All About Spines
What spine is -- and isn't
Let's start by an explicit definition of what spine is -- and a careful
statement of some things it is
not. Here is my definition:
Generically, spine in
golf club shafts is the directional variation of stiffness.
More specifically, the
spine is the direction of greatest stiffness of the shaft.
shafts cannot be built perfectly symmetrically, every shaft will be
stiffer (more resistant to bending) in some directions than in others.
This difference may be too small to be measured, or -- even if detected
-- too small to make a practical difference. But, given imperfect
the difference will exist.
does not sound like a very
radical definition. But the
ramifications that follow from it were widely misinterpreted. Because
of the way the spine's direction was typically determined, the
discussion of shaft spines since the 1990s has
been loaded with misconceptions. For instance, Bill Day has done a terrific
job in codifying
the description and terminology of measured spine in a shaft.
Unfortunately, almost everything in the document is based on notions
drawn from experience with instruments that do not properly measure the
spine. So it turns out that most of Bill's document is unnecessary. All
shafts, if they are measured for true spine as I defined it above, are
what Bill calls "Type 2" shafts. But most of the document deals with
characterization of things only
found in Type 1 and Type 3 shafts -- which are artifacts of a common
but generally incorrect measurement technique. This will be explained
in more detail later, where we compare the spine found by feel finders
with those found by FLO.
Having said that, it is important to note what spine is not:
Since Bill Day did such a good job of creating terminology, let's
adopt another of his terms. (Actually, we're adapting rather than
adopting. Our definition is in the spirit of his, but not based on the
misleading instrument that measured it.)
- It is not some obvious physical
like welded seam of steel shaft.
Actually, the name "spine" originally came from the belief that, since
shafts were welded tubes, the seam created an asymmetry in stiffness. But
the welding seam isn't the only cause of spine, nor
even the major cause. We won't worry yet about what causes it. First,
let's concern ourselves with the implications of the fact that
shafts have spine. That is the important thing for clubfitters, not how
the shaft got that way. But later we'll briefly look at the cause of spines in shafts.
- It is not residual bend.
Just as it is impossible to manufacture a perfectly symmetrical shaft,
it is impossible to manufacture a perfectly straight shaft. Some shafts
are straighter than others, just as some shafts are more symmetrical in
stiffness than others. The bend of the shaft at rest -- with no flex
forces on it -- is called the "residual bend". Why do we worry about
this? Because much of what is widely believed about spine is due to
measuring instruments that misinterpret residual
bend as spine. We'll see this in a later section.
And we will spend a lot of time discussing how Day's various shaft type
nomenclature is really a distinction based on residual bend more than
Bending Position (NBP)
is the direction of least stiffness of the shaft.
So we are left with the term "spine" for
the stiff direction and "NBP"
for the flexible direction of a shaft.
Let's finish with a definition of the size
of the spine. Remarkably little of the discussion of spine alignment
addresses the fact that there are big, serious spines and then there
are spines that are probably too small to matter.
size of the spine in a shaft is a measure of the difference between the
stiffness at the spine and the stiffness at the NBP. It can be measured
in CPM or in percentage difference of spring constant, depending on
whether your measuring instruments use frequency or deflection.
of frequency or 1%
of spring constant is a small or even negligible spine. 10cpm or 8% is
rather large spine. The work has not been done to quantify where,
between these numbers, is the "threshold" of spine importance, but it
makes no sense to talk about aligning spine without also talking about
the size of the spine.
The mechanics of
Spine is about stiffness. And stiffness is about bend. If the shaft
isn't bending, then the spine is having no effect on anything.
we need to understand what happens in a shaft when it bends. The field
of Engineering Mechanics deals with bending and flex. The golf shaft is
beam, flexing in response to forces imposed on it at the grip (forces
and torques transmitted from the hands to the grip) and the tip
(inertial forces from the clubhead). The result is a "bending moment"
that will try to curve the shaft in one direction or another.
a bending moment causes a beam to bend, the edge of the beam at the
outside of the curve is stretched a little longer, and the inside edge
is squeezed a little shorter. Since shaft materials are "elastic"
(technical terminology that means they act as a spring), the material
at the outside edge is in tension (just as if you were pulling on the
shaft) and the inside edge is in compression. Looking in a little more
all the material in the beam gets some stress (internal force) from the
bend. There can only be one plane through the beam that isn't either
longer or shorter than it was before the bend. Engineers call that the
Every fiber of material above the neutral plane in the diagram (that
is, toward the outside of the bend) is longer than it was at rest, and
therefore in tension. Below the neutral plane, the material is in
Quantitatively, the further from the neutral plane
we get, the greater the tension and compression. The blue triangle
represents the amount of tension at various distances from
neutral plane, and the red triangle the amount of compression. The
shape is a triangle because the amount of elongation or shortening is
directly proportional to the distance from the neutral plane; that's
The sizes of the triangles keep the tension and
compression in balance.They have to stay in balance if the shaft is not
to come apart into a lot of little pieces. The forces and
moments of tension, through the cross section, have to equal those of
description works just fine if the beam has a symmetrical
cross-section, like a perfect golf shaft. But this article is about
spine, which is what happens if the symmetry is not perfect. What
take an extreme example, shown in the diagram. Suppose the old Apollo
advertising campaign had been correct, and a welded shaft had a big
extra "bead" of steel down seam on the inside of the shaft. How would a
shaft like that react to bend?
Most people's first intuition
says that the shaft just became a lot stiffer if you try to bend it
downward, away from the added stiffening material. On the other hand it
be very slightly stiffer than it was before if you bend it upward. The
result is a serious spine, with the shaft much stiffer to bend
downward than upward.
That is certainly what I would have
thought before I took an Engineering Mechanics course, and it is the
immediate reaction of most clubmakers I have talked to.
But that is not what actually happens. Not even close.
What happens is the
neutral plane moves.
It repositions itself so that the material is better balanced on either
side of it. In the picture, the shaft is bending downward. The tension
triangle is smaller and the compressive triangle larger. But, since
there is more material above the neutral plane than below, we don't
need as much tension in each grain of the shaft in order to give a
total force balance above and below.
And suppose we bent the
asymmetrical shaft upwards? The neutral plane stays in the same place
as it did with the downward bend, but the red and blue triangles exchange
colors. The small triangle on top is compression and the larger
triangle on the bottom is tension.
What this means is the
stiffness is exactly the same whether we bend it upwards or downwards.
The welding bead will make the shaft stiffer overall in the up/down
plane than it was before, but it is just as stiff upwards as it is
downwards. True, we have introduced a spine.We have made up/down
stiffer than right/left. But we have not made down stiffer than up or vice versa.
brings us to a few rules for how the spine and NBP distribute
themselves in a shaft. These rules come from engineering textbooks
that have been around a long time. The rules work just fine on the
worst-spine golf shaft you will ever find; in fact, they work on much
asymmetrical beams than any golf shaft.
stiffness of the shaft in any direction can be represented as an
ellipse. (No, the shaft cross section does not have to be an ellipse.
It can be anything at all. The stiffness curve is an ellipse.) This
leads us to the rules, which apply as long as the shaft has enough
asymmetry that you can measure spine at all:
is a remarkable result -- and very different from the measurements that
led to Bill Day's terminology article. This says that every shaft is a
Type 2 shaft. If your measurement tools tell you differently, then your
measurement tools are wrong. (We'll see later why they are wrong.)
- The spines (directions of maximum stiffness) are two
in number and 180º apart from each other.
- The NBPs (directions of minimum stiffness) are two in
number and 180º apart from each other.
- The NBPs are 90º away from the spines.
How golf shafts bend
during a swing
again, let's remind ourselves that spine only means something if the
shaft is bending. When the shaft is straight, the spine has no effect
on anything. We just looked at what happens inside the shaft when
it bends. Now let's look at the bend behavior of the shaft during a
First off, the shaft bends in different planes. Part of
the reason for that is that the swing plane (which is ideally a slanted
version of the "target plane") is not the same as the reference planes
of the club. The club has a heel-toe plane which is perpendicular to
the "face-back" plane. In fact, the club planes and the swing plane
change their relationship completely during the downswing.
A few pictures
(from Jack Nicklaus' book "Golf
to show what I mean...
the start of the downswing, the club's heel-toe plane (green line) is
aligned in the same direction as the swing plane (blue line).
moment of impact, the clubface has squared and the face-back direction
is aligned with the swing plane.
all this complication, how can we possibly characterize enough about
shaft bend to be useful? An instrument called ShaftLab can be used to
measure the actual shaft bend in the club-referenced planes during the
instance, here is a "polar" trace showing the direction and magnitude
of the shaft bend during Greg Norman's driver swing (circa the late
1990s). The blue numbers at each data point on the curve refer to the
number of milliseconds before impact of clubhead and ball.
It turns out, of course, that everyone's swing is different.
But here are some worthwhile generalities about shaft bend during the
occurs while the wrists are just beginning to uncock. Most
of this bending
occurs while the heel-to plane is still fairly well aligned with the
swing plane, so this maximum bend is not far from the direction of the
- In the vicinity of impact, the bend
is a lot less less.
That means that any spine effects are likely to be greater earlier in
the downswing, unless there is some physical reason to assign more
importance to proximity to impact.
- Near impact, much of the bend is out of swing plane.
Much of the reason for this is "toe droop". Bend near impact
combination of toe droop (out-of-plane) and in-plane "rebound" from the
early in-plane bend. The angle of the total bend from in-plane depends
on the golfer. Tour pro swings exhibit angles of 25º-55º for a driver,
and about 5º more for an iron.
This is still too vague, and
too variable from golfer to golfer, to support a detailed analysis of
how spine affects the golf swing and vice versa. But it is enough to at least evaluate
the plausibility of the various theories of shaft behavior. And we will do that later.
- In February 2008,
Bill posted on the Spinetalker's forum, "For the moment, I find Dave's
article misleading, and it contains several errors. Dave, I
can use this forum or private e-mail to discuss the
errors that need correction, but at the moment I would suggest
that you remove the article from your web site until
corrections have been made." Bill and I exchanged more than a dozen
emails over the following five days, and found ourselves no closer to
agreement. Most of our disagreements stem from two very fundamentally
Both these controversies will become more clear later in this article.
Since I do not agree with the fundamentals underlying Bill's views, I
cannot call them "errors" and will not withdraw the article.
- Bill believes that every shaft needs to be spine-aligned, no matter how small the spine. I do not.
- I believe that just about every shaft that does not
behave as "Type 2" (in Bill's nomenclature) can be explained by
residual bend. Bill does not.
- Please don't
confuse "neutral plane" with NBP, which stands for "natural
bending position". I have seen NBP called "neutral position" or even
"neutral bending plane". Regardless of what you call it, it has absolutely nothing to do
with the neutral plane. Neutral plane and NBP are
competely unrelated concepts. Neutral plane
is the plane inside the
shaft where the tension and compression cancel to zero. NBP is the
direction of least stiffness when you bend the shaft.
And please don't
blame me for the confusion. NBP is relatively new terminology, around
since about 2000. Neutral plane has been in engineering texts since
well before I went to college in 1958.
- I have seen
this proved mathematically in S. Timoshenko's classic 1930 engineering
text, "Strength of
Materials - Part 1" Van Nostrand, 1930. See Appendix V for
the proof. If you want something more recent, try R. C. Hibbeler, "Engineering
Statics and Dynamics", Macmillan, 1983. See Sec. 10.7 -
Principal Moments of Inertia.
Last modified -- 2/23/2008