Selecting the Right Motor for Your RC Car

Along with LiPo battery technology, brushless motors have been one of the biggest technology leaps in our hobby.  With brushed motors, electric RC vehicles were quiet, simple, and clean to operate but the power could not match their fuel burning counterparts.  With the advent of brushless motors, the tables have turned and now electric cars have power surpassing what is available from a piston engine along with the simplicity that goes with an electric powertrain.

So what exactly is a brushless motor?  The inner workings of a brushless motor could be the subject of an entire article in of itself, but the basics are easy to understand.  All electric motors work on the principle of magnetism, and the fact that like-poles of a magnetic field repel each other.  The windings of an electric motor are constantly reversing their pole via electricity, and this pushes the magnets past them.  An electric motor is in a constant state of repelling magnetic fields which in turn spins the rotor inside of it.

Brushed motors

In a brushed motor, the motor housing contains the magnets, and the stator (windings), are on the rotor that spins inside the motor can.  Since the rotor is spinning, there needs to be a way to supply power to those windings without having wires connected to it.  This is where brushes come in; brushes are carbon pieces that conduct electricity and are physically contacting the end of the rotor to which the windings are connected. Spring pressure holds the brushes against the rotor so that as they wear they stay in constant contact with the rotor.  Have you ever seen sparks coming from a brushed motor during operation?  This is coming from where the brushes are contacting the rotor.  The downsides to brushed motors are quite obvious; the brushes physically wear out as well as the surface of the rotors where the brushes contact, and there is friction and heat created from the contacting surfaces.  Brushed motors require rebuilding during their life cycle which involves refreshing the rotor contact surface by cutting it on a lathe, and replacing the brushes when they are worn.  The advantage to a brushed motor is that they have very smooth operation at low speeds, the current is easy to control since there is a physical connection and this is why vehicles that normally operate at low speeds can still benefit from brushed motor.  RC crawlers are one class of vehicle that still widely use brushed motors.  Below is a diagram which illustrates the principle of a brushed motor.

Brushless motors (to the rescue!)

It’s pretty obvious that having so much friction inside something that we are trying to make power with is not a good thing.  Friction builds heat, robs power and greatly reduces the efficiency of the motor.  A brushless motor is, as the name implies, an electric motor that has no brushes.  A brushless motor ingeniously flips the conventional motor design around which eliminates the need to send electricity to a constantly moving part.  In a brushless motor, the magnets are on the rotor, and the stator windings surround the rotor inside the motor housing.  You may ask yourself why this was not the original design of the electric motor since it is much simpler than a brushed motor, and the answer to that is in the complexity of controlling the power to the stator.  In the brushed motor, all of the “switching” is simplified in the fact that as the rotor turns, different contacts are exposed to the brushes and only the ones in contact will receive power.  A brushless motor requires a specialized speed controller to run it capable of switching the power electronically inside of it and at incredibly high speed, and brushless ESC’s accomplish this through the use of forward energy transistors, or FET’s.  The brushless DC motors we use in our RC’s are actually closer in design to a 3-phase AC motor, hence the 3 wires on brushless motors.  Since the “driving force” of an electric motor is accomplished through magnetic fields, the only friction in a brushless motor comes from the bearings on which the rotor rides. The diagram below illustrates the operating principle of a brushless motor, the stator windings around the stator plates constantly alter the magnetic poles surrounding the permanent magnets of the motor causing the rotor to spin.

Motor Sizes

Motor sizes can be a confusing subject, as there is no consistent standard for motor sizing.  In general, there are motors for minis (1/18, 1/16 etc.), 1/10, 1/8 and 1/5 scale, with quite a variety of sizing within those scales.  Many motors are referred to by a number, such as 540, 4068, 4076 etc.  A 540 motor is 540mm long (and 36mm in diameter), a 4068 is 40mm wide by 68mm long, a 4076 is 40mm wide and 76mm long and so on. You will often see 1/8 motors listed as either 40 or 42mm wide, the difference is in the additional diameter that comes from the “fins” on the motor cases.  The actual can is 40mm wide but the fins add an additional 2mm to the overall width.  Since there is no standard for nomenclature it is confusing because some names refer only to the length, and some to the length and diameter of the motor. For 1/10 vehicles, the most common motors are 540 and 550 motors.  Both of these are 36mm in diameter but the 550 is 10mm longer.  The longer rotor helps products more torque, so these motors are geared towards heavier 1/10 vehicles like short course trucks. The most common 1/8 motors are either 68mm for buggies, or76mm for truggies and monster trucks (again, the longer length for additional torque in the heavier vehicles).

These are the basics of motor size, but it should be pointed out that there are a seemingly infinite amount of motor sizes out there from many different manufacturers with most of the variety coming in the lengths.  Below is an example of some of the most common motor sizes next to each other for comparison.


Motors are rated by either “turn”, or “KV”.  Before brushless motors came into our hobby, brushed motors were rated by how many times the wires were wrapped around inside the motor, or how many “turns” of wire there were.  When referring to brushless motors, the most common rating you will see is “KV”, which stands for “kilovolt”.  Some brushless motors, however, are still rated by “turn”, with the most common being 540 2-pole competition motors and they will still usually have a corresponding KV rating.  The only reason for this is that in racing, the “turn” rating was used for so many years that it is just a more common nomenclature when referring to motors.  There is no physical difference between motors that are rated by turn or KV, it is only a difference in name. KV has become the more common rating for a motor as it is more easily visualized than turns, higher KV’s mean higher RPM’s where with turns, the lower the number the higher the KV’s/RPM.  For every kilovolt, a motor will turn one RPM per volt of electricity supplied.  For example, a 4000KV motor on a 2S/7.4V LiPo will spin to 29,600RPM (4000 x 7.4 = 19,600).

Understanding how KV relates to a motor in use is absolutely critical to picking the correct KV for your car and keeping it running well once it is installed.  A higher KV motor can spin to a higher RPM at a given voltage than a motor of a lower KV, but the trade-off is in torque.  Higher KV motors have less torque, so they have to work much harder to propel a vehicle to higher speeds and the trade-off is in increased heat and power consumption.  A higher KV motor will also draw down a battery quicker than a lower KV motor.  Many guys make the mistake of thinking they can simply switch to a higher KV motor to go faster.  While this is true to a degree, it is unfortunately not that simple.  Visualize a car that is stuck in 1st gear, and a car that is stuck in 5th.  The car stuck in first gear will take off nice and quick and there is not much load on the motor, the car in 5th will have to work incredibly hard to get going, and the engine would get very hot due to this load.  Translated to an RC car, a higher KV motor needs to be geared lower to not overload/overheat.  Gearing down lowers your top speed, so while you can still go faster with a higher KV motor it will not be able to simply add as many RPM’s to your car as compared to the lower KV motor on the same battery.  Brushless motors make great power, but the laws of physics still apply and nothing, when it comes to energy, is free.  If you simply substitute a higher KV motor and  leave the gearing the same it is very likely that you will overheat your motor to the point of failure, and in extreme cases it can take the ESC out with it.

You will often see motor KV’s rated to a certain voltage (2S, 3S etc.).  There is nothing in a motor that physically limits or controls the amount of voltage you can run to it, a motor that is rated to run up to 2S would run fine on 3S, but it will likely overheat quickly and fail. For the majority of applications it is important to adhere to the voltage guidelines provided by the manufacturer.

The way to determine if your motor is operating in the “safe” zone is by monitoring the motor temperature.  The best way to do this is with a temperature gun, a general rule is to keep your motor under 180F.  If you do not have a temperature gun, you can feel the motor (use EXTREME CAUTION as an overheated motor can burn your finger very quickly).  If you can’t comfortably leave your finger on the motor for 2 or 3 seconds, it’s safe to assume the motor is too hot.  Motor temps and gearing are covered in more detail in our “Brushless System Set-Up” article.

Sensored vs. Sensorless

The most basic brushless motors are sensorless motors, they only have the 3 power wires running to them and the order in which these wires is connected does not matter.  If the motor turns opposite of the direction you need it to run in, you can simply swap any 2 of the wires.  In a sensored motor, there is an additional harness plugged into the motor, and the motor connections are labeled as “A, B & C”.  The wires must be connected in the appropriate order for the motor to operate correctly, if the rotation needs to be reversed it must be done via ESC programming.  To easily understand the difference between a sensored and sensorless motor, visualize the operation of a brushless motor as a chain reaction that needs to happen in a specific order.  Power starts flowing, magnetic fields are created and the rotor starts to spin due to the force of repelling magnetic fields.  The problem is that the ESC needs to apply the correct polarity to the correct section of the stator depending on which position the rotor is in for this chain reaction to work nice and smooth.  In a sensorless motor, the chain reaction starts “where it left off”, and sometimes the rotor is in the wrong position for the reaction so you get what is commonly referred to as “cogging”.  This is the jerky operation of a brushless motor when taking off from a stop and at low RPM.  Certain parts of the rotor may be repelled, but others may be attracted to the magnetic field so the motor is fighting itself until RPM’s build and the chain reaction begins to work in harmony.  A sensored motor uses a hall-effect sensor which can tell the ESC exactly what position the rotor is in, so the ESC is able to fire in the exact sequence it needs to for motor operation to be nice and smooth right from the start.


You may see motors referred to as 2-pole, 4-pole, 6-pole and we are starting to see even 8-pole motors.  This is referring to the number of magnets that are on the rotor.  With more magnets, the motor can make more torque as it is receiving more “pushes” per rotation of the motor.  The trade-off is in high RPM operation, once the rotor starts to spin very quickly it is harder to control the increased number of pulses per rotation and that has an effect on the smoothness and control over the motor at higher RPM’s.


Some race oriented motors have housings that are marked with different “degrees”, and you can adjust the timing by loosening some screws and moving the motor housing to the desired position.  When you adjust a motors timing, you are changing the relation of the rotor to the stator when the firing of the phase occurs.  Adding timing is like getting a head-start on the chain reaction each time that phase fires, so the motor can make more power with additional timing.  As usual, there is a trade-off and as is usually the case the trade-off is in heat and efficiency. 

Increasing timing is reduces the efficiency of the motor; the more timing the less efficient it gets, the more heat it builds and the quicker it will run down your battery.  It is important to make timing changes in small increments and closely monitor motor temperatures to ensure the motor is staying under 180F.