HorizonHobby.com How-To: Understanding RC Gearing

Please click "See More" for links and more information

One of the most common questions we hear about RC cars and trucks is, "how fast does it go?" While this seems like a common, ordinary and easy question to answer, the truth is that there are a number of different factors that contribute to a vehicle's speed, torque and acceleration. One of the big determining factors is the gear ratio and transmission of your vehicle. While things like installing a faster motor or higher voltage battery are easy things to understand, wrapping your head around gear ratios can be like taking an advanced course in physics, geometry, algebra and trigonometry. While there's a lot going on, there's some simple ways to get a basic understanding of what the gears in your transmission do, what changing them means and how things work. We've also put a list of commonly used terms together for you below for quick reference to help you as you progress.

External Gear Ratio -- The gear ratio of the gears that are visible or outside of a transmission case. This is the ratio of the pinion and spur gear.

Final Drive Ratio -- The gear ratio of the entire drive system in a car or truck. This includes the internal gear ratio of the transmission and the external gear ratio of the pinion and spur gears. You can calculate this ratio by dividing the spur gear tooth count by the pinion gear tooth count and multiplying that total by the internal transmission ratio.

Gear Ratio -- A gear ratio is the relationship between the number of teeth on two meshed gears. This relationship is expressed mathematically. For example, if one gear with 50 teeth is driven by a gear with 20 teeth, the gear ratio is 2.5:1. You would calculate this by diving 50/20 for a total of 2.5.In an RC Car, you have 2 different ratios you'll need to work with to achieve the proper gearing for a specific application and goal. The first is the external ratio and the other is the internal ratio.

Internal Gear Ratio -- The gear ratio of a vehicle's transmission gears separate from the pinion and spur gear ratios. The internal ratio is a fixed number determined by the manufacturer of your vehicle.


Module -- For countries that use the metric system, Module is the equivalent of pitch. It typically refers to the pitch diameter, in millimeters, divided by the number of teeth. The higher the number on the pitch, the finer the tooth profile is. The common module types are 1.0 module and 0.6 module.

Pinion Gear -- A small gear that directly attaches to the output shaft of an electric motor.

Pitch (AKA Pitch Diameter) -- In countries that do not use the metric system, pitch refers to the number of teeth on a gear with a 1-inch pitch diameter. The higher the number on the pitch, the finer the tooth profile is. The finer the pitch, the more efficient the gear; however, since they have less material, they are often more prone to stripping if the mesh is not set properly. The more coarse the pitch, the less efficient the gear will be; however, they are also more durable and less susceptible to damage. 48 pitch is the most common pitch in RC, however, 64 pitch and 32 pitch can also be used.

Spur Gear -- The large gear mounted to the top shaft of your transmission in an off-road vehicle, the center differential in an 1/8-scale or 4WD Short Course Truck or the main lay shaft in an on-road vehicle. The spur gear is driven by the pinion gear or clutch bell.


If you like our videos check out our podcast, the R/Cast, Thursdays

Horizon Hobby is now on Facebook! Visit us at

[krowe]

Referring to the math classes I thought I would never use, my very first thought when you said that was going to be using it here today was "I bet you're not". Because for me, that was Algebra lol. I worked for 25 years as a CNC programmer and machinist, and I never used Algebra one time after high school.

But this was a great video that really breaks things down. Good job, and thanks for sharing!

[MyHeap]

To determine the pitch of any spur gear use the following formula:

P = (T+2)/D

P = Pitch, T = number of teeth in gear and D = the measured outside diameter of the gear.

The resulting calculation will give you a whole number with a decimal. Disregard the decimal value on the end. The whole number is the pitch of the gear.

Example:

I have a pinion gear with 18 teeth. The measured diameter with a pair of calipers yields 0.415 inch. Using the above formula, P = (T+2)/D = (18+2)/.415 = 48.193, the pitch of the gear is 48

I hope that helps.

Joe

[craignorris]

I appreciate this type of refresher knowledge. Helps when you can “build and UN-build it in your head and logically”

[horizonhobby]

Generally speaking 48P gives you the best combination of durability and efficiency. What sort of car are you running?

[starRC]

It was like a math lesson, but very informative thanks :)

[horizonhobby]

As a general rule of thumb when you increase the RPMs of the motor you decrease the size of the pinion. So if you have a 13T motor you would gear it with a smaller pinion than say a 17T motor.

[horizonhobby]

@fieryguns Absolutely! Many people have learned on the Super Cub! And it's gotten even better in recent years with a battery upgrade to LiPo and a transmitter/receiver upgrade to Spektrum DSM2 radio gear. You may also want to check out the new HobbyZone Stratos as well.

[horizonhobby]

It all depends on where you're running, the size of the straight away, the battery configuration and more. It's really unique to each track.

[horizonhobby]

The lower the ratio will yield the faster top-speed but lower acceleration. The 32T pinion will give you more top speed. Cogging is caused by a number of reasons but the biggest is the motor/ESC themselves. The more you load a sensorless motor, however, can magnify that problem.

[horizonhobby]

The FDR (Final Drive Ratio) of the Rustler transmission is 2.72:1. So for that transmission you would divide your spur gear tooth count by the pinion gear tooth count and multiply that by 2.72 to get your overall gear ratio. So for wha you have now, 32 / 32 x 2.72= 2.72 for your gear ratio.

[jonathanjhl]

great video!!  i finally feel like i have the basics of gearing down pat.

[horizonhobby]

They are all relations. For the larger gear to turn once, for example, how many times will the smaller gear have to turn. So for your first example, 7.35... The pinion gear will turn 7 full rotations and .35 of an 8th rotation. In this same time the spur gear will turn a total of one time. Hence 7.35(rotations):1(rotation)

[Anthonylopez-rdnr]

Cool video, it can't get any better than this.

[clashybashy]

I know this is an old video but this helped so much! Thank you horizon hobby

[horizonhobby]

@charles1matt Yes, if you're overgeared (too large of a pinion or too small of a spur gear) the motor will overheat. If that's the case just start taking teeth off of the pinion 1-2 at a time until you hit an acceptable temperature.

[horizonhobby]

Sounds like your setscrew on the pinion gear backed off is all. Check and see if you can move the gear on the output shaft of the motor. If it moves you just need to tighten the little screw down. If that isn't it I would check to make sure the slipper clutch is tight.

[horizonhobby]

That is the overall desire, to have the best balance between top speed, acceleration and runtime.

[gonzodetroitify]

This is awesome for a new person to hobby, now because of you I now know what type of pinion I'll ever need just by looking, and also taking into account the application I'll be using💎💎💎💎❤️👊🏿

[DanieleGrecoUK]

I put like before ending see the video cause I already know how he's good explaining things, helped me once though... :)

[rokstar]

Thanks for the info I'm new to RC and bought a Losi Muggy and for now it's a little to fast for me so I'm trying to slow it down a little bit until I get use to it not to mention I'm hoping to get more torque for wheelies