Maximum Amount of Weight For Drive Train

Hello Again VEX Forums!

I have one more question for you. I want to find out the amount of weight a four motor drive train can handle. I would just like to know the calculation for this and nothing more. If you could tell me this piece of information it would be greatly appreciated.

Hello Again VEX Forums!

I have one more question for you. I want to find out the amount of weight a four motor drive train can handle. I would just like to know the calculation for this and nothing more. If you could tell me this piece of information it would be greatly appreciated.

Well, you can theoretically use 4 motor drive to drive a robot of any weight. It just matters how fast you want the robot to go. We used a 4 motor drive 30 lb robot for Starstruck. We had it geared down too. I can’t remember the exact ratio though.

4 HS drive with 4" wheels (Pretty much no friction) does 17+ pounds easy for me.

Is there any specific formula or calculation for this?

Because for our robot, we are using a 4 HS motor drive train, with 4" omniwheels, about 22 pounds, but it doesnt drive for as long as we want it to w/o burning out the motors too quickly

It also depends on how quickly you change requested values for the motors. You might want to try slew rate. It could also depend on how you have it wired? Where are the drive motors plugged in? What ports?

4 HS with 4" omnis safely drives ~16lbs of robot. If you search this forum, you will find this topic recurring often. an example.
Somewhere prior to starstruck, there is a thread where I document some real world testing that lead to the calculator you will find in the above link.

I’m on the same team. The 4 HS motors are Y’d into ports 2 and 9. We have tested it with slew rate and the problem still persists. The current formula we are using to calculate the weight it can handle is (( Stall Torque*4 )/Radius of wheel ). Is this the correct formula? If it is, then our robot should work fine with 3.25" wheels. With 3.25" wheels it can do ~22.6 in lbs, and our robot is 20 lbs.

I believe that is correct, although I am not 100% sure. For our robot, we were only driving 2 of the wheels.

There is a calculation.

(14.76inlb * number of motors)
---------------------------------------------- = weight it can move

If you want the ideal gear ratio based on the robot and game object weight, just switch gear ratio and weight in the formula.

This formula is not derived from any physics concepts like work or energy, but i made sure the units work out, and it works well in practice for a large range of all variables used, so this should help you.

is there a way to convert this to include high speed and turbo. Because the variable (number of motors) is just for torque if im not mistaken.

Iirc when Bryan told me, you just multiply that denominator by the internal ratio

This would be for normal geared 393 motors? So if I wanted the calculation for HS 393’s, then i would replace 14.76 with 9.2 correct?

If that is the case, then the robot could potentially hold ~ 36.8 pounds??? That doesnt sound right… Please let me know if i am misunderstanding anything

OH i see now haha, i didnt realize the dotted line was a division symbol…

That formula would calculate the horizontal force the wheels would exert when the motors are stalled, not the amount of mass they can safely accelerate. I’m not sure what the correct formula would be.

It also depends on power given to the motor. My team tested how many rpm you get from different power levels(0-127) and the amount of time it takes to burn out. We found that the rpm around 90 is almost the exact same as 127 but that it takes longer to burn out because there is less voltage heating up the motors

The power level set for a 393 does not directly relate to RPM. A power setting of 127 is not 127 RPM, but it is the maximum power the Cortex can send to a motor. A 393 with a standard set of gears is capable of 100 RPM, but will never reach that with a load. When you set the power to 90, you are setting it for about 70% max power, not 90 RPM. Actual RPM will be determined by battery levels, motor health and actual net load on the motor.

It is not voltage that creates heat, it is power, which is, for dc, volts times amps. Power is measured in watts, not voltage. Using 100% of the power setting will send the maximum possible power (without looking it up, something like 5 watts) to the motor, which will heat the motor faster than if are sending 70% power (roughly 90/127).

Interestingly, the V5 motors default to an RPM setting, not a power level setting. When you tell a V5 motor with a 100 RPM gear box to “go 70” you are setting it for 70 RPM, and the built-in motor control computer will do its best to get to and hold 70 RPM, no matter what the battery state is. The V5 motors aren’t just bigger, they are W A Y smarter, too.