- #1

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## Main Question or Discussion Point

Dear community,

For my electric vehicle project I want to select a gear ratio (single) of my gear box and send this information to the supplier. In order to calculate this ratio I thought of the following steps but am not sure and want some expert to recheck and answer my queries:

1. Calculate wheel torque by using the formula: Torque = Force x Velocity. Here I shall be calculating the Force =drag+rolling resistance+gradient

2. I will be checking the corresponding wheel RPM at the velocity at 1.

3. I will take this RPM and get the motors torque from the Torque vs RPM graph given in the datasheet.

4. Then the Gear ratio would be: Torque on wheels/Torque on engine at the given RPM in 3.

Now my concerns are:

a. For a normal street car, how many scenarios do I have to calculate? For instance if I calculate the resistive force at 50kmh and then determine the GR, would it also be optimal for other speeds such as 10kmh and 50kmh?

b. What about driving uphill as a worst case scenario? Do I calculate GR based on this?

c. In the formula I use in 1. I assume the car is driving at a constant V, what about acceleration cases? Would the GR supply the acceleration requirements I have?

Can someone explain this better :p

Thanks :)

For my electric vehicle project I want to select a gear ratio (single) of my gear box and send this information to the supplier. In order to calculate this ratio I thought of the following steps but am not sure and want some expert to recheck and answer my queries:

1. Calculate wheel torque by using the formula: Torque = Force x Velocity. Here I shall be calculating the Force =drag+rolling resistance+gradient

2. I will be checking the corresponding wheel RPM at the velocity at 1.

3. I will take this RPM and get the motors torque from the Torque vs RPM graph given in the datasheet.

4. Then the Gear ratio would be: Torque on wheels/Torque on engine at the given RPM in 3.

Now my concerns are:

a. For a normal street car, how many scenarios do I have to calculate? For instance if I calculate the resistive force at 50kmh and then determine the GR, would it also be optimal for other speeds such as 10kmh and 50kmh?

b. What about driving uphill as a worst case scenario? Do I calculate GR based on this?

c. In the formula I use in 1. I assume the car is driving at a constant V, what about acceleration cases? Would the GR supply the acceleration requirements I have?

Can someone explain this better :p

Thanks :)