# Which motor?

Is the following motor

pololu.com/catalog/product/1107

Suitable for 20lbs total robot weight in a 2 motor drive system? and what size of wheels do you recommend for it?

Hello.

They might work for you, but it all depends on what kind of acceleration/responsiveness you need, and you’re definitely getting on the heavy side of what I would consider reasonable. You’ll have to run the numbers and find out whether the torque from two of those motors (when combined with whatever wheel you choose) is sufficient for your needs. You might consider using a few of those motors in parallel instead of just one on each side.

You might find this blog post on force and torque helpful.

- Ben

I did the math using calculators for this and I would need 4 of those motors for a 25lbs total payload. So I would have a 4WD system. I think I will go with that. Also the stall current at 12V is 5A, so the 18v7 controllers would work fine, they are rated at 7A continuous.

The SMC 18v7 would be fine if you use one per motor, but I think you should use one motor controller per side, with each controller connected to two motors in parallel. In that case, you have a potential to be drawing 10 A if both motors on a side stall. It would be safer if you went with the SMC 18v15 (and the driver would generally run cooler), but you might be able to make the 18v7 work if you’re careful.

- Ben

Alright, the thing is it wouldn’t be 4WD, I would control 2 controllers that each control 2 motors instead of 4 controllers controlling 4 motors, right? But I see what you are saying, I’d be much more below the maximum of the controllers in Amps.

It would still be 4WD in the sense that all four wheels would be driven, but you would not have individual speed control over all four wheels. However, it’s not really useful to be able to make the two motors the same side turn at different speeds or in different directions, so using four motor controllers is kind of a waste (and it complicates things).

Have you seen our Wild Thumper Chassis? They are designed to be controlled in a similar way, with all the motors on one side connected in parallel to a single motor driver channel.

- Ben

Yes I’ve seen them, so by setting the -speed to 500 lets say and then --resume using the smc linux command tool both left (front and rear) motors would start at the exact same time and the same speed by connecting both motors in parallel with the controller?

That’s correct.

- Ben