Low rpm bicycle motor design

veloman

veloman

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Is there any way to build a motor that is about 5" diameter, brushless, that has max performance at 400rpms or so? DD hub motors do this, but they are too big and heavy.

I was thinking they should manufacture something like that, specifically for bikes. I guess it'd be the Stokemonkey motor, that's the closest thing out there to what I'm thinking of. That's the sort of drive that I think is best. Driving freewheel cranks is worth the slight complexity IMO, to get that perfect motor rpm.

Now, there is that cheap MY1018z that I have, it's just brushed and noisy with the gearbox. I think this can be done. Why not?

1000w rated at 36v, brushless, no reduction, 400rpm peak power rpm, 5" diameter, < 6lbs, lifespan - a long time, great reliability, with some nice mounting holes :lol:
 
Sounds a bit like Miles' "Alter-motor"

PICT0019comp.jpg


http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=5313&hilit=#p79670
 
veloman said:
Is there any way to build a motor that is about 5" diameter, brushless, that has max performance at 400rpms or so? DD hub motors do this, but they are too big and heavy.
Click to expand...
They are big and heavy because they need to be to get the torque required when you run at slow speeds. The motor above will put out 1000 watts cont. at 1600 rpm but, in order to do this at 400 rpm, it would have to be four times bigger.....

A motor with integrated helical planetary gear reduction seems to be the best option.
 
That perticular example is wound by making pre-wound coils that then get slipped into the slots as well, which is a semi-OK way to wind a motor with a wide stator, but a narrow stator with a ton of poles ends up with a lot bigger end-turn loss when it's wound with batch coils.

Kinda off-topic, but just an example of good things and bad things to look for in a motor's design.
 
This 270kv outrunner that I have can produce 2500watts at 10s, and it weighs 414grams.


I just thought that you'd be able to achieve 1000watts, weigh 6 times that weight, and 2.5x the diameter. - All with a kv like similar to a hub motor.

It doesn't need torque at 100rpms like a hub motor needs, since it is driving the cranks and would be above 350rpms 99% of the time.

If there's a reliable, quiet gear motor that acheives this, then let me know!
 
Miles said:
Off the shelf, direct drive, this looks promising:
Click to expand...
Promising ??? i didn't see nothing new here under de sun or did i miss something ?, i think a BMC torque motor is better no ?.

Good day!
Black Arrow
 
Miles said:
Off the shelf, direct drive, this looks promising:

http://www.selectricbikes.com/images/kits/falcoe/pdfs/8DxmMotor.pdf
Click to expand...

A 2kg, 400W quiet DD motor like that would be very nice in a stoke monkey like setup.
The pics in the PDF hint that it will be sold with integrated controller, so likely expensive and if one wants to go to the higher voltage needed for say a 4:1 or higher gearing on the monkey likely a separate controller has to be bought and the internal one disconnected.

In terms of power per kg it is not that different from a 9C though. My 9C gets barely warm to the touch (about 35C) when operating at 1500-2000W, and it weighs a bit over 5kg.
 
are we talking hub motor or just a dc motor with low Kv ? How low a Kv ? What about the colossus -75 Kv ?
 
jmygann said:
are we talking hub motor or just a dc motor with low Kv ? How low a Kv ? What about the colossus -75 Kv ?
Click to expand...


Well, I'm looking for a shaft output of around 450rpms max. So even 75kv would be too high with reasonable voltage.

I think hub motors are around 10kv, which would work. Maybe even 20kv.
 
veloman said:
jmygann said:
are we talking hub motor or just a dc motor with low Kv ? How low a Kv ? What about the colossus -75 Kv ?
Click to expand...


Well, I'm looking for a shaft output of around 450rpms max. So even 75kv would be too high with reasonable voltage.

I think hub motors are around 10kv, which would work. Maybe even 20kv.
Click to expand...

My guess is that you can likely go up to 1000rpm or a bit more and still make a stoke monkey type drive quiet. Swapping the chain for a belt will also help make it quieter.

In terms of the various RC outrunner motors the first question is how efficient they will be at low rpms. 450rpm is I guess too low, but 1000 might be ok. Low kv is also desirable of course to make matching a controller and battery easy.

I think a stoke monkey on a long tail frame makes a great utility bike like this one:
4741402970_92638417d9.jpg
 
The Stokemonkey's size is okay. It's just too expensive at ~$1000 I think.
 
Hi,

veloman said:
It doesn't need torque at 100rpms like a hub motor needs, since it is driving the cranks and would be above 350rpms 99% of the time.
Click to expand...
For driving the cranks you need a lower rpm than driving the hub (if you want to pedal).
 
I'm thinking what you don't yet realize you want is a little high RPM motor with a two stage planetary reduction box stuck on the end. Perhaps something with a pair of 5:1 planet stages, to make a 25:1 reduction.

IMHO, since you're not looking for more power than could be had in a small RC motor at high RPMs, it makes more sense to add a little length to a tiny motor in gearing to get the torque you want rather than to make the motor physically ~10x-20X larger size.
 
veloman said:
1000w rated at 36v, brushless, no reduction, 400rpm peak power rpm, 5" diameter, < 6lbs, lifespan - a long time, great reliability, with some nice mounting holes :lol:
Click to expand...
Reality check - this requires a torque density of 8.8 Nm/kg......
 
better to drive the rear wheel with larger sprocket ...IMHO

http://endless-sphere.com/forums/viewtopic.php?f=28&t=21638
 
MitchJi said:
Hi,

veloman said:
It doesn't need torque at 100rpms like a hub motor needs, since it is driving the cranks and would be above 350rpms 99% of the time.
Click to expand...
For driving the cranks you need a lower rpm than driving the hub (if you want to pedal).
Click to expand...

I"ll have another 5:1 reduction from shaft to chainring.
 
jmygann said:
better to drive the rear wheel with larger sprocket ...IMHO

http://endless-sphere.com/forums/viewtopic.php?f=28&t=21638
Click to expand...


This does not work for my interest. Driving through the bike's gears optimizes efficiency. If I were to do as you say, I might as well just ride a hub motor.
 
Miles said:
veloman said:
1000w rated at 36v, brushless, no reduction, 400rpm peak power rpm, 5" diameter, < 6lbs, lifespan - a long time, great reliability, with some nice mounting holes :lol:
Click to expand...
Reality check - this requires a torque density of 8.8 Nm/kg......
Click to expand...


That's why I started this thread, to ask if it was possible. What's the torque density of Recumpence's 14kw motor? I'm curious.


I'm not against a gear reduced motor, as long as it's quiet and would last a long time with no maintainence.
 
Toorbough ULL-Zeveigh said:
actually that's not up for opinion.
it's a fact.
Click to expand...


I know there are many who will not agree with that statement - that a relatively low power vehicle is better off with one gear than multiple.
 
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