MOI formulas - where you can put ferrule lengths too

[sms]

Has anybody done a calculator/formulas where the total section length can be inserted and also the ferrule length - so that there would not be estimate that all the ferrules are of the same length?

Grunde, Magnus or Eric?

Cheers,
Sakke

[Magnus]

Edit: not how the Swingweight thingy works

Not as far as I know.

[Eric]

Magnus and Grunde's MOI calc does each section separately then requires total assembled length as well, but it does not do anything with the ferrule The calculator works exactly as the article's formulas. Haven't seen anything elsewhere.

[grunde]

Should be straightforward to include variable ferrule length...

[sms]

Alright, now got everything sorted out in excel. I also included grip length. I do not know how much I trust the formula that evaluate the grip weight. I get pretty high weights with DH rods and zero length grips

[sms]

This is weird.
I get same results for all top parts (if I use assumption of equal ferrule lenghts)
When I set handle length to 0,16m I get the same results for butt section for most rods, but not all. For some reason I do not get the same for result for my 18' Daiwa rod butt.

[gordonjudd]

I do not know how much I trust the formula that evaluate the grip weight.

Sakke,
As Grunde mentioned his approach using an assumed uniform mass density for the reel seat and grip and a fixed length of 16 cm was:

This is certainly something that need some discussion and to be "standardized"

Most of my rods have combined grip and reel seat lengths greater than 16 cm and I would expect the mass density of a metal reel seat would be much higher than the density of a cork grip.

I have found that most grips on single handed rods use 14 cork rings so the mass of the grip is around 14*1.4 g/ring=20 g. The mass of the reel seat will vary from rod to rod, but can be determined using the approach Grunde used in his paper assuming you use measured values for the center of mass for the reel seat and and the center of mass and mass for the cork grip. I find that the mass of the stripper guide also impacts the cg of the second section, so I use the cg value for the third section as the "ideal" cg for the first two sections.

Using the equations Grunde gave in his paper I get this expected mass density curve for the "effective mass" of the blank.


I had measured the raw blank sections previously and found this mass density for the actual blank.


You can see the density values obtained with the approximations in Grunde's paper gives higher density values for the first two sections, and also gives a much higher mass for the blank (90 g) vs the actual measured value of 61.3 g.

Using better values for the mass and center of gravity of the reel seat, grip, and first stripper guide I get this mass density curve which is much closer to the measured values.


As Grunde noted mass near the butt does not have a big effect on the MOI, so my "modified" MOI value of 74.0 g-m.^2 was about the same as the 76.7 value given by Eric's spread sheet.

If you are looking for more accurate values you might want to just suspend the rod with a piece of thread and measure its physical pendulum values. You can use a video camera to measure the oscillating frequency of the resulting pendulum quite accurately. I use the procedure described in Grunde's paper to get the mass and center of mass values.

Here are the values I get for that approach:
%compute_moi_physical_pendulum will calculate the MOI based on the mass,
%center of gravity, and frequency of different physical pendulums
m_cg_freq=[
.1308 .495 .4648 %finished rod
.0614 .845 .4319];%blank
mass=m_cg_freq(:,1);
cg=m_cg_freq(:,2);
freq=m_cg_freq(:,3);
g=9.809278%acceleration constant at 34 N latitude
omega=2*pi*freq;
I_pendulum= mass.*cg ./ (omega.^2)*g*1000.,

That gives a measured value of 74.5 g-m.^2 for the rod, and 69.1 g-m.^2 for the blank. That compares to the 74 and 68.4 g-m.^2 values I got using better estimates for the effects of the reel seat, grip, and first stripper guide.

Gordy