Let's Make Robots!

How do you know if your motors will be strong enough?

Hello Everyone,

I'm new to the LMR community and want to build my first robot. I'm a linux enthousiast so for brains I have an beagleboard. This has an 500MHz arm processor and runs on 5Volt at 2 watt on full load. http://beagleboard.org/

It also has 3 serial interfaces so I would choose a serial motor controller.

But the problem is with the motors. I have my eye on the polulu 100:1 micro metal motor with 120 RPM and 0,7 kg/cm (or 10 oz/inch). http://www.pololu.com/catalog/product/992 and the 42*19 mm wheels and extended brackets.

I read that beginners should choose motors that do not go to fast, so I think this is a nice setup to start with. My only concern is how big can the robot be? Or better how much can the robot weigh? I mean, if I build this and see that the motors are underpowered, its a waste of good motors.

Or maybe have an example robot with this setup of motors? Or I can look at the size?

Many thanks!

Best regards,

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From some robotics info posted long ago, sort of "back of the napkin" calculations paraphrased by another robot builder from a magazine article a number of nears back.

Here are the equations (from Robert Nansel's Nov. 2000 "Amateur Robotics").

Fw = Tp / ((sin b + Cr * cos b) * r) where 


  • Fw is the weight the robot will be able to carry 
  • Tp is the torque of the motors 
  • b is the angle of incline 
  • Cr is the rolling resistance of the floor 
  • r is the radius of the wheels 
For a level surface this simplifies to: Fw = Tp / (Cr * r) Mr. Nansel lists variousvalues of Cr: 
  • 0.015 for concrete, 
  • 0.08 for wood, 
  • 0.3 for carpet. 


Be sure to use the continuous torque rating for Tp, and not the stall torque rating. He suggests using a third of the stall torque to derive the continuous torque when it's not known. In our case, Pittman has conveniently listed the continuous torque in tables. So for your motor:

continuous motor torque is 9.5 oz/in * 5.9 gear ratio = 56 oz/in output torque two motors so total torque is 112 oz/in;

Tp = 112 oz/in

Solving the first equation with Cr = 0.3, incline angle b = 15 degrees, wheel radius of 1.5 inches:

Fw = Tp / ((sin b + Cr * cos b) * r) Fw = 112 / ((sin 15 + 0.3 * cos 15) * 1.5)

Fw = 136 oz or 8.5 lbs

This is sort of a "worst case scenario" so you could probably relax some of these parameters. For example, most robots spend all their time indoors on level ground.

Fw = Tp / (Cr * r) Fw = 112 / (0.3 * 1.5) Fw = 249 oz or 15.5 lbs

Also, rolling resistance of 0.3 is pretty high, unless you have some really thick shag. Lowering our assumption for Cr to 0.2 results in: Fw = 112 / (0.2 * 1.5) Fw = 373 oz or 23.3 pounds


Thanks for the picture, its good to see how small these are indeed. Maybe its better to search for something a little bigger for a first robot.



Here is a picture that may help you deceide:


This is the yellow wheel and this is the white moto.

Notice how much cheaper the bigger motors and wheels are, you could get both motors and wheels for the same price of ONE Pololu metal motor.

The coin in the picture is a Deutsche Mark which is roughly the same size as a US Quarter.

Hope that helps.

nice to see the size of the small ones. They are perfect for a small robot im going to build

Hi, those motors you chose are great but very small. Check the specs but to get a rough idea it is about the size of the middle bone of your pointer finger. Using 2 of those to drive I wouldn`t go any bigger than a 10cm x 10cm 500g robot.

For your first robot the tamiya dual gearboxes, DAGU gear motors or solarbotics/pololu plastic ones give you good bang for your buck.