Motor sizing

I have a question about what size motor I should get for a project I’m working on. The robot will be 18" diameter with two 4" treads lining the base. I need two motors, one for each tread. It doesnt have to go fast but when maxed out it will need to lift 100-150 lbs.
If anyone could help me out I’d greatly appreciate it.


By 18" inch diameter I’m guessing your building a circular robot??? What is it going to do, just curious. For your motors you should probably using CIMs just because you won’t ruin them if you happen to stall them out. If you know how to make a good gearbox for what your doing though I would personally go with 775 Pro’s.

2 CIMs should be fine if you keep the speed down. You’ll need to gear them down to around 10fps max (6-8 would probably be safer).

the 775 pro will be able to handle 150lbs? And would I be able to purchase a gear box as well? I plan on 3D printing the tread gears because I have been having trouble finding them as well.

You’ll need to use gears to reduce the speed of the CIM to a more reasonable level. The “free speed” of the CIM is 5310 rpm, which is probably far more than you want for this. Vexpro and Andymark sell gears made to mount onto a CIM shaft, and gears for hexagonal shafts; a few pairs of gears will get you the speed you need.

what would you say would be the end goal for rpm. I was recommended previously to get a motor that would have cost me 150$ each and then I was going to have to get a motor driver as well which would have been another 100$ on top of that. This would definitely help the cost project if this ends up working out.

And I really appreciate your help with this. I know what I want to do, just not how to do it haha.

SPARK motor controllers should work fine.
Your rpm depends on your wheel size. A higher wheel size will require a lower rpm to go as fast as a smaller wheel with a higher rpm. A 2" diameter wheel at 500 rpm will go:
2 *pi inches/rotation * 500 rotations/minute * 1 minute/60 seconds
= 52.4 inches/second, or 4.36 feet/second (fps).
If you’re not very familiar with this sort of thing, finding somebody who is experienced or simply using easier, slower motors (like the motor/controller combo you were looking at) may be a better option.
Andymark and Vex also sell premade gearbox options so you don’t need to make anything.

Ahh so correct me if im wrong, but this $18 motor will work as long as I can reduce the speed of it through gearing, whether I purchase a gear box or build a system myself?

Lift? So you’re not asking about drivetrain motors, you’re asking for motors to power a lifting mechanism?

To select a motor (or motors), the first thing you need to do is determine the power required.

So… how high do you need to lift the 150 lbs and how fast? That’s the info you need to estimate the power required.

lift was the wrong word. It just needs to be able to hold and move 150lbs

That is correct. If you do get a gearbox I would suggest you use this one:

It is one of the cheapest gear boxes that can accept 2 CIMs, and the default options seems to be about the gear ratio you want. Check if this will work with someone who knows more about your project though.

For a motor controller, you should probably get SPARKs (like one of the previous posts stated). They are the cheapest motor controllers you can get, and are even on sale right now :stuck_out_tongue: .

just a question what is the purpose of a motor controller? and the gearbox will allow the independent movement of the two motors or would I need two of them?

You need two gearboxes. The motor controller lets you efficiently control speed of the motor without heat issues from the high current.
I highly recommend you find somebody who’s already worked with FRC or robots in your area to help you; this would be an expensive mistake to make.

To provide varying amounts of electricity to the motors, and to make them turn on, you will need a motor controller for each motor you use. A gearbox is just a set of gears packaged together that will reduce the speed of the motors’ output while increasing the torque. If you use the gearbox I linked, you will need one gearbox, 2 motors, and 2 motor controllers for each tread.


I’d definitely recommend that you call up Andymark. They have a phone number listed on this page here: You should be able to speak to one of their engineers and they’ll help you spec-out everything you need from their inventory for a one-stop shopping experience to finish up this project.

My best reference is someone who is young and doesnt really look at the dollar signs, this is just me getting another guide as well as to help me understand what I am doing rather than just taking advise and nodding blindly.

In terms of being able to handle the load though, would 775 pros motor and gearbox and SPARK motor controller theoretically work?

thanks again guys

Good to know you have somebody to work with, that makes me feel a bit better. :smiley:
775pros could work, but because of their smaller mass you may face heating issues if you run this thing for a long time. CIMs are much safer and only cost slightly more (and have more reliable stock).

Strictly speaking, any motor can move any load. It’s just a quesiton of how fast.

In terms of “handling the load”, these are the factors you’ll need to identify:

  • How fast does the robot need to move?
  • How much weight is it carrying (you already answered this)?
  • A bit more about the geometry of the drivetrain (to estimate how much it’s going to scrub/fight itself when it tries to turn)
  • The material of the tread surface
  • The surface it’s driving on

The first question will identify the speed of your drivetrain, and the others help answer the question of how much torque you’ll need the drivetrain to output. Multiply these together, and you get how much power you’ll need in your drive; that’ll allow you to pick an appropriate motor and gearbox for the drive.

So this instead?