Changing the RC gearing on your vehicle can make a lot of difference in its performance. Knowing how to gear it right is very important! Incorrect gearing can cause overheating issues and performance problems.
RC Gearing means making a compromise between acceleration, top speed, and duration. How do you find the right combination?
It is probably fair to say that finding the correct gearing for your radio control vehicle requires experimentation or experience, and sometimes both. The tech charts published in the radio control car magazines or on the Internet can give you a guide to the gearing used by some of the top drivers. These can be a good starting point.
However, it is only when your car hits the track or street if you are just running high-speed street runs for the first time that you will really discover whether it benefits from the required mix of top speed, acceleration, and duration.
An electric powered RC car uses an onboard battery pack, so there is always a limited amount of energy available to use up during a run. The gearing needs to be optimized to ensure the battery power is not fully depleted before the car has crossed the finish line or you are done with your run. If not, all of the cars you have carefully been driving with during the race/run will come blasting by on the final lap as your car crawls towards the finish with a dead battery. With a nitro RC car, you can always make a fuel stop, but get the gearing right and you can stretch out the time between pit stops, helping to build an advantage on a day out with buddies at your favorite spot or on the track.
There Is A Formula To Use
There are two things that affect the distance an RC car will travel for one rotation of the engine or motor. These two things are the gearing and the diameter of the tires. A simple mathematical formula can be used to bring this information together, allowing the millimeters per rev to be accurately calculated. Expressing the movement of the car in this way enables direct comparisons to be made between different makes of car, even when they are running on different profile tires and internal gear ratios.
The MMPR your car should be geared at depends on a whole list of factors. Not on the amount of time you are racing for and the size of the circuit. A high MMPR will typically give a greater top speed, but at the expense of acceleration and duration. Conversely, a low MMPR will really let the engine rev, giving good acceleration and improved duration, but at a slower top speed.
On a four-wheel-drive car, the gearing depends on the number of teeth on the chosen pinion and spur gear, and the internal gearbox ratio sometimes referred to and known as the final drive ratio. This ratio is typically documented in the instructions provided by the manufacturer and in the box with new models. To calculate the MMPR, you want to use this following formula:
MMPR = Tyre diameter (mm) x PI (3.14) / Number of Spur Teeth x Number of Pinion Teeth / Drive Ratio
Let’s choose a (now aged) electric powered Associated RC10TC3 touring car running on fairly low profile tires as an example. Using a pair of calipers, we find the tires are 58mm in diameter, so multiplying these by 3.1416 gives the circumference of the tire. We then divide that by the number of teeth on the spur gear and multiply by the number of teeth on the pinion. Finally, we divide the result by the final drive ratio, which the instruction booklet for the TC3 tells us is 2.5:1. So the MMPR for a couple of very different circuits could be:
Large outdoor tarmac track:
MMPR = 58 x 3.1416 / 72 x 32 / 2.5 = 32.4
Small indoor carpet track:
MMPR = 58 x 3.1416 / 72 x 22 / 2.5 = 22.3
Outside on the tarmac circuit, the 32-tooth pinion gives a high MMPR, which will make the car move further for each rev of the motor. This gives it a very high top speed, but at the expense of acceleration. This is fine on wide, flowing circuits where the speed of the car can be maintained through the corners, but throw in a few hairpins and coming down a couple of teeth on the pinion can provide a better compromise. Indoors on the small carpet circuit, the straights are much shorter and the corners are tighter. Acceleration, therefore, becomes more important than top speed, making it better to run a much smaller pinion, such as the 22-tooth one we fitted here.
The RC gearing you run will depend on where and how you run. You will need to take a calculator with you to work out the MMPR of your car unless you are a math genius.
However, most drivers tend to use the same car, the same diameter tires and the same spur gear throughout a race meeting. It is, therefore, a pretty easy task to calculate the MMPR for each of the opinions you happen to have in your pit-box. Record these in a table and it will be easy to see the effect of moving up or down a tooth. Record next to this the circuits on which a particular opinion was run and you will quickly build up a very useful gearing guide.
Does The Motor Influence Gearing?
With an electric powered RC car, your choice of a motor will greatly influence the gearing. Assuming the timing on the motors is similar, a stock motor will not rev as highly as a modified motor for example, so it needs to be geared on a higher MMPR in order for the car to reach the same top speed. If you have any doubts at all about the gearing to run a particular motor on, always stay on the safe side and run a small pinion. Under-gearing a motor simply allows it to rev higher and will do the motor no harm at all compared to over-gearing it. Running too large a pinion can put the motor under a lot of strain. The current draw from the motor will be high, the commutator will quickly get scored or marked, brushes will start to discolor and the motor can lose some magnetism and the motor will become inoperable. Such changes will eventually happen to a correctly geared motor too, but it will take many more races before the effects begin to show.
Two speed – Multi-Speed Gearboxes
The MMPR calculations get more interesting on a nitro-powered car fitted with a multi-speed gearbox. On these RC cars you need to calculate the MMPR for each available spur/pinion combination, and then decide the engine revs at which the gearbox should change from one opinion to the other. Get this right and you can obtain the perfect compromise between acceleration and top speed for any circuit. A two or three-tooth gap between the gears often works best. With more than a four-tooth gap, the RC gearing may be susceptible to damage.
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3 thoughts on “How To Choose RC Gearing Correctly”
Great advise on changing your rc gearing. I have changed my gearing and had to change it back, due to my rc got really hot and only lasted half the time it used to on a full battery. I wish I would have read this article before I messed with it. Thank you!
I am sorry to hear you encountered some issues with your RC gearing option. However a lot of finding the correct gearing for you is by experimenting to see what works and what does not.
Just think of this as you learned what does not work for your application.
Remember when changing your gearing, There are several factors that come into play in order to achieve the results you are looking for. Such as tire size, battery, vehicle wieght and even what motor your are using can be a factor (if you are looking for serious speed)