I posted this back in 2007 , I posted this at our forums.thetoyz.com but is always a good read for gearing
The majority of cars have two elements of gearing that can be changed, these are the pinion (the gear that attaches to the motor) and the spur, the gear that meshes with the pinion.
The pinion is typically a metal part, however some manufacturers such as GPM and others are now using delrin gears. The spur is always made of a softer material than the pinion, typically nylon or some other type of plastic.
Pinions and spur gears are supplied with a certain number of teeth at a particular pitch. Pitch is simply the distance between each of the teeth. Increasing pitch means the teeth get closer together.
Most RC’s have 48pitch gears (a notable exception is Tamiya who have 0.6 module gears)
Why do we need gearing?
The purpose of gearing is to control the power output from the motor, the motor in an RC car generally spins somewhere in the 20-50,000RPM range, the wheels however cannot spin that fast otherwise they would be ripped to shreds. So the gearing in the car reduces the speed transmitted to the wheels. This reduction in speed does however have a benefit and that is added torque.
The reduction ratio
The reduction ratio of a RC refers to how much the speed is reduced between the motor and the wheels.
What does this mean? well firstly the wheels spin slower than the motor, and the torque at the wheels times that produced by the motor. This may sound confusing but to gear a RC low, means to actually increase the overall drive ratio, while gearing a car high reduces the overall drive ratio.
There is no such thing as the optimum gear ratio for a particular RC, this will vary depending on the track, surface, material of your tires and weight of your RC. The majority of gear ratios are in the 6 to 8.5 range and this is irrespective of RC make and model.
The effects of gearing
Gearing has predominantly two main effects, one is acceleration and the other is top speed.
You may be wondering why does the acceleration of the RC reduce as it gets quicker? This is the effect of drag from air. Drag force squares with speed. so if you were going at 2m/s and then speeded up to 4m/s the drag force would go up by 4 times.
The distance a RC covers is the area under each of those lines. By some crude maths, if the longest straight at a particular track takes more than 3.1 seconds to cover, a better gearing option would be the pink line (high geared car). I should stress there are half a dozen other corners which are probably more important than the straight!
Because acceleration and braking are physically the same thing (a change in velocity), a lower geared car will have better brakes than a higher geared car.
Heat and run time
Some unwanted effects of gearing are on the run time of the RC. Increasing the gear ratio will cause the motor to get hotter as it has to induce more current to supply the same amount of torque, this in turn causes the batteries to drain quicker. There are many heatsinks with or without fans on the market now that can help reduce heat. Also, Thermal paste has shown to be a good help as well.
The majority of people out there suggest that you gear your RC so that it reaches maximum speed at the end of the longest straight, I personally don’t agree with that. You can gain or lose more time on the average track under braking or accelerating. For this reason I always suggest gearing the RC so it is very quick round the infield and sacrifice some straight line speed but that’s just me 🙂