newbie question about motor power in watts

[cajunjay]

Hi guys

Im about to build my first ebike and im in china right now so whilst im here thought I would source some parts as there much cheaper than back in the UK.

I will be running a rear hub motor rated at 36v but putting 48 volts through it but not sure which one to buy.

What is the actual difference in running these motors in this way if I can choose between a 250w, 350w or maybe a 700w? They mostly have 250w motors for sale, its much more difficult sourcing a more powerful motor.

The motor will be laced into a 20" wheel but at 250w will this be feeble? I want to use the bike for commuting and also some light off road stuff like bmx tracks and things like that.

I want to run along at 20mph+ but also have some power when pulling off (acceleration)

Sorry for the stupid question just need to clear a few things up before I buy anything


Thanks

 

[rui_fujino]

I will be running a rear hub motor rated at 36v but putting 48 volts through it but not sure which one to buy.

I would get 48v kit if you wish to go 20mph on 20"

What is the actual difference in running these motors in this way if I can choose between a 250w, 350w or maybe a 700w? They mostly have 250w motors for sale, its much more difficult sourcing a more powerful motor.

those wattages are to let us know the rating of each motor. just think of them as how much current you can put them through. higher the better if you want to upgrade in future.

The motor will be laced into a 20" wheel but at 250w will this be feeble? I want to use the bike for commuting and also some light off road stuff like bmx tracks and things like that.
I want to run along at 20mph+ but also have some power when pulling off (acceleration)

what sort of condition are you expecting to run? (flat/hills etc)
How long would you like to travel (distance)
are you going to pedal at all or just motor?

 

[LegendLength]

The wattage ratings for motors are nearly always too conservative. The only way to know the true rating is to look for that specific brand of motor on these forums and see what users have managed to put through them and for what amount of time.

 

[cajunjay]

I think around 10 miles would be okay to start with was gonna go with some turnigy batteries which will amount to a 44.4v 10ah pack.

I will be riding on all types of ground some is flat but there are hilly areas aswell, for arguments sake lets just say this is going to be a bmx. As for the pedaling I will definitely pedal along with the motor when travelling or even just to get upto a decent speed and use the juice so to speak to help me keep up a nice pace.


So im understanding its better I go for something more powerful than a 250w motor, I have the option of a 48v 250w but now that seems very weak.

Im no good at working out what I need to get the performance im after, just need pointing in the right direction.


Thanks for the advice so far

 

[Jethro56]

Check this out

http://www.ebikes.ca/simulator/

P.S. I'd buy from him If you want something to last.

 

[rui_fujino]

20" wheel base and going 20mph is not a good idea... As far as I know you bike will have so much torque but no top speed unless you get GM motor(Magic pie) with 48v.
I used to have 48v 1000w on 26" and that was only 26mph top speed with high torque.

Also I agree with Jethro56 about where you buy the motor from. (You could get them cheaper on ebay if you need to)

 

[cajunjay]

What kind of performance would I get from a 48v 800w in a 24" wheel or compared with a 26" wheel.

The off road stuff I mentioned isnt needed 100% would just prefer something to ride around on for fun or just a 5-10 mile round trip to the town.

 

[Ykick]

Are you talking about DD ( direct drive) motors or geared hub motors? Big difference. DD's can generally handle wider voltage and power range for modding.

Some folks believe smaller wheels are stronger and for DD motors they may allow the motor to run in more efficient RPM.

I wouldn't bother with a laced wheel from China. Their spokes/rims truly suck. Bring back a bare motor and find a spoke cutter and build it right with eyelet rim and quality SS 13/14 single butted Stainless spokes.

 

[dogman dan]

Some basic info might be helpfull. On flat level ground, a regular upright bike needs 300-500w to go 20 mph, depending on wind.

So you can at least strike the sub 300w motors off your list. 400-800w is the ballpark you need to shop in.

Pay more attention to the actual potential wattage of the setup. 15 amp controller + 48v battery = 720 potential watts of power.
Which a "350 w" motor can handle. It takes 1000w to really be happy with performance on hills with most hubmotors. So you might prefer a 48v battery and 20 amp controller a LOT, along with a more expensive (larger) battery to run it.