In brief, RC Cars utilize a transmission that contains a gear set. Different applications will require different “pitches”. A gear pitch given in imperial terms is related to the amount of teeth that would encircle a one inch diameter. This one inch diameter is known as the pitch diameter. The metric module is how pitches are defined in the metric system. The metric module is known as the ratio of the diameter of the pitch circle per tooth. Do not confuse the metric and imperial pitch sizes as they represent different sizes of pitch.

What Gear Pitches Exist

In particular, there are both metric gear pitches and Imperial gear pitches used in RC cars. Below is a list of the common gear pitches used:

Imperial Gear Pitches:

• 32 Pitch
• 48 Pitch
• 64 Pitch

Metric Gear Pitches:

• 0.5 Module
• 0.8 Module
• 1.0 Module

If you are unsure of which pitch your specific RC car vehicle uses, look up the parts in your manual. In any event that you can’t find the pitch of your spur gear or pinion gear in the manual, try out the gear pitch calculator.

As the imperial gear pitch increases, the size of the teeth and gear actually decrease. They get smaller. Furthermore, as the Metric Module increases, the size of the teeth and gear increase. Hence, the gears and teeth get larger

Can you Use a Metric Pitch in Place of an Imperial Pitch?

A Metric Module of 1.0 converts to an imperial pitch of 25.4. Gears with a Metric pitch are not designed to match an imperial pitched gear.  I would recommend not using a metric pitch in place of an imperial pitch. In short, this does not mean that you will never find a combination that is close enough to substitute. One could argue and even use a 0.8M gear in place of a 32P gear and it may work well. Even so, I would not recommend using the pitches interchangeably unless you are OK with an increase in gear wear that could result.

Why Use Different Gear Pitches?

Not all applications are alike, this is why we must use different gear pitches in RC. Generally speaking there are two main reasons to use a different pitch. The first reason being physical size. If you are operating a smaller indoor 1/18 scale RC car, it is not ideal to use a large gear with a big pitch. The size of the gear would be far too big to place in to the RC car. It’s also worth it to point out that moving to smaller gears also allow for a decrease in friction allowing more power to make it to the wheel. This is also important for smaller RC cars in order to get the limited power they make, to the ground more efficiently.

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.