Tutelman -- January 11, 2006
"Spine" refers to the fact that it is impossible to
make a shaft that flexes exactly the same amount in every direction.
Some shafts are so close to spineless that you can't measure the flex
variation with a NeuFinder, nor with the conventional frequency meter
that measures to 1cpm. Others have a very pronounced spine, that can
affect the performance of a club made with that shaft.
There is no doubt that a shaft spine that is big enough can affect the
performance of a golf club. It has been proven that it can lead to
off-center hits, and perhaps other problems as well. You don't want
good swings to result in off-center hits, and you especially don't want
an inconsistent set -- where you need different corrections for
different clubs in order to keep impact on-center.
These instructions tell how to use your NF4 to find and measure the
spine of a shaft. We will begin by covering some of the basics of shaft
spine. Then we will discuss the three major ways to find the spine:
- Flat-Line Oscillation (FLO), which you can't do with an NF4.
- Feel-Finding, which you can do with any NeuFinder of any vintage.
- Differential Deflection, which was one of the tasks the NF4 was designed to do.
Size of spineWhat does "big" mean in a spine, and how big is big
enough to cause problems? The way to measure the size of the spine is the difference in
stiffness between the stiffest and most flexible directions of the
shaft. If you are measuring with a frequency meter, the difference is
measured in cycles per minute, or CPM. If you are measuring it with an
NF4, the difference is measured as a load difference in kilograms.
How big is "big enough". There is no published data on that, and
opinions vary. It is pretty generally agreed that spines over 7 or 8
cpm will affect performance. Most informed opinion suggests that below some
number, you just don't worry about the spine. Depending on the
clubmaker, that number might be anywhere from 2cpm to 7cpm.
How big is that in Kg? We do not yet have a direct translation from cpm
to Kg, so we can't say yet. But a 5cpm difference is probably in the
range of 0.10 to 0.30 Kg. If you are measuring that sort of
difference or larger, you have a spine that may matter.
What should you do about spine?
There are three approaches to dealing with a shaft with a big spine:
- Ignore it. Most of the brand-name manufacturers take this
approach, and hope it doesn't come back to haunt them. It's much easier
and less expensive to assemble the club without bothering to find the
spine. Morevoer, if you know where the spine is, then dealing with it
costs even more. On top of that, they tend to pay bottom dollar for
their shafts, so the sort of quality control that minimizes spine is
- Discard it. A lot of clubmakers (myself included) will
send a badly-spined shaft back to the manufacturer rather than use it
in a club. But sometimes the customer just has to have that shaft they saw on TV, and the shaft has a sizeable spine. In that case...
- Align it. The problems caused by spine can be reduced by
aligning the direction of the spine in some relation to the heel-toe
plane of the club. Or maybe to the target plane. There are at least
four theories, and probably more, of how to align the shaft once you
measure it for spine. Since this is instruction for spine-finding, we
won't cover those theories here.
Basic structural considerations
There are some facts of life that every structural engineer learns in
college, but which leave a lot of clubmakers in denial. We'll mention
here the ones that affect the measurement of spine. There are others
that affect theories of alignment, but let's not complicate the issue
in a document that only covers measurement.
a cross-section through a graphite shaft. The shaft is not perfectly
round; it is a little bit elliptical. (No shaft I've ever seen is this
bad, but the drawing exaggerates, to make the point more intuitive.)
Obviously, the shaft is not going to be the same stiffness in all
directions. The wider the shaft is, the more it can resist bending.
lines show the widest and the narrowest axes of the ellipse. These
correspond to the stiffest and the "weakest" (most flexible) directions
of the shaft. A few points to note:
So we can say that the stiffest axis (the "spine") is at 25º and
205º, and the most flexible axis is at 115º and 295º.
This is the way most clubmakers talk about spine: they identify a stiff
or flexible direction as being some angle on the shaft. It is
convenient and precise, and we will talk about it that way, too. Be sure you are comfortable with this nomenclature, because differential detection depends on it.
- We've labeled the directions around the shaft by degrees,
so we can talk about a direction. For instance, the long axis runs from
about 25º to 205º.
- The two axes are at right angles to each other -- 90º apart.
So far, this is pretty sensible and intuitive. The next step is where
it gets counter-intuitive to people without engineering training.
Using the example of an ellipse above, it is easy to see that the spine
is symmetrical (the spine directions are 180º apart), and the most
flexible directions are also 180º apart -- and 90º from the
spine. But what about a figure that is not so symmetrical?
|Here is a cross-section that is about as lopsided as you could imagine. What is the "hard" and "soft" side of this shaft?
An intuitive guess would say that the stiffest side is at 0º, and
the most flexible at 180º. But that is not correct. There are
stiff sides at 0º and 180º, and flexible sides at 90º
In fact, that's a general rule. Leaving aside the reasoning (it's in textbooks, and I don't want to go off on a tangent here):
- The spine is a plane, showing up as "sides" 180º apart.
- The most flexible direction is also a plane, also showing up as "sides" 180º apart.
- The stiffest and most flexible planes are 90º apart from one another.
Always! The cross-section of the shaft does not matter.
There are a couple of consequences of this argument:
- If your instruments tell you something else (for instance,
that the hard and soft sides are 180º apart), then the instruments
or your measuring technique are wrong.
- As long as you have
good instrumentation, you don't need to measure around the whole
circumference (360º) of the shaft. If you scan 180º around
the shaft and locate the stiff and flexible directions, you already
know where the stiff and flexible directions are on the other side. Remember this! We are going to use it later in finding spines using differential deflection.
Why Does Spine Matter?
Why should it even matter if the shaft has a spine? Here's why.
If you bend a shaft, it tries to spring back. This is not a surprise to anybody.
If the shaft has the same stiffness in every direction, then the spring
force directly opposes the bending, in the same plane that the shaft is
bent. But, if the shaft is stiffer in some directions than others, the
force may not be in the plane of bending. It might be a little out of
That spring force is acting on the clubhead. If there is a significant
spine, and the spine plane is not the same as the bending plane, then
the spring force is shoving the clubhead out-of-plane. There are other
effects as well, that are a little harder to visualize. But you can see
that a big directional difference in stiffness can create problems for
a club's consistency.
- If you bend the shaft in its stiffest plane, the spring force will be in the bending plane.
- Same for the most flexible plane; the spring force will be in the bending plane.
- If you bend the shaft in any other plane, the spring force will
not be in the bending plane. The bigger the spine, the more
out-of-plane the force will be.
Flat-Line Oscillation (FLO)
FLO-finding consists of clamping the butt of the shaft securely,
putting a weight on the tip of the shaft, plucking it and watching it
vibrate. When the shaft is pulled and released, one of several things
The activity consists of rotating the shaft in the clamp and repeating
the pluck until you find planes that FLO, as described in #1 above.
There should be two of them. The shaft will FLO in the planes of
maximum and minimum stiffness. If there is a significant difference
between maximum and minimum, the shaft will wobble into an oval as
described in #3 above.
- If released cleanly, the tip vibrates back and forth in a single
plane, or flat line. This is called flat-line oscillation, or FLO.
- If not released cleanly, the tip starts out in an elliptical
pattern. Just stop it and start the test again so it starts in a
- The tip starts out in a flat plane (indicating a clean release),
but quickly vibrates out of that plane and starts to "oval". The ovals
get thinner and fatter, occasionally becoming a plane briefly before
going oval again.
Why should that be? Remember what we said earlier about the restoring
force when the shaft is bent. If you bend the shaft in a plane of
minimum or maximum stiffness, the restoring force will be in that plane
-- and the shaft will vibrate completely in the plane. If you bend the
shaft in any other plane, the restoring force will be outside the
plane. The force will pull the shaft out of the original plane, and
wobble will begin.
For most clubmakers, the butt clamp for finding FLO is the clamp for
their frequency meter. Not only did they give consideration to a secure
clamp when they mounted it (rigidity is a factor for both frequency
measurement and FLO-finding), but the frequency meter is a good adjunct
to finding FLO. When you find a plane of FLO, the frequency will tell
you which FLO plane you found. The plane of maximum stiffness has a higher frequency than minimum stiffness.
This document is a set of instructions for the NF4, and the NF4 cannot
measure FLO. So we won't go into the tools and techniques for FLOing a
shaft. For our current purposes, it's enough to know the what and why of FLO -- the how is covered elsewhere.
Last modified by DaveT - 1/11/2006