How much torque?


I'm designing a 2 wheel drive robot and I'm looking for suitable motors. How does one calculate how much torque(oz-in) is need from each motor. Is their an equation/formula where I can plugin my wheel size, weight of robot, max incline it has to go up, etc, and determine how much torque I need from each motor.


Here's an approach that is reasonably easy to do (without too many equations) for an incline:

Make an incline that's as steep as you want to be able to handle, and take your wheel and mark one point on the edge.

Start with the mark at the bottom of the incline, roll the wheel up one turn and mark the incline where the wheel-mark meets it.

Measure the height (vertically) of that point on the incline. This is the amount of vertical lift the motor does turning the wheel one turn.

Take the weight of your entire robot including payload (in kg), multiply by 10, multiply by the height you measured (in metres). This figure is the energy to lift the robot that amount.

The theoretical torque you need is that energy divided by 2 pi. The actual torque has to overcome friction, so double that figure for good measure.


wheel lifts robot 0.15m after one turn on incline. Robot weighs 4.5kg.

Energy = 4.5 x 10 x 0.15 = 6.75 joules.

divide by 2 pi = 6.75 / 2 / 3.14 = 1.075 (torque in newton-metres)

double the value to allow for friction and losses = 2 N-m (approx).

Note that none of this discussion concerns speed - the motor might have a reduction gear to get the torque needed and thus be quite slow. Power = torque x angular velocity, so more torque and more speed requires more powerful motor.

And the torque we are talking about (!) is that at the output of any gear system, direct to the wheel.

Thank you so much! That really helped.

I have one more question. In your example the amount of torque was 2 N-m. Would each motor need to deliver 2 N-m or should each one put out 1 N-m?

I know the amount of torque I need will be different then your example; I was just using your example to clarify what I need.

Thank you so much!

Yup, with two wheels and motors each has to provide half the torque since they share the work.

Thank you so much!