OpenRC Tractor FAQ

Frequently asked questions

Before asking questions, please check this BOM list of plastic parts and materials where you can find links to buy all you need to build your tractor.

Videos to help you with the assembly here

How do I connect the micro motors? / One motor is running forward and the other backward, what's wrong?

When using micro motors, one motor should be connected oppositely to the other to avoid one motor running forward and the other backward. As the motors are placed in an inverted position, in order for them to go forward or backward simultaneously their cables have to be inverted. If you have one wheel forwarding and the other going backwards, you should invert cables for ONE motor.

See picture 1 for more details


1-Micro-motor connections detail

Which radio transmitter and recevier should I use?

In order to be able to use the rear arms, you'll need a 3 channel radio transmitter and receiver. I have a pair of this one. They work ok and are cheap. The downside is that on the third channel for the rear arms, you'll only get a on/off button, meaning you can only put the arms up or down but not control them more accurately. If you want to do so, I also have this one that has 4 channels. It's more expensive but in addition to the on/off button channel, there is also one with a potentiometer that lets you control the rear arms (or the trailer ramps) more accurately.

I'm new to RC how can I connect the electronics?

  • Check BOM.pdf files and BOM.png image that you'll find attached to the design parts of the download
  • Check design pictures
  • I have written a post about components and connections with some extra instructions

I have problems assembling or connecting rear arms


Rear arms work with two micro-servos. One rear arm servo needs a servo reversed because the servos are mounted in opposite direction. If you don’t use one, one servo will go up and the other down. Once one servo has been reversed, both rear arm servos should be connected to one Y cable in order to join two cables into one. Now that we have only one cable coming from the rear arms, this should be connected to the 3rd channel.


When assembling rear arms, it is important the angle of the servo's arm. As both servos move simultaneously, they have to be in the same initial position, so when they are connected, they travel the same distance and have same endstops. So before connecting them, you have to manually (do it gently or internal gear may break) adjust them both to be aligned.

See picture 2 for assembly details


2-Rear arms detail

Which motor and speed controller (esc) should I choose?

In order to work, DC motors need a speed controller. The speed controller you need depends on the kind of motor you're using. There are brushed and brushless systems (motor and esc should be same type). In this case we don't need speed or lots of torque. I recommend the 56rpm brushed micro-motors from this link. For these motors, I'd suggest you buy this brushed speed controller. It's small and works well for a reasonable price.

I have the motor mod, how do I assemble the new parts?

If you want to use a standard 540 size motor, you’ll need to replace parts #1 (lower chassis), #2 (upper chassis), #23 right and left (fender arms) and the motor axle. Moreover, you’ll need to add pinion and gear parts that are not present on the original design.

I have written a POST with detailed pictures about assembly and connections.

I'm stuck with S5 (step 5) of the assembly instructions

On the original assembly instructions, there is a mistake in S5, as it seems that you have to mount servos on part 1 from the BOM picture. That's not correct, instead servos go on upper part of the chassis (part 2).
I have corrected the assembly instructions. Download updated assembly instructions

Download updated assembly instructions

Which arms, push rods and servos should I buy for steering and rear system?

I need help with the assembly of the fertilizer

I added a BOM pdf so you can check what kind of screws are needed. Download it from here (search for attached documents).


Which battery should I choose?

IMPORTANT! LIPO batteries must be treated with extreme caution, I take no responsibility of its use or any damage that could cause.

There are several batteries that would work with this project. I'm no expert, but I'll try to make it as easy and understandable as possible.

You can basically use LIPO batteries or NIMH batteries. You can use a 6 cell 7,2v Nimh/Nicd battery, they are easier to use and less dangerous, but as the battery space is limited, they are difficult to accommodate.

On the other hand, LIPO batteries have several different sizes and are easier to accommodate in the reduced space. However, LIPO batteries are dangerous if they are not treated with extreme caution. Here there are some common instructions when using LIPO batteries:

  • Never over-charge a LIPO battery or it may burn
  • Never over-discharge LIPO batteries or they will become useless
  • Don't put them beside a high temperature condition
  • Don't throw them into fire or water
  • Don't leave LIPO batteries unattended during the charge process
  • Always follow the manufacturer instructions

With that in mind, don't be scared, If you take some precautions, you shouldn't have any problems.

If you choose LIPO batteries, bear in mind that the battery space is limited, and a full RC lipo 2S or 3S battery won't fit. You'll need to use a "shorty" version or one with less mAh. The higher the mAh, the longer it will last, but we don't need long run times for this project. In addition to the battery, you'll need LIPO balancer/charger, as LIPO batteries have to be balanced. I recommend 2S 7,4v batteries but it depends on the ESC you use. With the one I recommend, I use this one and this one or (this one). Both work really good and I haven't had problems so far, just follow the instructions carefully! You'll also need a T-plug to JST adapter (choose model A) if you're using the ESC I suggest.

To charge them, I use an old LIPO charger that I already had, but seeing the comments, I think this IMAX B6 with this power adapter (IMPORTANT, you'll need both) seems good value for the money (haven't personally tried them though). Another option is this one that does not require an external power adapter.

It is also recommended to use a LIPO safe bag to prevent problems.

All in all, you'll need:

Hope that helps with your battery choosing!

I have problems with the trailer ramps

Trailer ramps work with two micro-servos. One ramp needs a servo reversed because the servos are mounted in opposite direction. If you don’t use one, one ramp will go up and the other down. Once one servo has been reversed, both rear arm servos should be connected to one Y cable in order to join two cables into one. Now that we have only one cable coming from the ramps, this should be connected to the 3rd or 4th channel of the radio. The cables are supposed to go through the tractor, that's why there is a hole between the rear arms.

See the bottom view picture for more detail:

OpenRC tractor trailer bottom view