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Calcs on rotational inertia and effective mass

 
Old Feb 12, 2019, 09:17 AM
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Calcs on rotational inertia and effective mass

Figured the track day crowd would find this info interesting. The flywheel accounts for a lot of the effective mass.
https://motoiq.com/where-are-the-bes...-and-find-out/
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Old Feb 16, 2019, 10:00 AM
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Interesting article. Has anyone sanity-checked those numbers, though?

They show a 3.78:1 ratio between flywheel speed and wheel speed in 6th gear, but they still have the flywheel contributing more rotational inertia than all four wheels combined.

How does a 22lb 11" (?) flywheel contribute 20% more to the rotational inertia than a 17" wheel? Maybe he has some incredibly light wheels? Or am I doing my math wrong somewhere?

The 15:1 ratio between flywheel speed and wheel speed in 1st gear is brutal no matter what, though.

Last edited by Construct; Feb 16, 2019 at 10:06 AM.
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Old Feb 16, 2019, 06:36 PM
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The article makes some gross assumptions on moment of inertia treating everything as a solid disc. In a wheel or tires case, there is huge error in it. Enough, I would completely invalidate his findings.
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Old Feb 17, 2019, 08:57 AM
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here is an different take on the same subject.

http://hpwizard.com/rotational-inertia.html
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Old Feb 17, 2019, 11:37 AM
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Just first-thoughts that come to mind,
When you get under the car and try to "rotate" wheels, driveshaft, axle shafts, anything, push the car around...

There is so much friction in diffs, drag by brakes, sticky tight-fitting resistance in gearbox/transfer case, and inertia by long drive-shaft assembly and axle shafts,
to question the value of lightening anything that cannot shed substantial weight:

Flywheel, its contribution to in between gear-change spininess in response I can see, but no contribution to overall acceleration. +/-5-8 lb there is minuscule compared to each wheel/tire combo, say 35-45 lb with mass concentrated on the perimeter. And there are 4 of them.

The 2 piece driveshafts, like Aluminum - shed only 3-4 lb off the overall weight, stock is 38lb and alu 34lb, I see no benefit.
It you can half- the weight, I can see the value, but that is possible only with 1 piece which is not functional upgrade.

Lighter wheels, brake rotors, and even lightweight calipers, I can see benefit, as its 4x repeated.
We are limited here by grip requirement = larger = more weight, and each small savings can mean a lot of $$$ additional cost.


It would be great if somebody had access to a model, even in Excel, where all the cals are entered, and variables for each component can be entered, including some diameters to define at least roughly the radius of mass-center, to more accurately calculate and show effects of saving some weight here and there on total inertia, and maybe some relative power-requirement to spin up that inertia(mass).
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Old Feb 25, 2019, 09:58 AM
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Originally Posted by Dallas J View Post
The article makes some gross assumptions on moment of inertia treating everything as a solid disc. In a wheel or tires case, there is huge error in it. Enough, I would completely invalidate his findings.
Actually, the tires were assumed as a hoop, and the wheels a hoop and disk. While not completely accurate (3d model with calculated inertia values would be accurate), it's close enough for ball park estimations. Feel free to do your own analysis and compare.
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Old Feb 25, 2019, 10:08 AM
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Originally Posted by Construct View Post
Interesting article. Has anyone sanity-checked those numbers, though?

They show a 3.78:1 ratio between flywheel speed and wheel speed in 6th gear, but they still have the flywheel contributing more rotational inertia than all four wheels combined.

How does a 22lb 11" (?) flywheel contribute 20% more to the rotational inertia than a 17" wheel? Maybe he has some incredibly light wheels? Or am I doing my math wrong somewhere?

The 15:1 ratio between flywheel speed and wheel speed in 1st gear is brutal no matter what, though.
Remember, the mass factor is to the gear ratio SQUARED. The wheels are 7.6kg each. As noted in the article, many assumptions had to be made on inertia calcs. So while not accurate enough to say the flywheel is 20% more inertia contribution than the wheels in 6th gear, it's good enough to say they contribute in the same ballpark as each other relative to say the tires or the drive shaft.
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Old Feb 25, 2019, 10:31 AM
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Originally Posted by spdracerut View Post
Actually, the tires were assumed as a hoop, and the wheels a hoop and disk. While not completely accurate (3d model with calculated inertia values would be accurate), it's close enough for ball park estimations. Feel free to do your own analysis and compare.
OK, I see where the error is. His column title for inertia is missing the "/2" for the disk measurements. I had to go back and check the equations. Sooo, I suppose the approximations are at least good enough for the exercise.
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