Hub motor 48 volt 500 watt in 26 inch rim / I plan to lace motor into 20 inch rim.

amberwolf said:

for the same voltage, a motor wheel will be lower rpm in a smaller wheel regardless of controller, but it will have proportionally higher torque. ****

if you want a motor to spin a certain speed in a certain size wheel, find out what rpm the motor spins in that size wheel (or calculate it bsed on the size wheel it's already in vs the size you want to put it in), at some voltage.

then you can do the math to see what rpm it will spin at at the voltage you are going to run it at, or conversely you can see what voltage you will need to spin it at a specific rpm.

the simulator at ebikes.ca may make this easier than doing the math all by hand, for motors already listed in the simulator. if nothing else it will show you how the whole system works and interrelates even if its' not with the speciifc motor you will be using.

you can also use it to see what controller size / amps is needed to provide enough torque to go up specific grade slopes with various wheel sizes and motors.

i'd recommend finding your actual worst case conditions you have to ride in, then see what it takes to overcome them, and build for worse conditions than that to be certain your system will do what you want without stress.


*** some foc controllers have field weakening that can work to spin a motor faster but it will waste (a lot of) power and create (a lot of) extra heat in hte motor to do this. exact results will depend on the controller's method of doing it, the specific settings you choose / tune the system with, and the motor itself, so it will be an experiment you'd have to try to find out if it would do what you want.

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docw009 said:

Your thinking is correct for #2. A motor specified for a certain rpm at 36V will spin faster at 48V and offset the smaller tire diameter. Motor speed follows voltage,

What motor depends on budget, and what the bike is used for. It's often less money in total to buy the motor/wheel together if you buy from China, SHipping charges can be over $100, but still less than buying the parts yourself and lacing it. Nonethless, I would rather buy name brand spokes/rim and build a better wheel than use the chinese spokes.

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gromike said:

I'm so sorry, but the 750 watt BBSO2 is a great motor, though I could live with 500 watts. But 250 watts is not enough. I suppose you can draw more than 250 watts on your motor, but for how long?

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amberwolf said:

were they extra, or did they just forget ot install them? :lol:

i doubt they meant them to be in there....

i can't imagine a bike manufacturer (especially one like the mongooses come from) including spokes on purpose with a bike, as nearly none of their customers would have any idea what to do with them, and most probably don't even know what they are (they might or might not connect their appearance with the ones already in their wheels)


maybe the bike you got was used and whoever had it beofre left the new spokes in teh box when they returned it, becuase the wheel kept breaking and they got tired of fixing it...




that said, there are a bunch of threads about wheel building, spokes, rims, etc., wiht more detail than this, but the basic reason the spokes break on many oem wheels is they are too thick for the rims used. they usually use 12g or thicker spokes, when they should be using 15g or 14g. so the spokes have to be tensioned higher which pulls too hard on the rim which deforms the rim which loosens the spoke which forces the other spokes to deform from overload whcih eventually breaks them which puts more load on the remaining spokes until suddenly you're riding on your hubs with broken legs. .

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