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Understanding gear selections for 1/10 Electric RC!

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Old 08-12-2005, 08:06 AM
  #1
SkrapIron
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Default Understanding gear selections for 1/10 Electric RC!

Unstickied by Foxy on 21/5/13 - Everyone knows this now, with the brushless revolution, and even if they didn't its referred to everywhere, plus this is not up to our more recent standards for sticky quality.

What is the best set-up for my truck? 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 off and on, without a real good understanding of how to properly set it up. To me, it was FM ( friggin magic). What pinion should I use? What if I change the spur gear? 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!

Question 1: What size spur gear should I run? What about the pinion gear?
Well……. When selecting the spur gear and pinion gear size, you need to
understand that there is a direct ratio between the tire circumference (referred
to as roll-out) and the final drive ratio. That ratio, in most cases, should be
as should be as close to 1 to 1 as possible. 1 to 1? What? By a 1 to 1 ratio, I
am referring to one revolution of the tire to one revolution of the motor. In
doing so, we do our best to maintain the overall efficiency of the motor by
keeping it in the ‘Sweet Spot’. 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 roll-out of the tire. Multiply the diameter of the tire by
pi. ( Ex: 3.25”xpi=10.2101”) Now, you need to consider the final drive ratio of
your drive train. Begin by dividing the number of teeth on the spur gear by the
number of teeth on the pinion gear. This will give you your external drive
ratio. ( Ex: 87/19=4.5789). Now multiply your external drive ratio with your
transmission gear ratio ( Ex: 2.4x4.5789=10.9893 ). This is your final drive
ratio.

Now, what do we do with these numbers? Subtract the final drive ratio from the roll-out of the tire . (Ex: 10.2101-10.9893= -0.77926) Redo the calculation adding another tooth on the pinion : ( Ex: 87/20= 4.35 ( drive ratio )x2.4 ( transmission gear ratio )= 10.44 ( final drive ratio ) Then subtract the final drive ratio ( 10.44 ) from the roll-out ( 10.2101 )
( Ex: 10.2101-10.44= -0.230).


So, what is this formula telling us? In order to maintain the 1:1 ratio between
your roll-out and your final drive ratio, you need to select a gearing
combination that is as close to a 0 margin as possible. In this case, the -0.230
is the optimal choice, since it as close to 0 as we can get, without going over.

How do we know this works? We can check our overall efficiency by dividing the
roll-out by the final drive ratio. In this case, the 10.2101 roll-out, and the
10.44:1 final drive ratio means that we have less than a 3% total loss in
efficiency. In other words, the tire turns 0.977 times, for every 1 revolution
of the motor. Generally, it is best for your motor to gear your vehicle within a
+ or - 10% margin. ( In the case of this formula, +1 to -1 is an acceptable
margin.)

Overgearing a vehicle will add to the speed of the vehicle, but it does so at a
tremendous cost. The additional strain placed on the motor by the shorter
gearing, will cause tremendous heat build up as the motor struggles to reach its
peak RPM. Undergearing a vehicle can be just as damaging. Since the motor will
operate at it’s peak RPM almost exclusively, it will accelerate the wear of the
components and dramatically shorten it’s service life.

This formula works best with 1/10 scale vehicles running stock to mid-modified
motors. ( 27 turn to around 12 turn motors ).
However, hotter motors 12 turn and lower may require you to reduce the margin by
as much as -1 to combat heat build-up. But never gear your vehicle below an
overall efficiency of 90%.

We hav simplified this equasion, by adding a Gear Wizard Calculator. All you need to do is add the right entries, and the calculator does the math for you!

http://www.rcuniverse.com/community/gearwizard.cfm

Question 2: How fast will it go? Well, we have half the equation already. Using the circumference of the tire divide that by the final drive ratio. ( Ex: 10.2101/10.44=.977797) multiply that number by the maximum working rpm that your motor is capable of. Most motors are rated at XXX RPM, XXX rpm/volt, or XXXk/v. With the peak RPM rating, simply use that number, unless you are using a higher voltage cell. For RPM/v (k/v) you simply multiply its number of RPM/volt by the number of volts supplied to the motor. I have a Trinity Jade 15 turn motor. It is rated for 28,500 rpm. @ 7.2 volts. ( Ex: .977797*28500= 27,867.2145 inches per minute )
Convert that sum to feet per minute by dividing by 12 ( 12 inches in a foot) ( Ex: 27,867.2145 /12=2322.2678 feet per minute ). Now multiply your feet per minute by 60 minutes ( Ex: 2322.2678 *60= 139,336.0725 feet per hour ). Now divide your feet per hour by 5280 ( the number of feet in a mile ). ( Ex: 2322.2678 /5280= 26.389 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 this, it is a pretty good estimate of just how fast you can go with a given motor!

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!
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Old 08-12-2005, 11:42 AM
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Default RE: Understanding gear selections for 1/10 Electric RC!

WHAT ABOUT THIS, we pm u skrap, asking 4 our top speed, and u jus give it 2 us?..LOL! thats what i do, cause im not good with numbers[X(]
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Old 08-09-2017, 09:19 AM
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Default This has helped

Your post has helped me understand what I am supposed to be looking for but then you threw me off at the end since my motor falls under the too much power category. Specifically I just bought a 5700kv motor running on 3s lipos. It is a Tamiya TT02 so it has a wheel diameter of 1.9 and an internal ratio of 2.6 according to Tamiya. I was just wondering what size pinions and spur gearing I needed to get the most out of all that power without destroying it. Any help would be greatly appreciated.

Thanks,
Gordon

p.s. sorry for reviving an old post
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Old 08-09-2017, 12:03 PM
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Quote:
Originally Posted by gordon7074 View Post
Your post has helped me understand what I am supposed to be looking for but then you threw me off at the end since my motor falls under the too much power category. Specifically I just bought a 5700kv motor running on 3s lipos. It is a Tamiya TT02 so it has a wheel diameter of 1.9 and an internal ratio of 2.6 according to Tamiya. I was just wondering what size pinions and spur gearing I needed to get the most out of all that power without destroying it. Any help would be greatly appreciated.

Thanks,
Gordon

p.s. sorry for reviving an old post
Yowsers! Serious power indeed.

For this type of situation, I'd strongly suggest investing in a infrared thermometer to know the motor's temp. Initial testing... I'd go with your smallest pinion gear you got. I'm assuming you are doing some speed runs 'cause something with stock gearing will be launched into the 50+mph range.
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Old 08-09-2017, 12:11 PM
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So I should just go with the stock sizes? I was wondering because the stock gears are plastic and I figured this much juice would melt through them. So I was looking at steel gears but wanted to get the best ratio I could so I'm not buying tons of different sizes.
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Old 08-10-2017, 09:39 PM
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jesus.... 5700kv on 3s..... my first thought...... check manufactures specs of that motor and ESC with relation to there watt limit
if you are using a combo like Hobbywing ezrun 35a combo id really advice against it.

5700kv is not for 3s.... thats over 60,000rpm... and with the torque these motors have... it will be so slow because it cant get traction and is impossible to drive...

i built this for giggles and ended up selling it before it destroyed it self.... but this thing was impossible to drive in a straight line.... and it was only 4300kv on 2s


Last edited by phmaximus; 08-10-2017 at 09:42 PM.
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Old 08-11-2017, 05:03 AM
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I'm using castle sidewinder 3 kit which says it is completely fine running 3s. I've already bought better parts to get that power to the ground and have a controller that lets me create my own torque curve so it's not all there at once and completely useless.
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Old 08-13-2017, 01:06 PM
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Quote:
Originally Posted by gordon7074 View Post
I'm using castle sidewinder 3 kit which says it is completely fine running 3s. I've already bought better parts to get that power to the ground and have a controller that lets me create my own torque curve so it's not all there at once and completely useless.
That's clever enough. But yeah, 5700kv on 3S is gonna be a handful fer sure. I have that motor in my 4wd 10th scale truggy and run 2S with the stock gearing. On 3S....pheew! I'd want to gear it down a little, and, make sure I had plenty of room! And no letting anyone else drive it, cuz it's gonna be a missile. Better be sure to monitor your motor temps with a infrared thermometer. May want a fan-sink for the motor, too. Gear it so the motor isn't getting over 180F, best kept below 160. Otherwise you risk cooking the motor, the magnets loose strength and/or winding insulation shorts.

Last edited by EXT2Rob; 08-13-2017 at 01:10 PM.
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Old 08-13-2017, 08:36 PM
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Thanks for the help I'll see what I come up with.
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