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All electric powered radio control trucks or cars require gearing. Gearing allows the change in RPM to better suit what is needed at the wheels. For example a typical Electric Motor Setups can potentially rev in excess of 30 000 RPM. The wheels however usually rotate at a much smaller rate. The way this is achieved is through gear reduction.

On road cars have smaller tires and travel at higher speeds. Gear ratios in these cars will be lower than average.
Monster Trucks have larger tires and travel at slower speeds than on road vehicles requiring higher gear ratios.

Gear Types

There are two typical gear types found in RC Cars or Trucks. These are often referred to as the pinion gear and spur gear. The pinion gear is usually composed of around 15 teeth and made from a metal material. This pinion gear is placed on the input shaft of an electric motor. A spur gear is commonly made from a nylon material. Spur gears tend to be around 48 teeth. The two different types of material cause minimal radio interference and ultimately create a quieter gearing mesh.

Gearing Mesh

It’s important to properly set the mesh between your pinion gear on the motor shaft, with the spur gear located on the transmission of the RC car or truck. A common method of properly adjusting gear mesh is to place a piece of lined standard paper between the pinion and spur gear. Carefully press the two gears together firmly to squeeze the paper matching the shape to the gears. This properly provides the couple millimeters of clearance that is required between the gears.
Secure the electric motor in place. It is highly recommended to use Loc-Tite for this process. It is critical for the fasteners securing the motor, to not loosen during operation. If they do loosen the gearing mesh clearance will grow increasing the chances of stripping the gears. If not caught immediately it does not take long until this may occur.

Gear Ratio’s

Gear ratio’s in an electric powered car or truck are adjustable. The transmission itself in the electric car or truck is set permanently. The only options that are common for choice of gear ratio is in some cases the spur gear and in all other cases the pinion gear.
When given the oppurtunity to select the best pinion gear to provide the desired performance, it is important to keep a few things in mind.

A larger pinion with more teeth will decrease the gear ratio.
A smaller pinion with less teeth will increase the gear ratio

A larger gear ratio will provide increased acceleration and a better powerband in rougher terrain. It’s overall top speed will be reduced
A smaller gear ratio will be the exact opposite. It will suffer in acceleration performance, have a more difficult time in rougher terrain and will have increased top speed.

It’s also important to understand there are limits to setting the gear ratio outside of what pinions are actually provided. Let’s say for example we can have the largest pinion custom made. What has already been said above, tells us that this should provide the highest top speed.
While this is true on paper, the lower gear ratio from the largest pinion size will provide the highest theoretical top speed. This may not be true in reality.
The motor must produce enough power to handle the extra load. In motor terms, the motor must be large enough to remove the excess heat. Loading the motor too much will certainly destroy the it, the ESC, or the battery.

It would be best to start with a very conservative small pinion and work your way up. If your goal is to achieve a high rate of speed, it may be wise to test out gear ratio’s using one of the determining speed methods. The gear ratio that provides the highest speed is ultimately the best for that particular application. There is no real scientific way to determine this as there are several factors that lead up to the end result. Through calculations you may get close but not exact. Testing is recommended.