Spine - Who Is Correct?

By Tom Kirkman

RodMaker Magazine This question is from Volume 5, Issue #3 of RodMaker Magazine:
Q. I was looking at the FAQ on and noticed you said the outside of the relaxed curve is the effective spine. In the Flex Coat book, it says the inside of the curve. What am I missing? Nick via email.


First, let's understand that what we call the spine is not an actual physical thing. Rather, it is an effect created during the making of the blank and can be caused by any number of anomalies during the manufacturing process.

So what we are referencing is that point in which the blank seeks to align itself when pressured or loaded. Once in that position, we can locate any point of reference that allows us to remember or find that exact same location and call it the spine. You could locate a point 26 degrees off that location and call it the spine, as long as you remember that the position of the effect you are concerned with is 26 degrees away. Get the picture?

So calling the spine the inside of the curve, or the outside of the curve, makes little difference as long as you remember how each reference point relates to the position the blank will locate itself in when pressured.

The problem comes when rod builders try to converse on the issue. Without common terminology, none of it makes any sense when being discussed among rod builders who may all be using different points of reference. Therefore, most custom builders will pressure the blank and allow it to roll/spin to its preferred location. This will be the position where the blank naturally locates itself and wants to stay put, under pressure, always returning to the same position any time you attempt to roll or turn it from that position. They mark the outside of the "relaxed curve" and refer to that axis as the "effective spine." Remember, this will be the relaxed position or curve which the blank naturally seeks when it is pressured or loaded. And again, the outside of that curve is generally referred to as the spine or effective spine.

I will add that some builders and a few books, continue to call the spine the point where the blank jumps or kicks. That point is the stiffest axis of the blank and is neither considered the spine nor the opposite side of the spine by most knowledgable rod builders. It is, simply put, the stiffest axis and is something altogether different. ~ TK

Publishers note:
If you have any tips or techniques, send them along! Help out your fellow rodmakers! ~ Publisher, FAOL

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