Quesion for R/C motor-saavy people

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I have a Stokemonkey-powered ebike. It works great and is very quiet. An awesome ride; the best part of my day.

I assume "RC" stands for "remote control" and these motors are presumably for models. Some of these motors look very large and apparently pack a lot of power in a small package compared to the larger, slower motors like the Stokemonkey's, which is basically a specially-built Crystalyte 400 series, or so I have heard. So, relative to RC motors, it seems to be large, heavy, and slow turning.

My questions are,
1) if these motors are meant for models, what kind of models need such a large motor that the motor can also be used to power a bike?
2) Also, assuming I am correct about the high power of RC motors, why is the power density (power per weight or volume) higher than something like the Crystalytes or other hub motors? I assume physics plays a role somewhere and probably the higher motor speed is a part of it.
3) Finally, what is the noise level like when implemented on a bike? It seems like, for example in geared hub motors, the smaller and faster the motor turns, the more noise. Seems like there will need to be some pretty severe gearing relative to a hub-type motor.

I really like my bike, esp. the ability to haul a full load of groceries, etc. but I am considering trying to get my kids into ebikes instead of cars as they get close to driving age and a smaller, cooler ride would be more the ticket for them. When I decided on the Stokemonkey, I had it down to two systems, both non-hub commerically available kits. The thing that finally got me off the fence was the nearly complete quiet of the Stokemonkey set up. It is large and looks a bit awkward and non-traditional (i.e., not cool for teens), but it is a hauling machine and is whisper quiet. Did I mention it was quiet? Well, it is.

 

[jmygann]

Quite is nice on a e-bike .. I agree

Is there a QUITE non hub motor ? or is it the reduction gears/chains that make all the noise ?

Any 1000-1500 watt motor to run direct to the wheel sprockets ?

 

[olaf-lampe]

When your kids would only drive an e-bike 'cause it's cool, they won't drive it very long. It's the wrong approach.

Only my 2 cent
Olaf

 

[spinningmagnets]

This particular model is an actual jet, with engines made from turbocharger parts (AMT jets), however, there are piston-engines models this size, and an increasing number of electric models. edit: added other pics...

http://i.ytimg.com/vi/21MyC82OYEU/0.jpg

 

[boostjuice]

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=12881

 

[pdf]

Wow. There are some very big models out there. I hope the people controlling them know what they are doing. Can anyone tell me:

1) why the power density is so much higher than a hub motor? Like 4kW in a 2 lb motor versus 0.5kW in a 10 lb motor for a hub motor?
2) what the noise is like from these high speed motors? Also, can you gear them down without a large noise penalty?

Thanks.

 

[spinningmagnets]

No wasted space inside an RC motor, but also they spin at very high RPMs, and may require some type of gear-down (pulleys/belts...sprockets/chains)

If you look at a hub motor thats been opened up (such as the popular 9C) there is a lot of space around the center with nothing in it. That is so the magnets are moved away from the central axle to give the magnets more leverage (torque per volts applied) since they are turning at such a low RPM. Perhaps an average of around 250-RPMs? RC motors are often configured to be turning near 5,000-RPMs at top speed.

Heres a pic of the inside of a Crystalyte 530X, and also an outrunner

 

[liveforphysics]

Efficiency.

All motors have an efficiency of 0% at 0rpm.

A motors ability to reject heat is almost singularly determined by the amount of air passing over it that is cooler than it's own surfaces.

Lets say a given motor has the surfaces and air speed etc needed to reject 500w of heating.

If a motor operating at 50% efficiency it can handle 1000w (500w of heating)
If a motor operating at 75% efficiency it can handle 2000w (500w of heating)
If a motor operating at 87.5% efficiency it can handle 4000w (500w of heating)
If a motor operating at 93.75% efficiency it can handle 8000w (500w of heating)
If a motor operating at 96.9% efficiency it can handle 16,000w (500w of heating)


RC motors aren't even close to the kings of power density. They are as good as they get for an RC friendly form factor, but ironless axial flux motors blow them away. Like the Launch point motor for example, at 7kw continous at 1.4lbs motor weight, 1" thick, 8" diameter (going from memory on specs, don't quote me).

 

[jmygann]

But in a street legal 1000 watt electric bicycle motor , the most efficient is ??

 

[liveforphysics]

But in a street legal 1000 watt electric bicycle motor , the most efficient is ??

For something anyone can just buy from a retailer for a reasonable amount of money, a well designed RC motor is as efficient as it comes.

If you have no budget constraints, than there is a hub-motor that offers a solution drastically superior to any RC motor. The CISRO hub-motor with it's 98.8% efficiency is pretty tough to beat. How does it do it? Ironless axial flux, just like the launch point motor, only made a hub motor. I think the pricing starts at $20,000usd if I'm not mistaken though (but I think that included a controller?)

 

[jmygann]

But in a street legal 1000 watt electric bicycle motor , the most efficient is ??

For something anyone can just buy from a retailer for a reasonable amount of money, a well designed RC motor is as efficient as it comes.

even with the efficiency losses from the speed reduction needed ?

 

[liveforphysics]

But in a street legal 1000 watt electric bicycle motor , the most efficient is ??

For something anyone can just buy from a retailer for a reasonable amount of money, a well designed RC motor is as efficient as it comes.

even with the efficiency losses from the speed reduction needed ?

Absolutely.

Remember, the efficiency at 0rpm is always zero thing? Gearing lets you climb away from zero rpm as quickly as you please, aka, develop efficiency as soon as you like, and in the speed range you wish to operate (a tranny can further expand this optimized speed range).

For stop-n-go city riding, a hubmotor likely averages <25% efficiency (just a rough guess here, obviously depends on a ton of factors). The same riding conditions met with a properly geared RC motor setup could be >70%.

For just cruising along at a good speed match for the hubmotor, it may reach 60-70% efficiency, the RC setup might be in the 85-90% area if using a single stage reduction.

 

[jmygann]

Okay ... next is to make it quieter

best rc motor ? 50V @ 20amps continuously (1000 watt legal bicycle) ... 40 amps max

to drive 24 in wheel ?

 

[SoSauty]

PDF,

I assume you plan to pass on the $20,000 CISRO For teenager/college age commuters, you might be able to build a reliable RC setup, yet you need to address issues pretty much resolved with hubbies, yet not with RC:

1 matching your ESC or controller to battery and motor,
2 with high efficiency you'll need greater battery performance (higher C discharge), doable yet price climbs,
3 safety and maintenance knowhow related to whirling belts/chains that reduce 5,000rpm down to bike tire revolutions,
4 remember, legal also means <=20mph, a motor big enough to run efficiently would likely easily exceed 20mph, you may want a BionX setup or CycleAnalyst as a limiter.

A BaFang climber(14wind hubbie, $110) using 48V batt and 20A controller may top out just over 20mph (??21-23mph) and have a respectable pull around 15lbs torque 20mph+ or 23lbs at 15mph (enough to overcome wind friction or climb mod hills). Then a BMC T2 might improve on that a tad ('bout $600 hubbie). Reliability should be right up there with a modern car with some basic spoke and bicycle upkeep. Both make a soft whrrr. . on acceleration yet fade to quieter than tire noise at cruise.

In any case, don't underestimate bicycle maintenance. An e-bike ridden 300mi a month needs 4X's the upkeep that a reg bike ridden 150mi a month needs. If you are techy, have the time and interest, you could build the RC setup. If the kido's are little engineers, they would love the RC

 

[pdf]

SoSauty,

Well, I am an engineer so an not too averse to DIY. However, like many here I suspect, time to tinker is more of an issue than ability. I myself have a Stokemonkey-powered Xtracycle and put on it around 250 miles a month. I only have 1800 miles on it so I don't know the whole maintance picture but so far, it has been pretty minimal. Biggest hassle has been tires; lots of sharp stuff on the roads and bike tires don't have nearly the tread depth of a car. Relative to a hub motor, the only additional problem so far is keeping the smal chain between the motor and the chainring sufficiently tight. It has required re-tightening twice. This takes about 10 minutes so it is not a big deal. I lube the chains about every two weeks and clean them about every month to a month and a half.

My interest here is really two-fold, personal and semi-professional. My oldest is 13, driving is in the near future. I believe that the secret to sustainable transportation is to get people to make that decision before they get a liscence. Once they get behind the wheel, its a hard habit to break. Cars are cool; US culture is nearly car obsessed. Our lifestyles are built around them to a large extent. Even my oldest, whose dear ol dad drives an ebike to work every day, can't help but comment on how cool all the drop-top muscle cars and sports cars are. Olaf's observations aside for a moment, I do think "image" can be an ally or a enemy. As a speaker at a meeting on sustainable energy I attended once said, "Teen-aged boys think of two things, and one is cars.". That is hard to overcome on talk of how much more efficient an ebike is.

The Stokemonkey is a kit build. I am comfortable with that level of tinkering, time-commitment-wise. It is very well-designed and has years of service history and improvements that I can benefit from. I don't want to build from the ground up. I have just been wondering why, if RC motors have so many advantages, there aren't more commercial examples. I can imagine that a commercial build with a tranny and an RC motor and perhaps an internal hub could be very maintenance free.

The Stokemonkey requires the Xtracycle extension to clear the rear-triangle for the motor. In the city where we live, we could move just a couple of miles over, and my kids would have about 4 miles to the high school, and that nearly 100% on a bike trail. However, in high school, what's cool and what's not makes a difference. I'd love to see all the kids arriving at school on ebikes, but such is not the case and I don't see my changing that in the near future. The parking lot is full of students' cars, some of which I can't reasonably afford and I have a pretty good job.

I ride the bike trail that goes right behind the school every day. I am trying to think ahead to what my kids would realistically drive that would give them the independence they crave but be more efficient and sensible than a car for a 4 mile commute. My kids actually adore the Stokemonkey, but I'm wondering if something like looks more like a regular bike wouldn't be a better choice. Yes, "cooler" if you like.

I think the bikes by hightecbikes are getting in the right area, but the motors seem pretty small. I would love to have test driven one, but there is nowhere to test one that I can find. We live in a very hilly area. Pumping up a 20% grade gets pretty old day after day. The Stokemonkey has made it fun again. In the applications my family would see, I don't think we need all the power of a 2kW continuous motor, and as you point out, you have to have a controller and batteries to deal with that amount of power. The Stokemonkey, which I think is rated around 500W continuous, has no problem with the hills even toting a full load of groceries and current limited to 15 amps. At 36 V, it keeps me right at the continuous rating of the motor, which is a very conservative way to go, but it keeps strain on both the electrical and mechanical down. I think where the hills are not large, a hub motor is clearly the way to go. However, the ability to drop the vehicle speed and maintain the motor speed is critical to good hill-climbing performance. Hence my love of the Stokemonkey and similar geared solutions. Just looking for something similar that doesn't require a major bike modification so the 'teens of the world can zip back and forth to school, pool-parties, soccer practice etc. without succumbing to the four-wheeled, 300 hp obsessed culture the rest of us have been sold and bought, hook-line-and-sinker.

So the semi-professional interest comes from my day job. I am a University faculty member and am working with a team of 10 students and 4 other faculty to build and test two light electric vehicles for the application of "near-urban" commuting; that is, less than about 10 miles one way. One is being built as a battery electric and one as a hydrogen fuel cell-battery hybrid. The project is sponsored by the US EPA and a campus sustainability program. I say "semi-professional" because as any faculty member at a research university will tell you, advising undergraduate projects is not the road to greater glory, or even promotion. I'm only doing this one because occassionally I like to get students involved in a project that involves a real world issue with real nuts, real bolts, and real problems. Unlike the hypothetical world to which we usually subject our students, you don't get to start over after each test. It either works or it doesn't and "not working" is not an option in the "real world". Well, put more accurately, its the option that leads to getting fired. The students are working on the motor selection and while I am accomplished in my own academic area, electric vehicles is not that area. I'm always trying to stay current on what's out there and this forum has been surprisingly helpful in that way. Many of the questions I pose, I think I know the answer to, from a theoretical perspective, but as "liveforphysics" tag line states "Experience is a hard teacher. First comes the test, then the lesson." There are many man-years of experience accumulated here which I hope to benefit from.

 

[jmygann]

I say any kind of 1000 watt motor to a SRAM DD internal hub ala Randy / Miles is a winner