The gear ratio of an RC car is a key parameter of the overall configuration and refers to the number of revolutions the motor makes for each revolution of the wheels.
The gear ratio of an RC car is provided by a geared transmission system, which in RC cars can be made in a single element (e.g., in 2-wheel drive cars) or be made up of several stages of reduction ( typically in 4-wheel drive cars).
Current and RPM relationship
Recall an article we have previously published that is useful in understanding the importance of gear ratio. in an electric motor powered by a battery, the current is maximum at the start and decreases as the RPM increases.
The gear ratio of a RC car protects the motor
If the motor is not connected to the transmission, it will take very little time to increase its RPM , and an obviously longer time if it is connected to the transmission, because of the applied load.
Increasing the gear ratio is the same as decreasing the effort to be exerted, just as happens in full-scale cars when using a low gear.
On the other hand, if the gear ratio is decreased the time to reach the maximum RPM will consequently increase, and the time required for the current to switch from the maximum value to the minimum value will also increase.
The current ratings in the first half of the graph are very high rates and can be tolerated by the motor for a very short time, otherwise the conductors in the windings may be irreversibly damaged (literally, they burn out).
The gear ratio must be high enough to allow the motor to increase RPM quickly without damage.
Criteria for action
The rule above is as valid as the one below.
The rule above applies to protect the motor, but the car must run, so the gear ratio must also be low enough to allow the car to get an appropriate speed.
The parameters to be considered for the optimization of the gear ratio are to be determined by the use of the model.
If the RC car is used for competitions, the gear ratio must ensure maximum performance to meet race time and component reliability. On the other hand, if the car is only used for playing, you can ignore running time and focus just on reliability to avoid motor damage.
A well-established method of safeguarding the motor is monitoring the operating temperature, which can be measured with an IR thermometer and maintained within 70°C (158 F); at a higher temperature the life of the motor could be quickly reduced by progressive damage to conductors, magnets and any sensors.
In conclusion
There is no universal correct gear ratio, and this is already clear in the user manuals, which leave room for rather wide ranges of values. The reason is that there are many variables and they depend on the motor used, the track, the weight of the model, and the diameter of the wheels.
Adjusting the transmission ratio can enhance the model’s performance but may shorten the motor’s lifespan. It is, therefore, up to us, considering what has been discussed so far, to find the right balance between model performance and the health of the motor.