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!

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(]

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

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.

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.

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

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.

Thanks for the help I'll see what I come up with.

Hey all,

I'm still struggling to understand all this. So after you figure out your gearing to be as close to 1:1 like SkrapIron posted, wouldn't the weight of the car matter? I get that you don't want to run your motor hot but, what do you do when your motor is hot? Everyone talks about gearing down but what if I can't? Can I change the motor? To what?

Thanks~

What RC do you have?
What motor?
What batteries?

If you can't find the right gearing, then something else is possibly wrong, like bearing(s) being worn out, or having the wrong motor for your platform.

I have a 1/10 Tamiya I use for Rallying and bashing but, I was more asking in general. I mean by calculations you can come up with 1:1 gearing then that means your gearing is right, in theory. So then if your motor is running hot that would mean you have the wrong motor? So if your gearing is right and your motor isn't broken then other factors play into this "gearing/motor combination", correct? Then assuming everything is working on your RC (i.e. everything that needs to be moving/spinning freely is doing so, etc.), something else is overworking the motor. I feel like the weight would be the major factor here. I'm not an engineer or physicist or mechanic so, I don't know. But, if trying to determine which way to go with the motor, which way would it be? Do you need a higher/lower kv/turn motor?

The only time you will have a 1:1 ratio is when you are running the same tooth count pinion/spur, and internal gear ratio is also 1:1. I don't know of any "normal" RC with a 1:1 ratio. I mean, sure, you have on-road cars with spur/diff right on the axle are considered to have internal ratio of 1:1, but I always see a pinion gear that is smaller than the spur gear being used.

I don't understand highlighted sentence. My gearing on my RCs are all wrong then ;).

My lightest off-road buggy (2.6:1 internal) has a final drive ratio (FDR) somewhere around 6.19:1 (29t/69t pinion/spur) while running a 17.5T motor. If I am using a "hotter" motor, one with more RPMs, I compensate the FDR so as to not overwork/overheat the motor...I gear lower.

The same buggy as above, running with a 5.5T motor, I use a FDR somewhere around 14.138:1. Way too much motor, but if I don't gear that motor correctly, I will see a rise in temps.

Sorry, I was referring back to this when I said 1:1. I've highlighted in bold the sections. So RustyUs, I get what you are saying about the pinion/spur and you can adjust that to the motor but, I was wondering about motor adjustment when not changing the gearing after you've set the 1:1 as above.

Certainly, the vehicle weight is a factor to consider when gearing. Note that it says in that sticky post that the given calculations are for 1/10 scale vehicles. Another factor is the physical size of the motor. You could have a 380-size motor rated at 4300kv and a 540-size motor rated 4300kv. Which one will run hotter? In the same vehicle, the smaller can of the 380 will run hotter than the larger 540. The larger the motor the more torque it has. The more torque it has, the less it has to "work", and the cooler it will run. Also note that while the calculator is optimized for tenth scale, the actual size and weight of any given vehicle can vary widely.

Thanks OP, this was eye opener for me!
:D

thanks op but i have found somethign rather interesting, i have viewed many calculators and using my T4 as an example with a 10.5T (3250KV) brushless the calcs all agree that a 17/84 ( 18/87 pinion spur is the way to go, HOWEVER, this makes esc and motor run hot, and is a poor gearing option. i am still playing with gearning however because the cold temps outside arent exactly a blessing LOL. right now i have a 23/84 in it and motor temps are 90 to 98 degrees F telling me i have more room and stay uner 150 F but that is with the weather being cool/cold. anyway, this gearing puts me way outside of the 17/84 18/87 ( or 1:1 ). i also recalculated for running an 8.5T (4000KV) brushless and found that gearing it 20/87 was the sweet spot for temps but was overheating slipper and ball diff under normal driving conditions.

it is also important to note that AE recommends a 21/84 for the T4.3 for a 10.5T 3500KV reedy setup.

now here is where things get interesting. if we factor in tire size, my tires are proline bowties and they physically measure 4" across and are what came on my T4, i also have another set of bowties that are brand new that also measure 4" across. now looking at the AE recommended gearing of 21/84 for a T4.3 it would be possible to acheive a 1:1 IF the bowtie tires measured 3.5". so tire size also plays a role however this recommended gearing is way conservative on the part of AE since the T4 stock tires are 4" not 3.5" and i would personally recommend a 22/84 combo for a T4, T4.1, T4.2, or T4.3 running the 4" tires but if you have 3.5" tires starting point would indeed be 21/84 for a 10.5T.

at the end of the day gearing isnt just pinion/spur, it is also tires, and any other gear ratios be it internal tranny ratio, multiple diffs with different ratios ( overdrive on front as an example). when it comes to brushless though and no standardization between brushless manufactures just gear for whatever works for your track and if you are just bashing gear to give you the best trifecta of speed, control, and temp, gear calcs can be useful for a starting point but most common reply i see when people ask for help gearing is ask the local racers at your track, followed by gear calc suggestions.

Don't forget that terrain, driving style, quality of motor components, timing, and tire ballooning will play into gearing/temps. That's why a little gadget like an infrared thermometer will be a great value in determining sweet spots for different motors.

exactly, my point is that gearing calculators are a good tool, but they cant alway calc all the variables. R-C calc is ok but it has its shortcomings like all other calcs in that it doesnt cover every vehicle or motor on the market. it is also important to note that it is best to use the KV over "turns" of a brushless, as an example the lrp 10.5T is listed as 3500KV, while the sky rc cheetah 10.5T is listed as 3250KV, i would however like to also point out that i have had better luck getting better speed, controlability and temps with the cheetah motor over the reedy/lrp sonic. if i had one complaint on the cheetah it would be the timing is all done via the programming card whereas the (which some tracks dont allow) but sky rc ares pro motors have it on the endbell but the motor alone costs almost as much as the entire cheetah system does or rather did the cheetah systems seem to be going up in price ( i guess due to a lot of people buying them up lately).

After rethinking what AznDrvr was asking, I realize now, it's the 1:1 Rollout Ratio gearing they were after.

I'm beginning to think brushed motors will run hot, no matter what. Plugging the numbers into a Gear Wizard, my T4.3 is undergeared by quite a margin (0.83:1 rollout ratio). 15/87 with a 2.6:1 tranny, and ST tires...I can't get more than eight minutes of run time before motor is scorching hot. I can pretty much hold the trigger at WOT around the whole track.

I will try using a 20T pinion next time out, and after I clean/lube the motor. I pretty sure I tried the 20/87 gearing before and got same hot results.