largest diameter hub motor for a bike?

I'm wondering if there's a hub motor design that utilizes the whole diameter of the bike wheel by putting the airgap diameter as close to the wheel diameter as possible. I think you could make the lightest and most powerful motor with such an approach and wondering why I never see it done. It would need many poles and teeth and winding/building the motor would be more work but otherwise ..why doesnt anyone make/sell such a hub motor?


no complexity, expense, and weight of the spokes.

i believe motors with many poles generally end up cheaper than with fewer poles as the magnets can be thinner and theyre the biggest expense.

doing a hallbach array and omitting all backiron would be worth doing and the rotating weight could be brought way down. ( a lot of magnets and more work but im willing)

i havent tried running emotor software in ages but tempted to get it going to see what the results would be and maybe get a stator or two cut and the other parts machined and build it. Surely someone has done this before. any links from the past?

maybe there's surprise hurdles such as getting a stator made that's so big...but could maybe build it out of segmented teeth all glued to an aluminum holder. maybe such a segmented tooth is already out there? or maybe with the very high erpm it would need to run a motor with so many teeth it would be worth doing a powder core...and that would be easy and cheaper to make. although my version would likely be low iron content.


like a washing machine stator:
https://www.appliancepartspros.com/whirlpool-stator-motor-36-wpw10419333-ap6021196.html?msclkid=01ffba0be8b51daeb1d7da54b1c04a47&utm_source=bing&utm_medium=cpc&utm_campaign=Campaign%20%2366%20-%20Bing%20Shopping%20(BSC)&utm_term=4577679226710413&utm_content=All%20Products

Hummina Shadeeba said:

..why doesnt anyone make/sell such a hub motor?

no complexity, expense, and weight of the spokes.

Click to expand...

Manufacturers don't do that because it would spend all their design and development on a single wheel size, and they want to be able to serve a range of applications. You can see this in the stupidly narrow flange spacing of most hub motors, which seriously weakens full diameter wheels, only for the sake of being able to lace small diameter wheels to the same hub shell.

What you're suggesting would be massively heavy, not only because of its physical size, but because it would not benefit from the strength-to-weight ratio or repairability of a wire spoked wheel. The same impact that puts a minor flat spot in a bike rim might dislodge magnets or jam the air gap closed in a wheel-sized motor. It would have to be sealed up enough to keep from ingesting steel parts and swarf, implying hub motor side covers the diameter of the entire wheel. And the whole thing would have to be built stiff enough to keep the rotor from slewing to and fro with respect to the stator. It would look like, and weigh as much as, a manhole cover.

By the time you got all the problems ironed out, you'd be left longing for the (tolerable) complexity, (modest) expense, and (minimal) weight of wire spokes.

You could go cast wheel where you have the cast piece bolted onto the hub, but like Chalo said, having spokes and a rim is far superior in strength, cost and ease of repair and cost of repair to cast wheels bolted onto a hub motor. Because you need to purchase a new cast wheel from the hub motor company which you'll be waiting 8 months by the way the Suez Canal blockage is turning out to be.

Leaf has a cast wheel but not in their 1500W motors, but in their 1000W motors and the cast wheel is a fixed diameter so no choice in the matter. Other manufacturers/sellers have cast wheel available, like QS Motors (not sure if they are a reseller/rebrander or actually have a hub motor manufacturing plant)

Another way to go is to use motorcycle (mc) or moped rims with bicycle spokes, or mc spokes. Your gaining more weight in all aspects from the rim to the mc tubes to the mc tires but they are super durable. That might want you to go thick spokes, which weakens the wheels strength and thats the beauty of the bicycle setup, the wheel is meant to flex because thats where the strength comes from.

You might want to look at the BionX-D motors which are no longer made, proprietary but the motors you can use.

But just imagine for a moment your wet dream does come true, and you have a hub motor measuring ~22" in diameter and you have no need for a rim or spokes, and you just install a tire. Then you ride it and bottom out that tire and mess up the hub motor itself, the bent tire seat flanges are the least of your worries and you get exactly what Chalo says, your magnets will break, and hopefully doesnt cause more damage inside the motor. The uniform nature of the magnet ring is now compromised with possible hidden defects like hairline fractures to become evident a month or more later, or days later. Now you need to buy some magnets that are not the same as the others, or buy all new magnets if your so lucky enough to find a supplier or lucky enough the manufacturer sells you theirs. Figure out the glue used. A whole mess of problems. Whereas you can just go down the block, buy a new $25 rim, buy $40 spokes, $15 tire, $5 tube and be riding again within a day, not a months waiting on magnets, tools to straight out tire seat flange, figure out how to make the magnet ring uniform again. But your still left with that unknown factor of hidden defects to later put you back to square zero because now you need to buy a new motor.

Surely a thick enough 22” clam shell can be machined out of enough aluminum that the magnets aren’t moving and you could even ride around with no tire. I don’t even think that would have to weigh much. Maybe it would weigh much. Maybe even a composite and be easy to add to the machined clam shells and add structure in key spots. I don’t think structure is a concern and just wonder on the contest:

the contest between the small vs large diameter motor:
Best conductivity at a given kv, and also weight comparison at the same conductivity and kv

With the air gap diameter increased 2x or 3x the magnetic field strength needed would be..let’s say greatly reduced(not knowing), so stator teeth need be like as thin as the wire. And thinnest magnets or at least a lot less strength needed in the magnets per volume with the big air gap diameter. ..I’m sure there’s more to it and I’m simplifying but the general in my mind principals of what’s needed for a given torque output.. bigger the lever the easier. I’ll have to crank up the program. I think I’m wasting my free year on there but getting the igs or whatever drawings needed is hard. Not easy to whip up a design and run it for me.

Comparing to the stiffest bicycle wheels I’ve ridden, while I do think the spokes add a lot of spring so things don’t break, I think the tires are good enough for that job. There’s full disk composite wheels out there, and if the weight on the edge is kept low the side wind is the worst downside and messes up handling.

ill see if i can do it on emetor.

Largest diameter hub motor probably the BionX D500 (no longer made, but they pop up from time to time on Craigslist and ebay etc.), seems to be well regarded with the exception of the proprietary built-in controller.



Recent comments in this revived thread:
https://endless-sphere.com/forums/viewtopic.php?f=3&t=93078&start=175#p1534282