All About Spines

Finding spines: the instruments

Feel Finders

For years, there have been clubmaking tools on the market called "spine finders". I refer to them as "bearing-based spine finders", and have seen them referred to as "feel finders". I like the latter term, and have adopted it. (Sorry, but I don't know to whom to attribute the term.) This is the technology that Bill Day was using when he wrote his "terminology" paper.

Here is how feel finders work:

A shaft is fit into three freely-rotating ball bearing assemblies, so it can rotate as it pleases. The bearings are not aligned (or can be forced out of line -- see the red force arrows on the picture), so the shaft must bend as it rests in the bearings. Now the shaft is rotated, and the fingers feel the rotation. There are two kinds of features that you feel:
  • A "bump", a position where it takes some pressure to get the shaft there from either direction. This is generally considered to be a spine, the direction of maximum stiffness of the shaft.
  • A "stable" direction, where the shaft rotates to naturally and rests there if not held. This is how the NBP got the name "natural bending position". It is the direction the shaft "wants" to rest if bent.
The principle is that the shaft will naturally settle into the position that takes the least force to produce the bend imposed by the bearings. That's a good principle. All other things being equal, it will find the direction of least stiffness. Where that is the case, spine finders do find the spine and NBP.

But sometimes -- often, in fact -- all things are not equal. If the shaft has any residual bend, it can throw off the measurement. Let's see why.

Consider a shaft that has no spine at all (that is, the stiffness is the same in all directions), but 3mm of residual bend as shown in the picture. You would never notice 3mm of bend unless you looked for it. Now let's put the shaft in a feel finder that deflects the shaft by 20mm (almost an inch).
The shaft will just snap into position in the feel finder, indicating a very strong spine. Remembering that we started out by saying there is no spine, let's understand why this happens.

  • If you turn it so the residual bend is in the same direction as the spine-finder-induced bend, then the shaft is only deflected by 17mm (20mm minus the 3mm of built-in bend).
  • If you turn it so the residual bend is in the opposite direction to the spine-finder-induced bend, then the shaft is deflected by 23mm (20mm plus the 3mm of built-in bend).
That's a difference of 6mm, or 30% of the total 20mm bend. So the shaft is deflected more in one direction than the other direction. 30% more, in fact. The result: it will take 30% more force to deflect it. So there's a lot more force in one direction than the other. And that difference in force is how a feel finder locates the spine.

Because of the forces as you turn the shaft, you will come to the conclusion that there is a spine on one side and an NBP directly opposite. There are two things wrong with this conclusion:
  1. We started out by saying the shaft has no spine, so the conclusion has to be wrong.
  2. We know from physics that, if there is a spine in one direction, the opposite direction will have a spine, not an NBP. So the feel finder is telling us something that is physically impossible.
This explains what Bill Day calls a Type 1 shaft, a shaft with a single spine on one side and a single NBP opposite. It also explains why he says this is common in steel shafts. The vast majority of steel shafts have very little spine, so any residual bend will overwhelm what spine there is. The residual bend will be all that the feel finder can measure.

What happens when there is both spine and residual bend? Unless the spine effects are large enough to overwhelm the residual bend, you will get spine and NBP directions that do not conform to the laws of phyics -- and therefore are wrong. We will see typical examples a little later. Really, the only time you can trust the output of a feel finder is when it tells you that you have a well-behaved Type 2 shaft, with 180 between the spines and 180 between the NBPs. If it says anything else about the shaft, it is paying too much attention to the residual bend, and coming up with a wrong answer.

Feel finders come in a wide variety of shapes, some of which use quantitative measures and not just the sense of feel. Let's look at a few variations:

It is not necessary for the bearings to be equally spaced over the shaft. Many of the commercial feel-finders have two bearings only 6"-10" apart, for the butt of the shaft to be inserted. This allows the device to be packaged as two bearings in a tube. The JB spine finder at the right is a prime example of this very common construction..
When the two bearings for the shaft butt are packaged together, the third bearing is often hand-held. Or it may be omitted altogether, leaving the fingers acting as the tip bearing. Here the JB finder is demonstrated by Jerry Ballard himself.
There are quite a few instruments designed primarily as deflection analyzers, but which also advertise themselves as spine finders. One might be led by their advertising to believe that they are not just feel finders, because they measure the load on the shaft due to a deflection -- the definition of shaft stiffness.

But one look at their configuration shows them to be identical to the "principles" diagram above. And, if we tested the same steel shaft (with no spine and 3mm residual bend), the instrument would tell us -- very precisely, in fact -- that it is a Type 1 shaft with a 30% spine. So, no matter how precise the output reading is, you still can't tell how much of the indicated load is the spine and how much is an artifact of residual bend.

The GolfMechanix Auditor (shown in the photo) is a good example of this type of instrument. Others that fit the description are the Flexmaster, the NeuFinder 2, and the MCC Multi-Match. I have heard such gadgets defended as being more precise than feel finders. And they are. But -- much more important -- they are not a bit more accurate than feel finders, which is not very accurate at all. (If you are confused about the difference between precision and accuracy, see my article on the subject.)


The "gold standard" for finding spine is FLO, raising two questions:
  1. What is FLO?
  2. Why does it find the spine better than other methods?
FLO stands for "Flat Line Oscillation". It refers to the direction in which the shaft vibrates back and forth in a flat line, as opposed to wobbling out of the line.

To test for FLO, place the shaft in a frequency meter clamp and weight the tip. (You don't need a frequency meter to find FLO, but you do need a very secure clamp and a tip weight to get the shaft to vibrate. And the frequency meter is handy to distinguish spine from NBP.) In the picture, David Dugally of The Golf Coast has attached a laser pointer to the tip of the shaft, so it traces out the path of the shaft on the cardboard in the background. The path is a nice, straight line. That's what you want to see; it indicates FLO. The plane in which the shaft is vibrating is either the spine plane or the NBP plane.

If the shaft has a signrificant spine, then the path of the tip will wobble from its original straight line into an oval shape -- unless the shaft is oscillating in the plane of the spine or the NBP. So, to find the spine and NBP, test the vibration of the shaft in different orientations. Where you get Flat Line Oscillation with no tendency to wobble into an oval, then the plane of vibration is either the spine or the NBP.

You can tell which FLO plane is the spine and which the NBP by using a frequency meter. The higher of the two frequencies is the spine. (In a pinch, a feel-finder may be useful to tell which FLO plane is which.)

And here is an actual video of a shaft in non-FLO motion. (Click on the snapshot to watch the video.) It starts in a good up-down motion, but "ovals" out of it in a few vibrations. After ovaling a while, it briefly flattens again in a different direction from the original. But it ovals again before returning to the original up-down motion. This shaft has a very prominent spine; most will take longer to go out of flat.

FLO finds the real spine and NBP, unpolluted by non-spine properties like residual bend. How does it do that? If a shaft is bent in any direction but a spine or NBP, the resulting spring force is not exactly in line with the bend. That causes the beginning of wobble. Let it vibrate outside the FLO plane for enough cycles, and the wobble becomes very visible. If you want all the details, they are available in another article.

In finding FLO, you can use any convenient tip weight, as long as it is secured solidly to the shaft tip. The FLO planes will not change with the amount of weight. They won't even change if the weight is off-center -- which means there's no advantage to testing for FLO with the actual clubhead. John Kaufman did an interesting experiment to confirm this. From the CSFA Tech Notes:

Flat Line Oscillation (FLO) vs Center of Gravity

TECH NOTE 29: In the spine alignment process we try to determine the two planes in a shaft that produce flat line oscillation, i.e. no wobble. This is very often done with a tip weight rather than the actual clubhead. The questions arises," will the FLO change when I install the head because its center of gravity is offset from the centerline of the shaft?" Golfsmith claims it will not change. I was curious. If a change in FLO does occur is it due to the change in cg or because it's hard to twang the club straight up and down when your finger is twanging somewhere out on the clubhead rather than on the centerline of the shaft?

To test this I built a tip weight with an arm sticking out of the side of the piece that attaches to the shaft. This piece weighed about 100 grams. I placed another weight on this arm and held it in place with a setscrew. This second weight also weighs about 100 grams. This arrangement allowed me to vary the cg of the test weight by more than an inch in a direction at right angles to the shaft. I attached a very small key chain laser to the tip weight. I built a trigger release system to eliminate any variations due to finger plucking. I placed a 10" disc on the butt end of the shaft. With degree lines on this disc it was easy to repeat alignments as I searched for the FLO. I determined the FLO plane as accurately as I could with the laser projected about 20 feet across the room. I then adjusted the side weight to move the cg about an inch. I did not find any variation in the FLO plane. I guess Golfsmith was right.

You may have heard of PURED shafts. They are shafts whose FLO has been found using a proprietary machine from SST PURE (Dick Weiss' company). The machine uses a computer-controlled process to home in on the FLO plane by examining the wobble quantitatively and deducing the FLO plane from the details of the wobble. To do that, you need a mathematical model of the wobble. Fortunately, the vibration of a clamped shaft with a spine is remarkably simple as mathematical models go. You can see the model and a derivation of it in my article on FLO.

Let's finish the section on FLO by repeating: One does not find FLO because FLO is important in characterizing the motion of the shaft during the swing. One finds FLO because FLO is a reliable way to find the stiffest and softest flex directions of a shaft.

Differential deflection

Is FLO the only way to find true spine? Does measuring the force needed to deflect the shaft always result in unreliable answers?

Not necessarily. Measuring the force needed to deflect the shaft is actually a very sound way to find the spine; it goes right back to the definitions of spine and NBP. But you have to be sure you are deflecting the shaft by the same amount in all directions. The reason feel finders are unreliable is that a shaft with residual bend is pre-deflected, and bearing-based spine finders don't allow for that fact.

But there are instruments around that can do it right. In order to do it right, you have to deflect the shaft by the same amount for every load reading in every direction. Since the shaft may have residual bend, this is tricky. The way to do it is:
  • Pre-load the shaft by some small deflection, and note the load.
  • Increase the deflection by a carefully controlled amount (call it D), and note the load.
  • The difference between the two loads is the force due to a deflection of D.
This is called "differential deflection" because you use the difference between the loads at two deflections. The important things in this measurement are (a) the shaft must be pre-loaded and "read" in every position and (b) the deflection D is very precisely repeatable. You can't use bearings for this, because the shaft must hold its position stably, even when it doesn't "want" to stay there. (We know from using feel finders that there are positions the shaft will rotate away from when deflected.)

Here are a few instruments that can find spine and NBP via differential deflection:

The NeuFinder 4 (a 2004 upgrade from the NeuFinder 2) was designed with differential deflection in mind. A load reading is taken by measuring the increase in load as the toggle lever moves from the pre-load stop to fully deflected. The distance D is set by the height of the pre-load stop, which is the most important part of the calibration of the instrument.

Important point: the shaft is resting in a V-block instead of bearings at its middle support. This provides enough friction so the shaft will not roll to the direction of minimum resistance. As noted above, the shaft must stay put in each position for differential deflection to work.

The NeuFinder 4, as all the instruments that do differential deflection, has a "Tare/zero" button on its digital readout. So you don't have to do arithmetic to use it. Pre-load the shaft, zero the readout, then deflect the shaft. The readout now shows the differential deflection -- no subtraction needed.
The Flexmaster can be used for differential deflection spine finding, with a couple of caveats:
  • You have to ignore the instruction manual. Its instructions for spine-finding amount to a feel finder with a digital readout.
  • The shaft will roll to a feel-finder NBP if you let it. You have to hold it to keep it from turning, which requires a careful touch to keep your finger pressure from affecting the reading.
John Kaufman home-built his own differential detection jig, which he called an Inverted Flex Board. It's pretty simple. You could build one yourself if you have a digital scale with a tare/zero button. He makes the deflection distance D repeatable by adding a spacer (the same spacer every time) under the V-block that supports the shaft. Very clever.

Which brings us to the question, "What measurements do you take, and how do you interpret them to find spine and NBP?"

Here is a sample set of measurements. I have taken an AJ Tech shaft, and measured it at 10* increments. I selected this shaft to start because AJ Tech is known for very large spines, and this shaft is no exception. It should be easy to see and find the spine and NBP.

Sure enough, the graph of load vs angle is roughly a sine wave. The peaks are at 120 and 300, and the valleys at 30 and 210. We know by definition that the spine is the direction of maximum stiffness (the peaks) and the NBP is the direction of minimum stiffness (the valleys). This certainly confirms the rules for the arrangement of spines that we learned from engineering mechanics.

The graph is not a smooth sine wave because of imprecision in the measurement. The digital readout of the NeuFinder 4 has a resolution of 0.01, and a precision (repeatability) of perhaps a bit more than that (0.01 to 0.02). On top of that, it is very hard to align the shaft to a precision of better than 10. So the sine wave has "jaggies" because of the slightly-off measurements. This is not a problem with a high-spine shaft like the AJ Tech, but...

Here are a few more shafts. (I have left the AJ Tech there for comparison.)
  • The Hireko shaft, with half the spine size of the AJ Tech, is still easy to read, and the spine distribution rules still hold true.
  • The TrueTemper EI-70 has only half the spine size of the Hireko. For this shaft, the jaggies are big enough compared to the trend that it is difficult to tell exactly where the spine and NBP are. Doing it by eye, as we did with the others, is probably inadequate. We would need some more sophisticated data analysis. Or, even better, more precision in our instruments.
So differential deflection has a problem in that it needs precise instrumentation and data analysis to find the spine and NBP.

It has another weakenss as well. It tends to take longer than FLO-finding to find the spine and NBP. You have to do a full reading -- that is, rotate to next position, pre-load, tare, load, record the reading -- for each angle of the shaft. 10* increments is probably right. That means 18 full readings for each shaft, before you begin the data analysis.

Before you ask -- and everyone asks -- yes, you have to do a pre-load and tare for every reading. If you don't, you get the numbers that a feel finder would give.

Bottom line

  • If you have a frequency meter (or even just a proper clamp), FLO is probably the best and quickest way to find spine.
  • Differential detection takes longer, and isn't very precise if the spine size is small. But, if you don't have a frequency meter and do have a differential deflection instrument, it gives correct answers for your efforts.
  • Feel finding -- to paraphrase H.L. Mencken -- is quick, simple, and wrong. It gives wrong directions if the shaft has any residual bend. And, for shafts with small spines, it confidently gives very clear spine directions -- unfortunately based on little but the residual bend itself. Its major value is to give a starting point to look for FLO and, if you don't have a frequency meter, a hint which FLO is spine and which is NBP.

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Last modified -- 11/30/2008