Good morning. I have been reading the threads about "Understanding required torque for a motor lifting a weight" in hopes it would apply to my situation. I would like to utilize an R/C servo to rotate a mass about 100 degrees. the specifics are for my 1959 Impala, I would like to automate the license plate bracket to rotate down when the transmission is put into reverse so the back up camera will be able to view behind the car.

  • The license plate holder and license plate weight approximately 20oz (0.57kg) Dim: 10"L x 6"H x 0.075"W)
  • The rotating shaft is 0.25" (0.00635m) stainless steel
  • The license plate holder (and license plate) are offset from the shaft by 0.5" (0.0127m)
  • The total rotational distance for the holder and license plate to travel is 0.27m in 3 seconds

Solving F=ma, I come up with 5.59N (gravity) plus .5N (up) => F = 6.1N

T=Fr solves to be 0.02Nm

But I am not sure how the offset plays into the calculations?? or does it?



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  • $\begingroup$ Or does the 0.5" offset get added to the shaft diameter? If so, I come up with 0.0969 or 0.1 Nm $\endgroup$
    – myke smith
    Jun 12, 2019 at 18:52
  • $\begingroup$ Torque is one thing sure. But you should also think about how fast you want the action to be. Then calculate the innertia. $\endgroup$
    – joojaa
    Jun 12, 2019 at 21:03
  • $\begingroup$ This sounds way too complicated for anyone but an expert. Send your car to me, I'll give you my silver econobox in return... $\endgroup$
    – TimWescott
    Jun 12, 2019 at 23:16
  • $\begingroup$ Draw us a sketch? I'm not sure that an RC servo is the best way to go here, but there are certainly some stout ones out there. $\endgroup$
    – TimWescott
    Jun 12, 2019 at 23:32
  • $\begingroup$ I'm sorry, you're new to StackExchange. Please edit your question with a sketch - this place likes to have a nice 1:1 correspondence between question and answer. $\endgroup$
    – TimWescott
    Jun 13, 2019 at 0:40

1 Answer 1


This is a pitfall I see all the time. You will not be able to calculate the required force analytically in a situation like this. Friction, and exterior forces will dominate.

Honestly the best approach short of turning this into a scientific experiment would be to eyeball it, and test the results. Hobbyking makes a large robotics servo that may be about the right size.

  • $\begingroup$ Drew - in a perfect world (no friction), how would I solve this problem? And what do you mean by external forces - gravity? $\endgroup$
    – myke smith
    Jun 14, 2019 at 12:26
  • $\begingroup$ I mean forces like: G forces during driving, wind loads, people snagging the plate as they're walking by, all sorts of stuff. To solve this analytically (which again will be wrong), you would calculate the torque applied by gravity in the worst case, and maybe double that. You can calculate the inertia too and decide what acceleration you need, but that's going to be so inaccurate it's not even worth the paper. $\endgroup$
    – Drew
    Jun 14, 2019 at 18:23

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