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Magnetic drag in relation to direct drive motor power...

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Magnetic drag in relation to direct drive motor power...

Old 08-01-14, 02:48 AM
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Magnetic drag in relation to direct drive motor power...

I've been doing a fair amount of study on this issue but can't really find any definitive answer.

I'll start by saying that I'm planning to build my own front wheel drive e-bike, and I'm probably going to go with cheaper Ebay kits on my first outing. I already have a quality steel frame that I plan to convert. I'm also probably going to go with a 36V ~20AH LiFePO4 battery from Ebay, knowing full well that the quality might be suspect and the AH rating will probably be lower than advertised. I'd like to get a battery with quality Japanese cells, but the cost would probably be a lot higher than the cheap Chinese stuff on Ebay. If anyone can suggest a battery with the best price/performance ratio, I'm all ears.

Now to my main problem. I've been reading that direct drive motors have a problem with magnetic drag. Since I plan to use my bike a lot on any given day, I have to assume that I'll run out of power on some days and have to pedal it myself. So, I have to build my bike in a way that it can still function as a normal bike without becoming completely impractical to use normally.

Most of the cheap kits on Ebay are direct drive motors. For instance,

26" Electric Bicycle Motor Hub Kit Front Rear Wheel EBIKE36 48V250 500 800 1000W | eBay

I probably won't need a great deal of power. I'm a lightweight rider. My bike will be between 60 and 70 lbs finished. I'll be riding paved roads and not many hills. I don't need a great deal of speed or acceleration. 10mph will be plenty for me. I'm guessing that 250W will be sufficient for me. But my two main issues are magnetic drag and motor efficiency.

Assuming equal build quality, can I expect the 500W motor or the 250W motor to provide more drag with human power and no battery power. Just looking at the size difference, one would expect the 500W motor to have more drag, but I don't know.

Also, the 500W motor is listed as being more efficient, so assuming that I ran each motor identically (speed, acceleration, etc.), I'm expecting the 500W motor to provide a little greater range. But, having never ridden an e-bike before, I don't know how easy it would be for me to ease up on the throttle for the 500W motor and run it like a 250W motor. Any thoughts on this would also be appreciated. If I were to get a 500W motor, I'd like to baby it and run it at low throttle, but I don't know how responsive these cheap Ebay kits are.

Any help would be appreciated.

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Old 08-01-14, 05:27 AM
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Well, 1st off, 36V x 20Ah @10mph on flat paved road should give you ~70 mile motor only capability ... without any pedal assist!
See - Speed vs Range
At that speed even a slight pedal assist should push your range past 100 miles per charge.
That would be 10 hours of cycling per charge!

However, if you still fear running out of juice and having to pedal ...
You should probably be shopping for a "geared hub" motor.
Most, please confirm in description, have a freewheel function that disengages the motor when not used.

A 250w geared hub motor is capable of ≤17-18mph by itself and should provide plenty of power for your needs!
See - Efficiency

Oh ... "magnetic drag" is similar to brakes dragging slightly or very soft tires ...

Last edited by DrkAngel; 08-01-14 at 06:34 AM.
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Old 08-10-14, 09:20 PM
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A Ping battery has a great reputation and is reasonably priced.

36 Volt LiFePO4 Battery Packs with BMS (Battery Management System)

I have and like direct drive. If you bought the 250 watt it would give you the 10 mph and not burn too much energy doing it. The drag on a 250 watt is marginal and not noticeable while riding. The direct drive is longer lasting and more durable do to the fact it has no moving parts.

It is very hard to ride an ebike with partial throttle. Most of us tend to ride all out. Using the 250 watt would amplify this tendency. So my guess is using the 250 watt would be best for you if you want to stay around 10 mph.

Have fun.
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