Using RC motors on E-bikes [Archive]

[lawsonuw]

OK guys, go gently as I'm about to propose what might seem to be a crazy idea.............

I can't help but notice that RC brushless motors are light, relatively cheap and seem reasonably reliable (I bought a couple to play with at my local model shop).

On the other hand, big motors, like the Etek, Mars, Perm etc are heavy and expensive, although they do put out more power (on a continuous basis).

High voltage is good for efficiency, but so would a parallel array of low voltage brushless motors. Building on the excellent work you guys have been doing, I've come up with the idea of making a multiple motor array, with in-built toothed belt primary reduction and individual motor controllers, that would emulate something like an Etek, but run from multiple low voltage batteries.

...

I'd recommend using gears for your motor module. I made a geared four motor module for one of my battle bots. Worked quite well and generated exactly the amount of power I expected. Pretty compact, lighter, and cheaper than the single motor I'd originally planned on using. (http://www.sdp-si.com has a large technical section in the .pdf catalog. Lots of stuff on gears) Make sure to do some gear strength calculations.

Marty

 

[Fumesucker]

If you're going to use gears then Losi makes 48 pitch Kevlar spur gears in 82 teeth and a couple of other sizes I think.

$4.00, try beating that price in the industrial catalogs.

http://roguerc.com/catalog/TEAM-LOSI-82T-48P-Kevlar-Spur-XXX-T-p-9369.html

You can get matching pinion gears all the way down to 10 teeth for very little money for 1/8" shaft motors, pinion gears over about 15 or 16 teeth can be bored out to 5mm for the larger motors.

You could get an 8.2:1 ratio for one motor for less than ten bucks.. And only another five or so for each additional motor pinion gear.

 

[Jeremy Harris]

Thanks for all the really useful info, guys, and sorry for the delay in responding (I'm a few timezones away from most of you - you guys are all posting whilst I'm sleeping.........).

I agree about not understanding the multiple motor power increase issue, but think that there might be a clue in the way the propellers (and rotors) absorb power in proportion to the cube of the speed. Matt may well have nailed it with his comment about the helo having all the power it needed with one motor.

I agree that gears would be a good, efficient, option, but they would complicate the design a bit, by needing lubrication and might also be a bit noisy. I can get alloy HTD pulleys in the right sort of sizes and in small quantities easily enough (although they are a bit expensive - around $15 to $20 each) from a local retailer here in the UK, so will probably use them, just because they will be simpler to integrate into a prototype.

Looking at that crazy 24 chainsaw motor drag bike has made me think about changing the configuration I was thinking of, by turning it through 90 degrees. I'd originally thought of building a modular motor that would replace something like an Etek, Agni, Perm etc, with an output shaft in the centre of the motor array. However, with belt drive a linear configuration might make more sense, as long as I can keep the belt runs sensible in terms of losses.

I think I'll start off with a double motor module as an experiment, and try to get some hard data on it's performance, then look at scaling it up depending on the level of success (or otherwise) I have. If the double motor with belts system works, then I might revisit the gearbox idea. There is a certain attraction in having a sealed, circular, gearbox module, to which maybe half a dozen motors could be arranged around a central output shaft.

I'm pretty amazed at the price/performance ratio for this stuff though - I can easily see the attraction in using it. Thanks again for the ideas and comments.

Jeremy

 

[eP]

The last time I asked anyone was 5 or 6 weeks ago. I spoke with Bernie from Castle Creations about my bike and the blown ESC. I asked him if I should just go with two smaller motors and two ESCs to prevent frying another ESC. His response was--------- "Running double motors typically nets a 25% gain versus one motor. So, that is normally not the best option unless there is no other way to gain more power." At that point, I remembered all my past experience with double motors.

Hi Matt
I see you have very little electrical background.
The 25% power max gain for double motor setup could be the true only for very deep gear ratio (very high rpm = high Fe_loss to Cu_loss ratio) and too small battery (relatively high voltage sag at power max).

So if you want to double your power max for double motor setup you need double batteries capacity ( Ah ) as well.
And you should keep Fe_loss close to Cu_loss by right gear ratio choice.

That way (described above) is possible to get better than 100% power gain for some cases.

Best regards

 

[recumpence]

Let me repeat myself,

From my experience, this is NOT a battery issue. I have always run all my RC equipment with more battery than necessarry. Also, merely going to one larger motor made my power go up (using the same pack).

You are correct that I have little electronic background. However, what I am thoroughly familiar with is Lipo batteries.

Anyway, go ahead and try doubling the motors. I have nothing against others trying it. As I said, this has been my experience with RC equipment. It may, indeed, be different with heavier bikes than RC cars and helis.

I still go back to my original comment about the price point, however. If you were going to spend (say) $100 each on two ESCs, it would be better to spend $200 on one very good one. The same goes for the motor.

But, again, I agree this may not apply to the higher load and longer accelleration time related to bikes versus RC models.

I find this kind of strange; when I built my bike, nearly every e-bike guy out there gave me this reason and that reason why it would not work------ Not enough power, not enough range, etc. When I installed my Plettenberg motor, I was told the efficiency would go down judging by the numbers. Well, my efficincy is up with this motor. I went though the same thing when I scratch built my first tandem rotor helicopter------- "It won't fly! Here are the numbers to prove it!" Well, it not only flew, it flew without any adjustments what-so-ever.

What I am trying to say is, not every variable can be accounted for with simple math. It is extremely common for me to hear engineers say something will not work. But, my experience is not always (usually not) in line with what they predicted.

I have alot of respect for everyone here. However, I also have alot of experience in certain areas that others do not. I do not mind being disagreed with. However, I am not talking out of the side of my mouth, here. I have been manufacturing and selling thousands of RC heli upgrades for years (and complete helis). I went though this for a long time trying to work it out as an item I could market (double motor setup). The fact of the matter is, in each application I tried, the gain way not worth the cost, complexity, weight, etc of merely going to one larger motor.

Now, when double motors are used in various applications, it is worth while primarrily because there is not one motor that will put out enough power for the application, so double motors is the only option, or for the shock value of seeing multiple motors on one vehicle.

I will say it again, yes I may be wrong due to the difference in application for a bike versus an RC model. But, this has been my experience. A power increase will result from twin motors versus one motor. But it is very non-linear.


Matt

 

[Fumesucker]

I still go back to my original comment about the price point, however. If you were going to spend (say) $100 each on two ESCs, it would be better to spend $200 on one very good one. The same goes for the motor.

The point being though that, at the moment, the very good motor doesn't really exist as an item at the present level of development of RC motors with respect to amortized R&D costs..

Not to mention that there is something to be said for redundancy and scalability.

Want more power? Add another motor/controller to the pack..

What I am trying to say is, not every variable can be accounted for with simple math. It is extremely common for me to hear engineers say something will not work. But, my experience is not always (usually not) in line with what they predicted.

Clarke's first law: 1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

Clarke's other two laws also have some bearing here:

2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
3. Any sufficiently advanced technology is indistinguishable from magic.

 

[Miles]

As Matt said, I think I would use the minimum number of motors to meet the power requirement - and if that's more than one, implement some kind of intelligent switching to match, on-the-fly, the torque needed.

 

[eP]

I will say it again, yes I may be wrong due to the difference in application for a bike versus an RC model. But, this has been my experience. A power increase will result from twin motors versus one motor. But it is very non-linear.
Matt

It is not the matter of application kind.
It is the matter of gear ratio and load level.

Let me explain. Let assume case A: one motor setup - overloaded motor 70% efficient (for the sake of heavy overloading) and two motor setup as an alternative 85% efficient each one as they are optimaly loaded.
As a result you get 170% power out versus 70% power out for single motor setup. So your relitive gain is 140% more power out for double motor setup.

However the same kind of dependency could work against you.
Let assume case B: one motor setup optimaly loaded 85% efficient versus two motor setup 70% efficient for the sake of light load.
As a result you get 140% power versus 85% , so relative gain is 65% only for this case.

These are coarse calculations/asumptions but i hope clear enough to show what is the real issue.

Piotrek

 

[eP]

As Matt said, I think I would use the minimum number of motors to meet the power requirement - and if that's more than one, implement some kind of intelligent switching to match, on-the-fly, the torque needed.

Minimum number of motors could mean maximum number of gears, so it may be not an optimal choice. Especially if these gears are based on manual switching.

Two motors could give us more flexibility then 3 gears.

 

[Miles]

How about 2 motors and 2 gears, then? 8)

 

[eP]

How about 2 motors and 2 gears, then? 8)

One gear for each one ? :wink:

Edit
Keep in mind Miles two motor setup is intrisically more powerfull than single motor one.
And more power available means less gears we need (smaller span at least).

 

[Miles]

Keep in mind Miles two motor setup is intrisically more powerfull than single motor one.

How so?

 

[John in CR]

Matt,

I don't doubt your real world results with multiple motors, however, I believe there are advantages here that weren't applicable to your use in copters. It seems to me that the primary advantage is in the ability to use only one motor or both. I don't see any benefit to running multiple motors all the time unless you have a specific space where multiple small motors can fit, but one larger one can't. I've got some tubular motors for projector screens, so maybe multiple small RC motors on a common driveshaft inside a frame tube might be worth trying.

John

 

[eP]

Keep in mind Miles two motor setup is intrisically more powerfull than single motor one.

How so?

You could buy more watts (for motors and ESC) for the same buks.
Two motors are able to dissipate more heat than single one as they have more surface (i assume they are both the same level efficient).

In shortest words two motor setup is for people whos want more (power/speed/acceleration).

 

[Miles]

Ah, ok.

Smaller motors are generally less efficient, though...... You'd have to use your 2 motors intelligently, to cover this...

 

[Fumesucker]

You could buy more watts (for motors and ESC) for the same buks.
Two motors are able to dissipate more heat than single one as they have more surface (i assume they are both the same level efficient).

In shortest words two motor setup is for people whos want more (power/speed/acceleration).

Yep, the path for the heat to exit a small motor is shorter and the surface area to volume ratio is greater than with a large motor.

For long, high power running you would probably be better off with two smaller motors. For quick bursts of power, the larger thermal mass of the single motor will absorb more heat but it also releases that heat more slowly.

It does seem that larger motors are somewhat more efficient than smaller ones of similar design.

But the Devil is in the details, as always.

 

[Miles]

Yes, we need specific examples, now.

 

[eP]

Ah, ok.

Smaller motors are generally less efficient, though...... You'd have to use your 2 motors intelligently, to cover this...

I don't propose two motor setup for the sake of small diameter or weight.

Two motor setup is the best choice if someone want avoid super duper 200A amps ESC or the same level expensive 5-6 kW large outrunner.

Each one smaller motor is enough efficient for such case (two motor setup).
And you still has the choice if you want to use both of them at once or only single one if one is enough for particular load.

That is the flexibility what i'm talking about.

edit
And keep in mind AI is cheaper now than ever before.
And we don't need very powerful AI to switch between them effectively in fact.

 

[eP]

Yes, we need specific examples, now.

So lets back to the numbers. What is the cost of Matt's Predator ?

What would be the cost of two (40 mm class ) equivalents in power ?
The same questions are valid for ESC too.

 

[Miles]

Yes, we need specific examples, now.

So lets back to the numbers. What is the cost of Matt's Predator ?

What would be the cost of two (40 mm class ) equivalents in power ?
The same questions are valid for ESC too.

I think the best comparison would be to only choose motors of different classes from, say, the AXI range - or the Plettenberg range, if you want....

 

[johnrobholmes]

One 2215 nuemotor is $500. Two 1910 motors is $360. Same power out roughly. This only works if the dual motors have smaller stator diameters than the single, as you can cut cost on the lamination's greatly with smaller diameter motors. If you stayed within the 22xx and compared a 2215 with two 2207's you wouldn't find a cost savings.


The cost of controllers eats the savings though. If we assume that a controller can be found for the larger motor, two controllers of half power are generally more expensive IF the larger controller has been mass produced and isn't a small run item.

 

[Miles]

Do we try to match Kv values?

 

[eP]

Yes, we need specific examples, now.

So lets back to the numbers. What is the cost of Matt's Predator ?

What would be the cost of two (40 mm class ) equivalents in power ?
The same questions are valid for ESC too.

I think the best comparison would be to only choose motors of different classes from, say, the AXI range - or the Plettenberg range, if you want....

I don't think so.

If you need really extreme high power - each one choice is very expensive as thay are all niche targeted expensive products.

But if we are going to two motor setup than we are much wider options spectrum. And many choices are really money effective.
For mainstream products are many brands available and the prices are more friendly as competitive pressure is higher.

So we should compare the different brands products as long as they are the same kind reliable.
BTW Preadator is not recommended as a choice for personal vehicle motor at producer site. So i don't think it is much more reliable then other reliable RC
motors.

One 2215 nuemotor is $500. Two 1910 motors is $360. Same power out roughly. This only works if the dual motors have smaller stator diameters than the single, as you can cut cost on the lamination's greatly with smaller diameter motors. If you stayed within the 22xx and compared a 2215 with two 2207's you wouldn't find a cost savings.

For e-bike purposes ultra fine laminations are not so critical as for RC ones.
We don't want rpm in 20k+ or 30k+ range.

The cost of controllers eats the savings though. If we assume that a controller can be found for the larger motor, two controllers of half power are generally more expensive IF the larger controller has been mass produced and isn't a small run item.

Controllers are different story.
They are so expensive as long as they are related to expensive niche motors.
They could become much cheaper as their parts are cheap now - if many e-bike folks would need them.
If most of guys would perfer two motor/ESC setup so twice of them would be wanted.
As a result prices should come down shortly.

 

[Miles]

Controllers are different story.
They are so expensive as long as they are related to expensive niche motors.
They could become much cheaper as their parts are cheap now - if many e-bike folks would need them.
If most of guys would perfer two motor/ESC setup so twice of them would be wanted.
As a result prices should come down shortly.

You can't have it both ways Either it's a snapshot of the present situation, or not....

I guess there's not quite as much scope for large motor prices to reduce as there is for controllers, but surely...

 

[eP]

You can't have it both ways Either it's a snapshot of the present situation, or not....

I guess there's not quite as much scope for large motor prices to reduce as there is for controllers, but surely...

If someone don't want wait so he/she could start with less powerful but avalable now ESCs (at decent prices).
But if more folks would be interested in this option than cheap homemade ECS is the same way possible in near future as Gary's BMS.
That is much easier manageable than decent homemade motors manufacturing i'm sure.

My point is: we don't need care about current high power ESC prices.
Instead we should be aware how they (prices) could be low if we really want them (high power ESC).
So we should be aware how cheap could be homemaded e-bike the same level powerful as Matt's one but based at cheap enough components.
I'm sure it could be much much cheaper than $5000