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All About Spines

Dave Tutelman -- January 30, 2008

Back in February 2006, John Kaufman (the head guy at Club Scout, the frequency meter company) asked on the SpineTalker's forum,

Hi Folks,
Since things are so slow I have a simple question to ask. What are you trying to accomplish by spine aligning a shaft? Or maybe I should say trying to prevent? From all the talk I've heard over the years maybe this question isn't as trivial as it sounds.

Shaft spines are interesting because there is evidence that "aligning the spine" can affect the feel and/or performance of the golf club. Aligning the spine means rotating the shaft in the hosel, while building the club, so that the spine of the shaft points in some specific direction. For instance, the spine can be placed in the heel-toe plane of the club. Obviously, this has to be done at the time the shaft is installed in the clubhead.

The next day after John posted his query, there were a few responses along the lines of "we are trying to optimise the performance of a golf shaft within the rules of golf". That caused John to elaborate a bit:

I guess my simple question was too brief. You have stated all the results we hope to acheive by aligning shafts. But what causes these things to happen when you align a shaft?

John is right that the question is very non-trivial. I posted my initial thoughts in the form of an outline within about a month, but it took almost two years before I was ready to write it up as an article. Here it is.

Be forewarned, this is not the usual clubfitter's view of spines. It is spines as viewed by an engineer, so intuition and hand-waving are going to take a back seat. And, unfortunately, too much of what is "generally known" about spines falls under the heading of intuition and hand-waving. I'll try to point out where my views deviate from the "common wisdom", but I may not have done it everywhere.

Because of all the fallacies and misinformation about spines, among both the public and custom clubmakers, I'm going to start at the beginning and work my way from there to John's question.

Executive Summary

First, if all you want is advice on how to measure and align spines, here is a summary of the results. If you want to know why these are the results, you will have to read further than this summary. (I added the first point in 2021 to reflect a bigger truth that you should probably see first in big blinking lights.)

Shaft spine is a defect! Alignment is not a construction step, it is damage control. Be aware that aligning a spine is just a way to avoid confrontation with your supplier of defective components.
Don't use bearing-based spine finders to locate the spine; they give wrong answers. Use FLO (Flat Line Oscillation) instead.
The high-frequency FLO plane is the pair of spines (separated by 180*); the low-frequency FLO plane is the pair of NBPs (again, separated by 180*). The spine plane and the NBP plane are separated by 90*.
Corollary: if your instrument tells you that the spine is 180* from the NBP (instead of the proper 90*), throw away the procedure or the instrument that tells you that. It is giving you wrong answers.
Measure the frequency difference between the FLO planes. If it's less than 3cpm (2cpm for tour caliber players), spine alignment is not going to do anything for performance or feel (though it may give you some peace of mind).
There is no provable best direction for aligning the spine. Theories differ, and the experimental evidence is not conclusive. But most experiments and practice say to place the spine in the heel-toe plane and the NBP in the target plane. The most likely theories also support this alignment.
If you want to ask whether to place the NBP toward the target or away from it, you weren't paying attention. Go back and read the points a-f again. The question makes no sense if you accept those points.

Now for a list of the points in the article, a sort of table of contents. Each consists of a link to the place in the article where the point is elaborated.

Basic definitions and physics

Shafts are not perfectly symmetrical. They have directions where the shaft is stiffer and directions where the shaft is "weaker" (as in "less stiff").
"Spine" is the term for the stiff direction.
"Natural Bending Position" (NBP) is the term for the weak direction.
Every shaft where the spine is big enough to be discerned has:

When a shaft bends during the golfer's downswing:

Finding and measuring spine

Bearing-based spine finders, sometimes called feel finders, don't work reliably. If the shaft has any residual bend (even too small a bend to notice without very careful measurement), then that bend has a significant impact on the direction the instrument finds.
FLO is the "gold standard" for finding the spine. FLO refers to finding the shaft's planes of "Flat Line Oscillation". These two planes are the spine plane and the NBP plane.
Differential deflection works reliably, but is more tedious than FLO.

Both analysis and experimental evidence are presented to support this position.

Spine alignment

Finally we get to John's question: what are we doing to or for performance when we align the shaft in the club?

"Spine alignment" means having the spine and/or NBP face in a specific direction when the shaft is installed in the clubhead.
There is some experimental evidence that it works (that is, it has an effect on performance and/or feel), though hardly universal agreement on what alignment technique works.
Residual bend without spine has no effect on performance. There is no reason to align shafts based on residual bend.
There are plenty of theories why spine alignment should work. We examine them. None fits the data perfectly, but some fit better than others.

You should align so that the direction the shaft 'wants' to bend is in the target plane at impact. This sounds nice and intuitive, but is never offered with a sound physical rationale -- and I could not find any. Not viable.
Placing the NBP in the target plane allows the hands to square the clubface at impact. This is plausible, based on the assumption that the shaft bend is in the target plane in the vicinity of impact. Unfortunately for the theory, the shaft bend is not in the target plane during the tens of milliseconds before impact. Not viable.
When the shaft bends during the downswing, any bend not in the spine plane or NBP plane produces forces that tend to move the clubhead out of the swing plane. This is definitely a true statement. The question here -- so far unanswered -- is whether those forces are large enough to produce the observed results of misalignment. Possibly viable.
Since the shaft bend at impact is in the vicinity of the clubhead's center of gravity (CG), align the NBP with the CG -- using the same rationale as #2 above. This theory suffers from an assumption contrary to fact. The shaft bend at impact is not in the direction of the CG. Not viable.
The advantage is in feel at and after impact, where the clubhead "rebound" from the ball is in the target plane. This is consistent with experimental results based on feel. It is not as clear in explaining experimental results reporting performance differences. Possibly viable.
Any consistent alignment strategy results in a consistent set of clubs, and that is the best we can expect to do. This does not explain why experiments tend to show that certain alignment positions seem to be better than others. Not viable.

Loose ends

Some things that don't fit neatly into the other categories:

Bottom line...

We don't know nearly as much about spine alignment as most clubmakers would have you believe. And much of what we "know" may not be true. In fact, some of it is demonstrably false.

Last modified -- 8/3/2021