Hey all!
This is my first build and I am trying to build a mid-drive by myself. I have worked out some calculations and now have the amount of torque and RPM I need. I need help choosing a proper motor for the build. I am not sure about the calculations being right so please do points out mistakes if any. The power restrictions for my area is 250W and my build should stay within this limit.
I''m assuming the total weight of the bike including the rider, battery and motor to be 105kgs. My max speed will be 25kmh.
I calculated the Air Resistance using the formula:
I calculated the Rolling Resistance using the formula:
I calculated the Grade Resistance using the formula:
Acceleration force required to achieve 25kmh in 5sec with the GVW of 105kg is 145.74N
The total torque required including the grade resistance is 83.6Nm
The torque required on flat roads is 37.15Nm
The RPM required to achieve 25kmh after considering a fudge factor of 20% is 240
Now since it is a mid-drive the motor must be small, high torque motors are bigger. So the best solution is to use a high rpm motor and use gear reduction to achieve the required torque levels.Am I correct?
I need help on choosing the right motor and gear reduction.
Thanks
This is my first build and I am trying to build a mid-drive by myself. I have worked out some calculations and now have the amount of torque and RPM I need. I need help choosing a proper motor for the build. I am not sure about the calculations being right so please do points out mistakes if any. The power restrictions for my area is 250W and my build should stay within this limit.
I''m assuming the total weight of the bike including the rider, battery and motor to be 105kgs. My max speed will be 25kmh.
I calculated the Air Resistance using the formula:
Code:
RA = rhoCDAV^2/2 where RA is the Air Resistance, rho is density of air and A is area under contact in m^2 and v is velocity in m/s
RA = 1.225*0.88*0.4*6.94*6.94/2
RA = 10.38NI calculated the Rolling Resistance using the formula:
Code:
RR = GVW*Crr where RR is the Rolling Resistance, GVW is total weight of the bike and Crr is co-eff of rolling friction which is 0.004 for bicycle tire and asphalt roads
RR = 1030.05*0.004
RA = 4.12NI calculated the Grade Resistance using the formula:
Code:
RG = GVW*sin theta where RG is the Grade Resistance, GVW is total weight of the bike and sin theta is 0.196 for a grade of 20%
RG = 1030.05*0.196
RA = 202.09NAcceleration force required to achieve 25kmh in 5sec with the GVW of 105kg is 145.74N
The total torque required including the grade resistance is 83.6Nm
The torque required on flat roads is 37.15Nm
The RPM required to achieve 25kmh after considering a fudge factor of 20% is 240
Now since it is a mid-drive the motor must be small, high torque motors are bigger. So the best solution is to use a high rpm motor and use gear reduction to achieve the required torque levels.Am I correct?
I need help on choosing the right motor and gear reduction.
Thanks
