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Mystery Solved! How to select the proper gearing for your electric RC!

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Old 01-26-2005, 04:42 PM
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SkrapIron
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Default Mystery Solved! How to select the proper gearing for your electric RC!

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? Well……. When selecting the spur gear size, you need to keep in mind that the ratio between the tire circumference and the final drive ratio should be 1 to 1. 1 to 1? What? 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 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 drive ratio. ( Ex: 87/19=4.5789). Now multiply your drive ratio with your transmission gear ratio ( Ex: 2.4x4.5789=10.9893 ). This is your final drive ratio.

Now the magic part. Subtract the final drive ratio from the running circumference of your tire . (Ex: 10.2101-10.9893= -0.77926) That is VERY close to a 0, but is geared a tad to the torque side of the motor ( anything below 0 is always geared towards more torque). Want more speed than torque? Redo your 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 your final drive ratio ( 10.44 ) from your circumference ( 10.2101 ) ( Ex: 10.2101-10.44= -0.2299 ) This is your optimum gear ratio, since it is closest to a 0 margin. Any ratio that is greater than 1 will run faster, but will overheat your motor, battery and esc, eventually damaging them.

Question 2. How fast will it go? Well, we have half the equasion 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. I have a Trinity Jade 15 turn motor. It is rated for 23,500 rpm. ( Ex: .977797*23500= 22982.504 inches per minute )
Convert that sum to feet per minute by dividing by 12 ( 12 inches in a foot) ( Ex: 22982.504/12=1915.2087 feet per minute ). Now multiply your feet per minute by 60 minutes ( Ex: 1915.2087*60= 114912.5239 feet per hour ). Now divide your feet per hour by 5280 ( the number of feet in a mile ). ( Ex: 114912.5239/5280= 21.7637 miles per hour ). Keep in mind that this number is theoretical and is affected my the age of your motor, condition and charge of your battery, friction and or slip from your tires etc. Despite this, it is pretty close to accurate!

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!

Enjoy!
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Old 01-26-2005, 06:09 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

The first part of that is about a scientific as I've seen! [X(]

The second part is a good basic speed calculator that will put you a bit on the high side up to 30 or 40mph, then it starts getting way off due to drag and wind resistance. Consider a speed record car capable of 110mph to 130mph. That equation would predict that car to reach closer to 200mph.

Agreed, for most of us running in the 20 - 40mph range, it's a great way to guess-timate top speed!
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Old 01-26-2005, 09:12 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

I think.......... I'm gonna like the new guy =)

I've gotta sit down and crunch some numbers tonight, but you might be on to something.. One thing I notice right off though is that your initial math is incorrect.. 3.25”x2pi=10.2101 is wrong..

(3.25) x (2pi) = (3.25) x (6.283) = 20.420

Or am I misunderstanding your formula?
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Old 01-26-2005, 09:25 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

He's got the right answer, but a misprint in the math...

Circumference = Pi x Diameter (not 2Pi x diameter)
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Old 01-26-2005, 09:30 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Now that I'll agree with. I was wondering why he'd be doubling the circ.. <evaluates the rest of this interesting formula>

Skrap, you'll find that people around here accept NOTHING wihtout picking it to little tiny pieces and driving the original poster insane =)
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Old 01-26-2005, 10:54 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

I'm not nit-picking, just trying to help!

I do have one question for the jury, though -- how about weight? This looks at circ., which is definitely a factor, but what about vehicle weight? Take an emaxx - you could put 2.2 truck tires on it, and come up with the same gearing for it as a T4, but the maxx is gonna weigh a lot more and require some different gearing to suit. Just something to ponder.
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Old 01-26-2005, 11:45 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

I meant *I* was picking hehehe

I'm not bothering with the speed calculation, cuz I don't care exactly how fast I'm going. I figure if I win the A-main, it was fast enough. If I lose, then it WASN'T fast enough. Simple equation =)
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Old 01-26-2005, 11:56 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

circumference=2*pi*r or pi*d =)
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Old 01-27-2005, 01:54 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

So if I have a torquey motor that has low revs I should run with the final equation coming out into a negative value, whereas if I run a motor that revs high I should run with the final equation coming out to a positive value? Obivously the type of motor has some impact on the gearing that should be used.

Pete.
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Old 01-27-2005, 08:32 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Using your e-maxx example, yes, you would end up with a different gear ratio as the t4. The RPM of the motors ideally should run X number of revolutions for X inches of circumference of the overall wheel, but weight is a huge factor. Remember that the emaxx is using 32 pitch gears ( 66x18 ) and the transmission is 28.9:1 in first and 17.9:1 in second gear..... the E-maxx is geared WAY LOW ( -10.8359 ). Starting off in second (17.9:1) is closer to the 1 to 1 ratio (18.0641" circumference), but the truck is a dog! If you switch to smaller diameter tires ( such as Pro Line Road Rage ) you need to raise the spur gear to compensate for the smaller diameter tire. Divide your old tire diameter by your new tire diameter ( Ex 18.0641/17.278= 1.04549 ) and multiply that by the number of teeth in your original pinion ( Ex 1.04549x18= 18.8189) Rounding to the nearest whole number shows that you should raise the pinion 1 tooth to keep the same final drive ratio. Just try to keep as close to the starting value ( -10.8359 ) when selecting a new pinion or you will lose the torque required to get this rig rolling.

So to answer your question, yes. Weight is a factor when determining where to start out. Obviously, tipping the scale at over 9 pounds requires the motors to be OVERLY torquey, low RPM and geared WAY LOW. To gear the truck at a closer 1 to 1 ratio would require a motor that is 10 times as powerful as what is supplied but would require more batteries, and thus add more weight!
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Old 01-27-2005, 08:52 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Before I get beat up, I said the transmission ratio is 28.9:1 and 17.9:1. I mistyped. These are the FINAL DRIVE RATIOS. In first gear, the RPM to circumference is nearly 2 to 1. The transmission gear ratio is final drive ratio/(Spur/pinion) ( 66/18= 3.6667) 28.9/3.6667=7.881

Pete, your observation is correct. the gearing of your truck is heavily dependent on the type of motor you are using and it's peak rpm as well as the size and diameter of your tires.

I ran the numbers on an e-maxx using 2.2" Proline Gladiators : 10.2101 ( Gladiator circumference ) - 28.9 ( E-maxx final Drive) = -18.6899. That's a huge drop that will rev the heck out of the truck without going anywhere. That would require a 31 tooth pinion to get up to the original ratios! at that point, you need to add a larger spur gear, and calculate your pinion gear from there!
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Old 01-27-2005, 09:34 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

By the way, you guys are right. Circumference is Pi times the radius squared, of Pi times the diameter. My Bad!

I ditched the Speed gems and the LRP ESC yesterday. I bought the Novak SS brushless 4300 kit. Holy Cripes is that thing fast!!!!!!! I used my magic formula to determine the proper pinion gear for the setup. I ended up with an 87x17, running my Pro Line Dirt Works. I came up with a final drive ratio of 12.28. According to the Novak web site, the optimum final drive ratio is 12.30. Pretty good for someone who dropeed out of college 15 years ago!

The theoretical top speed for my truck with this motor ans setup is 30.75mph. That is 2.57 mph faster than my over geared Speed Gems Chromium, that I melted.

Oh I'm using math! I hate math!!!!
[:@]
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Old 01-27-2005, 10:07 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Skrap, I'm still trying to wrap my brain around all the intricacies of your formula, but thus far, you are scoring points with me for your logic. I may have to give this formula my official stamp of approval! I mean, the numbers are nothing new, we've all used them before. But finding the correlation between the 2 is brilliant. I'm currently building a spreadsheet to make this all run more smoothly for me at the track. I take my PDA with all my spreadsheets. People look at me like I'm crazy when I'm sitting there doing "math" to figure out my gearing or picking which pack and motor to run together, but it obviously works =)
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Old 01-27-2005, 10:58 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Here is the spreadsheet. Not 100% final, but I think it's close. Just fill in the yellow boxes and get your result in the blue. The sheet is locked, so ya can't screw up the math if you type over it. Hope y'all got Excel!

Ok, the server here won't let me upload a .xls file. I renamed it to a .txt file. Just save it, and rename it to gearing.xls Should work just fine.

Comments are welcome =)

EDIT: Ok, I found a booboo... I had the final calculation going the wrong direction. If you got the old one, download the new one.
Attached Files
File Type: txt
Lj23538.txt (16.0 KB, 656 views)
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Old 01-27-2005, 08:41 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Speedmunkey! Dude! That spreadsheet is the bomb! I sat here and obsessed over this data for 3 days straight ( not much going on at work this week). My wife thinks I'm crazy! But the facts speak for themselves! ANY tire, ANY spur, ANY Pinion, ANY transmission : optimum gear selection!!!!! It's perfect!!!!! Do you thing we can get a trademark on this and publish it???

Where was this info when I started looking on monday...... And to think I told my college algebra teacher that I'd never use any of his silly class ever again! I owe him an apology!

Now, we must be prophets and spread this good news! Damn, dude. That is a great friggin spreadsheet!
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Old 01-27-2005, 09:42 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

and to think, i've never used anything harder than 2+2 in my life. and i was going to drop my calculus class today! (taking to many hours) now ill have to rethink that.
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Old 01-27-2005, 09:59 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Math is the single most important class you'll ever take in school. I use it all day long in my work.

Glad you like the sheet Skrap =)
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Old 01-27-2005, 10:15 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

speed...you nerd...haha..j/k
you take all that on a pda huh? i got all my stuff on a laptop, i bet i would really look like i nerd bustin out the laptop to check batteries, gearing, and all that stuff.

and yeah math is important, however, typically you dont use anything more than algebra,geometry, trig, and MAYBE a little calc.
heck i design machines everyday and tyipcally i do nothing more difficult than air flow and force calculations. though i had to take calc 1,2 and 3 along with differential equations 1 and 2....[&:]
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Old 01-27-2005, 10:16 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

ORIGINAL: aliens8mycow

He's got the right answer, but a misprint in the math...

Circumference = Pi x Diameter (not 2Pi x diameter)
heheh i learnt that in math 2 years ago!.

young me. lol! wut a random comment. ill have to try that someday. i still have no cloe how fast my car is. but it looks fast. im guessing 35 KMPH, yehhhh......[:-]
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Old 01-27-2005, 10:22 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Sleepy, yeap, all on a little PDA. It lets me enter battery data (like charge time, amp rate, run time and opinions on the run), setup data (I'm still working on that one... It's an Excel based version of a setup sheet.. Kinda neat..), rollout, and now gearing thanks to Skrap.. Plus it lets me track my parts inventory, both in my box and in the store.. I also track my qualifying time vs my practice time from week to week, to show me trends in my slacking and help get me motivated to practice more often.. I take this racing stuff pretty seriously, I couldn't survive without my PDA!
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Old 01-27-2005, 10:37 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

oh i feel ya on that, i dont really race yet, but i take my data seriously. i have almost every single part on my t4's weighed and tagged by location and part number, working on my formulas to calculate all the rotating mass.....rollouts, gears, its all fun stuff
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Old 01-28-2005, 10:58 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

You can figure out the speed, just use the formulas above and plug in your relevant numbers. Even better, use speed munkeys spreadsheet, since it does most of the math for you.

The only trick to calculating your "top speed" is to know what RPM your motor is turning with your supplied voltage. You can really skew things by using an 8.4volt pack instead of a 7.2 volt pack.
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Old 01-28-2005, 11:11 AM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

Yeah, I didn't bother with the speed calculation. Myself I find it to be useless cuz I honestly don't care how fast I COULD be going, as long as I'm in the lead on lap 35.

My only concern is that people will take this as gospel, 100% of the time, and cook a motor. For example, a really hot 9t is going to want a smaller pinion than a mild 19t, but this tells them to always use a xxt pinion with certain tires.. Myself, I'm going to use it as a starting point, then check motor temps from there.
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Old 01-28-2005, 06:32 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

To you and I, munkey, speed is irrelevent. But to alot of guys I talk to, it is their holy grail! Nothing else matters to them but "How fast will it go?"

I ran my Novak SS tonight. The pack in the truck has been there all week, an dI've been using it to set the differential and slipper clutch to maximize the new motor. Well, I ran it for about 12 minures ( 3000mah NiMh ) and still had pretty good punch getting it back in the garage. The pack was warm, but the motor and ESC were cold. I mean cold! My Chromium would have been close to thermonuclear as hard as I was running that thing tonight. Oh how I love brushless!!!

Perhaps, for the edification of the masses, you could use your copious knowledge of excel and devise a "theoretical top speed" program, so those whose quest is 50+mph can get a better feel for how they need to gear their trucks?
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Old 01-28-2005, 08:41 PM
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Default RE: Mystery Solved! How to select the proper gearing for your electric RC!

oh dude, there is a ton of stuff to do get an even remotely close speed. you can easily do it based on gearing and motor rpm, but not realistic. i think i am close to having mine done, but its for a t4 only cuz i am basing it on the tranny gearing and the rotating mass
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