I’m thinking about building a robot just with some old parts I have. The main thing is the motors. I have 2 DC Motors
Specs below. (It’s a MFA975D Series with gear ratio 49:1)
Operation range(Voltage): 12V
No load - Speed 147RPM
Nm = 1,8

The tracks of the robot will be caterpillar from old tires, driven by chains onto a gear.

My robot might gonna weight approx 40kg

How do i calculate of the motors are strong enough and what speed would I be travelling in? Only horizontal no degrees inclined.

EDIT Thanks for helping me with my issues guy, i appreciate it a lot! The 2 motors have to run in a controlled loop. So when I want to go forward both motors go forward, and same as backwards. When I wanna turn, the motors gonna work in opposite direction of each other.

The things I want to know is , could it be possible to even use that motor standard, or should it need to be gear lower, for more torque? And how I’m calculating the speed of a my machine with this motor, my weight, the friction (Rubber vs Asphalt).

  • 1
    \$\begingroup\$ Ah, you need to know the torque of your motor. You might find some references about torques in Appendix B of this Q&A: electronics.stackexchange.com/questions/510755/…. Cheers. \$\endgroup\$
    – tlfong01
    Jan 24, 2021 at 11:36
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    \$\begingroup\$ You need to work out what friction losses you have at a target full speed. You also need to reveal what power the motors can produce and how long you are prepared to wait for the thing to reach that full speed. \$\endgroup\$
    – Andy aka
    Jan 24, 2021 at 11:38
  • \$\begingroup\$ Hi guys - Thanks for the answers. @andy - Do you know if there’s any formula for that? I very new in this topic. Br Christoffer \$\endgroup\$ Jan 24, 2021 at 11:55
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    \$\begingroup\$ That's a mechanical question so speak with a mechanical engineer. \$\endgroup\$
    – Andy aka
    Jan 24, 2021 at 11:57
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    \$\begingroup\$ If it's only horizontal, friction is generally going to be so low that almost any practical motor will move it, but the performance may be rubbish. There are so many variables like size of the track rollers, and whether it's going to look 'sluggish' to you, that the only answer is for you to throw one together and look at how it behaves. \$\endgroup\$
    – Neil_UK
    Jan 24, 2021 at 12:40

1 Answer 1


I would say you would go by trial and error, since there are factors like how the weight is distributed on the motors, will each motor run independently or will you have a closed control loop?

As @tlfong01 said in the comments, you need to know your torque. Higher torque = bigger force of the motor but slower speed.

And as @Andy aka said, you also need to know your frictions.

Technically, any motor can lift/drag any weight, if it has enough torque, but it will be really slow

  • 2
    \$\begingroup\$ Yes, high torque means high gear ratio and low speed. I learnt this lesson after carelessly overloading and crashing a couple of expensive metal gear boxes. :(. Now I am wiser and have a habit of buying several spare gear boxes (for different speed and loading for trial and error) for the same type of motor. \$\endgroup\$
    – tlfong01
    Jan 24, 2021 at 13:33

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