In two previous articles we have talked about the gear ratio of a RC car, its effects on reliability and performance and how to calculate it, but we had not yet addressed how to adjust the gear ratio.
How to adjust the gear ratio
Whatever is the RC car, the gear ratio can be adjusted by replacing the motor pinion with one of a different number of teeth. Another operation which is almost always possible, is to replace the spur gear that meshes with the motor pinion.
In very few other cases, the gear ratio can be adjusted acting on the number of teeth of the differential gears or pulleys; the last is the case of scalers, where diversifying the front and rear gear ratio is used to achieve Overdrive/Underdrive.
Effects of adjustments
Adjusting the gear ratio is an extremely simple task. In particular, the gear ratio decreases by increasing the pinion teeth and/or decreasing the spur gear teeth; on the contrary, the gear ratio increases.
It is worth noting that a lower gear ratio is associated with higher top speed, lower acceleration, higher battery consumption and thermal stress on the motor and ESC. With a higher gear ratio you get exactly the opposite.
The perfect gear ratio
For each model, there is a range indicated by the manufacturer that depends on the actual size of the pinion and spur gear that can be installed.
The choice of gear ratio depends on the model type, its weight, the motor and batteries used, the track and the conditions of use (onroad, offroad, bashing, amount of grip...)
The gear ratio is therefore to be set in the allowable range, considering that the motor temperature should stay within 70°C to avoid damaging the conductors, sensors and magnets.
Conclusions
As you can guess, there is no a priori proper gear ratio, but it should be set as the best trade-off between motor health and car performance.
Last but not least and especially if you are using the RC car for competitions you must monitor and account for discharge times, which must be well suited to provide maximum battery performance in qualifying and race times.