# 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.

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.

http://combots.net/wiki/index.php/Optimum_gearing

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 .

Simplfy:
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.

-Nick

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