Ask DH – A Primer on RC Gearing

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Ask-DH-Gearing

Sometimes there are questions you have about RC you’ve always wondered, but for one reason or another, never asked.  Whether you’re a seasoned veteran or brand new, we’re going to bring you up to speed. Have a question you’ve been wanting to ask? Let us know in the comments!

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We often get asked “What gearing should I use, and how fast is this truck?”. RC Truck gearing is as much an art as it is a science. While there are some things you need to do so your gears don’t disintegrate, like making sure they fit and mesh correctly, reaching the balance of acceleration and speed depends entirely on your desired performance. Typically, when talking about changes to “gearing”, people are talking about the pinion gear and spur gear. The ratio and tooth-count of these two external gears will affect the torque and top speed of your truck.

We’ll get into the numbers in a bit, but let’s start in first gear…

The Basics

Let’s put this in terms of the great equalizer: the bicycle.

The larger the gear that’s inputting power (the gear driven by pedals, aka ‘Pinion Gear‘), or the smaller the gear that’s being driven (the wheel sprocket, aka ‘Spur Gear), the harder you’ll work, but the faster you’ll go. A larger pinion gear means less ‘pedals’ of it are required to turn the spur gear/wheels. Don’t go too large though, or hope you’re not trying to go up a hill; your couch-tuned pasty thighs probably can’t produce enough torque to keep going. It’s like trying to drive out of the parking lot in 5th gear – great for the highway, but terrible from a stop. In RC terms, that means high top speed (once you get going) but a lot of strain (read: heat) on the gear teeth and motor.

Conversely, if you make the input (pinion) gear smaller, and/or the driven (spur) gear larger, you’ll have less top end speed, but better/easier torque and acceleration. Too small though, and you’ll end up like that guy who’s pedaling really fast and moving 0.1 mph. In RC terms, this means high motor RPM and acceleration, but a slower race.

GearingSpurPinionAskDH

Your final gearing ratio will be a combination of the external pinion/spur gearing (changeable) multiplied by your model-specific internal gearing that doesn’t change (input gear, idler gear and differential gear that turns the drive shaft). That ratio will ultimately show how many turns of the motor translate into turns of the wheels.

So which one should I use?

The technical answer? It depends.

In the graphic above, we have a 20-tooth pinion gear and a 68-tooth spur gear for an external gear ratio of 3.4 (68/20). This means the pinion gear turns 3.4 times for each rotation of the spur gear. This example is an optional set up included with the Traxxas E-Maxx Brushless Monster Truck that takes it from a 28+mph top speed to 40+mph top speed.   Since it requires more power than the stock gearing with a smaller pinion (17-tooth), you must run LiPo instead of NiMH. Remember, a larger pinion gear means you need to “pedal” harder but you can ultimately go faster. Just make sure the motor and ESC stay below 160 °F after running a few minutes.

The final gearing ratio is the external gear ratio (in this case, 3.4) multiplied by the model-specific internal gear ratio (in this case, 5.22) = 17.748. That means the motor/pinion must turn 17.7 times to turn the wheels/axle once.

Want to get more run time from your battery? Consider gearing down a bit and using a smaller pinion. You won’t go as fast, but the motor won’t work as hard.

Most trucks are able to handle many different combinations of spur and pinion. Just make sure the gear pitch fits. This is how to get a rough estimate:

  • Measure the diameter of the gear across the peaks of the teeth
  • Measure the diameter from the valley to valley
  • Take the average of those to measurements
  • Count the number of teeth and divide by that average, then round to the nearest whole number

More Info

But wait, there’s more!

Want to see how everything fits together? Check out this video from TLR.

Want to know more about gear pitch and make sure your gears fit together? Check this out and scroll to the bottom.

Want to find out your top speed? Use this awesome calculator.

Want to break the RC speed world record? Find the strongest motor you can and then try a 1:1 gear ratio and let us know how it turns out…

 

 

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