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help me understand different ways to create FDR

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help me understand different ways to create FDR

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Old 02-16-2019, 11:52 PM
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Default help me understand different ways to create FDR

so im looking at fdr at the moment. im trying to understand the difference in the different ways you can create the same fdr.
for example the 48p gears with an fdr of say 3.75 i can use 39 pinion and 77 spur vs fdr of 3.74 using 34 pinion 67 spur.
fdr is basically identical would there be any benefit to running one of those combinations vs the other? I.E the smaller pinon vs the larger pinion setup, will it effect the torque of the motor ? the efficency ? or will it make no difference at all?
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Old 02-17-2019, 12:25 AM
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This question is asked and answered many times

Basically you will shift the weight of the motor and then there is the gyroscopic effect of largere gears. But basically the numbers are low that they only can be noticed by good drivers.
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Old 02-17-2019, 12:27 AM
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Originally Posted by Roelof View Post
This question is asked and answered many times

Basically you will shift the weight of the motor and then there is the gyroscopic effect of largere gears. But basically the numbers are low that they only can be noticed by good drivers.
i had found some info on this but not enough to suggest that there was any benefits or losses in either way. i was thinking that it may have affected the power delivery or driveability of the motor?
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Old 02-17-2019, 12:50 AM
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The larger the gears you use, the lower the losses, especially from the "small gear". Once you're past say... 15-20 tooth, you're not going to see a lot of returns going to bigger gears. Bigger gears will have "some more" inertia. But with all the torque our motors make, I doubt that's a factor you can actually measure.

What you might notice, is that by moving the motor forward or backwards, you're going to change the CG of the car a bit. In some of my cars, I'm more limited by the motor adjustment range, than I am gears. I need to pick my gearing based on what I can actually reach using my motor mount, versus run what I've got handy, or what I'd prefer.
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Old 02-17-2019, 02:54 PM
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I agree with Nerobro's second paragraph. The big question for me is what gears can I fit in my car. Then my second thought would be the smaller the better. I don't have any proof that smaller is better but it works for me. Throttle seems more sensitive to my finger.
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Old 02-17-2019, 06:43 PM
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Larger the pinion gear. The hotter the motor will run. So you have to decide the trade off.
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Old 02-17-2019, 07:02 PM
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Originally Posted by massenb203 View Post
Larger the pinion gear. The hotter the motor will run. So you have to decide the trade off.
That assumes changing the FDR. For the same FDR, the motor will see virtually the same load.
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Old 02-17-2019, 08:35 PM
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Efficiency should be the same for any pinion spur gear combination of the same fdr. Inertia mismatch (rotor inertia-gear ratio-drive traine inertia) is as important as power it the ratio is less than 30 to 1. Based on the acceration rates Ive seen you are looking at something probably around 45:1. If someone would model a ceramic cylinder 12.5mm in diameter x length and then assume a 45:1 through the fdr and add the gear inertia to the drive train side it would be a simple way to see if it makes any appreciable difference.

less than 30:1 and the drive train should be fully accelerated in a couple seconds. 60:1 maybe 30 seconds. 1:1 500ms. It doesnt take the place of torque but below 30:1 means you will feel cogging more but it be less smooth if the motor isnt smooth it wil also be more controllable.

Below 30:1 and the drive train is driven by the motor. Above 30:1 and the rotor will have less control over the drive train which means it will dampen a lumpy motor.

in the world of gear ratios spur gears are not very efficient. Efficiency is like a friction across all rpms. I would suspect something less than 80 percent.
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Old 02-17-2019, 10:05 PM
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You're misinformed. The bigger the diameter of the meshing gears, the smaller the changes there are in each tooth to tooth contact. Spur gears are very efficient, depending on gear form, mesh, etc... If it sounds good, you're in the 97 to 99% torque transfer range. Belts are right up there too.

Why are you talking about gear ratios in the 30:1 range and higher? Almost nothing in r/c racing is more than 10:1 FDR. In the context of this discussion we're talking about the primary reduction between the motor and the center shaft (or input shaft to the transmission). It's really common to be in the 2.5 to 5:1 FDR. This means the primary reduction is in the 1.1:1 to 2:1 range.

If you are trying to compare motor rotor, pinion, and spur inertia versus chassis inertia.. well that's not crazy to calculate. And when you have 15g of spur and pinon versus 1300g of car... well that's not even in the noise.

Or.. did I completely misunderstand what you were trying to get at?
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Old 02-18-2019, 07:15 AM
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Originally Posted by Nerobro View Post
You're misinformed. The bigger the diameter of the meshing gears, the smaller the changes there are in each tooth to tooth contact. Spur gears are very efficient, depending on gear form, mesh, etc... If it sounds good, you're in the 97 to 99% torque transfer range. Belts are right up there too.

Why are you talking about gear ratios in the 30:1 range and higher? Almost nothing in r/c racing is more than 10:1 FDR. In the context of this discussion we're talking about the primary reduction between the motor and the center shaft (or input shaft to the transmission). It's really common to be in the 2.5 to 5:1 FDR. This means the primary reduction is in the 1.1:1 to 2:1 range.

If you are trying to compare motor rotor, pinion, and spur inertia versus chassis inertia.. well that's not crazy to calculate. And when you have 15g of spur and pinon versus 1300g of car... well that's not even in the noise.

Or.. did I completely misunderstand what you were trying to get at?

take a look at the coefficient of friction for dry nylon on nylon. Also take a look at straight cut ring and pinion efficiencies. I work with these physics everyday but maybe we arenít communicating effectively.

I didnít mention gear ratios of 30:1. The topic was inertia. Inertia mismatch is not common knowledge but it is common physics that most people minimize because the physics priorities of electric motors are pretty unique.

Donít take my answers as anything other than answers. You are a smart guy. But we are talking about different things. We disagree on efficiencies but even if I supported my claim it wouldnít make much of a difference to anyone else and I like what you do on here.

small changes in mass on the input side of a ratio are like squares of 2 and 4 on the output of the ratio. The same is true in the opposite direction.

Of course the calculations are easy for the rotor. Iím assuming allot of what I wrote made no sense to anybody but the guys who normally understand. I continue to poke because every time I do 1 more guy learns. I enjoy helping but what happens is people flame hard before they understand and then they donít go back and clean up the opinion pole that is created. I donít need to feel special but those who know donít have to say allot but should say something so that understandings arenít lost and misunderstandings become the understanding.

If you know you may be potentially misinformed wouldnít it make sense to gather data before evaluating me? Im not offended but Iíve read your stuff and you have a creative mind. Let it eat.
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Last edited by Bry195; 02-18-2019 at 11:46 AM.
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Old 02-18-2019, 08:35 AM
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Originally Posted by Nerobro View Post
Or.. did I completely misunderstand what you were trying to get at?
It's always an option. Hah! And i'm totally willing to talk about it in public. :-)

I heard "The efficiency of a spur gear-train is closer to 80%" which might be true of the peg and cogs in a windmill, but definitely not for the generated gear teeth we're blessed with in modern transmissions.

It's been a while since I was asked to look up coefficients of friction.. Nylon on nylon is .15 to .20. Sadly, because I didn't find an easy reference, that's the static coefficient, and we really should be treating this as sliding. This is one of the two places that "bigger gears" having less friction comes from. Bigger gears means theres less pressure on each gear tooth, so your friction is lower. Also bigger gears have flatter gear profiles and move less across their faces. (Now I did get a set of gears from 3 racing that look like they were pulled off the stack before hobbing was finished... One wonders how good the gear tooth shape generation is from 64 pitch hobs... )

You say "inertia mismatch" and I'm hearing "Torsional vibration", or in the end, elasticity in the drivetrain. The sort that destroyed early, big, radials, and threatens to shake apart every flat plane v8 in existence. That could be an interesting path to go down, given how many springs are in r/c car drivetrains. And then, the places for excess loads to bleed out. The belts, and their interface with the pulleys is a place energy can get dissipated (overload the belt, and it'll try to skip a tooth, as the belt resets into it's groove you're blowing energy on the teeth against the pully) also, some energy can escape through the rear diff, and impulses causing the tires to slip. There are so many places for energy to get out, do we need to worry about resonances?

Unlike a lot of people, you can tell me i'm wrong, and I will listen. ;-)
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Old 02-18-2019, 04:50 PM
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This same discussion came up a few years ago. I actually did the math, and here's what I posted:

As they say on Mythbusters...

WARNING - SCIENCE CONTENT

I did a little bit of calculating. Using gear combinations that will actually fit (on a TC6) that both are 4.0 FDR, the inertia values for small vs. large gear combos are as follows (assuming 48P aluminum pinions):

33P/66S = 0.0000002908 kg-m2
40P/80S = 0.0000005054 kg-m2

So it sounds like smaller is waaaay better, right? Only one problem. The reflected inertia of the entire car (1400g) is approx. 0.0007879 kg-m2. Which is 1559 times the inertia of the gears. So if you think a 0.06% increase in acceleration will help...... go for the small gears. Otherwise, I'd suggest going with a spur size that gives you some adjustment range to either side of where you start at.
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Old 02-18-2019, 07:49 PM
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Originally Posted by Kevin Marcy View Post
This same discussion came up a few years ago. I actually did the math, and here's what I posted:

As they say on Mythbusters...

WARNING - SCIENCE CONTENT

I did a little bit of calculating. Using gear combinations that will actually fit (on a TC6) that both are 4.0 FDR, the inertia values for small vs. large gear combos are as follows (assuming 48P aluminum pinions):

33P/66S = 0.0000002908 kg-m2
40P/80S = 0.0000005054 kg-m2

So it sounds like smaller is waaaay better, right? Only one problem. The reflected inertia of the entire car (1400g) is approx. 0.0007879 kg-m2. Which is 1559 times the inertia of the gears. So if you think a 0.06% increase in acceleration will help...... go for the small gears. Otherwise, I'd suggest going with a spur size that gives you some adjustment range to either side of where you start at.
That is good stuff. The small gears are half the inertia of the large. The reflected inertia ratio to consider is at the motor shaft. The sum everything forward through the ratios as seen by the rotor. Rotor inertia is anything attached to the rotor shaft back.

drive train inertia and mass + gear inertia /ratio=rotor inertia
1559 ish /ratio<30(rotor inertia)
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Old 02-18-2019, 08:06 PM
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Nerobro-you are pretty close. Its a ratio that keeps the mechanical system within range of the control bandwidth.

Here is some brain food for an application that positions (<10:1).
https://www.motioncontrolonline.org/...content_id/404
30:1 is the rule for dynamic velocity control and 60:1 is the ratio for constant velocity and then everything between these ratios is a blend of the control strategy.

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Old 02-18-2019, 08:23 PM
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Originally Posted by Bry195 View Post


That is good stuff. The small gears are half the inertia of the large. The reflected inertia ratio to consider is at the motor shaft. The sum everything forward through the ratios as seen by the rotor. Rotor inertia is anything attached to the rotor shaft back.

drive train inertia and mass + gear inertia /ratio=rotor inertia
1559 ish /ratio<30(rotor inertia)

It is good stuff insofar as it is mathematically correct. However, the really important part is: The inertia of the car is 1600 times higher than the inertia of the gears. Running smaller gears (with the same FDR) will do fuckall for your car's handling -- EXCEPT allowing you to change the position of the motor if that's an issue for you.

Don't run out and replace your gears with smaller ones just because their inertia is smaller. It isn't enough smaller to actually make a difference. Even the pros wouldn't notice a change that small -- though I'm sure their sponsors would play it up so they could sell more unnecessary tuning parts to rubes like us.
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Last edited by fyrstormer; 02-18-2019 at 08:34 PM.
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