Using RC motors on E-bikes [Archive]

[safe]

The Ideal Electric Mountain Bike

Is there ANY disagreement?

The Ideal Electric Mountain Bike will have the following properties:

All the best suspension that already exists.
The best materials... carbon fiber frame, etc.
The top of the line battery.
A light weight RC sized motor that revs to high rpm and delivers big power.
Geardown of the motor and connection to the derailler system so that the motor power and pedal power all operate through the same transmission system, thus allowing the motor to have "gears" as well as the rider.

...can anyone think of anything to make it any better?

Or can we declare the spec "finalized"?

So far no one has achieved the full spec... (who will be the first?)

As a way to pass the "test" for real world mountain biking you should try to race the same downhill that the regular downhill riders race on. If your bike breaks (or is significantly slower than non-electric) you fail, if you set or approximately equal present times you succeed. This is the true test of success because you need to be able to prove that the electric really is competitive.

And if you really want to impress the downhill racers you could race UP the downhill after the race is over just to show what an electric bike can do.

(make sure the track is clear of course)

 

[johnrobholmes]

I just don't like using the drive gear for pedal and E power. Although I really want to have changeable gears, I haven't yet thought of a way to accomplish this without driving the motor to the front sprocket with a freewheel on the output of the motor gearbox. There would be a bit of parasitic drag, but worse yet is that the motor forces the pedals to rotate.


The other option is a multi speed built into the motor reduction unit. Crappy part about missouri is that we can't have manually changed gears

 

[safe]

Crappy part about missouri is that we can't have manually changed gears

Well you can't have a "clutch". Multiple speed gears are okay, but they need to be shiftable without a clutch and a bicycle derailler is clutchless so it's okay.

I think the problem is that people have thought power through the pedals the wrong way. The "drive" gear should definitely be connected to the motor. So the motor to rear wheel should be fairly direct. It's the pedal to the motor that should be freewheeled... I've never seen anyone try that and yet it's the most logical way to do it.


And let me add that the swingarm pivot location and the motor could be the same spot so that long travel suspension has less chain slack.

We care most about motor efficiency... the pedaling efficiency is of secondary importance on an ebike.

 

[johnrobholmes]

I am thinking about using standard Bike parts. Front freewheels are not common, except for trials bikes (which are limited to 18t mostly). Your third diagram would probably work best, and the limiting factor will be chain line and the strength of the freewheel. You would need two freewheel mech's however, and a way to link them together and allow the motor to still freewheel. Lots of thinking.

Ignoring common parts:
If the first diagram were changed to include a freewheel on the motor too it would be the simplist way I think. This way the motor could freewheel apart from the pedals, the pedals could freewheel when the motor or wheel moved faster, and the motor power could still transfer through the driveline.


At any rate two freewheels must be used to link the motor to the driveline directly and allow gears to be changed without having parasitic drag or cranks that are linked to either wheel or motor rotation.


The law in missouri states automatic transmissions only for moped/bicycle consideration: http://dor.mo.gov/mvdl/drivers/faq/general.htm#q16 IF you know of a resource for e-bikes let me know.

"Any two-wheeled or three-wheeled device having an automatic transmission and a motor with a cylinder capacity of not more than fifty (50) cubic centimeters, which produces less than three (3) gross brake horsepower, and is capable of propelling the device at a maximum speed of not more than thirty (30) miles per hour on level ground."

 

[Miles]

We care most about motor efficiency... the pedaling efficiency is of secondary importance on an ebike.[/color]

Speak for yourself, sunshine. We aren't all designing a motorbike for teenagers....

By the way, your "direct pull" prejudice is not worth the firing of a single neurone

 

[Miles]

John,

Did you see this thread?: http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=4930

 

[safe]

You would need two freewheel mech's however, and a way to link them together and allow the motor to still freewheel.
The law in missouri states automatic transmissions only for moped/bicycle consideration: http://dor.mo.gov/mvdl/drivers/faq/general.htm#q16.

"Automatic Transmission" is the opposite of "Manual Transmission" that requires a "clutch". As long as your transmission does not use a "clutch" it is automatically shifting without the need of a "clutch". Make sense? I can't imagine any other interpretation for that... they put that in place because children have a hard time dealing with the "clutch" and so that's why they added it. It makes sense on gas powered mopeds.

As for the two freewheels... you don't NEED the freewheel for the motor, but you do need to allow the pedals to not spin when the motor runs for safety reasons. If the batteries dies and the motor was unable to operate you could still pedal through the BackEMF and get home. A double freewheel would be good on a true production bike, but for a home built you could easily skip it. (I plan to)

 

[safe]

Speak for yourself, sunshine. We aren't all designing a motorbike for teenagers...

Like I just said on the previous posting, on a true production vehicle sold for the masses you would make that special two way freewheel mechanism so that everything is perfect, but for the practical aspects of home built machines a single freewheel that favors the "Direct Pull" system makes more sense.

When a chain pulls directly from motor to rear wheel it's known to be about 98% efficient. If you have to spin a bottom bracket under load you are adding much more friction (pressure) than if the same bearing is unloaded and spun with excess chain slack.

"Direct Pull" seems the way to go... less friction...

On my bikes you don't always pedal and so it makes the most sense, so for me it's a "no brainer" decision, but for someone with a mountain bike I can see having doubts.

One Argument:

My argument would be that if your motor poduces 750 watts of power and your pedaling produces maybe 200 watts (average for typical non-athelete) that the frictional losses are best dealt with on the larger power number than the smaller. You have more to lose with the motor being inefficient than the pedals. (in actual power)

 

[Miles]

Like I just said on the previous posting, on a true production vehicle sold for the masses you would make that special two way freewheel mechanism so that everything is perfect, but for the practical aspects of home built machines a single freewheel that favors the "Direct Pull" system makes more sense.

The decision as to what pulls "directly", depends on the needs of the design, which is certainly not a given. Therefore what is "good" or "bad" will also vary.

When a chain pulls directly from motor to rear wheel it's known to be about 98% efficient. If you have to spin a bottom bracket under load you are adding much more friction (pressure) than if the same bearing is unloaded and spun with excess chain slack.
"Direct Pull" seems the way to go... less friction...

Yes, there will be a small difference. Have you tested/quantified it? After all you're telling us the way to design things based on it.

My argument would be that if your motor poduces 750 watts of power and your pedaling produces maybe 200 watts (average for typical non-athelete) that the frictional losses are best dealt with on the larger power number than the smaller. You have more to lose with the motor being inefficient than the pedals. (in actual power)

The total losses are not directly proportional to the torque throughput, therefore the difference may be less than you think. Also, as above, it depends what you consider the primary drive.

 

[safe]

The total losses are not directly proportional to the torque throughput therefore the difference may be less than you think. Also, as above, it depends what you consider the primary drive.

"Primary Drive" is the one that produces the most power... at least that's my argument. Since the motor can produce more power than the human any design that makes that most efficient should be taken.

How could the losses ever be less if you did it the other way?

(the "other way" being that the motor has to go through the bottom bracket location)

With a "Direct Pull" design the main power source get's optimized. The secondary pedal power assist has to lose a little more because it has to go through a secondary chain and extra bottom bracket.

Think of it this way... when you are not pedaling the motor does NOT even spin the bottom bracket freewheel and chain (see picture at bottom) when you use "Direct Pull".

I'm just expecting some solid argument that advocates otherwise... :wink:

Describe a scenario where "Direct Pull" was not superior...

...in Option #1 there are two chains and the motor must drive both of them. In Option #2 there is only one chain. In Option #3 there are two chains again, but the motor only needs to drive one of them.

 

[Miles]

"Primary Drive" is the one that produces the most power... at least that's my argument. Since the motor can produce more power than the human any design that makes that most efficient should be taken.

The primary drive is a free choice of the designer.

I don't think the difference in frictional losses between a loaded and unloaded bearing are as significant as you are making out.

As I said above, the greater the torque going through a drive, the greater the percentage efficiency. This partially mitigates the difference in the absolute losses, between the pedals and the motor.

If my design concept was a bike that was pedalled for 80% of the time, the motor being reserved for moderate hills, the decision as to which drive was "direct pull" would also be a "no brainer".

 

[safe]

As I said above, the greater the torque going through a drive, the greater the percentage efficiency. This partially mitigates the difference in the absolute losses, between the pedals and the motor.

That's an interesting assertion. :?

You would figure that the higher the load the higher the friction, but I can imagine that maybe it's more complex than that. As a percentage you are saying that if "X" friction exists at 250 watts the same "X" friction exists at 750 watts in absolute terms, but that as a percentage it's less. (so there is a "startup stickiness", but runtime rotation is good)

1 + 1 = 2

I still have to fall back on the most basic math here... if you can divide the friction between motor and pedals and can allow the motor to use "1" unit and the pedals to use "1 + 1 = 2" units then at least from the motors perspective there is less friction. But if you reverse that and make the pedals be direct drive then they get the "1" unit and the motor gets the "1 + 1 = 2" units.

Applying our 250 watt and 750 watt values and using a value of 10 watts (constant) for losses (just to make it easy) per chain:

Option 1:

Motor Alone - 10 watts + 10 watts = 20 watts
Pedals Alone - 10 watts
Combined - 10 watts + 10 watts = 20 watts

Option 2:

Motor Alone - 10 watts (?)
Pedals Alone - 10 watts (?)
Combined - 10 watts (?)

Option 3:

Motor Alone - 10 watts
Pedals Alone - 10 watts + 10 watts = 20 watts
Combined - 10 watts + 10 watts = 20 watts

...either way it's insignificant compared to other issues like proper gearing. (a 1% efficiency difference in the motor wipes this clean) So on that alone I can see your point, it might not matter that much in the "big picture".

 

[cerewa]

Safe - my bike is a good example of a setup where pedaling being the direct-drive and motor being "secondary" is a good idea. It is the standard cyclone 500W setup except that I left out the crank-freewheel, which would have allowed me to stop pedaling when the motor is operating.

So on my bike, the rider must turn the pedals when the motor is operating but the motor does not need to turn while the pedals are turning. The bike weighs less than 50 pounds with motor and battery included and the motor is only in use 50 percent of the time, while the pedals are always in use except when slowing down or going down a steep hill. The battery BMS does not allow power out of the battery above 480W (24V20A).

 

[Miles]

You would figure that the higher the load the higher the friction, but I can imagine that maybe it's more complex than that. As a percentage you are saying that if "X" friction exists at 250 watts the same "X" friction exists at 750 watts in absolute terms, but that as a percentage it's less. (so there is a "startup stickiness", but runtime rotation is good)

Not quite that.

The data from the testing of derailleurs and hub gears shows that the percentage efficiency rises with the torque throughput. To give an example: a simple chain drive to a 19t cog is shown as 96.4% efficient @ 100 watts but the efficiency increases to 98.4% for a throughput of 400 watts. So, losses increase with load, but not as a direct proportion.

 

[dirty_d]

the force of friction is the coefficient of friction * the pressure between the surfaces, the power generated in the form of heat is coefficient * pressure * velocity, so doubling pressure or velocity should be the same.

pressure is actually just the force between the two surfaces, the surface area doesn't matter. if its a large surface area its a lower pressure per square inch, a smaller area and its a higher pressure per square inch, it all works out the same.

 

[EVTodd]

Ok, this is probably a really dumb question but can you power one of these motors with sla batteries? I'm thinking about picking one up to try some testing with my little friction drive system. I know slas defeat the purpose of having a small lightweight motor but I would like to try it out before plunking down the cash for more expensive batteries.

I would like to find a motor that can spin around 6500 rpms at 24 to 36 volts and provide at least as much torque as my Kollmorgens. Is that realistic? Any motor suggestions?

 

[Drunkskunk]

Ok, this is probably a really dumb question but can you power one of these motors with sla batteries? I'm thinking about picking one up to try some testing with my little friction drive system. I know slas defeat the purpose of having a small lightweight motor but I would like to try it out before plunking down the cash for more expensive batteries.

I would like to find a motor that can spin around 6500 rpms at 24 to 36 volts and provide at least as much torque as my Kollmorgens. Is that realistic? Any motor suggestions?

Voltage is voltage. Motors don't care where the voltage comes from, as long as its the correct voltage.

 

[EVTodd]

Voltage is voltage. Motors don't care where the voltage comes from, as long as its the correct voltage.

That's what I thought but I just wanted to make sure there wasn't something 'special' about these outrunners that would make an sla explode or something.

 

[recumpence]

Hey Guys.

Can any of you motor gurus tell me if my Plettenberg Terminator will run on 60 volts? I have the Kontronic Power Jazz ESC coming for it and that control will run up to 63 volts (15S). But, info on my motor is scarce. I have never found anyone who has run this motor on anything higher than 48 volts.

Any thoughts (judging by the specs)?

Matt

 

[johnrobholmes]

Hey Guys.

Can any of you motor gurus tell me if my Plettenberg Terminator will run on 60 volts? I have the Kontronic Power Jazz ESC coming for it and that control will run up to 63 volts (15S). But, info on my motor is scarce. I have never found anyone who has run this motor on anything higher than 48 volts.

Any thoughts (judging by the specs)?

Matt

Do you trust the motor to keep together at 12k rpm? Will you gear it low enough to keep your final drive speed the same VS 48v? That is a 25% increase in voltage, a pretty big step.

 

[TylerDurden]

Without knowing the kV... I'd say 60V should be fine, if it's an outrunner; the magnets can't fly off.

 

[johnrobholmes]

The bell can warp under speed though, and the bearings may not be meant for the speed. The kv is around 200 if I remember correctly.

 

[dirty_d]

recumpence, maybe you can email them asking for all the detailed motor specs., what would be your goal in using a higher voltage?

 

[recumpence]

The KV is 215. That is no load KV. It runs roughly 8,000 RPM at full throttle under load. I only see 3 volt sag at full throttle. So, that is at 45 volts.

I emailed them. It is a German company. It is tough getting a decent answer out of them.

Anyone speak German here that would be willing to translate my email to them?

Matt

 

[recumpence]

what would be your goal in using a higher voltage?

I just wanted to see how much I could get out of this thing. Part of it is wanting to up my watt-hour capacity without buying 12 new cells (another $435).

Lastly, I would be keeping the ratio the same. So, my top speed would go up from 40mph to 50mph (theoretically) doing this. I have enough power to just gear up for that speed. But, I like the tractability of the gearing I have now. Plus, running at the edge is cool!

No big thing, though. Admittedly, it is plenty powerful and fast enough. But, more is always better! :wink:

Matt