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Most Powerful 21.5T, 17.5T, and 13.5T Motors?

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Most Powerful 21.5T, 17.5T, and 13.5T Motors?

Old 06-16-2019, 04:54 PM
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I feel like you guys have picked up some literature on inertia matching and run with it without understanding why. Inertia matching is mostly useful in motion control systems where position is important (eg servos). It doesn't really apply to simply accelerating a car as fast as possible. And even if it did, the total inertia of the car is part of the load, you can't just look at the rotational inertia of the drivetrain.
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Old 06-16-2019, 05:28 PM
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Originally Posted by gigaplex
I feel like you guys have picked up some literature on inertia matching and run with it without understanding why. Inertia matching is mostly useful in motion control systems where position is important (eg servos). It doesn't really apply to simply accelerating a car as fast as possible. And even if it did, the total inertia of the car is part of the load, you can't just look at the rotational inertia of the drivetrain.
I was just thinking about this bolded part. We have weight minimums so ultimately the inertia ratio has a limit.
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Old 06-16-2019, 06:46 PM
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.... and this is why the hobby has gone downhill folks.
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Old 06-16-2019, 08:00 PM
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Originally Posted by gigaplex
I feel like you guys have picked up some literature on inertia matching and run with it without understanding why. Inertia matching is mostly useful in motion control systems where position is important (eg servos). It doesn't really apply to simply accelerating a car as fast as possible. And even if it did, the total inertia of the car is part of the load, you can't just look at the rotational inertia of the drivetrain.
Ive designed motion control systems for 25ish years. I hope it makes you feel like a read a book or two. How come i feel like you are trying to have something to say in a topic you dont understand by feeling something? A google search will provide you with lots of results around positioning and inertia matching in motion control because the guys posting DIY topics for robot people may have read about it or had someone explain it to them. It applies to all electric motor powered devices but the ratios are different for each class of product. But it always has to be followed whether someone knows its applied or not.

If you want to know then ask. Here are my feelings on your comments;
it doesnt apply to accelerating a car as quickly as possible- its importance depends on the motion profile and the exact ratio along with it. If an electric motor starts and stops and is coupled to a drive train which obviously has a mass including the mass of the motor then it is the 1st most important piece of physics. If the motor and the rest are at a constant velocity its the second most important piece of physics.

Its true you can put a 10hp electric motor on a conveyor that needs 1hp. What happens every time a coil aligns with the rotor and induces a torque? What happens to gears? What happens to belts? What happens to wheels? What happens if the motor doesnt shake into failure? So you are feeding a big pulse into a weak system. What if it can handle that pulse and magically no other problems like traction get in the way? You are winding up a rubber band that is going to feedback into the motor and collide with the opposing forces while all the while one side has a very high inertia and the other side doesnt.

I used horsepower to visualize the issue only. You can have a very heavy rotor with very little torque and vice versa.

Originally Posted by waitwhat
I was just thinking about this bolded part. We have weight minimums so ultimately the inertia ratio has a limit.
I suspect the gear train is too heavy but I clarified that too much or too little is a problem. But you are still right. We have weight limits. You can tune your power train like you tune the rest of the car. You can make it easier to drive or you can make it wildly powerful. Its like measuring camber. Once you know what you need for a track and how it feels you can go back to it every time. Once you figure it out you can repeat it and do other things. Once you find that a 10:1 ratio feels right at your favorite track you wont be going to 1:1 to go faster at that track.

if you had a track that never had a straight and was continuous acceleration followed by deceleration something below 10:1 would save you a couple 100 ms per accel/deceleration. A track that is just a straight line could be 60:1 if it never need acc/dec. a track that is equal time at acceleration/deceleration as opposed to continuous speed 30:1. But the right inertia mismatch adds milliseconds to every single accel/deceleration.

there is no way around practice to build consistency. You can push the mechanics as you get better or you can limit the mechanical system to build confidence and consistency but either way...

​​​​​​​
Originally Posted by Racermac73
.... and this is why the hobby has gone downhill folks.
downhill racing puts more emphasis on inertia matching.

I know electric motor applications. its not a requirement. Go have fun. If you want to know something I’ll share but keep in mind that Im not here to make any difference to the hobby. i race for fun and practice to get better. My points are to specific questions for specific people, if someone else gets something then thats even better but I dont mind people saying things i dont understand or disagree with and I dont need everyone to get caught up in what I write. It hits some people in a good way and some people in a bad way. im not here to discourage anyone.

Last edited by 1/8 IC Fan; 06-16-2019 at 09:41 PM.
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Old 06-16-2019, 08:45 PM
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Originally Posted by Bry195


Ive designed motion control systems for 25ish years. I hope it makes you feel like a read a book or two. How come i feel like you are trying to have something to say in a topic you dont understand by feeling something? A google search will provide you with lots of results around positioning and inertia matching in motion control because the guys posting DIY topics for robot people may have read about it or had someone explain it to them. It applies to all electric motor powered devices but the ratios are different for each class of product. But it always has to be followed whether someone knows its applied or not.

If you want to know then ask. Here are my feelings on your comments;
it doesnt apply to accelerating a car as quickly as possible- its importance depends on the motion profile and the exact ratio along with it. If an electric motor starts and stops and is coupled to a drive train which obviously has a mass including the mass of the motor then it is the 1st most important piece of physics. If the motor and the rest are at a constant velocity its the second most important piece of physics.

Its true you can put a 10hp electric motor on a conveyor that needs 1hp. What happens every time a coil aligns with the rotor and induces a torque? What happens to gears? What happens to belts? What happens to wheels? What happens if the motor doesnt shake into failure? So you are feeding a big pulse into a weak system. What if it can handle that pulse and magically no other problems like traction get in the way? You are winding up a rubber band that is going to feedback into the motor and collide with the opposing forces while all the while one side has a very high inertia and the other side doesnt.

I used horsepower to visualize the issue only. You can have a very heavy rotor with very little torque and vice versa.
My comment was aimed more at Bert who has a long history on this forum of running with concepts without understanding them properly. However you haven't said much that addresses my comment about total inertia vs rotational inertia. The drivetrain inertia is fairly negligible compared to the rotational inertia of the wheels or the total inertia of the car.

Regarding Berts plan to use a tungsten rotor shaft to prove the theory - just use a heavier pinion. That's actually legal.
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Old 06-17-2019, 05:41 AM
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Originally Posted by gigaplex
My comment was aimed more at Bert who has a long history on this forum of running with concepts without understanding them properly. However you haven't said much that addresses my comment about total inertia vs rotational inertia. The drivetrain inertia is fairly negligible compared to the rotational inertia of the wheels or the total inertia of the car.

Regarding Berts plan to use a tungsten rotor shaft to prove the theory - just use a heavier pinion. That's actually legal.
Anything that can be felt by the motor in the form of inertia is ratio’ed to the rotor primarily through gearing. I suspect you are correct though.
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Old 06-17-2019, 07:38 AM
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Originally Posted by gigaplex
My comment was aimed more at Bert who has a long history on this forum of running with concepts without understanding them properly. However you haven't said much that addresses my comment about total inertia vs rotational inertia. The drivetrain inertia is fairly negligible compared to the rotational inertia of the wheels or the total inertia of the car.

Regarding Berts plan to use a tungsten rotor shaft to prove the theory - just use a heavier pinion. That's actually legal.
Using a heavier pinion is not a bad idea at all if it was made of 14grams heavier tungsten , but it can only be done legally with the muchmore titanium rotors that will eliminate any steel shaft magnetic interference, causing an extra gain of top end Rpms while weighing more...That would mean a total rotor+pinion weight of 36.2grams vs 24gram(with aluminum pinion)...You guys keep forgetting that I already lost 52grams of drivetrain mass, and adding a little 14gram to the rotor is just an effort to balance out both rotor and load inertias...Maybe adding a tungsten pinion to a tungsten rotor shaft for a 50+gram rotor will yield better results....Thanks gigaplex for the recommendations....Lots of good ideas in this thread....
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Old 06-17-2019, 07:41 AM
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Didn't you say going from a steel pinion to aluminum pinion yielded 0.4 secs a lap faster? Now you want a tungsten pinion?
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Old 06-17-2019, 08:47 AM
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Dan, there are multiple rotor+pinion combinations available out there for tuning, and they are all on the light side...What about the heavy side?? I just wonder what are we all missing ??? Only one way to find out: build and track test.....
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Old 06-17-2019, 09:00 AM
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Originally Posted by bertrandsv87
Dan, there are multiple rotor+pinion combinations available out there for tuning, and they are all on the light side...What about the heavy side?? I just wonder what are we all missing ??? Only one way to find out: build and track test.....
Well ok, provided the time you can test any nonsense. But if you have that time, it may be more productive to invest it in simple dynamics understanding. Anyway, if you need slower acceleration there are number of much more convenient alternatives to enlarging the moment of inertia. Like slowing your ESC signal input from the the transmitter for example.
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Old 06-17-2019, 09:22 AM
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Downhill racing.... my mind is blown.
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Old 06-17-2019, 09:55 AM
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Bert just go get some practice time. It will give you better results than any mad science project you have planned in your basement.
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Old 06-17-2019, 10:06 AM
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Originally Posted by bertrandsv87
Dan, there are multiple rotor+pinion combinations available out there for tuning, and they are all on the light side...What about the heavy side?? I just wonder what are we all missing ??? Only one way to find out: build and track test.....
I thought the only thing you tested at the track was the durability of the pipes?
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Old 06-17-2019, 11:43 AM
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Originally Posted by Marcos.J

I thought the only thing you tested at the track was the durability of the pipes?
I tested that too: the reduced total car inertia failed to break the pipes and decreased the total parts broken by alot......Guys, we got to test everything and not just the stuff that makes us win...I guess I will have to do all the R&D...Stay tuned.....
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Old 06-17-2019, 06:26 PM
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May I offer a suggestion not only to Bert but to anyone thinking they need the latest and greatest motor.

Work on your bloody driving

When you can run each and every race and get at LEAST 95% consistency, then perhaps look at motors / lightened pinions and whatever to get that last tenth. Until then YOU are your own biggest obstacle, if you spent half the time practicing as you did flooding internet forums with half baked theories you'd be a world champ in no time!

I've started my son with a fixed timing Justock 21.5 motor and he's not getting anything faster until he gets the consistency down.
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