Electric Bikes - Why not use lightweight R/C aircraft motors?

Hi

I notice that all hub motors weigh a massive 1-4 kg. Could someone explain why people don't gear down high revving motors used by the model aircraft world?

For example, you can buy cheap 800W motors that weight only 280g.

I'm looking to develop a motor assist & energy recover system for my bike. For my slightly odd application, I'll only need bursts of power, say 1-2 seconds at a time. Would these lightweight motors handle this load?

Thanks in anticipation.

Power is not the only specification of a motor. It has more than just volts and watts. For example torque is an aspect you need to consider and the higher the rpm, the lower the torque, given no change in power.

Here you have all formulas for motor : http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

And here is something similar to use an rc motor on a bike and it seems more complicated than you probably think : http://endless-sphere.com/forums/viewtopic.php?f=28&t=8503

I think cooling has to be considered too. Bikes are relatively slow moving compared to r/c projects, much less air flow, hence the need for larger surface areas to dissipate said heat.

They don't have the torque to move a human and they're pretty stretched to the limit already on a rc car. I used to playwith rc cars and in 5 minutes of running they are already hot to the touch (can't even touch the heat sinks). Plus the good motors are over like a hundred bucks already you'll probably kill it in 30 minutes on a bike.

Thanks everyone.

I can't see torque being an issue since Power = force (torque) x velocity(rotational vel.). Two motors of equal power but with totally different torque ratings can be made equivalent through a gearbox. I'd lose a bit of efficiency through a reduction 'box, but it's a price worth paying for a motor at 10% the weight.

Interesting point on the heating. My application of sparodic bursts should give ample cooling time. Given that these motors are so tiny, it should surely be fairly simple to fit a decent heat sink around it just in case.

Thanks for the link about the chap planning to use an R/C motor.

As mentioned previously, torque is regained through gearing so this is a non-issue. I wouldn't hesitate to try this. However...

A bigger issue with these motors is reliability unless care is taken in selection and use. Consider bearing life for these high-speed motors. You may have to de-rate a bit thermally for this application unless duty is quite intermittent. Consider also where the motor will be operating on its efficiency curve. For example, too small a motor running at a high current and/or at too low a voltage will get hot very fast as it will be operating above its peak efficiency.

RC motors work and they can hold up; but like all other motors you have to use them the way they were designed to work.
The link provided above will take you through months of developement that Matt Shumaker has done to adapt a high power RC Helicopter motor to his recumbent bike. It (finally) works beautifully.
He had motor/controller compatability issues, lots of development gearing it down (it wants to run a 6,000 rpm+), trouble with too abrupt starts (wanting to pull things apart) and other issues.
Bear in mind that he was looking for the holy grail; super low weight, super power, super efficiency.
If you want to go his way you can plunk down a few hundred dollars for his belt reduction unit, $X for what ever size motor you want, $$$$ for a proper contol unit and several $hundred for LiPo batteries and if you get the most powerful (finicky-dangerous) batteries, another few hundred or more for a special charger.
When you look at what he has done the price is reasonable, because he has gone top grade all the way.
I'm thinking in terms of one of his, or an uglier home brewed reduction drive with less ambitious motor, controller & batteries, but with some of the principals he has used, which are are good.

I wonder the same thing. Why aren't a few people using german made motors? I agree that chinese motors are probably the most cost effective, but dang! what ever happened to high quality?

I think most people getting into an electric bike first time are afraid to invest too much into them, until they see how they work out. Then when they have problems or disappointing performance they give up on the whole idea.
Enthusiast who pay high dollar for a bike are spending all they can afford on something lightweight and don't want to add batteries to weigh them down.
-Andthe dept store sell a lot of cheap bikes. The high end stuff is not where the biggest market is.
the German motors I have seen are an improvement in quality and reliability, but not in performance or versatility for what I want.
frankly I don't have much money these days, but I will do without rather than buy junk (although I did buy an e-bike a while back that is not what I would have bought had I known better)
I don't see an add-on system that is quite what I would build for myself out there. I have been looking at various systems for a few years, but not seen quite what I want. Given the time and a bit of $ I will be building a system for my road bike, or preferably, for a trike.

A very high rpm motor does have a very good power to weight ratio. The gear reduction box that would be needed to make it usable on the bike would pretty much ruin the power to weight ratio.

Here's an idea. Just mount the motor with propeller on the back of your bike! :)

Why not use that high rpm motor with a friction drive?

Because you would probably shred tires very quickly.

A friction drive generator is hard on tires and you are trying to make 10 watts or so. Imagine what happens when you try to put 300 watts through that interface.

Not to beat this to death, but Matt Shumaker (not an aquaintence, friend or business partner) has solved most of the above problems very well. (except for cost, and I think that is not too bad when you look at what is out there.)

Weight; his single stage reduction unit is less than two pounds and you can mount motors weighing from around 10 ounces to over two pounds for a range over of just over 2 1/2 pounds to a bit over 4 pounds - very light by comparison.

cost; 1 stage reduction unit something over $200 if I remember right ($225?) Motors on the net $70- $800. Many of his customers are running the Astro 3120 which can put out 3-4Kw and costs $400. Batteries - the sky is the limit.

One guy had results from a DF bike with what he called a "cheap" motor and I think LiPo batteries from Ping Battery. He was clocked on radar at 43mph IIRC.

When I can, I intend to make something similar, maybe more DIY to save money. Matt posted that he would sell components of his drive kit for folks who wanted to build their own.

I would recommend that anyone who wants a light weight but powerful drive at least skim the blog. this gut has done a lot of development work that you might benefit from. http://www.recumbents.com/WISIL/shumaker/edrive.htm this is his site about his kit, but does not have the latest info. you can e-mail him or follow the link to the LOOOOOONG blog. (74 pages as of today.)

Respectfully submitted.

CB

In your original post, you say you will only need bursts for 1-2 seconds. How is that? What are you planning to do with 1-2 seconds of power?

Not to beat this to death, but Matt Shumaker (not an aquaintence, friend or business partner) has solved most of the above problems very well. (except for cost, and I think that is not too bad when you look at what is out there.)

Weight; his single stage reduction unit is less than two pounds and you can mount motors weighing from around 10 ounces to over two pounds for a range over of just over 2 1/2 pounds to a bit over 4 pounds - very light by comparison.

cost; 1 stage reduction unit something over $200 if I remember right ($225?) Motors on the net $70- $800. Many of his customers are running the Astro 3120 which can put out 3-4Kw and costs $400. Batteries - the sky is the limit.

One guy had results from a DF bike with what he called a "cheap" motor and I think LiPo batteries from Ping Battery. He was clocked on radar at 43mph IIRC.

When I can, I intend to make something similar, maybe more DIY to save money. Matt posted that he would sell components of his drive kit for folks who wanted to build their own.

I would recommend that anyone who wants a light weight but powerful drive at least skim the blog. this gut has done a lot of development work that you might benefit from. http://www.recumbents.com/WISIL/shumaker/edrive.htm this is his site about his kit, but does not have the latest info. you can e-mail him or follow the link to the LOOOOOONG blog. (74 pages as of today.)

Respectfully submitted.

CB

Something else to consider, is John Tetz's philosophy of providing just enough assist to keep your heart rate out of the red-line zone. His RC Outrunner motor thru a 19:1 planetary gearbox is noisy, but it's extremely light, and gives short bursts of boost needed to get up hills.
http://www.recumbents.com/WISIL/tetz/E-AssistMetric/default.htm

Following Tetz's philosophy get you exercise, which most of us do need...

Another hardware option for a partial assist, is some motor geared down by the DeWalt XRP 3-speed planetary drill transmission, with low gear being 45:1 reduction. The FIRST robotics teams use these, but frequently strip out the low range gearing.

See what you think... http://www.chiefdelphi.com/media/papers/1592

The only real problem here is hub motors are not produced on an industrial level that allows for more research and developement or lower cost. 98% of them are from china as well. Geared hubs are all total trash and last less then 200 miles on average before they spider out. Non geared hubs are generaly heavier but they do distribute the pounds well as they are in the center of the bikes gravity. Standard chain drive kits are messy and offer more chances for bodily harm. They require alot more upkeep as well. I dont think if this industry ever takes off you will see a large amount of chain drive kits. It will be no upkeep gearless hubs. They are zero noise and zero injury risk.