Using RC motors on E-bikes [Archive]

[TylerDurden]

Yeah, we discussed the croc and other stuff, jeeze .... about a year ago. Suitable laminations are an issue IIRC.

Another soultion is a dual-motor setup in series: high torque motor for acelleration, high efficiency for cruising.

A HTX is about 55 bucks and puts out good power:
2400W 6000rpm (noload) @ 24V

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5144

...for 150 you can get a 6500W HTX @ 130KV
3100rpm @24V

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5142

 

[dirty_d]

WOW that thing is a beast and the price is amazing, in the specs is shows 32ohm for resistance but they must mean milli-ohms, im gonna plot it and see if the specs seem real or not.

about the mechanical stuff, i think it would be doable to go with a one stage reduction(if you don't count the freewheel and chain). you could use a 12t GT2 timing pulley on the motor and then a 60t pullet on a .5" jackshaft along with a bmx freewheel. the first stage will give you a 5:1 ratio, then you could easily go the rest of the way with the sprockets, you only need a 4:1 there to get 20:1 overall. you could use a bigger timing pulley if you need, the 60t on is only 3.7" in diameter. the only problems i see for someone without machining equipment is making the 1.125" holes in the frame for the .5" shaft bearings, you would probably need a specially drill like one with 3 flutes instead of 2 so it centers itself. large diameter 2 flute drills drill horrible holes in thin material(at least thats my experience), i think because the point needs to be pushing into something to stay centered. you might get away with a metal cutting hole-saw, but i think the tolerance on those is horrible, the bearing would likely not fit in the hole or be too loose. and the other problem being mounting the freewheel on the .5" shaft, i saw an adapter but its for 5/8" shaft. also you need a way to secure the larger timing pulley to the shaft. they have 2 set screws, but i don't think those would cut it you would likely need a key way cut into the pulley.

maybe some other people have some ideas or know of parts to make this more buildable without much more than a drill press saw and maybe a welder.

 

[dirty_d]

basically the same as the AXI motor except you get 4300W at 90% efficiency at 5500rpm instead of around 8000-9000rpm for the AXI.

 

[safe]

Efficiency Argument Often Shallow

A defender of the "Big Iron" will often cite that the efficiency of the little RC motors is diminished by the need for the extra geardown to make it work.

"Well gee, if you have to reduce 92% by 2% and yet again by 2% you are just suffering so bad..."

...the only problem with this logic coming from someone who has become accustomed to carrying 25 lbs of metal in their rear wheel is that there is no NEED to carry all that weight. Had the bike owner bought 15 extra pounds of batteries instead of the "Big Iron" that would also expand the range and power possibilities.

So getting "real" if we take:

25 lbs of "Big Iron" @ 90% efficiency + 20 lbs of batteries

...you could have instead:

10 lbs of RC motor @ 92% -2% -2% = 88% eff + 35 lbs batteries (that's 75% more battery and about the same extra range for the same weight)

...and if you add gears you get:

25% overall efficiency improvement
25% overall power improvement
33% less heat

(this is when you take into account that all riding is not steady state and you have to go up and down through the rpms all the time)

So in the end the advantages to the "Swiss Watch" RC motor will outweigh any negative criticism by a large margin.

It's the "smart" solution... however... for the moment I'm also at a loss for how to fabricate it as a "do it yourself" project... you need machining tools like Recumpence has. I have a welder and a drill and a grinder and can get some reasonable precision... so if I'm clever I might try it... in fact I can say with near certainty that this is the next motor I'm buying.

 

[Drunkskunk]

Efficiency Argument Often Shallow

A defender of the "Big Iron" will often cite that the efficiency of the little RC motors is diminished by the need for the extra geardown to make it work.

Only Safe would take a discussion as a personal attack, and make inflamitory statements like that.



Safe, here's your gentalman's chalange: Go build it. No tech problem listed here is insurmountable. It should be doable. Go do it.

 

[Dee Jay]

Yeah, we discussed the croc and other stuff, jeeze .... about a year ago. Suitable laminations are an issue IIRC.
Another soultion is a dual-motor setup in series: high torque motor for acelleration, high efficiency for cruising.

This had been my vision all a long. Dual light weight motors (or even several) acting as gears!

J

 

[Miles]

(or even several) acting as gears!
J

Ugh!

 

[Miles]

"Well gee, if you have to reduce 92% by 2% and yet again by 2% you are just suffering so bad..."

safe,

We simply don't have the data, at the moment. 2% + 2% seems wildly optimistic.....

 

[dirty_d]

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5141&Product_Name=HXT_80-85-B_170Kv_Brushless_Outrunner_(eq:_70-40)

about that motor, on the bottom i don't know what they are talking about with Rm = 0.097 Ohms, the resistance is 0.037 Ohms, Rm is the symbol for the motors speed regulation constant in units of (rad/s)/Nm. Rc sites seem to never use standard specs, Kv is supposed to be in units of V/(rad/s) and Kt Nm/A, which when used with those correct units are the same numerical value. rc people use rpm/V.

 

[Miles]

They do 4 versions : 6t & 8t and 85 & 100 lengths (40 & 55 stator lengths):

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5141&Product_Name=HXT_80-85-B_170Kv_Brushless_Outrunner_(eq:_70-40)

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5140&Product_Name=HXT_80-85-A_250Kv_Brushless_Outrunner_(eq:_70-40)

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5139&Product_Name=HXT_80-100-A_180Kv_Brushless_Outrunner_(eq:_70-55)

http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5142&Product_Name=HXT_80-100-B_130Kv_Brushless_Outrunner_(eq:_70-55)

 

[Drunkskunk]

Yeah, we discussed the croc and other stuff, jeeze .... about a year ago. Suitable laminations are an issue IIRC.
Another soultion is a dual-motor setup in series: high torque motor for acelleration, high efficiency for cruising.

This had been my vision all a long. Dual light weight motors (or even several) acting as gears!

J

Didn't NASA do that with a Mars Rover? The design I remember made for infinatly variable gearing up to 1:1. I remember how it worked, but can't remember if they ended up using it.

 

[dirty_d]

like this thing? this is pretty cool, it uses 2 motors and a planetary gear set to make a sort of motor/CVT unit. http://www.solomontechnologies.com/wheel.htm

 

[Miles]

like this thing? this is pretty cool, it uses 2 motors and a planetary gear set to make a sort of motor/CVT unit. http://www.solomontechnologies.com/wheel.htm

Neat.

Similar idea to this: http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=US5242335&F=0

 

[Drunkskunk]

like this thing? this is pretty cool, it uses 2 motors and a planetary gear set to make a sort of motor/CVT unit. http://www.solomontechnologies.com/wheel.htm

Exactly!

Fitting that in a bike hub and making it strong enough to survive what most of us put the bikes through would be a real feat, but should be possable.

 

[The7]

like this thing? this is pretty cool, it uses 2 motors and a planetary gear set to make a sort of motor/CVT unit. http://www.solomontechnologies.com/wheel.htm

This method of CVT is used by Toyota in their "Hybrid" Camry and Prius. Toyota calls them "Snergy" drive. The high power motor is ICE and the other low power motor is electrical.

 

[John in CR]

... in fact I can say with near certainty that this is the next motor I'm buying.

For the price per watt it will be difficult to resist. I even have some aluminum housing wheelchair gearboxes that are 24:1 reductions that would be very simple to mate with the output shaft of the motor. Too bad the larger RC motors don't have cooling blower attachments as options like some of the smaller motors. It might be easy enough to trim an RC prop to fit in a cylindrical housing as a makeshift blower though. For me the real appeal is that the small size and weight makes placement easy. I have a 4.5hp motor that I'd already have on my bike, but it's just too heavy to find a good place without major frame modifications.

John

 

[dirty_d]

doe anyone know anything about this motor?

does that prop mount thing come off easily, and can the shaft be loosened and moved back and forth so it would be easier to mount something on the mount side? i cant see the other side of the shaft, is it just round or is there a flat spot on it so you would mount a sprocket on it?

 

[TylerDurden]

doe anyone know anything about this motor?
does that prop mount thing come off easily, and can the shaft be loosened and moved back and forth so it would be easier to mount something on the mount side? i cant see the other side of the shaft, is it just round or is there a flat spot on it so you would mount a sprocket on it?

The spinner comes off with a hex key. I highly doubt the shaft can move fore/aft.

I don't recall any flats being mentioned/shown. (It has been awhile.) other models may provide clues.
:?

 

[Miles]

It would be better to take the drive off the other end, anyway, I think.

 

[dirty_d]

actually i don't think the shaft not having a flat would be a problem, with a 65A motor current limit you get a constant torque until about 90% no-load of 5 Nm(3.7 lb/ft) that's nothing. so just the set screws on a round shaft should do it. as for the current limiting, it might be tricky. if you use an MCU to feed the pwm throttle signal there is going to be delay between when you measure the current and when you apply the calculated PWM signal to the throttle so i don't know how well this would work, there would probably be some bad oscillation or something. im trying to find a cheap 70-100A ESC that can take 50V, there are a lot that meet the current requirements, but most are only rated for 20V.

Miles, what do you mean, you need to put the pulley on the end with the mount on it, there's too much of a load if you use the prop mount side. that whole thing needs to be free floating because it all spins. maybe i misunderstood you.

 

[Miles]

Sorry, dirty_d. I slightly misread your post.

Anyway, I was agreeing with you, the drive has to come off the mount end because of the radial force (which you don't get with a prop). It should be possible to move the shaft through, but otherwise just cut it off - looks like there's plenty at the mount end.

 

[Jeremy Harris]

A quick question aimed at those of you who have some experience with these motors.

One of my other hobbies is paramotoring and I've been watching the electric PPG developments for the past year or so with interest. I built a PPG trike a couple of years ago (a very light composite thing, it weighs just 9kg without the paramotor). Two of these motors would be just enough power, the primary requirement is to get enough thrust for take off; cruise power is only about half take off power. My present PPG weighs about 30kg when fully fuelled, so it's not too far adrift from the sort of weight of an electric unit. The price of these big motors has come down to the point where it now seems viable to buy a couple to experiment with.

One option would seem to be to connect two motors together in series, by aligning their shafts axially and radially and fixing them with a solid connection. An HTD belt reduction of around 2.5:1 would drive the prop, pretty much like the drives we use already with petrol engines.

Am I right in thinking that I might be able to get away with a single controller, provided that I aligned the motors correctly, so they were both "in phase"? I can't see an obvious reason why this wouldn't work, other than the challenge of finding a suitable controller.

Thanks in anticipation.

Jeremy

 

[Miles]

Hi Jeremy,

This is a question that comes up quite a lot on the RC forums, might be worth searching there.
eg: http://www.rcgroups.com/forums/showthread.php?p=6736594&highlight=two+motors+one+controller#post6736594#

 

[Link]

You could tie the axles together, but you'd have to be very careful about doing it. You'd be walking a razor thin line between it working and blowing on you. Not something I would want to try myself. :?

 

[Miles]

Is there really a need to lock them mechanically, anyway? If you don't, it should work ok. Take the outputs through 2 synchronous belts to a common shaft via overunning clutch bearings...