Spine-Finding

Dave 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:
  1. Flat-Line Oscillation (FLO), which you can't do with an NF4.
  2. Feel-Finding, which you can do with any NeuFinder of any vintage.
  3. Differential Deflection, which was one of the tasks the NF4 was designed to do.

Spine Basics

Size of spine

What 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:

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.

Here's 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. So...
The 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:
  • 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 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.

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 and 270º.

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:
  1. 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.
  2. 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 plane.
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.

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 could happen:
  1. If released cleanly, the tip vibrates back and forth in a single plane, or flat line. This is called flat-line oscillation, or FLO.
  2. 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 straight line.
  3. 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.
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.

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.


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Last modified by DaveT - 1/11/2006