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Required Electric Motor Torque For Sm Car

  1. Dec 7, 2011 #1
    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.
  2. jcsd
  3. Dec 12, 2011 #2
    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, lets 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!
  4. Dec 12, 2011 #3
    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.)
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