# RPMs, Power, Torque.. DC vs AC Questions



## Lauris_K (Feb 25, 2013)

Well for a starter, power(hp, KW) is torque multiplied by rpm, in straight forward way. So low power high torque motor (DC one in your case) will have high torque at low rpm and torque decreases when rpm increase. And (AC one) high power low torque would prefer constant torque for longer rpm range, and only at certain point torque will start to drop.

For meaning of all that in practice in straight forward way, lets consider 3 motor types, AC, DC and internal combustion. all these 3 types have different max torque range, DC is at start, internal combustion more at middle (diesel lower, TD bit higher than diesel, petrol higher than TD) and AC more to equal in all rpm range till max power is reached and then it is starting to drop. So in each case is you will sit on accelerator pedal from start:
DC - massive acceleration at beginning and after that constantly dropping,
Internal combustion - max acceleration at range 2000-4000 rpm, depending on fuel type, motor and many other factors,
AC - around same acceleration until certain point (like 3000 rpm) when it will start to decrease.

this is excluding many real life factors, like aerodynamics, road surface and ect.


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## ruckus (Apr 15, 2009)

Torque (in Newton Meters) X RPM, divided by 9549 = KW

9549 is just a conversion number. But it also means Nm=KW at 9549 rpm.

Conversely, kw X 9549 / rpm = Nm.


Ratings on motors vary wildly. A huge industrial motor designed to work without ventilation may only be rated at 10kw. The same motor in a car with lots of cooling would be rated at 'peak' output (likely only good for 10-30 seconds before melting). It could be rated at 150kw or more. Same motor, different cooling apparatus and duty cycle.

Industrial motors are often rated at a 'continuous' duty cycle. That means 24 hours a day for months or years. 

Each motor type has advantages and problems.

Brushed DC are cheap and produce huge starting torque but have low continuous hp without overheating.

AC induction have lower starting torque, but usually have fairly high continuous hp for their size since they can hit high rpm's. 

Brushless Magnet motors have high torque like the brushed DC AND high continuous hp like the AC, but can be more expensive and more sensitive to overheating.

Are you looking at a particular motor?


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## mbman88 (Feb 17, 2013)

Lauris_K said:


> Well for a starter


BAHAHAHAHAHA. Now that was a good pun. Wait.. you probably didn't intend that, did you?


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## mbman88 (Feb 17, 2013)

ruckus said:


> Are you looking at a particular motor?



I've been looking at motors for a while online.. I always get confused about why certain motors are for certain applications, and I get confused about the ratings which I'm glad you helped clarify if only a little. It seems that the world tends to stick with the manufacturing process in order to obtain their own motors. I wonder how much GE is involved... 
For my Honda Prelude I would obviously prefer the motors that AC Propulsion has available. Tesla Motors, according to my limited research, buys their motors from them.. but they are custom, obviously. AC Propulsion then sells a motor very SIMILAR but not the same. And I think you can only get the motor through having an entire installation done by AC Propulsion themselves. I'm okay with that, since I can modify things afterwards.. although not a whole lot seems needed to modify. 

But as for the cheap route... Everytime I consider it I will get confused by the motors. I DID try looking up forklift motors but it seems that they are difficult to find.. since a lot of forklifts use ICEs (Internal Combustion Engines) that run on propane.. I guess you're supposed to get a motor that operates the lift? A lot of them use hydraulics so that confuses me again. 

The Forklift motor seems like the best idea.. for power and expense... But otherwise, no I am not looking at any specific motor immediately.. except I AM.. but it's for a bicycle... and.. well.. this is an electric CAR forum.. lol


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## PStechPaul (May 1, 2012)

For an electric bicycle you may want a BLDC or PMDC motor, since they have a high power to weight and size ratio and there are many sources for small motors. You probably can use a 500 watt to 1500 watt (2/3 to 2 HP)continuous rated motor for a decent bicycle that doesn't need to achieve high speed or compete in drag races. You can figure out the torque you need based on the weight of the bike and rider and the maximum acceleration (or the maximum grade to climb, which is equivalent). 

If you want to be able to climb a vertical wall the acceleration is 1G, or 9.9 M/s/s or 22 MPH/sec. That's 0-66 in 3 seconds. More realistically you might want to be able to climb a 25% grade, which is about 0.25G. An e-bike might weigh 50 kg and the rider maybe 70 kg, so 120 kg or 264 lb total. On a 25% grade you need thrust (forward force) of about 72 lb. This translates to wheel torque based on the diameter. A 24" dia wheel has a radius of 1 foot, so torque in lb-ft is the same as the thrust, or 72 lb-ft.

Power is only defined based on speed, so holding the bike on the 25% slope but not moving does not use any power. It is only torque, or force. Of course if you use an electric motor or a human rider, exerting this force does use some power because of the losses in the motor or the metabolism of the rider's muscles to maintain the force. Now consider the case where you are moving up the hill at 40 km/hr (25 MPH). I used my http://enginuitysystems.com/EVCalculator.htm to find that this requires about 5 HP. If you expect to encounter such a steep hill that is 2 miles long, you will need that power for about 5 minutes at 25 MPH, and depending on the motor design, it may be able to do that at a peak power level which may be 2-3 times the continuous rating. Thus a 2 HP motor should be sufficient.

Now you also need to figure the gear, belt, or sprocket ratio between the motor and the wheel. The 24" wheel turns at 348 RPM at 25 MPH, and a motor usually has a maximum RPM of about 3600 RPM, so a 10:1 ratio would be good. Now you know that you need 72 lb-ft at the wheel or 7.2 lb-ft at the motor. At 3480 RPM that is 4.7 HP (3.5 kW), which corresponds to the power determined above.

The actual power also includes losses due to aerodynamic drag and rolling resistance, which depend on weight, speed, and frontal area, and I estimated them in my calculations, so actual performance may differ. There are also losses in the mechanical drivetrain, motor, controller, and batteries. But most EVs are about 80% efficient. For this example, the calculator shows energy use of about 150 Wh/mile. If you use 4x12V12Ah SLA batteries, the current would be 3500W/48V=73A. The batteries have a rated capacity of 576 Wh, but using the Peukert calculator the capacity drops to about 4 Ah or 192 Wh, and you should only use about 80% of that, so you would only be able to go about 1 mile at that level of performance.

I hope this helps. It took me a long time to "grok" the concepts of power and torque. Consider that you can get essentially an infinite amount of torque by using enough gear/pulley ratio, as Archimedes remarked that with a lever long enough, and a fulcrum strong enough, he could move the earth. But it would not move very far, or very fast, unless you also have a lot of power.


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## puddleglum (Oct 22, 2008)

mbman88 said:


> But as for the cheap route... Everytime I consider it I will get confused by the motors. I DID try looking up forklift motors but it seems that they are difficult to find.. since a lot of forklifts use ICEs (Internal Combustion Engines) that run on propane.. I guess you're supposed to get a motor that operates the lift? A lot of them use hydraulics so that confuses me again.
> 
> The Forklift motor seems like the best idea.. for power and expense... But otherwise, no I am not looking at any specific motor immediately.. except I AM.. but it's for a bicycle... and.. well.. this is an electric CAR forum.. lol


ICE forklifts don't have anything you can use( only electric motor is a starter motor), you have to find an electric forklift. Unfortunately, they are very rare in many areas, and the ones that are out there, are kept in service for as long as possible. Only certain models have motors the right size for a car as well, and even then, some of those will have shafts that are hard to couple to. I think it would be a good choice for a low budget conversion, IF you can find a good one for a good price. I would do it if the price was right, but otherwise, I would get a good EV motor. So, you are building an ebike first?


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## mbman88 (Feb 17, 2013)

PStechPaul your response was awesome. I will keep my eye on it for reference. thanks a lot

Puddlegum
Yes I am building an E-bike first. What I'm REALLY trying to do is master the art of building electric vehicles cheaply.  lol

Okay... I think I have a grasp of it... after seeing an Isuzu VehiCROSS from hell mudding and plowing through snow in Russia via YouTube... that V6.. (I've seen a comparable Chevy Blazer and Ford Ranger die from these events) it shows me the capability of ANY engine to do the same thing as long as the gearing is proper (or appropriate to the weight of the vehicle). And if it's too heavy it just means it's going to move at a slower rate (possibly too slow to overcome entropy)... hence the importance of power.


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## mbman88 (Feb 17, 2013)

PStechPaul said:


> For an electric bicycle you may want a BLDC or PMDC motor, since they have a high power to weight and size ratio and there are many sources for small motors. You probably can use a 500 watt to 1500 watt (2/3 to 2 HP)continuous rated motor for a decent bicycle that doesn't need to achieve high speed or compete in drag races. You can figure out the torque you need based on the weight of the bike and rider and the maximum acceleration (or the maximum grade to climb, which is equivalent).
> 
> If you want to be able to climb a vertical wall the acceleration is 1G, or 9.9 M/s/s or 22 MPH/sec. That's 0-66 in 3 seconds. More realistically you might want to be able to climb a 25% grade, which is about 0.25G. An e-bike might weigh 50 kg and the rider maybe 70 kg, so 120 kg or 264 lb total. On a 25% grade you need thrust (forward force) of about 72 lb. This translates to wheel torque based on the diameter. A 24" dia wheel has a radius of 1 foot, so torque in lb-ft is the same as the thrust, or 72 lb-ft.
> 
> ...


according to this calculator.. in order to even decently accelerate.. such as from 0-20km/h in 6 seconds, I will need 1.5 hp. My only question becomes "How on earth do electric-assist owners even get ANY assistance with their 300 watt, or less, motors?" Somethings doesn't seem right... am I really going to have an underpowered bicycle if I stick with 1000 watts for motor and battery? I assumed it would be plenty... now I feel the opposite feeling.


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## PStechPaul (May 1, 2012)

Well, I used 150 kg and 0.9 m/s/s and an end speed of 20 km/h and I get 946 watts (1.2 HP). But that is the power at the end of the six second run, so to get average power, use 1/2 the top speed, and you get 460 watts (0.6 HP). 

Mobility assist vehicles (wheelchairs and scooters) are usually used on fairly flat surfaces and are geared down to a top speed of perhaps 8 km/h (5 MPH). At a moderate speed of 5 km/h and a total weight of 150 kg and a 5% slope, you need only 145 watts or 0.2 HP. This also corresponds to an acceleration of 0.5 m/s/s on a level surface or 0-8 km/h in 3 seconds.

My own mini-EV riding mower was able to move me at a speed similar to a mobility vehicle with a power input of 24 VDC at 15 amps, or 360 watts (1/2 HP). It uses a 2 HP 2 pole three phase induction motor and a VFD, so it seems to be fairly efficient. Under those conditions, it is probably only 50% efficient, with 180 watts delivered. So that seems to validate the estimates from the calculator.

You might even consider an AC system for your bicycle. You can get a 1 HP motor for under $50 and VFD for under $100, and a 1000W 24V to 220 VAC inverter for about $50. I have one (a 12V version) which I opened up and I brought out the internal 270 VDC bus, which is perfect for a 220 VAC VFD.

http://www.ebay.com/itm/Reliance-Electric-1hp-3phase-ID-P14H1448S-/170618721650
http://www.ebay.com/itm/Marathon-El...7D2092K-Great-working-condition-/350721142805 (only 25 lb)
http://www.ebay.com/itm/Leeson-1HP-3PH-RPM-1725-1425-Electric-Motor-Model-C6T17FC2B-/140877638741 (This is a C-face motor which may be easiest to mount)

http://www.ebay.com/itm/fuji-vfd-variable-frequency-drive-220-240-vac-3-phase-2hp-/230929761401 (2 HP for $100)

http://www.ebay.com/itm/181102816053 (24V to 220V 1200W inverter 2400W peak, $45)

To get started you could use a couple of 12V 12Ah SLA batteries which are about $22 each:
http://www.ebay.com/itm/UPG-2-Pack-...h-Sealed-AGM-Battery-MK-BATTERY-/130829112225

Of course you will have to be especially careful with 220 VAC and 270 VDC out of the inverter. And an AC system like this won't have regen. But the entire system will only weigh about 60 pounds and these are standard industrial components which are rugged and dependable, and can be easily serviced or replaced. Only about $250 total. I think you will be happy with a 1 HP system which can get 2-3 HP peak (but you will need stronger batteries). These little SLAs will only give you about 1/3 HP continuous and 1 HP peak at 3C, and a range of about 5 km of gentle use. Easy enough to expand from this, mostly with 8x20 Ah lithium cells for about $250-$300).


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## mbman88 (Feb 17, 2013)

PStechPaul said:


> Well, I used 150 kg and 0.9 m/s/s and an end speed of 20 km/h and I get 946 watts (1.2 HP). But that is the power at the end of the six second run, so to get average power, use 1/2 the top speed, and you get 460 watts (0.6 HP).
> 
> Mobility assist vehicles (wheelchairs and scooters) are usually used on fairly flat surfaces and are geared down to a top speed of perhaps 8 km/h (5 MPH). At a moderate speed of 5 km/h and a total weight of 150 kg and a 5% slope, you need only 145 watts or 0.2 HP. This also corresponds to an acceleration of 0.5 m/s/s on a level surface or 0-8 km/h in 3 seconds.
> 
> ...


It's almost as if I don't even need to try! lol thanks for the help

since batteries have been brought up.. I was wondering if a wheelchair battery would be effective. They have deep cycle batteries and I looked some up last night that say 55 amp-hours. The problem is that, because they are essentially a part, they don't give any useful information. So I don't know if they are powerful enough or not. I figured sticking with SLAs would be cheapest.. but using deep cycles, and that's where I found myself.


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## puddleglum (Oct 22, 2008)

I think for an ebike, a car too for that matter, lithium is the best and the cheapest in the long run. This is the advice I was given from the ebike guys. Small SLA's sag quickly and don't have a good lifespan. I built my son a lightweight mobility scooter last summer with a 24v 300watt motor, 35amp peak controller and a 15ah lithium pack. It's geared low to limit the speed so it limits at 14kph and weighs 54lb (about the same as an ebike). I've never timed it, but I know, if you cracked the throttle, it would spin the drive tire and reach top speed too quickly for what I had designed it for. I have since went to a 250 watt motor and 30 amp controller and it's still not slow. When looking at specs. don't confuse continuous ratings with peak ratings. My 300 watt motor was easily putting out 800 amps peak. Also, if your talking pedal assist, don't forget to add the output of your legs.


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## mbman88 (Feb 17, 2013)

puddleglum said:


> I think for an ebike, a car too for that matter, lithium is the best and the cheapest in the long run. This is the advice I was given from the ebike guys. Small SLA's sag quickly and don't have a good lifespan. I built my son a lightweight mobility scooter last summer with a 24v 300watt motor, 35amp peak controller and a 15ah lithium pack. It's geared low to limit the speed so it limits at 14kph and weighs 54lb (about the same as an ebike). I've never timed it, but I know, if you cracked the throttle, it would spin the drive tire and reach top speed too quickly for what I had designed it for. I have since went to a 250 watt motor and 30 amp controller and it's still not slow. When looking at specs. don't confuse continuous ratings with peak ratings. My 300 watt motor was easily putting out 800 amps peak. Also, if your talking pedal assist, don't forget to add the output of your legs.


Yes I definitely won't forget about pedal-power.. lol But yes. I am counting on leg performance as well. Lithium.... that comes in many forms I've been discovering. There is the infamous Lithium-Ion.. but also many others. 




ALSO.. I am having some serious problems with mounting and more importantly... gear reduction. What I'd imagine as ideal would be a planetary gear for reduction.. and then using the output of the planetary gear to connect to the wheel hub using a wheel-mount sprocket. 
The EZIP Trailz bike comes with what I want for a mount and gears and even the sprocket. But the gears are attached to the motor and I'm not willing to chance buying the entire motor just to take it apart and discover that I cannot mount a different motor.


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## Siwastaja (Aug 1, 2012)

mbman88 said:


> Lithium.... that comes in many forms I've been discovering. There is the infamous Lithium-Ion.. but also many others.


No -- lithium-ion is currently the only feasible re-chargeable lithium-based battery technology. However, there are many varieties of li-ion, such as LiCo, LiMn and LiFePO4.

What might be the "infamous" lithium ion you are talking about? LiCo, the one that has enabled the wireless world and works just fine, better than any battery technology ever before? Of course, other li-ion technologies are even better in regards of safety and lifetime.


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## mbman88 (Feb 17, 2013)

Siwastaja said:


> No -- lithium-ion is currently the only feasible re-chargeable lithium-based battery technology. However, there are many varieties of li-ion, such as LiCo, LiMn and LiFePO4.
> 
> What might be the "infamous" lithium ion you are talking about? LiCo, the one that has enabled the wireless world and works just fine, better than any battery technology ever before? Of course, other li-ion technologies are even better in regards of safety and lifetime.


Alright then, I guess I didn't catch on to that. Maybe I'm looking at the ones for EVs in the wrong light. And yes.. it would be the "infamous" LiCoO2 that I was thinking of... which gets categorized as though it were all of Lithium Ion batteries thanks to advertising.


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## puddleglum (Oct 22, 2008)

mbman88 said:


> Yes I definitely won't forget about pedal-power.. lol But yes. I am counting on leg performance as well. Lithium.... that comes in many forms I've been discovering. There is the infamous Lithium-Ion.. but also many others.
> 
> 
> 
> ...


Do you have a motor already that you are trying to mount with a gear reduction drive? How are you planning to use the bike? From what I've read over at endless sphere (great info on ebikes in case you haven't been there), mid mount or gear reduction drives are best for off road bikes, but a geared or direct drive hub motor might be a better choice on a commuter bike. Wish I could speak from personal experience, but I can't.


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## mbman88 (Feb 17, 2013)

puddleglum said:


> Do you have a motor already that you are trying to mount with a gear reduction drive? How are you planning to use the bike? From what I've read over at endless sphere (great info on ebikes in case you haven't been there), mid mount or gear reduction drives are best for off road bikes, but a geared or direct drive hub motor might be a better choice on a commuter bike. Wish I could speak from personal experience, but I can't.


I currently have no motor. I'm trying to stay true to the DIY ideology and NOT buy into some manufactured electric bike... but like a DIY engineer living in a technician's world would have it... it's proving very challenging. Almost all parts are replacements for specific products. 

Now to get my head out of the clouds.. I would prefer to differentiate between "hub motor" and "direct drive" since 1 is placed IN the hub (inside of it) and another can possibly be used with a chain and gear thereby being another form of gear-reduction.. however in some cases not nearly as much reduction has a mid-mount would be if attached to the cranks. 

I initially wanted to make it as simple as possible and attach a chain from the rear gears/freewheel (mine has 6 gears) on the largest gear (1st gear) and run that to the motor sprocket attached to the motor shaft. I would adjust the derailleur spring so that it does not allow the main bike chain to use 1 and 2 gears. Suddenly one of the biggest problems with this is the problem with gear ratio.. any motor that is cheap is highly unlikely to be ready for low rpm.. From what I've seen on the web people will use really large gears on their rear wheel (half the size of the entire wheel) connected via chain to a fairly small motor drive sprocket. Another thing I've seen is a motor connected to a rubber belt, connected to a larger belt-gear which reduces to a shaft that has a chain sprocket on it.. and that chain then attaches to another sprocket/gear modified to fit on the left-side of the rear wheel.. which is about the same size as the largest gear on the multiple-speed freewheel. 

From mid-mount (driving the crank-gears), we have a guy who built his own freewheel crank gearset using sickbikeparts dot com. They have a freewheel that connects to a range of sizes of gears that then connect to the cranks on a typical engine-driven bicycle (2 stroke engine), you keep the pedals in this case. He modified his crank gearset so that the 3 "stock" gears connect in-harmony with the motor-driven gear. This allows the pedals to still propel the vehicle while allowing the motor to propel the vehicle without spinning the crank pedals. This required grinding, shaving, and drilling. All things that I do not have the privilege to use right now (finance-based.. not authority-based). 

Considering the fabrication that is starting to seem necessary.. it's almost easier (or at least more gratifying) to build an electric motorcycle. Unless I want to "sell-out" and buy a manufactured bike. 

That's why I'm looking for parts that make things easy such as the parts for the EZIP Trailz bike which are available at electricscooterparts and electricsuperkids ... including the motor mount plate.. which seems to only work with the specially modified Trailz bike... 

It's really coming down to this "If there was a way, it would be a revolution." Well it seems that hub-driven is the way these days. Buy a wheel.. attach it and go. That doesn't leave me freedom of ingenuity...


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## mbman88 (Feb 17, 2013)

PStechPaul said:


> These little SLAs will only give you about 1/3 HP continuous and 1 HP peak at 3C, and a range of about 5 km of gentle use.


Everytime I re-read this I seriously believe you meant to say that 1/3 HP would be the peak at 3C... because those batteries only say 3.6 amps for cyclic usage on the side of the casing. at 24 volts that's a minuscule 86 watts. at 36 volts I'm seeing 130 watts.. and at 3 times I'm starting to see the 300 watt arena that equates to 1 human power. I think I'll need a slightly stronger starter battery.. maybe 5 amps continuous. 
Maybe I'm wrong here?


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## PStechPaul (May 1, 2012)

Here are the specs:








The 3.6A seems to be for charging. You can get as much as 100A for 5 seconds, and at 1C (12A) the discharge time is 30 minutes, which is half capacity, as predicted from the Peukert estimation. At 3C (36A) you can get 5-6 minutes, which is about 30% capacity. 

BTW, the weight is actually 4 kg (9 lb), and not 12 lb as I had thought.

Also, you should check out this resource for e-bike performance:
http://www.ebikes.ca/simulator/


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## puddleglum (Oct 22, 2008)

mbman88 said:


> Now to get my head out of the clouds.. I would prefer to differentiate between "hub motor" and "direct drive" since 1 is placed IN the hub (inside of it) and another can possibly be used with a chain and gear thereby being another form of gear-reduction.. however in some cases not nearly as much reduction has a mid-mount would be if attached to the cranks.
> 
> I initially wanted to make it as simple as possible and attach a chain from the rear gears/freewheel (mine has 6 gears) on the largest gear (1st gear) and run that to the motor sprocket attached to the motor shaft. I would adjust the derailleur spring so that it does not allow the main bike chain to use 1 and 2 gears. Suddenly one of the biggest problems with this is the problem with gear ratio.. any motor that is cheap is highly unlikely to be ready for low rpm.. From what I've seen on the web people will use really large gears on their rear wheel (half the size of the entire wheel) connected via chain to a fairly small motor drive sprocket. Another thing I've seen is a motor connected to a rubber belt, connected to a larger belt-gear which reduces to a shaft that has a chain sprocket on it.. and that chain then attaches to another sprocket/gear modified to fit on the left-side of the rear wheel.. which is about the same size as the largest gear on the multiple-speed freewheel.
> 
> ...


I think you can still stay DIY (not buy a prebuilt bike) and work within your constraints if you select your own parts and put together your system. If you don't have a way to manufacture your own specialty parts, an externally mounted motor is going to be extremely difficult and probably more expensive, if you have to pay someone else to do it. 
You misunderstood my reference to geared vs direct drive hub motors. Both are hub motor designs. In direct drive hub motors, there is no internal gear reduction and the wheel hub is the outer part of the motor. In a geared hub motor, a small motor inside, drives a planetary set which drives the wheel hub. 
I was thinking about doing an external drive very similar to what you are describing (I have a spare scooter motor from my other project) but haven't because of the complexity of it all. Another thing I found when doing the mobility scooter is that external drives are noisier as well (at least mine is with a high speed motor), I don't think i would want that on a bike.


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