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Ultimate lightweight wheel-motor concept


10 MW
Nov 14, 2012
new england
Idea here is to build a consensus to what would make up the ultimate light weight wheel motor and follow on with modeling and prototyping. We are not restricting any ideas. Cost is not a issue within reason.

Goals is to not add more than 2 - 3 pounds to a standard 700c bicycle wheel. Power is unlimited with the idea being that 1kw continuous output should be a realistic target and motor designed to have exceptional torque / acceleration and maximize efficiency. Understandably we can scale results for different wheel sizes and customize for off road or for road racing in follow on builds/wheel sizes.

Sky is the limit, but we should consider the lightest, simplest, most efficient and reliable as key to what is explored. Please, no debate on hub vs mid drive. Discussion of transmissions/ reduction gearing or the like should be handled elsewhere. Ultimate is a simple two part design. Rotor / stator. If we can share wheel walls (eliminate spokes) with the motors rotor/housing and make a disk wheel out of it within the weight targets that would be perfect.

Some have already given this topic tremendous amounts of thought and would appreciate some further design goals they would deem crucial to such a motor. Does it need to have regen capabilities? Should it free wheel? No cogging? What am I missing on the wish list?
1KW would match nicely with 30mph.

I do like the previous suggestion of simply widening the motor to achieve greater power handling/torque. I'm pretty ignorant of motor design, but if it were simply a case of taking the design and adding width to everything (rotor, windings, magnets, flux ring) with everything else staying fundamentally the same, that would be awesome :) Just keep adding to the lamination stack until you get a width that matches the closest available size of magnets :)

Looking forward to see what you clever chaps come up with :)
Miles said:
What maximum rpm do we need, for the prototype?

400rpm gets you to 32mph on a 675mm diameter wheel.

Hi Miles

With a standard type controller/ motor topology 400 - 450 rpm sounds good. Ideally, thinking we should optimize this for a lower speed/ better climbing / acceleration and build in some sort of controller/ timing advance to electronically gain higher RPM's / speeds at riders discretion when we get further into mating suitable controller technology. I am much too green to talk intelligently about that side of things just yet. Anyone here is welcome to take over any end of that side and help steer us out of the weeds. There will be a significant swing in overall diameters going from a bit over 20mm tires to the bigger 35 and even the 40mm types, so the lower end of the rpm range may be a better place to start.
Miles said:
I think we need to start thinking in terms of torque.... Even at 400rpm, 1kW adds up to 17.6Nm/kg, if you want to retain the 3 pound limit

There is some radius at which this works fine. :)

Stubby tiny teeth made of a cobalt alloy. Very narrow, very large diameter stator, very thin magnets, very thin magnet back iron, perhaps in mumetal or whatever makes the most sense.

I think it would be awesome to target possible lacing into a 20in rim. Spoke flanges on the sides near the axles rather than the outside of the motor, so we can still have good spoke angles and length to make strong wheels.
Hi Miles

The sky is the limit. Iron less / axial / pcb stator /rotor etc etc. I am thinking motor can share many of the structural wheel components stealing some weight in the process. A normal bullet proof rear wheel is in the neighborhood of 2 1/2 -3 pounds without tire and gearing. Lightest ones in the 550 - 600 gram range. Thinking something in the 6 pound range for the entire wheel / motor is realistic without resorting to the space craft materials.
liveforphysics said:
There is some radius at which this works fine. :)
Maybe we need to harness the competitive spirit? :)

Let's leave the 400rpm. The next question is..... What fundamental frequency?

At 400rpm, 100 poles gives you 333hz.
Maybe we need to harness the competitive spirit?
Thats a great idea :mrgreen:
My current axial flux design is easily scalable to fit a 20"wheel. It works out to 30 pole prs & 63 teeth.(how do you like 200mm for a radius 8) )

Shall we set a cut off date for entries? :wink:

But seriously, a composite wheel with a few relief passages to negate the disc wheel syndrome in cross winds.....no problem.
Miles said:
I think we need to start thinking in terms of torque.... Even at 400rpm, 1kW adds up to 17.6Nm/kg, if you want to retain the 3 pound limit

yasa hits 30Nm/kg
specific torque is over 40Nm/kg now with the halbach version of the yasa

We should avoid water cooling but leave provisions

Halbach array would be awesome but not practical so forget it

it is all about air gap surface area
maximize motor diameter to fit in 20 inch wheel
adaptors for larger
Could you move from hub motor to rim motor? Placing the magnets within a double wall alloy and the coils on the frame. I think static torque would be greatly improved, but could speed to simply addressed with higher commutation frequency.

I keep coming back to the idea, but by now I'm usually washing my hands.

Edit: Perhaps induction rather than permanent, with coils on both stays. Hmm
Maybe. Depends on whether it looks like a dorky magic pie. I'm working on a coulple other linear motors right now. Much simpler but harder in other ways
flathill said:
maximize motor diameter to fit in 20 inch wheel
adaptors for larger
So, we're talking about something with an airgap diameter of 350mm, now?

I had a try with a 500mm airgap diameter design but I couldn't get the active weight down to anywhere near 1.5kg.
No rush guys. I posted the opener this AM. I am up for discussing all of the above. Frame mounted stator, rim mounted magnets, halbach array, spoke mount / flange -disk mount etc. We are not looking for cheap and easy. The solutions to many of the costly manufacturing problems, I am certain can flourish and be worked out here with many good possible options.

Lots coming quickly and find myself in serious catch up mode. Some things that were posted previous thread, axial being best with bumps possibly may be a serious advantage here. Also like the idea of a porous disk setup.

Let's leave the 400rpm. The next question is..... What fundamental frequency?

At 400rpm, 100 poles gives you 333hz.
What ever you suggest is a reasonable trade off for weight, complexity and torque.
"Sky is the limit." Stop saying that! :?

So ya got room to pack a wing-sail? Or a balloon or a "lighter than air" inflatable wing (that also floats)?

Well, you did say "Sky is the limit." :mrgreen:
flathill said:
Maybe. Depends on whether it looks like a dorky magic pie. I'm working on a coulple other linear motors right now. Much simpler but harder in other ways

I see a lot of aero looking rims that could hide the magnets in the wheel well kinda area. It is a bad place to put weight though.

I have decided the design is probably unsuitable for feather-weight use. It means carrying a lot of magnets, but using only a few at a time. For a lightweight design you surely use the best magnets and keep them busy.
Nice work! How much did that cost you to run? Just curious

What happens when the "back iron" is changed from hyperco to a low carbon steel?

When considering total motor weight does it always make sense to maximize diameter? When do you start growing wider? I'm not talking ideal calculation from textbooks but real world hub motors that need bearings and support structures to become a wheel. Radial flux probably has an advantage here unless we are talking aircore

As you figured, 2-3 pounds total is not going to work due to all the non active weight needed for large diameter motors

I think 5 pounds total may be a more reasonable goal for a large diameter direct drive motor. What do you all think?
Miles! Beautiful work! It can be made lighter still though. :) I noticed it said that simulation used 1.2 credits. How can we buy you credits? I want to buy you 20 credits right now. :)

93% efficiency direct drive very light hubmotor is so possible to do.
Great work miles. It is a few pounds more than target but still close, it is super efficient and powerful. Conventional motor design may not get there unless we can take advantage of stealing some weight from spokes / rim and hub design. I am also in to contribute to the cost of the simulation runs.

Great question on the optimization trade offs. At some point the factors cross and we may be better off with a smaller package for a given weight target at a given power level. Have done some mechanical optimizations in solid works for weight and other physicals but nothing like this. May be worth doing a simple doe for diameters, pole counts and several other of the mentioned core and magnet features to better see relevant response factors in optimizing torque - power to weight.
Miles, can you sim a magic pie as a comparision to set the baseline if we can gather the dimensions?

Anyone know the weight of a magic pie with no controller? Are the side covers steel or al?

Luke, can you sketch your spoke idea? Some solar car motors have side covers like that but no spokes
'... i got a barn we can use & becky can bake up some pies!"

honestly, you think you're gonna invent a better wheel that's had invested millions of dollars & thousands of man-hours with math PhD's working on it?
or is it simpler to spend the extra bux & face the music of how dumb you are for doing so?

flathill said:
Luke, can you sketch your spoke idea

really lucas, you're lapsing into 'safe-mode' not referencing your sources, taking implied credit by ommision.
"lukes idea" which received a HUGE YAWN from the E*S blu-rues when posted here nearly six months ago.
how are those royalty checks coming?