Robotic Car Gearing

I have students doing a robotic car design challenge in class. Some of my students are doing high speed ratios and even compound gearing to make those ratios higher for speed. They work well when you’re holding the car but don’t move once they’re set on the ground. Could it be because there’s so little torque? Wondering if anyone has done something similar and has any suggestions on what to do with programming and whatnot to make these high speed ratios work on the ground.

Definitely torque issues. Make sure the cars are as light as possible and if that doesen’t fix it, they need to lower the ratios. VEX motors have a stall point of 19 pounds I believe (its listed on the website) and if you’re gearing up to something like 1:21 it likely wont move

If you are using aluminum and the cars are still lightweight, then your students could change the internal gearing of the motors to the high speed option for the wheels. This should provide enough torque while also making the cars faster than just using regular 393 motors. This is definitely a lack of torque issue. Torque is needed to accelerate and move the robot. The heavier the robot, the more torque that is required to move it.
Even if you are using steel, I’m sure the high speed motors would still work.

You also could just use more motors. For example, a 2 motor 5:1 drive would not work but 12 motor definitely would work for that (as long as the robot is reasonably light)

Thank you! I was thinking torque was the problem so I appreciate the confirmation. I have a constraint on the design challenge where they are only allowed 2 motors to make it more of a challenge (and also because of limited parts). I was thinking upgrading the gears in the motors would be a good idea. I’ve never done it before but have thought about trying it. Does anyone have a resource that shows how to do it? Want to make sure I don’t break anything (still pretty new with this).

There is a video that “thatstemclass” posted on Youtube. It is with the vex 101 videos he is making. The video covers all the basics of changing internal gears and should be more than enough to get you through this.

Note that if torque is the reason the cars won’t propel themselves (and it seems likely it is) then changing the standard gearing to high speed gearing will reduce available shaft torque, not increase it. There are no internal gearing options available to increase torque. In fact, a 393 motor with standard gearing is often called a “torque motor” because of this.

Two 393 motors will move a lot of mass. How heavy are the cars? What gear ratio are they going for?

Considering the fact that they said compound gearing occurred, I would assume they attempted at least a 1:9 ratio lol

Good analysis; can’t be compound with our standard gears at less than 1:9.

If they are attempting 1:9 I don’t believe it will work with only two motors. Batteries and structure are simply too heavy and motors aren’t super strong. I’d recommend like a 1:3 ratio

1:9 and two motors might let you move a two pound vehicle on a very smooth and level surface.

True, but the battery, motors themselves and cortex are about 1.4 pounds, and adding 4 wheels will push it to about 2 pounds (that’s not including hardware). However, if you used 2.75" wheels it will strain the motors less. I recommend small wheels if you have them, though it will slow the robot down (because technically the wheel size to the ground is a gear ratio)

Using 3.25 in wheels can help because they are lighter than the 2.75 in wheels and can still provide more torque than if you used 4in wheels without sacrificing too much of the speed

I doubt weight is a problem in this situation, considering these are class project and probably small with only a few motors and one battery. I suspect you just can’t really do a compound gear ratio with two motors at all.

My freshmen year, I built a transmission bot that shifted through ratios of 5:3, then 3:5, then 1:7. I didn’t really know a whole lot about VEX at the time, so obviously those ratios aren’t ideal. It used only 2 high strength motors, built mostly of aluminum, and once you shifted through the second gear you could gain a bit more speed in the 1:7 before it topped out on not having enough torque to go any faster. However, there’s no way that the robot would start driving on its own in third gear, and it certainly will not go with a 1:9 and only 2 high strength motors.

@RuizK1 what are the exact rules for the competition?

Are you allowed to use elastics for storing energy?

Do you need to turn or drive in a straight line?

What is the length / shape of the track?

It’s a straight line drag race. Students are only allowed to use two motors. No elastics. It’s just a 10 meter long track.

Based on these guidelines, it seems the fastest gear ratio (probably 5:3 in this case, maybe 1:3), the lightest robot, and the drive with the least friction should win the race comfortably.

Ya, in that short a distance torque is that much more important. You can’t get going super-fast in such a short distance. Even with about ideal acceleration, you’re limited to about 14 m/s top speed. With only two motors, I’d probably look at multiple gears, but my first gear would be to get more torque, looking toward a maximum acceleration.

There is a whole science devoted to finding the best gear ratios for the race cars and the optimal time to shift gears. There are scientific papers, online calculators, tables, graphs, etc… However, they all apply to vehicles with internal combustion engines and multi-gear transmissions which have totally different power torque curves from what we try to model here.

Electric motor has a limit of maximum power it could provide and by choosing the gear ratio you could either maximize the torque or the velocity you get from it, but their product is limited P= τ*ω


Lets try to find the optimal gear ratio that will give you the fastest time to cover the distance S.

First, the vehicle accelerates during time t1, up to the maximum velocity Vmax, then it takes additional time t2 to reach its destination at constant speed. The shaded area on the graph corresponds to the total distance vehicle covers. The slope of the first step corresponds to the acceleration of the vehicle. Effective force out of drivetrain minus the force of friction goes to accelerate vehicle of the mass m according to Newton’s law F=m*a. Note that Vmax is always less than Vidle that you would get if there were no friction.

Since by picking optimal gear ratio you can choose between max speed and max torque (force) we need to find the optimal ratio between t1 and t2 to minimize total time it takes to cover distance S.


According to my calculations (which you should definitely check, because I could have made a mistake) the total time of the trip Ttotal = t1+t2 = (m*Vmax^2)/(2P) + S/Vmax. If you plot both components on the graph you could see that minimum travel time time corresponds to some value of Vmax = Voptimal. My guess is that vehicle needs to accelerate for the first 30-50% of the trip and then drive at that constant speed Vmax. Exact values would depend on the distance S and amount of the friction in the drivetrain and the mass of the vehicle.

Another important note is that as the vehicle accelerates you don’t want to sent max power to the motors right away, because they will overheat PTC and lose power before reaching the destination. Instead you want to apply a slew rate control by gradually increasing the motor power. For example, in 10 power unit increments per step. Where optimal time of the step could be determined experimentally.