Newton-meter confusion.

elmo78

1 mW
Joined
Nov 12, 2016
Messages
12
Hello

I am building my e-bike and my choice is a mid-mount motor because of better handling and low unsprung weight of the rear wheel. However...

I am having problems when it comes to choose the right motor parameters. I want a bike that can go around 50km/h on a 24 inch wheel at the back. No pedals. My weight 75kg. I do not want a monster which will bend my frame, break my chain and stretch my spokes, but it needs to be responsive and accelerate like a healthy mountain biker.

I have found an motor which has the following power curve...
BLT-500W.jpg

Enough Nm to suit my needs? Should I look for a more powerful motor? I am aware that I need max cadence around 120RPM to work with my rear hub sprocket cassette, so dividing RPM by 4 would multiply Nm by 4, right?

I can do all the fabrication, welding, gearing without problems, I just don't know if 8Nm peak at the motor is enough for real-life riding?
 
You could look at the http://ebikes.ca/simulator at various motors in different wheels and conditions and see what kind of torque levels they get at various speeds and power levels, and see if any of that info helps.


A bit of googling for "newton meter healthy biker" (or cyclist) finds things like this
https://bicycles.stackexchange.com/questions/12518/what-is-the-optimal-cadence
that may have useful info, too.
 
8.18 newtons at 1000 RPM is 68n at the 120rpm you plan to gear it down to. So that's 50 foot-pounds of torque.

50 foot-pounds of torque at the crank is the same force as a 100 pound girl standing on a pedal (175mm crank arm = 6 inches)

So that motor would peak out with all the power of an angry pint sized Ex girlfriend chasing you down on a bike. :mrgreen:

This isn't frame bending or chain breaking power. it's on the low side for an adult, but still decent power. Your goal of 50kph may be hard to reach with this motor without some aerodynamic considerations.
 
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