- #1

evbkezz

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- TL;DR Summary
- Need help finding the correct motor and gear configuration for a 250W mid-drive.

Hey all!

I am working on my first ebike build and posted a question about friction drives earlier. Now, I have decided to go with a Mid-drive. I made calculations and found out the torque and rpm required for the build. But I am having difficulty find the right motor and gear ratio. I am trying to build something like the Brose S Mag, the size of the drive is so small but the power it delivers is insane. It delivers a torque of 90Nm. They said they use a planetary gear box. As per my understanding, they have used a high rpm motor and geared it to achieve the required torque and RPM. The place I live has a legal power limit of 250W and speed limit of 25Kmh for ebikes.

These are the specs of my bike:

GVW= 105kg

Wheel radius = 330mm

Max Speed = 25kmh

I calculated the air resistance using the formula:

RA = rhoCdAV^2/2 where rho is density of air, Cd is the drag coefficient(0.88), A is area of contact in m^2(0.4) and v is velocity in m/s(6.94)

RA = 10.38N

I calculated rolling resistance using the formula:

RR = GVW*Crr where GVW is total vehicle weight(105Kgs) and Crr is rolling resistance of tire on asphalt road(0.004)

RR = 4.12N

I calculated grade resistance using the formula:

RG = GVW*sin theta where sin theta = 0.196 for 20% gradeablity

RR = 202.09N

The acceleration force required for 25kmh in 5 secs is

The torque required when considering grade resistance is

The torque required on flat roads is

The RPM required for 25kmh with a wheel dia of 26" is

I should now select a motor that can achieve the above figures and the rated power has to be below 250W and size of motor should be really small. I think using a high rpm motor with planetary gear reduction can help me achieve these numbers. I am planning to use a 36V 10A controller so the peak power can be till

Idk what motor should I choose and what gear ratio I should use. Will RC motors work as they provide very high rpms and are small too?

A user on the other forum told me that I would have to design a gearbox such the peak power of the motor is delivered at 60RPM since the bicycle drivetrains are built for the average human rpm which is around 60. But when I look for motors online, manufactures aren't giving the peak power inform. How do I find the rpm and torque at the peak power? I should build a gear to reduce it to 60 RPM? What should be the ideal torque or RPM values before reduction? If I reduce the RPM to 60 will still be able to achieve 25Kmh?

Should I design it such that it exceeds the

Please do let me know if my calculations are right.

Thanks

I am working on my first ebike build and posted a question about friction drives earlier. Now, I have decided to go with a Mid-drive. I made calculations and found out the torque and rpm required for the build. But I am having difficulty find the right motor and gear ratio. I am trying to build something like the Brose S Mag, the size of the drive is so small but the power it delivers is insane. It delivers a torque of 90Nm. They said they use a planetary gear box. As per my understanding, they have used a high rpm motor and geared it to achieve the required torque and RPM. The place I live has a legal power limit of 250W and speed limit of 25Kmh for ebikes.

These are the specs of my bike:

GVW= 105kg

Wheel radius = 330mm

Max Speed = 25kmh

I calculated the air resistance using the formula:

RA = rhoCdAV^2/2 where rho is density of air, Cd is the drag coefficient(0.88), A is area of contact in m^2(0.4) and v is velocity in m/s(6.94)

RA = 10.38N

I calculated rolling resistance using the formula:

RR = GVW*Crr where GVW is total vehicle weight(105Kgs) and Crr is rolling resistance of tire on asphalt road(0.004)

RR = 4.12N

I calculated grade resistance using the formula:

RG = GVW*sin theta where sin theta = 0.196 for 20% gradeablity

RR = 202.09N

The acceleration force required for 25kmh in 5 secs is

**145.74N**The torque required when considering grade resistance is

**83.6Nm**The torque required on flat roads is

**37.15Nm**The RPM required for 25kmh with a wheel dia of 26" is

**240**after including a fudge factor of 20%I should now select a motor that can achieve the above figures and the rated power has to be below 250W and size of motor should be really small. I think using a high rpm motor with planetary gear reduction can help me achieve these numbers. I am planning to use a 36V 10A controller so the peak power can be till

**306W**assuming a 85% efficiency of the motor.Idk what motor should I choose and what gear ratio I should use. Will RC motors work as they provide very high rpms and are small too?

A user on the other forum told me that I would have to design a gearbox such the peak power of the motor is delivered at 60RPM since the bicycle drivetrains are built for the average human rpm which is around 60. But when I look for motors online, manufactures aren't giving the peak power inform. How do I find the rpm and torque at the peak power? I should build a gear to reduce it to 60 RPM? What should be the ideal torque or RPM values before reduction? If I reduce the RPM to 60 will still be able to achieve 25Kmh?

Should I design it such that it exceeds the

**83.6Nm**torque at the highest gear?Please do let me know if my calculations are right.

Thanks