another method to find flywheel inertia

[luca-219]

The physics link has a link with D.Vernooy doc https://dvernooy.github.io/projects/ergware/
With Dave I tested another way to find Flywheel Inertia, I tested my flywheel in both way and the result is quite the same, but this solution is more simple (in my opinion).
The formula derived from the "parallel axis theorem". (provided me from Dave Vernooy)

My test
using my stopwatch I get 5.28 seconds for 2 swings back and forth. So this means T = 5.28/2 = 2.64 seconds. With M = 1.91 kg and L = 1.71M, here is what I calculate from my formula:
I = 0.249 * M * L* T^2 - M * L^2
= 0.2491.911.712.64^2 - 1.911.71^2
= 0.2491.911.712.642.64 - 1.911.711.71
= 5.66808 - 5.585031
= 0.083 kg*m^2

So I = 0.08 kg*m^2 for flywheel. Very similar to spreadsheet.

video_2023-01-13_08-55-32.mp4

I hope it can be useful for someone

[JaapvanEkris]

We are aware of the method, and it is pretty cool indeed. However, most people are not willing to take their machine apart to test the inertia of the flywheel. For most commercial machines, people choose to adjust the inertia experimentally to match their expected power/speed.

[KjellVerb]

I tried this method and I got a negative value, do you have any idea what could be wrong with my measurement?

M = 5.2 kg (not super accurate unfortunately, my kitchen scales error at 5kg so this is measured on my bathroom scale)
L = 3.085
T = 3.5 (10 swings in 35sec)

==> I = -0.5575 ???

[Abasz]

If you took it apart already why not to use Dave Vernooy's original method?

What i did in my rower is that i did Dave's method with approximation. Then I went to a gym a few times and did a few different exercises (with max sprints like pirmaid of 200, 500, 1000, 2000 metres) on concept 2 then on my rower and tweaked the inertia so the power for the split distances are similar/equal. This takes a bit of time but results in very good accuracy.

[KjellVerb]

Which original method are you referring to? I did not find another method apart from adding up bikewheel components (mine does not use a bikewheel) or this formula.
I tried it in excel but the formula is extremely sensitive to variations in L and T, L is quite easy to measure accurately but T even with 10 or 20 swings can be hard to nail down to 2 decimal places.

I guess that indeed I will have to compare to a concept2 but I will have to find one first. I'm new to rowing so was trying to avoid this method for now.

[Abasz]

Yes, adding app the components. Its not just for bike wheel but any flywheel. Yours is good because its essentially a solid disk. There are formulas for each shape acutally: http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

The way I did (and my flywheel, as an air rower, is a bit more complicated) measured each component and calculated that component's inertia formula (depending on the shape) , then added up (this is dave's original method, measuring the parts individually and then add them up). For a simple disc its quite easy:

You use solid cylinder as a starting point (I know you have holes in it but mass distribution even in case is equal so it would behave like a solid disk in essence and reduced weight will account for these holes). And then fine tune with concept2 comparison.

I would weight it with the spindle in for simplicity as essentially that is a solid cylinder.

The swing method I dont like that much as small measurement errors tend to accumulate. So to avoid such errors one needs to have long ropes (on 1mm error on 10cm is 1% while on 100cm 0.1%) and long swing time (hitting the correct moment when the cycle is complete is the most challenging part actually...) so the measurement errors are comparatively lowered.

[KjellVerb]

Actually the flywheel on my machine is more like a hollow cylinder with thick walls. I have found another method but this may cause thread drift so I will document it here: #171

[luca-219]

Hi, I don’t remember very Well all step. But i suggest you to Try With more accurate value M,L,T And double check the formula. Il giorno sab 26 ott 2024 alle 01:33 KjellVerb ***@***.***> ha scritto:

…

I tried this method and I got a negative value, do you have any idea what could be wrong with my measurement? M = 5.2 kg (not super accurate unfortunately, my kitchen scales error at 5kg so this is measured on my bathroom scale) L = 3.085 T = 3.5 (10 swings in 35sec) ==> I = -0.5575 ??? — Reply to this email directly, view it on GitHub <#113 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/A5AGSVAKMKTQDSXQOL5F2KDZ5LIKZAVCNFSM6AAAAABQUGC23GVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTCMBVG42TCNY> . You are receiving this because you authored the thread.Message ID: ***@***.*** .com>