Which motor to use?

I have been trying to determine what motors to use (269, 363 torque, or 363 speed) for our drive train. Our drive train will hopefully be a 4-wheeled tank drive made with mecanum wheels. It will also be geared up 18:12 (3:2) for speed.

To determine which motor to use I am trying to find out what is the max weight for our robot to weigh, yet only use 50% of my stall torque for each motor. Which ever one weighs closes to what I think our robot will weigh is the one we will probably go with. I have been looking around with both the vex search tool and google to try and find a way to find this out but I have not succeeded. Here is as far as I got (not far):

For short m1=269 motor, m2=363 motor (torque), and m3=363 motor (speed)

Gear Conversions (speed)
m1: m1.speed * 1.5 = output.speed or 100 * 1.5 = 150 rpm
m2: m2.speed * 1.5 = output.speed or 100 * 1.5 = 150 rpm
m3: m3.speed * 1.5 = output.speed or 160 * 1.5 = 240 rpm

Gear Conversions (torque)
m1: m1.torque * 2/3 = output.torque or 8.6 * 2/3 = 5.733 in-lbs
m2: m2.torque * 2/3 = output.torque or 13.5 * 2/3 = 9 in-lbs
m3: m3.torque * 2/3 = output.torque or 8.4 * 2/3 = 5.6 in-lbs

Now all I need is some magic formula where you input speed, torque, and wheel radius (about 1.84 inches) and it ouputs the maximum weight.

Well I only have a basic idea of what you need.

Wheel speed
Circumference=Diameter X PI
C=3.68 X 3.14
C=11.5552 inches

11.5552/1rotation X 100rotations/1minute= 1155.52inches/1minute

1155.52inches/1minute X 1minute/60seconds= 19.2586inches

Robot Weight

of Motors

im gonna go with 4 269s direct drive
Their output is usually 4.3
4.3 X 4= 17.2pounds

Correct me if im wrong i did this kinda fast

Torque is measured in inch-pounds. If you have 4 269s at 4.3 inch-pounds apiece, the total torque is 17.2 inch-pounds. If you have a 2 inch radius, the total force exerted is 8.6 pounds. Now, this doesn’t mean that those motors can move a 8.6 pound robot - you need to take into account the friction between the wheels and the playing surface, etc.
See this thread for more info: https://vexforum.com/t/black-c-channels/46011/1

I would figure out how much your robot is going to weigh and then adjust the gear ratio accordingly. It’s a lot easier to change a gear ratio on a drive train then it is to try to reduce weight on the robot. However this doesn’t change that you need the formula.


Very useful link for this topic. However, you need the coefficient of friction to figure all of this out, which I don’t believe anyone on the forum has been able to supply yet.

We’re working on the coefficient of static friction… https://vexforum.com/t/anyone-have-friction-coefficient-of-vex-wheels/19261/1
Specifically this post right now: [https://vexforum.com/showpost.php?p=192601&postcount=19

Trying to get more wheel configurations, and possibly more accurate data?


Thank you every one for the links here is what I have found out.

Relevant formulas from links:
Torque = Wheel Radius * Robot Weight * Coefficient of Friction

Wheel Radius can easily be determined, as well as Torque, and as stated above the Coefficient of Friction will be somewhere around 0.7 .

m1: 22.933 = 1.84 * Robot Weight * 0.7
m2: 36 = 1.84 * Robot Weight * 0.7
m3: 22.4 = 1.84 * Robot Weight * 0.7

Solving for Robot Weight:
m1: Robot Weight = 17.81
m2: Robot Weight = 27.95
m3: Robot Weight = 17.39

But those numbers are with 100% torque and now that I see those numbers I might be willing to go for 70% instead of 50%.

With 70% torque:
m1: Robot Weight = 12.47
m2: Robot Weight = 19.57
m3: Robot Weight = 12.17

Why do you have the torque for each setup at 2/3 the stall torque???

That is because it was for my 12:18 for speed gearing. Sorry I forgot to specify that.

ahh makes sense.

So a better way to write the equation would be:

(Desired % of torque from motor) * (Total Stall Torque from motors)
_________________________________________________________ = (wheel radius) * (weight) * (CoF)
(gear ratio)

I think we can lock that equation in.


ps- that line is division in case that’s unclear.