For every mating pair of gears in an RC car, there exists a gear ratio. The gear ratio is simply the number of teeth of one gear divided by the number of teeth of a mating gear. Gear ratios are important as they allow us to better control speed and torque. We are able to sacrifice speed in order to benefit from increased torque.
This relationship allows us to achieve optimal output RPM by adjusting the gear ratio. Another point worth noting, it is much easier to extract power from a high RPM brushless motor vs a slower turning motor. It is all made possible by utilizing the gear ratio.
How Gear Ratio is Calculated
The gear ratio is calculated using the number of teeth on a gear. The other option instead of using the number of teeth can be using the overall diameter of the gear. Either way will work, for simplicity and consistency, we will use the number of teeth as our preferred method. Watch the video for a more visual demonstration.
Determine the number of teeth on the two meshing gears that you are calculating the ratio between. Use the number of teeth on the spur gear and divide by the number of teeth on the pinion gear. The resulting value will more than likely be greater than 1.0. A good way to always remember this relationship is dividing the Output gear by the Input gear.
Multiple Gear Sets and Ratios
Gear ratios can be confusing when there are multiple sets being used. In the example below, the output shaft of gear set number one is connected to the input shaft of gear set number two. The gear ratio is determined in each set first. The above method is used to calculate the ratio in each set. Next, the calculated gear ratios for each set are then multiplied together. The resulting value is the gear ratio for the entire system.
You may also use the RCI gear ratio calculator.
Determine Torque Multiplication from a gear Ratio
Torque multiplication always occurs when there are two meshing gears of different number of teeth. The torque multiplication can be calculated directly based off of the gear ratio. As an example, we will use a gear ratio of 10:1. For every 10 turns of the input shaft, the output shaft turns once. We can use this exact number and multiply it by the amount of torque the motor produces. For example, if we say the brushless motor can deliver 0.05 ft-lbs of torque at the input shaft, we would expect 10 times this at the output shaft. The resulting value would be 0.50 ft-lbs of torque at the output shaft.
Putting it together – Application of Gear Ratio Sets in RC
In many RC vehicles, multiple sets of gears are used. Quite commonly, one set will almost always be found directly on the motor itself. The pinion gear on the motor is the first gear that is in a set. The next set of gears in a transmission is typically the differential on a shaft driven drive train. The output shaft going to either the front or rear differential contains a pinion gear. This pinion gear mates with the differentials ring gear, that ultimately drives the differential. In order to calculate the gear ratio, the Motor pinion gear mated to the spur gear must have the ratio calculate first. Next, calculate the gear ratio of the differential setup. Lastly, take the 2 resulting gear ratios and multiply them together.
The total value that you have as a result is what would be considered as your final drive ratio. For every one turn of the output shaft leading to a tire, the final drive ratio represents how many times the pinion gear on the motor shaft must turn. You could then use this value to determine the total amount of output torque produced as long as you know the total input torque.