Spine-Finding 1

Dave Tutelman -- January 11, 2006

Feel-Finding

The essence of feel-finding is letting the shaft "tell you" where its spine is. The idea is that you take a shaft and bend it, but allow it to rotate freely on its axis. The shaft will naturally gravitate toward the least stressful position -- the position where the minimum stiffness is in the plane of bending.

You probably noticed that I just described a NeuFinder. It bends a shaft by applying a deflection to it. The force for deflection is transmitted to the shaft by bearings that allow the shaft to rotate very easily. If this is any substantial spine, the bearings will allow the shaft to rotate to the position where the plane of minimum stiffness is vertical -- the same as the plane of the bend.

History and Terminology

Spine-finding started with feel-finding, and lots of clubmakers continue to use it today. Just about every instrument sold under the name "spine finder" works by feel-finding. If you are going to speak with clubmakers who use feel-finders, here is some terminology that you need to know that arose in the 1990s (early in the most recent spine-alignment game) and persists to this day:

Natural Bending Plane or NBP	The direction the shaft settles when bent in the bearings. In spine diagrams, the NBP is designated "N".
Spine	The neutral but unstable direction for the shaft to rest. When the shaft is bent at the spine, it is balanced but precariously so.
N1, S1, N2, S2, ...	When a clubmaker audits a shaft for spines with a feel-finder, he typically records his findings as NBPs and spines found. The strongest or most prominent spine is labeled "S1", the next-strongest spine "S2", etc. Similarly, the NBPs are labeled "N1", "N2", etc.
Type 1 Shaft	When people started feel-finding shafts, they discovered that almost all shafts fell into two categories, which they called "Type 1" and "Type 2". A Type 1 shaft has a single N and a single S. They are typically opposite or nearly opposite one another, roughly 180º apart.
Type 2 Shaft	A Type 2 shaft has two NBPs and two spines. The spines are typically opposite one another, as are the NBPs.

At this point, you should be wondering what's going on here. According to what we said earlier, all shafts are Type 2. In fact, all shafts are very well-behaved Type 2 shafts. The directions S1-N1-S2-N2 should sequence in that order, exactly 90º apart. So why should there ever be a Type 1 shaft?

The answer is that the instrument is deceiving you. It is influenced by factors other than stiffness, as we will see below. Suffice it to say that all feel-finders are subject to this deception, and thus find wrong spines and NBPs in a lot of shafts. But they can work well for ideal Type 2 shafts.

Feel-Finding with an Ideal Shaft

Let's start by describing the process of feel-finding for spines. We will assume that the shaft is perfect except for one thing: it has a spine that is too big to ignore. Here's the process:

1. Set the beam length for the section of shaft you want to spine. The most natural thing is to use the same beam length you do for matching shafts. (This is not the only choice. It may or may not be the best choice, depending on which theory of spine alignment you believe. And it matters, because the cross-section of the shaft changes over the length of the shaft; in particular, the spine in a graphite sheet-wrapped shaft may well spiral down the shaft -- so the direction of the net spine will depend on what section of shaft you measure.)

2. Secure the toggle board in its lower position, for maximum throw of the toggle.

3. Place the shaft in the bearings, with the tip against the tip stop and the middle bearing block in the "bearings" position.

4. Add a strip of masking tape around the shaft, directly under the shaft marking guide. The easiest way to do this is to place the end of the tape against the shaft, then wrap it on by spinning the shaft in the bearings.

5. Use the toggle clamp to load the shaft. The now bent shaft will rotate to the nearest NBP -- "snap" is probably a more apt description than "rotate".

6. Using your fingers, rotate the shaft a full turn. You will notice that it will take some effort to turns the shaft in some orientations, and in others you will have to restrain the shaft from running away from you. The shaft clearly "knows what positions it likes."

This full turn is sort of a preliminary scan of the shaft, to see what you have. A Type 2 shaft has two favored positions opposite each other, and has two "bumps" that are hard to turn the shaft through. The favored positions are the NBP directions, and the bumps are the spine directions.

7. Slowly move the shaft through another full turn, recording your findings directly on the shaft. As shown in the photo, you can use the marking guide to make a mark on the masking tape at the exact top of the shaft. So orient the shaft at an N or S and make the mark, then label it "N" or "S". (Charlie Badami uses fine-point colored markers, and uses a color-coding for N and S -- so he never has to actually write letters on the tape.)

It is worth noting how to orient the shaft:

For an NBP, allow the shaft to snap to position. Typically there will be a "dead space" of 10º to 30º, depending on the size of the spine. (If the dead space is 30º or more, you probably don't have a spine worth aligning -- but that just my opinion.) Feel-find the edges of the dead space, then orient the shaft in the middle of it. Mark the shaft while you hold it in this position.
For a spine, turn the shaft until you feel the "top of the bump". Now try to find spot near the top where your fingers require minimum effort to hold the shaft in position; it doesn't want to spin either way -- though you know it will as soon as you let go. So don't let go until you have marked the position.

I can't describe it any better in words and pictures. You just have to try it until you get the feel. I guess that's why it's called "feel finding".

Feel-Finding with Less-Than-Ideal Shafts

When you have a Type 2 shaft with everything normal, feel-finding works fine. S1-S2 defines the plane of the greatest stiffness, and N1-N2 defines the plane of least stiffness. That's exactly what spine-finding is supposed to do.

But not all shafts are perfect except for spine. And some imperfections will throw off the spine measurements you get from feel-finding. To make matters worse, those imperfections are pretty harmless when it comes to performance, so they are no reason to discard the shaft -- but they will make it impossible to use feel-finding to assess the spine of the shaft.

The worst such "problem" is the straightness of the shaft. Many steel shafts, as well as quite a few graphite shafts, are not perfectly straight when they come out of the box. They have a small amount of what is called "residual bend", which plays absolute havoc with feel-finding. It is almost always the cause of Type 1 shafts. Consider the case of a shaft with little or no spine but a residual bend. As soon as you place a load on the shaft in the NF4 (or any other bearing-based spine finder), the shaft swivels to align the bend it already has with the bend the NF4 is trying to apply to it. The pre-existing bend clearly defines the stable orientation for the shaft to sit in the bearings.

As a result, when you try to feel-find a bent shaft, the convex side of the bend snaps to the NBP position and appears as the NBP. And, when the concave side is rotated to the top, it feels exactly like a spine. There is no way to tell if you're dealing with a real spine or residual bend. Well, no way except one; you know this shaft is defying the laws of physics if that's a real spine -- because it has a lone spine on one side and a lone NBP 180º away.

For example, I measured a True Temper Dynalite R/S combination flex, a steel shaft from a company that I know is not very careful about straightness. Here is the tale of the tape:

I went through a feel-finding, and found a clear NBP at 100º, and a pronounced spine at 280º. Type 1 shaft!
I even measured the "size of the spine", taking a load reading from the digital scale at the NBP and another at the spine. (I did not pre-load the shaft, so it wasn't differential deflection. Instead, I tried to simulate the sort of measurement you would get from an NF2, which has no preload nor tare capability.) The difference between the spine and the NBP was 0.13Kg. Not huge, but certainly not negligible. Anyone bothering to find this spine would probably bother to align it.
I then went around the shaft measuring its actual stiffness using differential deflection. (We'll see how to do this in the next section.) The difference between the lowest and highest readings was only 0.03Kg. And the difference between the supposed spine and NBP was only 0.01Kg -- barely even measurable by an NF4.
Finally, I clamped the shaft in my frequency meter and looked for FLO. I got good FLO in every orientation I tried.

In other words, the shaft was basically spineless for all practical purposes. But feel-finding showed a significant spine. And an NF2-type measurement confirmed that the spine was significant. So feel-finding can lead you to very wrong conclusions about the size, location, and even the very presence of a spine.

So which is the result you want? Which information should you trust in order to decide how to treat the shaft? The actual spine, or the combination of spine and residual bend that you get from the feel-finder? It may not be an open-and-shut case yet, but the studies so far are convincing to me. Consider:

Studies of Type 2 shafts show a definite difference in performance depending on the orientation of the shaft. Specifically, these studies focused on the orientation of either a measured stiffness spine or measured FLO.
There is a physical reason for the actual spine to affect performance; shaft bend at an angle to the spine will force the clubhead out of plane.
There have been a few studies of spineless shafts with a small residual bend; none that I know of have shown any effect on performance or feel.

My conclusion from this is that you want to make shaft decisions based on the location and size of the actual stiffness variations -- the true spine and true NBP, unpolluted by the effects of residual bend. In the next section (on spine finding using Differential Deflection), we will see that feel-finding often does a very poor job in finding the real spine, even when the shaft has one. If all that is true, then feel-finding is useless except in the case of a very well-behaved Type 2 shaft.

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Last modified by DaveT - 1/14/2006