Brushed motors are commonly used for micro RC vehicles. The standards set out here are commonly accepted throughout the micro RC community and industry. However, please be aware that manufacturers may not always strictly follow these generally accepted guidelines.
The red wire or the blue wire? Well we all know the answer to that question, but when it comes to choosing the correct RC motor for your application a bit more thought is needed. These days, brushed motors commonly come with blue and red wires or black and white wires, but each is very different. Both motors are specifically designed to run in one direction only.
You may say that the motors are tuned to be more efficient in one direction. But more importantly than this, the brushes inside are augmented for a shaft turning in one particular direction. Because of this, if you run a motor the reverse direction for long enough it will certainly ruin the motor (the brushes). This image shows the correct direction of rotation.
Note that once you remove the back plate of a brushed motor you will not be able to re-attach the back plate without damaging the brushes.
With the advent of quads, motor sets have come to be developed with motors tuned to rotate in different directions. This is why we have motors these days with different coloured wires and as these motors are tuned to run in one direction, it’s important to understand which direction that is.
To determine yourself which direction your motor should be run you can simply test the motor. Use the black or blue wire as earth and the red or white wire as positive.
When talking about direction we first need to separate motor from propeller so to speak, because direction has a different meaning for each. Let’s start with motors.
Motor direction is stated as either clockwise (a positive motor) or counter clockwise (a negative motor). But most importantly, this is determined by looking at the front of the motor with the shaft pointing toward you. Note that a positive motor has blue and red wires and a negative motor has black and white wires.
Propeller direction is always stated looking from the back face of the propeller. Imagine sitting in the cockpit of an airplane. The prop in front of you turns clockwise and that’s a “standard” or “positive” propeller. The slight right and down thrust you see on the motor of most RC planes is correct for a clockwise rotating motor.
So here’s where is gets difficult. A Negative motor (rotating counter clockwise when viewed from the front) will actually turn clockwise when viewed from the rear and that’s why this Negative motor will need a standard “positive” prop.
So if you are wanting to set up a standard plane with motor at the front you will need a negative motor with black and white wires. But this is only the case if you are attaching the prop directly to the motor shaft.
A geardrive actually reverses the direction of rotation because you have a pinion gear on the motor shaft turning a spur gear attached to the drive shaft that the propeller is attached to. So now everything is opposite. You will now need a Positive (clockwise rotating motor) which will turn the geardrive’s drive shaft counter clockwise. This is suitable for “standard” or “positive” propeller.
Of course if you have a pusher prop or a plane with contra-rotating motors mounted on each wing then you will once again need to re-consider the above as relative to your needs. Just keep the main points in mind and work out which motor is needed by thinking about motor and prop rotation directions. Remembering also that a pusher prop configuration will require the prop to be attached in reverse on the drive shaft.
Most 4mm motors will have either 0.6 or 0.7mm shafts. 6mm and 7mm motors will generally have either 0.8mm or 1.0mm shafts. In particular, on a 7mm motor with a long shaft, the diameter will usually be 0.8mm. 8.5mm and 10mm motors will almost always have 1mm shafts. For direct drive applications, you will mostly need to drill out the bore of a small propeller to allow it to fit on a 1mm shaft.
Almost all motors will be 1S (3.7volts) compatible. However for 8.5mm motors there is also a 2S version available which is suitable for 7.4 volts. These look identical to the 3.7volt 1S version.
Great care is needed when pushing anything onto the shaft of a brushed motor because the rear end of the shaft is supported by nothing more than the plastic end plate at the back of the motor. And that may not be held in firmly to the casing. Applying a reasonable amount of pressure to the shaft may break or pop the rear plate off the motor and render it useless. When pressing a prop adapter, gear or propeller onto the shaft always support the back of the motor.
Choose something to support
the back of the motor that
fits between the wires.
Press the propeller, gear or
prop adapter onto the shaft
No damage to the back plate.
The images below summarise both positive and negative motor
types and describe what props should be used for each.