How to Measure RC Car Motors: A Comprehensive Guide

When it comes to optimizing the performance of your RC car, understanding how to measure and evaluate your motor is essential. The motor is the heart of your vehicle, influencing speed, torque, and efficiency, and knowing how to properly assess its key metrics can help you get the most out of your setup. In this guide, we’ll cover the critical aspects of measuring RC car motors, including KV rating, RPM, current draw, and size, so you can ensure your motor is running at its best for any terrain or race.

Tools Needed to Measure RC Motors

Multimeter

A multimeter is an essential tool for measuring various electrical properties of an RC motor, such as voltage, current, and resistance. When testing RC motors, a multimeter is primarily used to measure the current draw and voltage levels, both of which are critical for evaluating motor efficiency and power consumption. By attaching the multimeter in series with the motor’s power supply, you can determine how much current the motor is drawing under different load conditions. This helps in identifying whether the motor is operating within its safe limits or drawing excessive power, which could lead to overheating or damage.

In addition to current and voltage, some multimeters can also measure motor resistance. This is useful for diagnosing potential motor issues such as internal short circuits or faulty windings, which can affect performance. Regularly measuring these electrical properties with a multimeter ensures that the motor is running optimally and helps prevent unexpected failures during operation.

Tachometer

A tachometer is used to measure the revolutions per minute (RPM) of an RC motor, which is a direct indicator of its speed. The RPM is crucial for understanding how fast the motor can spin under various conditions, which in turn affects the speed and acceleration of the RC car. There are several types of tachometers, including contact and non-contact versions. Non-contact tachometers are particularly useful for measuring the RPM of small, high-speed motors like those found in RC cars, as they can measure the speed without physically touching the motor.

By pointing a tachometer at the spinning motor or its output shaft, you can obtain an accurate reading of its speed. This is important for performance tuning, as different motors have different RPM characteristics, and knowing these allows you to match the motor to the right gear ratios, ensuring the RC car achieves its optimal performance on the track or terrain.

Calipers

Calipers are used to measure the physical dimensions of the motor, such as its diameter, length, and shaft size. These measurements are essential when fitting the motor into the RC car’s chassis or when upgrading to a new motor. Most RC car motors come in standard sizes, but there can still be variations that affect compatibility with different models. By using calipers, you can precisely measure the motor’s external dimensions to ensure it fits within the motor mount and does not interfere with other components of the car.

Calipers are especially useful when working with custom builds or when replacing parts, as even small differences in motor size can affect the car’s balance, weight distribution, and overall performance. Accurate measurements with calipers also help when comparing different motors for upgrades, ensuring you choose a motor that fits both your car and performance requirements.

Power Supply Unit (PSU)

A power supply unit (PSU) provides the necessary and stable voltage and current to the RC motor during testing. Instead of relying on a battery, which can fluctuate in power output, a PSU allows for controlled and precise measurements under different voltage and current settings. This is particularly useful when testing the motor’s performance at various power levels or when trying to simulate real-world conditions without actually driving the RC car.

A PSU allows you to adjust the voltage to see how the motor responds at different levels, providing insights into its efficiency and power output. For example, a motor’s RPM will increase with higher voltages, but you need to ensure that it stays within safe operating limits to avoid damaging the motor. Using a PSU also makes it easier to measure the motor’s KV rating (RPM per volt) and other key parameters in a controlled environment.

Measuring Motor KV Rating

Definition of KV Rating

The KV rating of a motor refers to the number of revolutions per minute (RPM) the motor will turn when supplied with 1 volt of electricity without any load. In other words, the KV rating is a measure of the motor’s speed capability at a given voltage. For example, a motor with a KV rating of 2000 will spin at 2000 RPM for every volt applied. This rating is critical in RC applications because it helps determine how fast the motor will turn when paired with a battery, directly affecting the speed and performance of the RC car. The higher the KV rating, the faster the motor will spin, but it may produce less torque compared to a lower KV motor.

Step-by-Step Process to Measure KV

Prepare the Motor for Testing:
- Mount the motor securely to prevent movement during the test.
- Ensure that no load (such as a propeller or gear) is attached to the motor shaft. The motor should spin freely.
Set Up Your Power Supply:
- Use a stable, adjustable power supply unit (PSU) to provide a controlled voltage to the motor. Ideally, use a voltage that is within the motor’s operating range (e.g., 6V or 12V).
- Connect the positive and negative leads of the power supply to the motor’s terminals.
Measure the Voltage and RPM:
- Set the PSU to a specific voltage and record this voltage using a multimeter.
- Use a tachometer to measure the motor’s RPM at that voltage. Point the tachometer at the motor shaft or any reflective part of the motor to get an accurate RPM reading.
Calculate the KV Rating:
- Once you have the RPM and the applied voltage, divide the RPM by the voltage to calculate the KV rating. The formula is: KV=RPMVoltageKV=VoltageRPM
- For example, if the motor spins at 24,000 RPM with a 12V supply, the KV rating would be: KV=24,00012=2000 RPM/VoltKV=1224,000=2000RPM/Volt
Cross-Check the Results:
- Repeat the measurement at different voltages to ensure consistency in the KV rating. The value should remain the same regardless of voltage, as long as the motor operates without a load.

Importance of the KV Rating in Motor Performance

The KV rating is one of the most important factors when selecting a motor for your RC car because it directly influences the vehicle’s speed and torque characteristics. A higher KV motor will provide more RPM per volt, making the car faster, but it might sacrifice torque, which is essential for acceleration and handling in off-road or rugged conditions. Conversely, a lower KV motor generates more torque, providing better control in rough terrains or when towing, but it will offer less top-end speed.

Knowing the motor’s KV rating helps in matching it to the correct battery voltage and gearing for your specific RC car setup. For instance, a motor with a high KV rating may perform best in high-speed track races, while a motor with a lower KV rating might be ideal for off-road RC cars where torque and control are more important. Properly understanding and measuring the KV rating allows you to optimize the motor’s performance to suit your driving style and terrain, ensuring a balanced setup between speed, torque, and efficiency.

Measuring Motor RPM

What is RPM and Why It Matters?

RPM, or revolutions per minute, refers to the number of times a motor’s shaft completes a full rotation in one minute. In RC car motors, RPM is a critical measurement because it directly affects the vehicle’s speed and overall performance. The higher the RPM, the faster the motor spins, which translates to higher speed for the RC car. RPM also helps determine the motor’s efficiency and load-handling capability. By measuring RPM, you can assess whether the motor is performing optimally or if it’s under excessive strain, which can lead to overheating or premature failure.

Knowing the motor’s RPM is especially important when tuning your RC car for specific driving conditions. For example, high RPM motors may be better suited for flat, high-speed tracks, while motors with lower RPM but more torque might perform better in off-road environments where acceleration and control are more important than outright speed.

Tools Needed (Tachometer)

To measure the RPM of an RC motor, you’ll need a tachometer. A tachometer is a device that measures the rotational speed of the motor shaft. There are two main types of tachometers:

Contact Tachometer: Requires direct physical contact with the motor’s rotating shaft. This type is less commonly used for small, high-speed motors like those in RC cars because the contact can affect the motor’s performance during measurement.
Non-Contact Tachometer: Uses a laser or optical sensor to measure the RPM by detecting the motion of the rotating object. For RC motors, a non-contact tachometer is ideal as it allows you to measure RPM without interfering with the motor’s operation. It’s fast, accurate, and doesn’t require direct contact with the motor’s moving parts.

How to Measure the RPM of a Motor?

Prepare the Motor for Testing:
- Securely mount the motor on a test stand to ensure that it remains stable during operation. Make sure there is no load attached to the motor (e.g., no wheels or gears).
Connect the Power Supply:
- Power the motor using a reliable and controlled power source, such as a power supply unit (PSU). This ensures that the motor runs at a steady and known voltage, which helps in obtaining consistent RPM measurements.
Set Up the Tachometer:
- If using a non-contact tachometer, attach a reflective strip to the motor’s shaft or a part of the rotating assembly. This will allow the tachometer to accurately detect the rotations without direct contact.
- Turn on the tachometer and point it at the reflective strip. Make sure the sensor has a clear line of sight to the rotating part of the motor.
Measure the RPM:
- With the motor running, aim the tachometer at the rotating shaft or reflective strip. The tachometer will measure the number of revolutions per minute and display the value on its screen.
- Ensure that the motor is running at a consistent speed when taking the measurement. Record the RPM for analysis.
Cross-Check with Different Voltages:
- To ensure the accuracy of the measurement, test the motor’s RPM at different voltage levels. A higher voltage should result in a higher RPM, as the motor spins faster with increased power. This helps verify that the motor is functioning correctly and allows you to observe its performance characteristics under varying conditions.
Interpret the Results:
- Compare the measured RPM to the motor’s specifications. If the motor is running significantly below its rated RPM, it might indicate issues like wear and tear, overheating, or electrical problems. High RPM measurements, on the other hand, may signal that the motor is functioning efficiently or being pushed beyond its ideal operating range.

Measuring Current Draw

Definition of Current Draw in Motors

Current draw refers to the amount of electrical current (measured in amperes or amps) that an RC motor consumes while running. It is a key indicator of the motor’s power consumption and efficiency. The current draw depends on factors such as load, voltage, and the type of motor being used. A higher current draw indicates that the motor is consuming more power, which can lead to increased heat generation and potential stress on the motor, ESC (Electronic Speed Controller), and battery. Monitoring the current draw is essential for ensuring that the motor operates within safe limits, avoiding damage to the electrical system, and optimizing overall performance.

Excessive current draw can lead to overheating, reduced motor lifespan, and poor battery performance. Understanding and managing current draw helps prevent these issues, making it a critical parameter to measure, especially during motor tuning or testing under different loads.

Step-by-Step Instructions on Measuring Current with a Multimeter

Gather the Necessary Equipment:
- You’ll need a multimeter capable of measuring current (in amps) and a power supply unit (PSU) or battery to run the motor. Ensure the multimeter can handle the expected current range (typically 10A or more for RC motors).
Set the Multimeter to Measure Current:
- Turn the multimeter dial to the DC current (A) setting. Most multimeters have dedicated ports for measuring high current; you may need to plug one of the probes into the “10A” or “A” port on the multimeter.
Connect the Multimeter in Series:
- To measure the current, the multimeter must be connected in series with the motor and power supply. This means that the current flowing through the motor must pass through the multimeter.
  - Disconnect one of the motor’s power leads (either positive or negative) from the PSU or battery.
  - Connect the multimeter’s positive (red) probe to the motor terminal where the lead was disconnected.
  - Connect the multimeter’s negative (black) probe to the disconnected power lead. This forms a series circuit, allowing the multimeter to measure the current flowing into the motor.
Power On the Motor:
- Turn on the PSU or connect the battery to power the motor. The motor should start running, and the multimeter will display the current draw in amps.
- If you’re testing the motor under load (e.g., with wheels attached), make sure the motor is operating under realistic conditions to get accurate measurements.
Record the Current Draw:
- Observe the multimeter reading and note the current draw. For most RC motors, you’ll want to measure the current under no-load conditions (without a load) as well as under load (with a propeller, wheels, or another accessory attached). This gives you a complete picture of how much current the motor is drawing in different scenarios.
Turn Off the Power:
- Once you have recorded the necessary data, turn off the power supply and disconnect the multimeter to prevent accidental shorts or damage.

Understanding the Results and Their Implications

Low Current Draw:
- If the motor is drawing less current than expected, it may indicate that the motor is not working under enough load or that there is a mechanical issue, such as poor connections or insufficient voltage supply. Low current draw can lead to underperformance, meaning the motor may not reach its full potential in terms of speed or torque.
Normal Current Draw:
- A motor drawing current within its specified range means it’s operating efficiently. This is the ideal scenario, where the motor is running smoothly without drawing excessive power, ensuring a balance between performance and durability. The ESC, battery, and motor will also stay within their safe operational limits, reducing the risk of damage.
High Current Draw:
- A high current draw indicates that the motor is consuming more power than it should under normal conditions. This is often a sign of an excessive load (e.g., too large of a propeller or wheels), inefficient motor performance, or overheating. Over time, high current draw can damage the motor, ESC, and battery by generating excessive heat. It may also lead to shorter run times as the battery drains faster under heavy loads.
Overheating Risks:
- Continuously high current draw can cause the motor to overheat, which may lead to thermal shutdown, motor damage, or even failure. If you notice a consistently high current reading, consider reducing the load, improving cooling (e.g., using a motor heat sink), or switching to a motor better suited for your application.
Battery Life Impact:
- High current draw drains the battery more quickly, reducing overall run time. If the motor draws excessive current, it can cause the battery to overheat or degrade faster over time. Monitoring the current helps ensure you’re using a motor that is compatible with your battery’s capacity.

Measuring Motor Size and Weight

Importance of Motor Dimensions for Fitting in RC Cars

Motor dimensions play a crucial role in determining whether a motor will fit properly within an RC car’s chassis. Different RC cars have varying motor mounts and space limitations, so the motor’s physical size—its diameter and length—must be compatible with the vehicle’s design. If a motor is too large, it may not fit into the motor mount or may interfere with other components such as gears, drive shafts, or suspension parts. On the other hand, a motor that is too small may not provide adequate power or may lead to imbalance, affecting the car’s overall performance.

Additionally, knowing the motor dimensions is important when upgrading or replacing motors, as mismatched sizes can lead to inefficient heat dissipation, vibration, or even damage to the car’s internal components. Ensuring the motor fits snugly in the designated motor space guarantees proper alignment and performance, reducing the likelihood of mechanical failure during operation.

How to Use Calipers to Measure the Motor’s Diameter and Length

Calipers are precision measuring tools used to accurately measure the diameter and length of the motor. To ensure the motor fits correctly in the RC car, follow these steps:

Measuring the Diameter:
- Use the caliper’s jaws to measure the motor’s diameter. Place the motor between the two jaws of the caliper and gently close them around the widest part of the motor (usually around the stator or the motor casing).
- Read the measurement on the caliper display or scale. Ensure you are measuring the motor at its widest point to get the correct diameter, as this will help ensure it fits into the motor mount without causing interference.
Measuring the Length:
- To measure the motor’s length, open the caliper’s jaws wide enough to fit the full length of the motor, from one end of the casing to the other.
- Close the jaws carefully until they make contact with both ends of the motor. The length measurement is crucial for determining whether the motor will fit within the available space in the RC car’s chassis, particularly if there are space constraints for motor mounts or adjacent components like gearboxes.
Measuring the Shaft Diameter:
- If needed, use the calipers to measure the motor shaft diameter. This measurement is important to ensure the motor shaft is compatible with gears, pinions, or pulleys used in the car’s drivetrain.

By using calipers, you can obtain precise measurements of the motor’s dimensions, ensuring that the motor will fit properly within the RC car’s designated space and align correctly with the drivetrain.

Weighing the Motor and Why It’s Relevant for Balancing the Car

The motor’s weight can significantly affect the balance and handling of an RC car, especially for competitive racing or high-performance models. RC cars rely on precise weight distribution for optimal handling, acceleration, and cornering. A heavier motor can shift the car’s center of gravity, affecting its stability and making it harder to control, especially at high speeds or on uneven terrain. Conversely, a lighter motor might improve agility but could result in reduced traction or power output.

To weigh the motor, you can use a digital scale:

Place the motor on the scale:
- Make sure the scale is set to zero before placing the motor on it. Record the motor’s weight in grams (or ounces, depending on the scale).
Consider the Weight in the Overall Car Setup:
- If you are building or upgrading an RC car, consider the weight of the motor in relation to other components like the battery, ESC, and chassis. A heavier motor might require adjustments in other parts of the car, such as stronger suspension or more powerful shocks, to compensate for the added weight.
Balance the Car:
- For competitive racing, balancing the car by evenly distributing weight across the chassis is crucial. An unbalanced car will not perform as well when cornering, accelerating, or braking. By weighing the motor and considering its impact on the overall weight distribution, you can fine-tune the RC car for better handling and performance on different terrains.

Understanding Efficiency

What is Motor Efficiency and Why It Matters?

Motor efficiency is a measure of how effectively a motor converts electrical energy into mechanical energy (motion). In simple terms, it’s the ratio of the useful mechanical power output to the electrical power input. The more efficient a motor is, the less energy it wastes in the form of heat and other losses. This is especially important in RC cars because high efficiency means longer run times, better performance, and less heat generation, which prolongs the life of the motor and other electrical components.

Inefficient motors consume more energy than they convert into useful work, which not only drains the battery faster but also causes the motor to overheat. Over time, this can lead to reduced performance, increased wear and tear, and eventual motor failure. On the other hand, a motor with high efficiency will allow for better energy utilization, meaning the RC car can achieve faster speeds and better acceleration without putting as much strain on the electrical system.

How to Calculate Efficiency Using Voltage, Current, and RPM?

To calculate the motor’s efficiency, you need to measure both the input electrical power and the output mechanical power. These can be calculated using the voltage, current, and RPM.

1. Calculate the Input Electrical Power:

The input electrical power is the total power supplied to the motor from the battery or power supply. It is calculated using the following formula:

Input Power=Voltage×CurrentInput Power=Voltage×Current

Where:

Voltage (V) is the voltage supplied to the motor.
Current (I) is the current drawn by the motor in amperes (A).

For example, if the motor is operating at 12V and drawing 3A of current, the input power would be:

Input Power=12V×3A=36 wattsInput Power=12V×3A=36watts

2. Calculate the Output Mechanical Power:

The output mechanical power is the actual power the motor produces in terms of rotational motion. This can be calculated using the motor’s RPM and torque. The formula for mechanical power is:

Mechanical Power (Watts)=RPM×Torque (Nm)9.5488Mechanical Power (Watts)=9.5488RPM×Torque (Nm)

Where:

RPM is the revolutions per minute of the motor.
Torque is the rotational force produced by the motor, measured in Newton-meters (Nm).
The constant 9.5488 converts the rotational speed and torque into watts.

For example, if the motor is spinning at 10,000 RPM and producing 0.1 Nm of torque, the mechanical power output would be:

Mechanical Power=10,000×0.19.5488≈104.72 wattsMechanical Power=9.548810,000×0.1≈104.72watts

3. Calculate the Efficiency:

Once you have both the input electrical power and the output mechanical power, you can calculate the motor’s efficiency. Efficiency is expressed as a percentage and can be calculated using the following formula:

Efficiency=(Mechanical PowerElectrical Input Power)×100Efficiency=(Electrical Input PowerMechanical Power)×100

For example, if the motor’s mechanical power output is 104.72 watts and the electrical input power is 150 watts, the efficiency would be:

Efficiency=(104.72150)×100≈69.81%Efficiency=(150104.72)×100≈69.81%

This means the motor is converting 69.81% of the electrical energy into useful mechanical energy, while the remaining 30.19% is lost as heat or other inefficiencies.

Why Efficiency Matters?

Longer Run Times: Motors with higher efficiency draw less current from the battery for the same level of performance, extending the car’s run time. This is especially important for battery-powered RC cars, where maximizing battery life can significantly improve user experience.
Less Heat Generation: Efficient motors produce less waste heat, which helps keep the motor and other components cool, preventing overheating and increasing the lifespan of the motor.
Better Performance: High-efficiency motors make better use of the available power, allowing the RC car to achieve higher speeds and better acceleration without requiring additional energy.
Cost Savings: Efficient motors reduce the strain on batteries and ESCs, potentially lowering long-term costs by minimizing the need for frequent replacements or repairs due to overheating or excessive wear.

Enjoyed this guide of how to measure RC car motors? Then be sure to check out our other RC Rating guides.