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Homework Help: The torque of motor required to start a car moving.

  1. Aug 22, 2011 #1
    Hi, everyone. I am assigned a school project to design a off-road car from scratch. In order to do that, i need to size the motor effectively. I understand that power depends on the following factors: static friction, kinetic friction, air drag, gear efficiencies and orientations of planes. In addition, i know that in order to start a car moving, i need to overcome the static friciton, and from there, i need to find the starting torque required from the motor to start the car moving.
    Below is my assumed working:

    total static frictional forces (for 4 wheels) = coefficient of friction*weight of car
    total torque = r*F = radius of wheel * frictional force.

    This would be the torque i need to size my motor. Am i correct? Please advise me. Thanks in advance!
  2. jcsd
  3. Aug 22, 2011 #2


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    Homework Helper

    To keep a car moving, you need to overcome rolling resistance, aerodynamic drag, and losses within the drivetrain. To start a car moving, you need to overcome some issues related to compression type deformations that can occur when a car sits for a while. For example, the tires get a mild type of flat spot, more so if it's cold. Then there's something we in the USA call "stiction" where parts sort of get stuck together after not moving for a while, mostly if there's some compressive load on the surfaces. I'm not aware of the math for this.

    Static friction just refers to the coefficient of grip at the tires when they're still or rolling and doesn't refer to the rolling resistance that opposes rolling motion.
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