# Required Electric Motor Torque For Sm Car

• mdinitto
In summary, the conversation is about how to calculate the torque required for a small DC motor to move a shoebox-sized car, given the car's weight, wheel diameter, axle diameter, and speed. The conversation also touches on the effects of gear ratios and the number of wheels on the torque calculation.
mdinitto
I am trying to spec out a small dc motor for a shoebox sized car.

If I have a motor with a gear on it rotating a second gear with a gear ratio of 1:1 that is attached to the axle with a wheel at each end, how can I calculate how much torque the motor needs to be capable of supplying to move the car?

Car weight: 10lbs
Wheel diameter: 1.75"
Axle diameter: 0.125"
Car speed: 1ft/s
Wheel RPM (from 1ft/s and diameter): 131RPM

Experimentally I found that the car requires 0.8lbs of horizontal force to start rolling from rest.

I understand the effect of gear ratios (so I made them 1:1 for this question), but I am unsure how how the wheel diameter:axle diameter plays into the equation (if at all). I am also unsure of how directly to apply the force required to move the car from stop. Can this be applied to the tangent of the wheel to give a wheel torque of 0.8lb*wheel radius? Does this need to be multiplied/divided by 2 to take into account the 1 motor supplying torque for 2 wheels?

Thank you for your help! Please let me know if there is any other missing piece of information needed.

Alright well, why don't I take a stab at my own question and maybe someone else can take a look and see if if they agree or not... worth a shot, right?

If the force was applied to the front of the car, and the car started to move, that means that the equal but opposite force was applied by the wheels on the table...since the wheels contact the table at the tangent, then the 0.8lb was also applied by the tangent of the wheel to the ground, thus being the tangential force of the wheel.

Now if we apply an energy (power) balance and consider friction throughout the gears and such negligible, then we can take a look at each part...

Power = torque * RPM

Then you can look at each section…so if we had a gear ratio you would take that into account with its power staying constant, but the rpm and torque will shift depending on gear diameters/#teeth….but for the sake of this post, let's assume that the gears are 1:1, so the wheels are spinning at the same RPM as the motor….
Therefore, the wheels will have the same torque as the motor.

I am still unsure of whether I need to take into account the the fact that there are 2 wheels though.

Any thoughts?

Thank you!

Generally speaking, the number of wheels is irrelevant. (If you need more accuracy, you would have to account for the various losses in bearing friction, rolling resistance, etc.)

I'm not very clear on what specifically you are trying to determine/solve. You mention axle diameter, but that's not a factor, unless the axle is itself one of the "gears". Just consider the wheel diameter as another gear. (On a real vehicle with pneumatic tires you have to determine the rolling radius, which changes with inflation pressure, payload, wear, etc.)

## 1. What is the definition of required electric motor torque for a small car?

The required electric motor torque for a small car is the amount of rotational force that the motor must be able to produce in order to move the car forward. It is typically measured in units of Newton-meters (Nm) or pound-feet (lb-ft).

## 2. How is the required electric motor torque calculated for a small car?

The required electric motor torque for a small car is calculated by dividing the total weight of the car (including passengers and cargo) by the wheel radius and the gear ratio of the motor. This calculation takes into account the force needed to overcome the car's inertia and to move it at a desired speed.

## 3. Does the required electric motor torque vary for different types of small cars?

Yes, the required electric motor torque can vary for different types of small cars depending on their weight, size, and aerodynamics. For example, a small sports car may require a higher torque to achieve the same level of performance as a small sedan.

## 4. How does the required electric motor torque affect the acceleration and top speed of a small car?

The required electric motor torque directly impacts the acceleration and top speed of a small car. A higher torque will result in faster acceleration and a higher top speed, while a lower torque will result in slower acceleration and a lower top speed.

## 5. Are there any other factors that can affect the required electric motor torque for a small car?

Yes, there are other factors that can affect the required electric motor torque for a small car, such as the type of terrain the car will be driven on, the number of passengers and cargo, and the desired driving range. These factors should also be taken into consideration when determining the appropriate torque for a small car.

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