I've been attempting to calculate how much torque a motor needs to produce in order to start a stationary object on wheels moving. (The torque is being applied to the rear 2 wheels, the front 2 are on bearings.)

I keep seeing Torque = Force * Radius (of the torqued wheel). I can't figure out how to calculate the Force in this equation though, so that I may find the Torque.

The radius of the wheel is 3cm.

The weight of the entire object (including wheels and everything else) is 5kg.

I don't need an incredibly accurate result, so I haven't even been trying to factor in friction from the non-powered front wheels' bearings.

I tried something with a friction coefficient and gravity, but the more I read the less I believe that my calculation was correct.

Can anyone point me in the right direction?


If you ignore friction in the drive train even the smallest torque will start the obect moving.

Your calculation of force is correct, and from Newton's first law the acceleration will be the force divided by the mass. So even the smallest force (i.e. torque) will cause the object to accelerate, albeit very slowly.

However experience suggests that if you apply a small force to your car it will just sit there and smile at you. This is because there is static friction in the gearbox, axle etc and you need to apply a force great enough to overcome this static friction. I don't know any easy way to calculate what the static friction will be; I think you will have to measure it.

  • $\begingroup$ In order to measure the static friction, can I just pull the object with a spring scale and see at what point on the scale it begins to move? $\endgroup$ – JonathonG Jan 22 '12 at 20:08
  • $\begingroup$ Correction: In order to measure the force required to overcome the static friction. $\endgroup$ – JonathonG Jan 22 '12 at 20:22
  • $\begingroup$ Yes. You'll probably find the rolling friction is lower than the static friction, so once the car has started moving the force needed to keep it at a steady speed will be less than the force needed to start it moving. $\endgroup$ – John Rennie Jan 23 '12 at 8:17

protected by Qmechanic Feb 12 '13 at 9:42

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