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

RE: Mystery Solved! How to select the proper gearing for your electric RC!
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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:09 PM

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"When your gearing systems starts winning races, then I’ll believe. Until then I’ll use my simple formula. "

I guess you are so wrapped up in the idea of disproving Skrappy's formula you missed the part about me setting the unofficial (it was in practice, so it doesn't count..) track record by using his formula. You can say what you want, but this has worked for me and I see no reason to doubt it. Yeah, it ain't perfect, but it gets me within a tooth either way, where as before I started with the assinine method of using the same number of teeth as the motor has turns, and then trying again when I couldn't find a 12t pinion for my 12t motor. Yeah, I always got there eventually, but it took many laps, many overheated motors, many pairs of brushes and many cuts to find the perfect gear. With his formula, I'm 90% there before I ever put it on the track.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:14 PM

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You missed the point of the exercize entirely.

WHY is a 19t pinion the recommended stock gear for the 27t motor? Why does the E-Maxx have an 18t pinion if it has 2 motors? Why is the B4's starting gear ratio at 83/21?

The answer is the Final Drive Ratio's. It's as plain as day!

Look at EVERY recommended setup. If you will pay close attention, you will see that the Final Drive Ratio for MOST manufacturer is selected based on the circumference of the stock tire. The exception to this is Traxxas and DuraTrax. They outfit their trucks and buggies with such poor performing motors, that they deliberately overgear the chassis, to compensate for the lack of performcance. Once you change the stock motor in either chassis, then you have to adjust your gearing .

So, we both arrive at the same answer, using two different approaches. You figure out the difference between the two, and mathematically arrive at the conclusion. That's VERY handy if you have ALL of the recommneded ratio's printed out and available to you at all times. Myself, I understand that the determination of the Final Drive Ratio is affected by the circumference of the drive tire. I base my calculations on that observation, and accordingly arrive at a good baseline setup.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:30 PM

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quote:

ORIGINAL: Graystar

quote:

I've got an Idea. Take a T4. with an 85t spur. Take off the stock M3 Hole shot tires. Put on, say some masher 2000's. You are running a Novak SS 4300 motor. What pinion will you use? Hmm? Nothing in this list is stock. What pinion do you use, wise guy?

Come on, I want an answer, and an explaination of why.........

No prob...

I would increase the spur/pinion ratio by the increase in tire size.

from the tire specs...
M3 diameter = 3.75"
2000 diameter = 4.9"

Increase in tire diameter = 4.9/3.75 = 1.3067 or 30.67%

From Novak specs: "the SS4300 Brushless Motor provides performance similar to a 27-turn racing stock brushed motor."

So, from the T4 manual...stock motor = 87T spur/19T pinion

ratio = 87/19 = 4.5789

just multiply by the increase...

4.5789 x 1.3067 = 5.98324863

That's my new spur/pinion ratio

Finally, I divide 85 by 5.98324863 and get a pinion size of 14.

The formula is:

[Actual Spur] / (( [new tire dia.] / [stock tire dia.]) X ([sugg. spur] / [sugg. pinion])) = actual pinion needed

Gee, if you'd taken a moment to step back from your trying to prove me wrong, you'd find out that you might be on to something.....

What is the Final Drive Ratio for this above setup? 15.786:1

What is the circumference of the Masher 2000's? 15.394"

Conincidence?

Try it again. Stock T4, M3 Hole Shot, 87t Spur, 19t pinion:

What's this Final Drive Ratio? 11.905:1

What's the Hole Shot M3 Circumference? 11.781"

So, by your own formula, You proved me right. The only thing that you overlooked, or failed to comprehend is the DIRECT relationship between the circumference of the tire and final drive ratio.

As for "When your gearing systems starts winning races, then I’ll believe. Until then I’ll use my simple formula." It already is winning races.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:35 PM

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quote:

with the assinine method of using the same number of teeth as the motor has turns, and then trying again when I couldn't find a 12t pinion for my 12t motor.

OMG! Who told you to do that?? That IS asinine!

On any modern racing truck or buggy there's only a 5-6 tooth range between a stock motor and a 10 turn motor. How many motors could a person burn out trying to find the right pinion?? It shouldn't take more than 4!! (actually, it shouldn't take any because you should start with a small pinion and work your way up.)

Doesn't your car manual have any gearing recommendations? If it does, did you try them? If it doesn't, how far away are you now from the stock pinion?

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:40 PM

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The difference is, with Skrappy's method, I have everything I need to gear a truck I've NEVER touched before or read up on the factory suggested gearing. I have tires, a caliper to measure them with, i have the pinion, the spur and, if need be, I can open the tranny and figure out the internal gearing. With Gray's method, I'd need the manual (which no one ever has) or access to the web to find out how the factory says to gear it. Then, I'd have to do just as much math to figure out the same end result.

So, simple equation with no real work other than counting teeth and measuring a tire, or simple equation that I may or may not have all the facts for on raceday..... ? Yeah, that's a no brainer..

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 1:43 PM

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"OMG! Who told you to do that?? That IS asinine!

On any modern racing truck or buggy there's only a 5-6 tooth range between a stock motor and a 10 turn motor. How many motors could a person burn out trying to find the right pinion?? It shouldn't take more than 4!! (actually, it shouldn't take any because you should start with a small pinion and work your way up.)

Doesn't your car manual have any gearing recommendations? If it does, did you try them? If it doesn't, how far away are you now from the stock pinion? "

When I first got into racing, the common way of finding your gearing was to start with the same number of teeth as turns, and then work from there based on motor temp. And generally, the trucks I buy don't come with pinions. RTRs come with pinions, the good stuff makes you buy your own

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 2:11 PM

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quote:

Look at EVERY recommended setup. If you will pay close attention, you will see that the Final Drive Ratio for MOST manufacturer is selected based on the circumference of the stock tire.

Like I said before, that only seems to work for stadium trucks. The circumference of my stock B3 tires is 11 inches but my FDR is 8.83

quote:

They outfit their trucks and buggies with such poor performing motors, that they deliberately overgear the chassis, to compensate for the lack of performcance.

Oh so they intentionally risk burning out their customers electronics?? I don't think so. They use that gearing because that’s the gearing required for those motors. Stock motors are always geared higher than modified motors.

quote:

So, by your own formula, You proved me right.

No, it only proves what I said before...this it only seems to work for stadium trucks. If I were to put those tires on my B3 then the FDR wouldn’t match.

In fact I just found Novak recommended gearing for the T4 when using an SS4300. Using their recommended gearing as a basis, I would go with a 13T pinion.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 5:40 PM

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quote:

ORIGINAL: Graystar

Like I said before, that only seems to work for stadium trucks. The circumference of my stock B3 tires is 11 inches but my FDR is 8.83

Actually, Graystar, your STOCK B3 tires ( Proline M2 Holeshot p/n 6825) are only 3.20" in diameter. That is a circumference of 10.053"

You are running a 22 tooth pinion in your B3, giving you your 8.836:1 final drive ratio.

The base-line gearing for your B3 would be a 20t pinion.

You are running a 22t. Surprise! That falls well within the +/- 1 to 2 tooth adjustment that is permissable for fine tuning the performance of the vehicle!

So, yes, the formula works JUST FINE for gearing your buggy too.

The gear calculator is designed to give you the best balance of speed, and acceleration as a baseline for fine tuning your performance.

It is IMPOSSIBLE to OVERGEAR any vehicle using this calculator, because the Final Drive Ratio is BALANCED with the tire circumference.

There may instences where the vehicle might be slightly undergeared, but it is permissable to compensate by adding 1-2 teeth to the pinion. Doing so, will not signifigantly add to the load put on the motor. You have already done that. You have your buggy geared at the Top End of the safe range. Any higher, and you may pick up an mph or two, but will risk damage to your electronics.

quote:

ORIGINAL: Graystar

Oh so they intentionally risk burning out their customers electronics?? I don't think so. They use that gearing because that’s the gearing required for those motors. Stock motors are always geared higher than modified

Have you ever run a Stock Stinger or Photon motor? They last all of about a month, before they need to be thrown away and replaced. They get VERY hot after running them, and DO NOT last.

They are junk motors, deliberately overgeared, to make the stock truck faster. When you change from a Stinger to say, a P2K2, you will need to change the stock pinion gear as well. Running the stock gear, will cause the new motor to overheat, and will shorten it's life expectancy. Use your calculation and figure it out for yourself. Running the stock 4.75" tires, and an 87t spur, you'd need to drop the pinion 4 teeth, to keep the motor cool while running it.

< Message edited by SkrapIron -- 4/8/2005 5:53 PM >

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 7:20 PM

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quote:

Actually, Graystar, your STOCK B3 tires ( Proline M2 Holeshot p/n 6825) are only 3.20" in diameter. That is a circumference of 10.053"

Yeah, one would think that from looking at the specs…wouldn’t one? Except that the car actually came with the tires that are pictured on the manual...Bowties. And Bowties have a diameter of 3.5”, which gives a circumference of 11”. So, surprise! Any dissents should be directed towards my tape measure.

quote:

That falls well within the +/- 1 to 2 tooth adjustment that is permissable for fine tuning the performance of the vehicle!

It isn’t quite as permissible as you make it seem. A 2 tooth adjustment in pinion size can take you from dialed-in to way overheated. Of course, 2 teeth smaller will leave you running cool but spinning out all over the place. That's why you have different size spur gears...because sometimes the adjustment range is within a single tooth, and they don't sell 19 1/2 tooth pinions.

quote:

It is IMPOSSIBLE to OVERGEAR any vehicle using this calculator, because the Final Drive Ratio is BALANCED with the tire circumference.

It may very well be impossible to over-gear any vehicle because it appears that your formula will under-gear trucks by a small margin, and buggies by a large margin. I guess that's a good thing for guys who dropped a hot motor and a pinion 3 teeth bigger than recommended into their car because they wanted to go really really fast, and are now complaining about burnt out motors. I'll just stick to my system, thanks.

quote:

They are junk motors, deliberately overgeared

You know, in a previous post you said the Photon was under-geared to compensate for the lack of performance of the Photon motor. And then you said...

quote:

The Stampede/Rustler/Evader are deliberately UNDERGEARRED to compensate for ANEMIC motors.

But now you’re saying they’re over-geared. So...um...which is it?

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 8:44 PM

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He also said earlier that it was a mis-type.. I knew when he first typed it that his brain was going the wrong way. It happens.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 9:07 PM

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You and I have arrived at the same answers for various setups, using two entirely different methods. Yours requires a knowledge of the target final drive ratio, as well as some basic math. Mine determines the target final drive ratio based on some simple observations, and some basic math.

As you finally observed;

" It may very well be impossible to over-gear any vehicle because it appears that your formula will under-gear trucks by a small margin, and buggies by a large margin. I guess that's a good thing for guys who dropped a hot motor and a pinion 3 teeth bigger than recommended into their car because they wanted to go really really fast, and are now complaining about burnt out motors."

That is the point of the calculator. It is to assist people without a good understanding of how they should gear their vehicle, to do so, safely, without risk of damage to their equipment. It gives a good reference point for a base-line gearing based on the optimum final drive ratio.

The method has been tried, tested and approved my many racers, since I introduced it in January. It allows you to do 95% of the setup work, before you even set out for the track. Once you are in the Ball Park, you can quickly adjust your performance from there.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/8/2005 11:15 PM

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quote:

Yours requires a knowledge of the target final drive ratio

Take another look. All that is required is knowing the current spur and pinion, and the diameters of the old and new tires.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/11/2005 6:56 PM

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(Moved here in the interest of maintaining context and not hijacking a thread)

quote:

ORIGINAL: SkrapIron

quote:

ORIGINAL: Graystar

Don’t you have the manual? Get it from Associated here:

http://www.teamassociated.com/shusting/CatalogHub/cathub_offroad.htm

The B4 manual says to use a 22T pinion on Spec 19 motors with the 81T spur. From the recommended size, you can add 1 tooth for more speed or subtract 1 tooth for more acceleration. If you add 2 or more teeth then you risk burning out the motor. Also, you probably won’t go any faster because motors are limited by the torque they can ultimately supply. A motor has its limit...and no amount of teeth can overcome that.

Associated says that the top speed of the B4 will be from mid-20 to mid-30 mph. A Spec motor is on the slower end, so I’m guessing that you’re probably looking at the high 20s.

If speed is your only concern then get something like a 10 turn modified motor and add 1 tooth to the recommended pinion, which would be 19 +1 = 20. That would be fast. Your acceleration would be so so, but you’d be fast. If you add 2 teeth then you’d be getting into that area where you’re not going any faster...you’re just creating more heat.

I think this is what you and I have been discussing for the last few days, Graystar. Most of these guys who ask these questions either: 1: Do not yet have the model in question. 2: Cannot find their manual. 3: Have their manual, but are desperately confused by it. 4: Have the gearing so screwed up by experementing that they have no clue how to get the setup back to normal.

To quote an article by Racing Lines Magazine:

" (If in doubt) It is better to err by gearing nitro's tall, and electric short. Most car manufacturers include a recommended gear ratio chart and it is a good policy, at least initially, to follow their guidelines. They built the car for all-around performance and some experementing on your part will enable you to work out which way you should go."

In the same article, they addressed the importance of knowing the overall tire roll-out of the vehicle, when selecting the pinion gears, and how it affects the final drive ratio. While their article dealt exclusively with racing foams, it stands to reason that the same rule can be carried over into rubber tires on ST's and buggy's.

That's where my calculator come into it's own. By balancing the final drive ratio be closely as possible to the tire's roll-out, you can guarentee two things. 1. The resulting gear selection is err'd on the side of caution. 2. While providing a good balance of speed to acceleration, it allows room for experementation. It is by providing a base-line gearing solution which is in-line with manufacturer recommended ratios, that someone can select a good gear ratio, without having to keep track of said ratios at all times.

Well, it’s only in line with manufacturer’s recommendations for stadium trucks. Still, if you put it in that context, that’s fine with me. I don’t think I made myself clear initially, but I was mostly taking issue with some of the claims you had stated, such as:

quote:

"Any ratio that is greater than 1 will run faster, but will overheat your motor, battery and esc, eventually damaging them."

"Optimally, your final drive ratio (spur gear/pinion gear*transmission gear) should equate as close to the circumference of your tire as possible."

"...and you'll be assured of a balanced, , predicatble, well handling car"

"Pan car gearing should follow the same concept,"

"It does not matter what motor you select. Follow the gearing selector. The speed of your truck/buggy is determined by the RPM of the motor, not the gearing selected."

"…is deliberately undergeared…"

"The Stampede/Rustler/Evader are deliberately UNDERGEARRED to compensate for ANEMIC motors"

"They are OVERGEARED to compensate for their rather pathetic motors."

These statements simply aren’t true. And while over-gearing is bad, under-gearing creates problems as well, mostly damaged tires and handling issues.

Let me tell you what I really think. I think that your gearing suggesting is probably just fine for trucks and buggies, with just the slightest possibility of over-gearing a hot 10t motor on a truck, while under-gearing buggies with stock motors. What bothers me most is when concepts are thrown out without a solid basis in facts or reasoning, and when statements are made that are clearly wrong or that require some substantiation that’s not provided (see list above.) You don’t *know* that the gearing is ideal. What you know is that your system works well for your trucks. Beyond that, you shouldn’t speak authoritatively on subjects you don’t know about, such as pan car gearing and the reasons behind gearing decisions made by RC car manufacturers.

Here’s a rule of thumb that I would suggest to people wondering about the gearing on their buggy or truck...place a ruler next to your car and measure how far it moves with one revolution of the motor. If it’s 1 inch (+ or – 1/8”) you’re probably in a safe range. Can’t see the pinion spinning? Feel for 6 “bumps” while rolling the car. If the car moves more than 1 1/8 inches then you might be over-geared. A buggy with a stock motor can go to 1 1/4" and be okay. Got a 10T motor? Your truck better be at 1” or less. I find it easiest to perform this test by rolling the rear tires with my hands, instead of trying to push the car by the chassis.

This does pretty much the same as your calculations, with about the same risk, but without the calculations.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/11/2005 8:06 PM

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Why do you insist on polluting this thread with alternative methods? This thread is about Skrap's formula, which does work and works very well. Yes, your method might also work, and I encourage you to go make your own thread about. But this thread is Skrap's, please show his the same courtesy and respect you would want.

and now, you come up with this....

"This does pretty much the same as your calculations, with about the same risk, but without the calculations. "

The calculations he's given here are super simple, and RCU has even provided a calculator to do them for people with only a few variables to fill in. Instead, you suggest they set up a test that isn't nearly as accurate, to try to get the same answer, that does the same thing. First you say it won't work, now you give another method of getting the same answer. Personally, I'd like to see this thread pruned of about the last week, then locked for good. It was a good thread up until you decided to ruin it.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 12:48 AM

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Speedmunkey,

The more he argues against this thread, and my observations, the more he confirms them as true. You can see that in his change from the first post ( It can't work) to now ( It MIGHT work for trucks and buggies ).

What I find comical is now he's suggesting that someone take a ruler, and pace off the roll out of the tire, to essentially duplicate the formula.

I'm going to e-mail keith and have him replace this thread with the article that I wrote for the magazine. Once it is published, and made sticky, I'm going to ask him to lock it, so next time, it can't be pulled so far off subject.

Graystar,

If you have a system or an idea that works for you, then I'd encurage you to publish it in your own thread. That way, we can analyze and critique your methods and reasoning.

As for you assertation that I don't know of what I'm speaking about, I have been running electric RC cars since 1983. We did not have RTR's back then, so you had to learn by doing. I've been at this long enough to know what works and what doesn't.

Now, If you please, confine your ravings to your own thread, and thank you for substatiating mine.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 1:19 AM

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quote:

You can see that in his change from the first post ( It can't work) to now ( It MIGHT work for trucks and buggies ).

Ah, now I see the problem here. You're not fully reading my posts.

In my first post I simply questioned the rational behind the formula. I never said it can’t work.

In my second post I said that it works only for stadium trucks, which I still believe. It will under-gear a buggy, but that shouldn't damage anything.

And it doesn’t matter how long you’ve been in RC...wrong information is wrong information, and those statements you made were wrong. Bad information only increases confusion.

quote:

What I find comical is now he's suggesting that someone take a ruler, and pace off the roll out of the tire, to essentially duplicate the formula.

So you're just dismissing it without trying it? Interesting...

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 3:35 AM

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Why is it worth trying? You said yourself it's simply a way to work Skrap's formula without doing the calculations.. If that's the case, it works! Case closed. You seem like a very intelligent guy, and obviously we need intelligent guys around here to help people. I'm NOT trying to discourage you, please don't read that in my posts. All I'm asking you for is to respect Skrap's thread and his system, which has been track proven by myself, to be a great starting point for a properly geared vehicle.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 3:45 AM

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The rationale behind the formula is this. A given motor produces a set number of RPM per volt. ALL motors are rated exactly the same. The only difference is the number of RPM produced per volt.

That RPM is then divided through the gear train to produce torque, that is otherwise not available to the motor by itself. By the time that the gear reduction is finished dividing the RPM of the motor, you are left only a percentage of the original RPM. ( a 30000 rpm motor, running through a gear train with approximatley 12.30:1 of reduction produces only 2439.02 RPM at the axle). Are you with me so far?

A 17t pinion on the example motor, with a diameter of say .40", will theoretically run forward 1.256" per revolution. That drive momentum is then divided by the reduction ratio of the gear train. By the time that division is completed, the axle will travel a staggering .105" per revolution.

For the sake of efficiency, it is only fitting that you select a tire with a roll-out that is as close to an equal with the drive reduction ratio. That way, 1" of forward momentum travelled by the pinion will equal out to 1" travelled by the drive tires. (.105" per revolution X 12.881" roll-out = 1.352" traveled per revolution of the axle. That tire also will turn 30255 RPM or about 30mph. That means that the example is slightly overgeared.

It's that simple. That is why it is considered an optimum ratio.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 3:56 PM

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quote:

ORIGINAL: SkrapIron

The rationale behind the formula is this...

Well I guess that says it all.

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RE: Mystery Solved! How to select the proper gearing fo... - 4/12/2005 5:48 PM

Speedmunkey

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You want to explain that?

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RE: Mystery Solved! How to select the proper gearing fo... - 4/14/2005 4:50 AM

kab1sg

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Gentlemen,

Kudos to all for well thought-out points of view and theories. I commend anyone who attempts to make something easier for everyone. Good job SkrapIron. This has turned into an argument that is not necessary. If you race, you want gearing that allows you to get your car around the track as fast as possible...the only run-time issue is the time of the heat or main. Usually if you end up with lots of runtime after a round of racing, you have left a lot on the table. If you just want to have as much fun as you can for as long as you can, then you seek ways to make that happen. Go to any race and you will probably find several effective ideas about gearing. I have my own opinions and I welcome input that is helpful to me or others. The bottom line is: If it works fot you, DO IT! If not, don't use it, try something else. Manufacturers will tell you that their recommendations are starting points and not set in stone. That's why they have engineers and designers constantly working to improve their products. I have been involved in this hobby for several years and know only one thing for sure. It's fun! Enjoy!

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RE: Mystery Solved! How to select the proper gearing fo... - 4/24/2005 12:32 AM

S.A.E.W.O.T.

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R U TALKIN?....2ME?

ok nice thats kooL!, i donno this thread is 2 long so i desided not 2 read it!....lol im not ignurant, im jus lazy! and i can't read!.....lol!

, so yea, i jus experiment with gears and such, and if i find 1 that works i stick with it!

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RE: Mystery Solved! How to select the proper gearing fo... - 4/24/2005 10:51 AM

scottmattson

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Skrap- I just finished reading the entire thread and re-created speeds original spreadshhet while reading that post. It works exactly as you say. I did find the optimal gearing for speed is with parts not available to me in Iraq, so Tower here I come. Graystar is jealous that he didn't come up with the thread first or as simply stated as you put it. He might be on to something but it takes into account information I cannot findsitting in my barracks in Iraq. I even figured out what gears I need for my nitro truck sitting in the states and HPI does not list gearing recommendations in the manual. Keep up the good work Skrap, I enjoy your insight in many threads on this site.

Scott in Baghdad

< Message edited by scottmattson -- 4/24/2005 10:53 AM >

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RE: Mystery Solved! How to select the proper gearing fo... - 4/24/2005 6:43 PM

jaejw1

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forgive me for not reading ALL of the posts on this thread,, just way too long,, but i did read the first several posts,,,

my question /discovery is this,,

while playing with my rc18t mini stadium truck with the mamba 8000kv motor inside,, i had it geared 60/12 (5:1)with an internal gearing of 2.5:1,,, the car ran really fast,, very fast infact,,, played with it with 6cells,7 cells, 8 cells and also 10 cells,, all of which were much faster than the previous,, my mamba then died,, and i replaced it with a new motor,,

now i have 55/11 gearing again 5:1 the same as 60/12 BUT,,, this current gearing set up is much faster than the previous 60/12,,,, eventhough both have the same ratio,,, they do not share the same speed,, maybe im dreaming but i have tried to get this answered because i would like to make my xcellerator faster than it is now but in using the same ratio or close to it,,

xcellerator is currently 90/18 (5:1) with an internal of 2.6:1... i was thinking of doing 87/17 which gives almost the same ratio,, of 5.117:1

question,,, will using smaller gears,, both pinion and spur make the car faster if the ratio is the same as stock,, or what the current usage is,, it worked for my rc18t can it work for my 1/0th stadium truck,,, my guess was that it was less rotatig mass on both ends,, allowing the motor to spin with ease..

thanks,, and again sorry for not reading the entire thread and if this has been answered before please let me know,, ......

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RE: Mystery Solved! How to select the proper gearing fo... - 4/28/2005 12:23 PM

ronnerwin

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At first I couldn’t figure out how or why SkrapIron’s tire/gear ratio works, and was pretty much in the same boat as Graystar. The problem I had is that a scalar value (tire circumference measured exclusively in inches) is subtracted with a constant (final drive ratio) to yield another constant (the tire/gear ratio). By the way, it should be called tire-gear difference or something since a subtraction is being carried out, but the name doesn’t really matter. Normally, you can’t do this kind of math because the units cannot cancel out to produce a non-scalar result; that is why the “ratio” calculation doesn’t apply to metric units without first converting to imperial. However, just because the math doesn't make sense, doesn’t mean the method doesn’t either.

As mentioned in the original post, 1 to 1 (rollout value of 1 inch/rev or 25.4 mm/rev) is ideal. This rollout value is equivalent to a tire/gear value of 0, but this calculation is easier. Rollout is important because it takes into account both final drive ratio and tire size. For example, you can change your RC’s spur gear to half the size (giving it half the final gear ratio) and still have it usable with nothing melting as long as you reduce the size of your tires to half as well because the rollout will stay the same. In fact, top speed and acceleration may improve since rotating mass in the tires and spur gear would be reduced. Hey, there’s an idea. Therefore, final drive ratio doesn’t mean much by itself. I think the main reason why tire size is normally ignored is that tire sizes are fairly standardized, especially in touring cars.

By the way, I made a spreadsheet (see attachment RC ratios calculator.v1.txt) similar to Speedmunkey a while back, but it did not have SkrapIron’s tire/gear ratio. I recently incorporated the tire/gear ratio in a newer version (RC ratios calculator.v2.txt) since it is useful and have been tested by many people in this forum with good results. Thanks for the insight.

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Attachment.txt

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