Using RC motors on E-bikes [Archive]

Discussions related to motors other than hub motors.
This includes R/C motors, botttom bracket, roller and geared drives.
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dirty_d   10 kW

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Using RC motors on E-bikes [Archive]

Post by dirty_d » Jun 05 2008 12:37am

i thought this was crazy until i saw recumpent's build log of his e-bike, i did a little more research and these motors are actually pretty spectacular. i got the info for the motor he used here http://www.espritmodel.com/index.asp?Pa ... rodID=5421. then i used that info to plot the motors performace. all i needed was the no-load speed at a certain voltage and the resistance(0.027 Ohms) of the windings, with that you can find all the other characteristics of the motor.

here is the plot of the motor taking all the amps it wants at a constant 48V from 0 - 10000rpm
Image

WTF! those are some big numbers, these motors are designed to be grossly underpowered(electrically i mean, not construction wise). at stall this motor will draw 1777A at 48V, thats 85kW!, the max output power would be about 21kW. of course this isnt possible because the motor windings would melt instantly. its designed to be run at very conservatively which allows it to still produce a lot of power while being very efficient. you can see the site does seem to publish the real peak efficiency as it matches up exactly on the graph, its hard to see, but in the graph on my computer i could see exactly and it was 92.8% at peak.

heres how it runs with a 100A motor current limit.
Image

now this is where the motor shows its beauty, with a 100A limit the motor is over 85% efficient over more than half of the rpm range, and its already 70% efficient at about 1/10th no-load speed. and there is still a respectful amount of power available. between 1500-4500W the motor is running 80-90% efficiency that is pretty amazing.

ive known for a while that using a more powerful motor than you need is more efficent because you can run it under max power and get good efficiency while still having all the power you need. i always though if the motor was to be more powerful it needed to be physically larger, but it seems you can get the same effect by using low resitance windings and strong magnets in a tiny motor. of course the motor wont actually be able to run at more than maybe 10% of the stall current without burning up, but it will run more efficiently at low power over a wide rpm range.

the only problem is the high rpms, its doable but there are problems, recumpent had problems with the belts slipping and breaking. and with 2+ chains/belts you do lose efficiency, how much you lose i really have no idea but im sure he will post his findings after analyzing the data from his logger. and you need a lot of skill and tools to do this stuff yourself stuff has to be precise and perfect, its not like by bike with a 10lb motor with a #35 chain going straight to the rear wheel, i didnt need anything more than a welder, grinder, and a saw for that. i might have a go at it someday though :D

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Re: Using RC motors on E-bikes

Post by voicecoils » Jun 05 2008 2:56am

I wonder if something like this could help for these high RPM motors:

18.75:1 gearbox for motorized (ICE) bikes:
http://www.staton-inc.com/Details.asp?ProductID=2363
$259, weight not listed.

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Re: Using RC motors on E-bikes

Post by docnjoj » Jun 05 2008 8:13am

Whats the conversion fromm radians/sec to rpm?
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Re: Using RC motors on E-bikes

Post by fechter » Jun 05 2008 9:11am

According to my math, Take radian/sec X 9.55 = rpm.

Those are excellent graphs since they cover the entire operating range. I wonder if they are from actual measurements?
"One test is worth a thousand opinions"

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safe   100 GW

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 9:17am

Dirty_D I'm glad you posted some charts.

Yes, I've spotted the "genius" of these RC motors too after Recumpence showed up. These things are freaking incredible. For something that weighs 5lbs or so you get the power of a 25lb hub motor.

I tried to get a "Developer Zone" thread going to talk about the gearing issues, but it was trashed four times in a row and then locked down after being trashed a fifth time.

:arrow: Maybe we can discuss it here?

It's almost like you have to carry on discussions in a "stealth" manner these days because of the trashers.

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Re: Using RC motors on E-bikes

Post by rkosiorek » Jun 05 2008 9:40am

docnjoj wrote:Whats the conversion fromm radians/sec to rpm?
otherDoc
warning for those who suffer from ADD "HIGH SCHOOL MATH LESSON AHEAD".

given : 1 radian = 180/pi degrees = 180/3.1416 = 57.296 degrees.
and : 1 revolution = 360 degress
therefore : 1 radian = 57.296/360 = 0.15915 revolutions

it follows that : 1 radian/second = 57.296 degrees/second = 0.15915 revolutions per second
given : 60 seconds = 1 minute
so: 1 revolution per second = 60 revolutions per minute.
therefore : 1 radian/second = 0.15915 revolutions/second * 60 seconds = 9.5493 rpm

so multiply radians/second by 9.5493 to get rpm.

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Re: Using RC motors on E-bikes

Post by Miles » Jun 05 2008 9:41am

Here are the specs on the AXI website: http://www.modelmotors.cz/index.php?pag ... gorie=5345

They're also happy to do custom motors/windings and the cost seemed quite reasonable, to me.

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Re: Using RC motors on E-bikes

Post by rkosiorek » Jun 05 2008 10:24am

i love the machining that was done to make that RC motor work on an e-bike. but questions still remain - like - How much does all of that gearing affect the efficiency of the motor? what are the losses through the transmission?

he has a solution that qualifies more as a work of art. since he has his own machine shop he can produce these beautiful machines. now how about we ask him for an estimate of what he would charge to do a small production run of say 50 units. do you think you could afford to buy one? really how many of the readers could build something similar? they are out there but they are far from the majority.

i don't think that the idea was trashed as being impossible. i think it was criticized as impractical for most people to reproduce.

but if you do have the expertise and the equipment i would love to see your end result. in my mind i think of this as our e-bike equivalent of a custom chopper by those guys on American Chopper or a Hot Rod built by the likes of Boyd Coddington or Lil' John Buttera (may he rest in peace). lots of incredible fabricated pieces assembled into a functional machine. i do appreciate the beauty of it.

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Miles   100 GW

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Re: Using RC motors on E-bikes

Post by Miles » Jun 05 2008 10:44am

We've covered a lot of this ground, speculatively, in this thread: http://www.endless-sphere.com/forums/vi ... f=2&t=1853

It's a pity eP doesn't contribute anymore...

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 11:04am

Miles wrote:It's a pity eP doesn't contribute anymore...
It's become such a hostile place these days. In the beginning there was very little conflict, but I think the volume of people now just makes people act like monkey's. Crowd a bunch of monkeys into a tight cage and they turn into a fight club.

The RC motor is simply a wonderful idea and it's good that the person that delivered it (in a big way) as a concept wasn't me.

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 11:08am

More Tech Talk

I was thinking that a single gear-to-gear pair for a geardown of 1:3 combined with a secondary planetary geardown of another 1:4 would deliver a 1:12 reduction.

It's multiplication (not addition) when you combine gears right?

:arrow: So:

1:3 + 1:4 = 1:12

:arrow: Example being:

22:66 to 25:100 results in the equivalent of 550:6600 or 1:12


Planetary Gears

http://en.wikipedia.org/wiki/Epicyclic_gearing

:arrow: One turn of the sun gear results in S / P turns of the planets

:arrow: One turn of a planet gear results in P / A turns of the annulus

So, with the planetary carrier locked, one turn of the sun gear results in S / A turns of the annulus.

The annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear. This configuration will produce an increase in gear ratio, equal to 1+A/S.

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Re: Using RC motors on E-bikes

Post by dirty_d » Jun 05 2008 11:23am

fechter, made the graphs through calculations and a program called gnuplot. the motor specs match up perfectly with the plot. the first graph isnt really that important as the motor in reality couldn't run like that because of physical limitations, but it does accurately show its electrical characteristics, the second plot should very accurately predict the motors realworld performance with a 100A limit because none of the motors physical limitations are being exceeded. well actually the amperage might be a little high, the site shows 110A as being the max current, so maybe it would be better to set the max at 60A for continuous operation, you still get a whole lot of power though.

as for the gearing, how would regular gears work? are they more efficient than belts? if they are it seems like that would be easier because you don't have to worry about anything slipping. one problem is that its hard to change gear ratios, i suppose you could just make long slots for the motor to be moved along to allow different sized primary gears.

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 11:31am

dirty_d wrote:how would regular gears work? are they more efficient than belts? if they are it seems like that would be easier because you don't have to worry about anything slipping.
I'm assuming you meant "chains and sprockets" and not gears.

The problem with chains is that they can't be run above a certain speed before all kinds of pathological conditions develop. Basically as I recall the chain speed is not supposed to exceed about 100-200 rpm or you develop the problems.

True "gears" can be spun at any speed and the planetary gear is the most efficient of all types. The worm gear can most radically lower the gear ratio, but the efficiency is terrible. Usually a drop of about 1:3 or 1:4 can be done easily without much in the way of losses... and if you did two of them they multiply. So 1:3 plus 1:4 could yield 1:12 which is all you need. By using a straight cut gear first and then a planetary gear you make that first drop in the strongest way and the second in the most efficient. That way you use the best approach for each step.

Make it small enough and you have a "Swiss Watch" transmission for your bike. :P

Mass produce it and you have a manufacturing career. I would think that if someone dedicated themselves exclusively to developing this transmission they could dominate the market and maybe even have a virtual monopoly on it. If "greed is good", then this is "green greed" of both types. :)

("going green", "green greed", money is green... you get it)

The funny thing is that it might even be possible to convince the manufacturers of RC motors and gearboxes to build a custom adapted gearset for ebikes. It might end up staying within the existing manufacturing base and not come from the outside.

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Re: Using RC motors on E-bikes

Post by dirty_d » Jun 05 2008 11:45am

lol, no by "regular gears" i mean regular gears.

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Re: Using RC motors on E-bikes

Post by Miles » Jun 05 2008 11:50am

dirty_d wrote:lol, no by "regular gears" i mean regular gears.
:lol:

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Re: Using RC motors on E-bikes

Post by Miles » Jun 05 2008 11:53am

safe wrote: Basically as I recall the chain speed is not supposed to exceed about 100-200 rpm or you develop the problems.
Umm, not exactly..

For #35 chain over 17t to 25t sprockets the maximum velocity would be about 4000 rpm

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Re: Using RC motors on E-bikes

Post by JennyB » Jun 05 2008 12:43pm

safe wrote: I was thinking that a single gear-to-gear pair for a geardown of 1:3 combined with a secondary planetary geardown of another 1:4 would deliver a 1:12 reduction.

It's multiplication (not addition) when you combine gears right?

:arrow: So:

1:3 + 1:4 = 1:12

:arrow: Example being:

22:66 to 25:100 results in the equivalent of 550:6600 or 1:12
That's right, but as a bikie from WAY BACK I'm more used to thinking of gears in inches. That's the diameter of a directly-driven wheel that would produce the same speed for the same revs. 100 rpm on a 100" gear gives 30 mph (more or less). If you want your RC motor to rev 6000 rpm at 30 mph then you need a gear of 100" * 100/6000 = 1.66". The motor sprocket would need to be smaller than that to do the reduction in one stage, otherwise the output sprocket would be bigger than the wheel!

Your 1:12 reduction would work best with a 20" wheel (1.66" * 12).

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Re: Using RC motors on E-bikes

Post by Drunkskunk » Jun 05 2008 2:00pm

Its partly my experiance with these RC motors that got me into Ebikes.
The only problems with them are the RPMs, bearing size, and the cooling.

Cooling is almost a non issue now. a year ago motors were sold rated at the bleeding edge of there capacity. now they are underrated a bit and run cooler.

The bearings are tiny, able to handle 100-300 hours of axial load, but not ment for high latteral torque loads. it would take carefull management of torque, probably with clutches and freewheels to keep the bearings from being overstressed.

Gearing is the biggest issue. anything over 10:1 will start having binding issues. at the 50 to 100:1 needed to make some of these work, the normal fluctuations of wheel speed going over bumps will bind up the motor quickly.

But its more than the technicle hurdles of a functional gear set. its also an efficancy problem. a single speed bike, with no derailer loses around 5-10% of the rider's power through the chain. add in the derailer and you lose another 5-10% (numbers taken from 2 bike shops) and thats at a 3 or 4:1 ratio. if you drive the ratio up to 50:1, your efficancy loss becomes significant. At this point, a friction drive starts looking more reasonable.
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safe   100 GW

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 2:09pm

Drunkskunk wrote:The bearings are tiny, able to handle 100-300 hours of axial load, but not ment for high latteral torque loads. it would take carefull management of torque, probably with clutches and freewheels to keep the bearings from being overstressed.
:arrow: Well, I have the answer for this.

You take the controller and modify it to do Armature Current Limiting and that guarantees constant torque across the entire spectrum. That's one of the big advantages of doing it.

Next you use as many gears as is necessary to cover the span of slope and speed that you need to cover.

Finally, you really don't need more than 1:10 geardown if even that. For example:

10,000 rpm taken down 1:8 gives 1,250 rpm
100 rpm pedal power taken up 10:1 gives 1,000 rpm

Take the 1,250 rpm down 1:2 and you get 625 peak rpm at the rear hub. You can then go up and down on that with something like an internally geared hub which does geardown oriented gearing.

...so the big issue is that 1:8 geardown coming off the 10,000 rpm, that's the part that most chains and belts have trouble with.
Last edited by safe on Jun 05 2008 2:14pm, edited 3 times in total.

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Re: Using RC motors on E-bikes

Post by Miles » Jun 05 2008 2:10pm

Drunkskunk wrote: But its more than the technicle hurdles of a functional gear set. its also an efficancy problem. a single speed bike, with no derailer loses around 5-10% of the rider's power through the chain. add in the derailer and you lose another 5-10% (numbers taken from 2 bike shops)
This is a bit pessimistic....

An average efficiency for a derailleur drive might be around 95% all up, I think.

Ref: http://www.ihpva.org/HParchive/PDF/hp52-2001.pdf
Last edited by Miles on Jun 05 2008 2:22pm, edited 2 times in total.

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safe   100 GW

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 2:11pm

Miles wrote:For #35 chain over 17t to 25t sprockets the maximum velocity would be about 4000 rpm
That's true. The #35 is different than the bicycle chain though which has a much lower recommended maximum.

Go-Kart sprockets are cheap and go up to 114 teeth.

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Re: Using RC motors on E-bikes

Post by safe » Jun 05 2008 6:19pm

An Example

http://www.rctoys.com/rc-toys-and-parts ... HIMAX.html

Image

If you use 6 LiFePO4 cells that's 19 volts so:

Max RPM - 10,000
Peak Power - Just over 800 watts
Top Speed (road racer style estimate) - About 40-44 mph

...this thing only costs $116 and it's brushless. :P

280 grams = 0.62 lbs 8)

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Re: Using RC motors on E-bikes

Post by cerewa » Jun 05 2008 6:51pm

Some of those big RC brushless motors have enough torque to work really well at moderate power (750W and down) with only one stage 1:8 gear reduction to a 20" or 26" bike wheel.

The only thing stopping me from doing that is that I don't know how to put a suitable sprocket on an RC motor or on my bike.

(well, that and concern about the RC motor bearings)

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Re: Using RC motors on E-bikes

Post by Lock » Jun 05 2008 7:10pm

Thought RC motors came up previously, and the argument was that they're just not designed for EV starting torques... more for spinning props in free air. In any case, `cause so many here are into power/speed, I still think ya might keep an eye on the ePPG folks (for ppl that haven't seen these yet):

http://www.electricppg.com/

Never thought I'd fly again, but exchanging wheels for 24 meters of cloth would change my commutes considerably. Sorta like base jumpers. Hard to catch <hehe>
tks
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Re: Using RC motors on E-bikes

Post by John in CR » Jun 05 2008 8:49pm

I'm just waiting for a shared diy project describing the build of these types of motors, but meant for our type of use. Forgive my ignorance, but wouldn't it only take an extra couple of turns on the windings and better bearings? Here's a good example of how easy a high power high efficiency motor can be to build http://www.flyelectric.ukgateway.net/croc.htm .

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