Finding the right motors (torque)

I have 4 micro metal gearmotor (5:1 HPCB) and I’m planning to build a small & fast car robot (10cm long) and to use 32×7mm Pololu Wheels (2 motors each side)

Each motor has 2 oz-in (0.1 kg-cm) torque.

  1. What would be the ideal weight of the robot if I want to use those 4 motors?
  2. In my experience the weight will be in the area of 250-300 grams. If I want to use 4 micro metal gearmotor, which motors you would recommend me to use?



It sounds like your questions can probably be answered with a few calculations explained in our “Force and Torque” blog post. Essentially, the force that one of those motors can exert on the robot is the torque divided by the radius of the wheel. Since stalling brushed DC motors like these can easily damage them, a practical limit for the torque is around 20-30% of the stall torque, which leaves you with:

F = (30% of stall torque) / radius of wheel
F = (2oz*in × 0.3) / 0.63in
F = 0.95oz

You can multiply this by 4 since you have 4 motors and wheels, so a robot that weighs around 3.8oz (~108g) would be a good fit.

If your robot is closer to 300g, the 30:1 HPCB micro metal gearmotors would probably be a better fit for you. We carry 6V and 12V versions of those gearmotors.