up the links, but I'm fairly certain you've already seen them.

Best of luck.
 
Thank you!
The reduction is 12:1, I calculated that at a rotor speed of 2600rpm it would propel a 26" wheel bike at 15mph. This is a pretty reasonable speed I think and dual rotor axial flux is a configuration which gives very high torque as the opposing magnets produce high flux density. I think it will be quite easy to play around with the turn count too, the coils should just pop in and out.

I have no experience in motor design, this was just something I did to pass the time during the covid lockdown and I thought maybe it deserves to go further, by I don't really have the capability; if anyone would be interested to pursue it let me know.
 
Your not into speed, so what size tire/rim you using?


spinningmagnets said:
A high reduction means you can use a very fast winding in the coils. It's also called a "low turn count", and that means filling the available stator-coil space with fewer turns of thicker wire. Luke Workman has persuaded me that the lower resistance of the thicker wire is worth the trade-offs, so...I think your idea has merit.

I've seen several cycloidal reductions, and other than some noise, they are a great option. Having two of them configured to 180-degrees is what Wankel did with the rotary engine, rather than having one larger rotor. I'd look up the links, but I'm fairly certain you've already seen them.

Best of luck.
 
That was just a rough calculation for the motor with a 26" wheel. When I say it's a reasonable speed, I mean the rotor speed of 2600rpm, which is in the normal range of conventional motors, that's all I know. I'm pretty sure it will go a lot faster than that, the limit will be how hot it gets I guess.
 
Your rotor RPM may have to be considerably higher for the motor to be efficient with air cores, or switch to iron cores. If you could get away with a nylon "gear" that could cut down on vibration considerably as well, I have seen some attempts at 3D printed Cycloidal gearboxes running on all bearings and they seemed to work OK. I think tolerances still need to be tight so the rollers are always in contact with the gear to have good lifespan at this speed though.
 
DesignCycle said:
ebike-motor-cycloidal-gear (1).gif

apologies for the jerky animation, I had to just screen record and crank it by hand as my computer isnt powerful enough to render the full animation

You'll probably want a balancer of some sort (counterweight) to stop massive vibration from that cycloid thing spinning round.
 
There are two of those cycloid things offset to each other by 180 degrees, and made of nylon so the weight should be pretty low, hopefully vibration will be minimised. The jerkyness of the animation is just my mouse movements being slightly erratic.
 
With the two being at opposite sites, it Will tend to rotate around its center.

What is the quality og the hub motor? Is it very powerful or quiet or light?
What does it bring to the table?
 
The objective of this design is to match or exceed the high torque delivered by direct drive 'pancake' motors (which are large and heavy and can affect road handling) in the small light form factor of conventional geared hub motors, with lower friction and longer lifespan than conventional planetary gearing.

Yes there is the possibility that a vibration will develop along the shaft around the center point but I am hoping that with nylon cycloid wheels the weight will be low enough for it not to be significant.
 
Alright. I get where you are going, but maybe it's not quite there yet.

Here are sine questions that I think need answering to improve on the design.

1 What are the advantages of this over a compound planetary reduction ala cute q100, which can achieve same reduction ratio but cheaper?

2 Will the air core motor produce sufficient power for it to be significantly more powerful than already available hub motor designs?
I think the benchmark here is cute q128h for light weight @800w cont. And 2,8 kg.
For higher power it will be the Mac, which is rated at 1000w cont with high torque available but weighs 4,3 kg.

3 is the noisiness of the epicyclic reduction acceptable in the type of hub motor you want to make?

4 what sort of price point is this motor aimed at?

Personally, I can see the coreless axial flux as a really nice candidate for a geared hub motor as it has good efficiency and eliminates the need for a clutch in the hub.
I don't think nylon planetary hub gears are as unreliable as you do, so I think the appropiate reduction can be obtained by compounding those. I think the appropiate reduction for this to be powerful is significantly larger than 10:1, more like 40:1 as the strength of the coreless motor is that IT can be spun very fast with minimal losses.
For high performance, one could switch to metal gears and have a splashing oil bath inside the hub, which will also help cooling.

I hope this can be helpful to further refine your design :)
 
Thanks Vbruun, yes this design is definitely not quite there yet, it's dimensionally accurate but not much more than a vague concept really. I'm appealing for collaborators, maybe there is someone out there who would be interested to take it forward. I'm not looking to make lots of money from this since it's really just a concept, and I don't have the facilities or funding to take it further myself, I'm just a washed-up bum who lives in a caravan.

1) I see the main advantage as the reduction in friction which I hope will improve efficiency and thereby range, and also durability.

2) I am hopeful that the dual axis axial thrust motor will be significantly more powerful than conventional radial flux types due to the higher flux density and better flux alignment. https://www.magnax.com/magnax-blog/axial-flux-vs-radial-flux.-4-reasons-why-does-axial-flux-machines-deliver-a-higher-power-density
I think the next stage would be to perform electromechanical modelling to determine estimated torque output. Someone with real expertise and experience in the field (and appropriate software) would be able to do this much more easily than I.

3) Noise levels will probably be determined by machining tolerances. Using nylon for the cycloid gear may help.

4) I would imagine this would start life as a premium product in the $500+ range but quickly become more economical to build and allow more competitive pricing as manufacturing techiniques are honed and the motor becomes more widely adopted. It certainly won't be able to undercut conventional geared hubmotors.

The splashing oil bath idea is good, it would help with the noise as well as general efficiency. I had considered it at one stage but was concerned about how to separate the gears from the motor without leakage. It would be interesting to investigate further though.

My concern with nylon planetary hub gears is more about how much torque they can handle. I understand that these days they can be quite reliable but are less suitable for high torque output eg. when hill climbing under high load.

Thanks again for your thoughts
 
I need to correct you here.

A cycloidal reduction drive is not significantly more efficient than a gear reduction. Wiki claims Up to 96% effiency on cycloidal and typically 97% on epicyclic/planetary gear sets.

https://en.m.wikipedia.org/wiki/Cycloidal_drive
https://en.m.wikipedia.org/wiki/Epicyclic_gearing

In effect that means that the amount of friction in the two systems are similar. If durability is your concern, oil lubed gear reductions usually last some 300.000 km if they are made of steel.
Where the cycloidal drive may shine here is that it has good torque density and so can be made very thin, leaving more room for the motor.
 
So please if somebody knows better correct me but after some curiosity and research I think the motor design you currently have would actually not be that efficient. The same design with steel lamination probably would be but thick air cores like that seem to suffer from high losses due to large airgap (I believe the empty core is also air gap here). This is why most coreless motors have very thin windings, axial versions being a thin flat disk of compressed windings and radial versions being a thin shell or cup shape. Also large coreless motors like that I believe can be very challenging to run as their inductances are very small. I'm certainly no motor expert so I would be curious to here what other more experienced users have to say on this.
 
The idea is certainly interesting. As far as I understand, with 'air core' it has no 'iron losses', but suffer from poor torque capacity, but it can benefit from massive reduction and obtain good efficiency AND torque?

I think, a 'fixed gear' left sided middrive is a much more viable alternative - you can add like 6 times more reduction using a chain or a belt with very high efficiency, and combination of no iron losses AND recuperative braking is very, VERY cool.
 
Vbrunn- I didn't mean the cycloid gear is more efficient, the improved efficiency comes from the axial thrust motor. I am hoping that the cyloid gearbox will suffer a little less friction than conventional planetary types because every contacting surface is formed by a bearing.

Scianiac- there's no real way to predict the efficiency without complex electromechanical modelling or a prototype. The main argument is that dual rotor axial thrust motors can be much more efficient than radial flux motors, and that friction will be reduced in the gearbox by using bearings as gearteeth. Another point to highlight is that this is a brushless design - since the coils remain stationary, they can be wired directly to an electronic commutation circuit on a circular pcb in the center and the supply cable passed through the hollow axle.

BalroNG- the choice of air core was mostly for manufacturing simplicity but it does reduce some losses. Yes there are many other viable alternatives, but here I'm discussing this particular configuration. I'm not really interested in mid-drive I'm afraid.
 
I want to see the air-vore prototype run, and get actual numbers.

Once it's run, you can even swap-in iron core stators to see how much of a difference there is, and which you prefer.
 
Back
Top