Gearmotor & Controller for RC18T


I have an old RC18T RC car that I’d like to convert into a self driving car robot. It’s a 4 wheel drive 1/18th scale car. Off the shelf it has a “Super 370” motor and an “XPS micro ESC” speed controller and a 1100mAh NiMH battery, and it’s way too fast! I’ve done some experiments with an arduino. Here’s a video of the first one, and the second one. Those worked OK, but I couldn’t get smooth driving motion at low speed.

For the next build, I’m planning to use my raspberry pi (with a PWM board) for more powerful control options and a gearmotor and motor control board to slow it down and get more precise motion control.

Mechanically, the 25D mm gearmotors look like they will fit really well. They are the same diameter as the “Super 370” motor, same mounting holes, just a bit longer (which is no problem) and the drive shafts are thicker (4mm instead of 2mm). I think I can just drill out the pinion gear to make it fit, but I’m looking for feedback on which of these gearmotors might be the best fit.

I’m thinking I’ll need 1045 RPM from the motor under load to get to my ideal max speed of 12in per second. A bit slower would be OK too. With the current 14T pinion gear the car has a gear ratio of 10.71:1. Wheel circumference is 7.38" See attached excel sheet

Based on the RPM’s it looks like the 9.7:1 Metal Gearmotor 25Dx48L mm HP might be a good fit, but I’m not sure how to evaluate how much torque I need. Maybe I don’t need an “HP” gearmotor? And what speed can I expect under load?

Assuming that gearmotor is a good fit, would the VNH5019 Motor Driver Carrier work as control board for it?

I’m really looking forward to getting this set up, but these parts are new to me so a sanity check would be great. What do you think?

Thank you!
RC18T Motor Speed Calc 1-10-2015.xlsx (10.5 KB)


The speed you will get from the motors under load is dependent on your specific setup and something you will have to determine. You might find this force and torque blog helpful. In general, we recommend using our motors at 10% - 30% of their stall torques.

The VNH5019 motor driver carrier should be fine for those motors. You might also consider the Simple Motor Controller; it is also suitable for driving the motor you are considering and has several more features than the VNH5019 motor driver.

- Grant

Hi Grant,
Thanks for your feedback - that post on force and torque is really helpful! I’m working through applying it to my setup and will post back with the results.

Order placed!

#1582 9.7:1 Metal Gearmotor 25Dx48L mm

  • Should have plenty of torque, not too much current draw (2.2A stall), and should get the car moving up to around 6 in/sec, which would still be plenty fast.

#2135 DRV8835 Dual Motor Driver Carrier

  • With 3A max output (wired parallel) this should be beefy enough. plus, existing libraries for the pi! Almost got the one that’s made to fit on the pi, but wanted better access to the remaining pins.

#2574 Pololu 5V Step-Up/Step-Down Voltage Regulator S18V20F5

  • 2A of 5V to power the pi (and future goodies) from onboard batteries

See attached for the updated motor calc spreadsheet.

Looking forward to testing it out!
RC18T Motor Speed Calc 1-18-2015.xlsx (12.4 KB)

I took a quick look at your spreadsheet and it looks like when you calculated the force at the wheel you multiplied the diameter by two instead of dividing it to get the radius. However, you are probably fine since your calculated value underestimates the performance from the motor.

When you get the car running we would be interested in seeing it in action. Feel free to share it in the “Share Your Projects” section of our forum.

- Grant

Thanks for checking it out. I’ll have to double check the sheet, but it’s good to hear that it won’t be an issue. I’m looking forward to the parts, and will post again once it’s up and running. Thank you!