5 Comments

1. Grunde says:

   November 8, 2009 at 5:16 am

   Jason,

   thanks for posting the link. The MOI calculator should work for both blanks and fully assembled rods

   Cheers,
   Grunde
2. Beebe says:

   November 11, 2009 at 10:37 am

   After reading the detailed calculations section, this calculator uses some assumptions that are relevant only to fly rods, and multi-piece ones at that.
3. Grunde says:

   November 12, 2009 at 5:44 am

   Beebe,

   yes that is true, this method would probably never work well for 1pc rods/blanks. But for multi-piece blanks (2 or more pieces) it doesn’t matter what kind of blank it is.

   It’s only for fully assembled rods the “single handed fly rod” assumption kicks in.

   For other types of (multi-piece) rods it should be possible to come up with other kinds of corrections for the handle and reel seat than the one used in the current MOI calculator (and new ideas are always welcome ). I would also like to point out that the typical contribution from say the butt section of a 4pc rod is much smaller than from the other 3 sections. So even without using a “tailored handle correction” you can use the current calculator to compare swing weights for the parts of the rod that really matters

   Cheers,
   Grunde
4. Beebe says:

   November 12, 2009 at 10:15 am

   Grunde,

   I haven’t done any calcs, but how do these multi-piece parallel-axis theorem results compare to results that would be obtained by weighing only the assembled rod assuming it is a single cylinder?

   I’m curious if the percent difference is enough to worry about.

   I graphed MOI vs Weight for the rods listed in the data PDF, and they group pretty linearly. There’s not enough scatter to say that “Sage builds a 100 gram 5# rod with lower moment of inertia than Winston” etc. Mass and MOI appear to go hand in hand.
5. Grunde says:

   November 20, 2009 at 4:14 am

   Beebe,

   sorry late reply (the story of my life).

   Since the MOI of an object goes as the distance squared changing the mass center in the estimate would change the estimated MOI quite a bit. So I think my method is a vast improvement over the “single cylinder” assumption.

   To get a feeling for the difference you can try to plug in mass centers at the middle of each section in the online MOI calculator.

   To answer your last comment please take a look at the two 9′ 5wts in Table. 1 in the PDF. They are both top of the line latest and greatest rods from well known manufacturers. The overall weight of the Orvis is 16g lower than the weight of the Sage. Despite this the MOI of the two rods are really the same. This nicely demonstrates that our MOI estimate is not “fooled” by the light reel seat and handle of the Orvis rod.

   Cheers,
   Grunde