Originally Posted by dean_mt
Good, a little rod building talk. The fishing is **** around here and I'm going a little nuts.
I'll just say that I do not have anything to add on which side of the spine to mount your guides, I've read the arguments for both and while I wish I did, I do not know which is better or why.
I would however like to comment on the idea that there is a "strongest" part to a rod. I am assuming that when people make an argument about the stronger side that they do not mean stiffness rather in regard to breakage strength of the rod. The idea that there is a "stronger" side to a long shaft maybe true, there is more material on one side, or the there is a seam (point of weakness) along one side where the cylinder was closed. But when a rod breaks it breaks around the shaft, perpendicular to the length. I can see no way that putting pressure along the curve of a rod either with the spine up or down will have any effect on the strength of the rod.
I do not know the Physics of the problem at hand. This is only my assumption. I wish there was someone who could explain the Physics of the problem, because that is the only way to answer it.
Otherwise, if the concern over where the spine is placed in relationship to the guides is not about the breakage strength but about the casting ability of a regular fly rod, I don't think it is anything to lose sleep over. As said, 95% of fisherman will not be able to tell the difference.
I believe the "strength" that we (or most people) refer to when they say a rod is stronger on one side is really better described as the rod's resistance to bend.
As I mentioned, take the human body for example. The spine runs down our body, but obviously internally closer to our back than our front. You can think of the spine as a human bending forwards and backwards. If you build on the inside of the spine (on our chest), the rod will bend easier, and absorb more pressure (protecting light tippets more). If you build on the spine itself, our back, there is more resistance to bend and you will have more pulling/leverage power, as it is more resistant to bend at that angle.
As countless people have said, the difference here probably isn't very noticeable, but it certainly does exist.