RCU Review: RCU Forums: Proper Pinion Selection

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    Contributed by: John Messner | Published: April 2005 | Views: 34051 | email icon Email this Article | PDFpdf icon

    by: SkrapIron

    “Proper Pinion Gear Selection”

    The Scene is a Saturday morning at a track in Somewhere, USA.
    "Excuse me, sir. I'm new to this sport and I was wondering, what is the best set-up for my truck, and just how fast will it go?"

    These are questions that have haunted me for nearly 10 years. I have been running 1/10 scale R/C trucks and buggies all that time, without a real understanding of how to properly set it up. To me, it simply was all voodoo. What pinion should I use? What if I change the spur gear? Do my tires matter? It was all trial and error that resulted in several melted motors, blown ESC's and damaged batteries.

    But I have found the answer! And it is good!

    Let's tackle the first question. What size pinion gear should I run? Well……. When selecting the pinion gear size, you need to keep in mind that there is a ratio between the tire circumference and the motor. This is called the final drive ratio. What does it mean? For a proper, well balanced vehicle, your 540 sized motor should ideally turn X number of times for every 1 revolution of the tires. In a perfect world, our trucks and buggies would be geared directly off of the motor. But we're not in a perfect world. While our hobby-grade 540 motors excel in creating RPM's, they lack in creating sufficient torque to get the mass moving. That is overcome by dividing the work created by the motor over the entirety of the gear train, thus dividing our overall RPM's, but multiplying our torque.

    Here's how it works. We'll use my RC10t3 as the example vehicle. The first thing to consider is the diameter of your tire. It is used to calculate the circumference of the tire. Multiply the diameter (D) of the tire by pi. (DxPi=Circumference) ( Ex: 3.25"xpi=10.2101")

    Once you have the circumference of the tire, you need to consider the final drive ratio of your drive train. Begin by dividing the number of teeth on the spur(S) gear by the number of teeth on the pinion(P) gear.( S/P=E) This will give you your external gear ratio(E). ( Ex: 87/19=4.5789). Now multiply your external gear ratio with your transmission gear ratio(T).. (ExT=FDR) ( Ex: 2.4x4.5789=10.9893 ). This is your final drive ratio (FDR).

    Most, if not all manufacturers publish the transmission gear ratio either on their websites, or in the documentation that came with your vehicle. It is usually presented as an X.XX:1 transmission

    Now for the magic.

    Subtract the running circumference of your tire from the final drive ratio. I call this your motor to drive ratio (MDR). (Circumference-FDR= MDR) (Ex: 10.2101-10.9893= -0.77926)

    Ideally, you want to get the sum of these 2 numbers as close to Zero as you can. Anything with a negative value will be geared towards the torque and acceleration side of the performance envelope. Everything with a positive value is geared towards the top speed side of the envelope.

    This calculation will also work for pan cars, or others without an internal transmission. Simply divide your spur (S) by the pinion (P) to get your overall gear ratio (G). Then subtract the circumference of the tire (C) from the overall gear ratio (G) (C- G=MDR)

    Ironically, this setup works with all hobby-grade brushed and brushless motors, regardless of the number of turns on the motor. The secret is that the gearing does not determine the overall speed of the vehicle in question. That is determined by the number of RPM's that the motor can generate. By overgearing a low turn motor, or even a brushless setup, it is possible to squeeze a few more MPH out of the top end, but at what price? Overheated electronics, damaged batteries, and shortened run times.

    There is an old saying in racing: "Speed Costs." It's even more true in Electric RC.

    Now we can answer the second question. How fast will it go? Well, we have half the equation already. Using the circumference (C) of the tire, divide that by the final drive ratio (FDR). (C/FDR= Drive Efficiency Ratio) ( Ex: 10.2101/10.44=.977797) Multiply that number by the maximum working RPM that your motor is capable of. This will give you your speed in Inches per minute. I have a Trinity Jade 15 turn motor. It is rated for 28,500 rpm. ( Ex: .977797*28500= 27867.21 inches per minute )

    Convert that sum to feet per minute by dividing by 12 (12 inches in a foot) (Ex: 27867.21 /12=2322.27feet per minute ). Now multiply your feet per minute by 60 minutes (Ex: 2322.27*60= 139336.2 feet per hour). Now divide your feet per hour by 5280 (the number of feet in a mile). (Ex: 139336.2/5280= 26.3894 miles per hour). Keep in mind that this number is entirely theoretical and is affected by the age of your motor, condition and charge of your battery, friction and/or slip from your tires etc. Despite all that, it is still a pretty good place to start when you want to know.

    So, the key to speed and longevity is a high rpm motor coupled to a properly geared drive train. It will make for many a happy afternoon of backyard bashing with your truck!

    To Read about/Discuss this in the RCU Forums click here.


    Comments on RCU Review: RCU Forums: Proper Pinion Selection

    Posted by: raymanh on 02/03/2008
    Page: 1

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