Pinion Gear Selection
Scene is a Saturday morning at a track in Somewhere,
"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?"
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.
I have found the answer! And it is good!
tackle the first question. What size pinion
gear should I run? Well
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
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
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
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
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.
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!