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All About Spines
Spine alignment: theories
On to John K's question:
What are we hoping to do by aligning the shaft
to the club?
Spine alignment: explanation
So far, we have some experimental data (some more anecdotal than
others) that spine alignment "helps". But why does
spine introduce problems, and why should alignment help?
Stated in terms of scientific
inquiry, we now have a few laws
about spine alignment. They are based on occasionally contradictory
evidence, but at least they show definite trends. Now we need a theory
to explain those laws.
Here are the
theories I have heard or have come up with myself. Let's
see how the theories answer our question, in light of the
evidence at hand. For the theories below, I have assigned thumbs-up or
-down based on how each agrees with the empirical evidence -- and
occasionally a thumbs-down because it doesn't address the question we
are trying to answer.
Theory 1:
stable at impact
This one I would characterize as "the common wisdom". It is generally
stated, "Of course you want the shaft to have its most stable
position in the target line at impact. And that means the NBP in the
target line; just look at what happens to the shaft in a spine finder."
I don't get it!
Maybe because I don't find it rational.
Arguments for and against this theory:
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Intuitive and hand-waving. No
physics involved. Basically, it is a statement rather than a theory,
and does not address John's original question. It doesn't say why you should want it in a "stable position", just states that you should.
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If true, then spine-to-target should be a very bad
position, because the spine is among the least stable positions in a
spine finder. But it's
not; in fact, a sizable minority feel it's the best position. So does
the most successful company in the spine alignment business. |
Theory 2: NBP in
target plane reduces torque needed to square
clubface
This is a more mechanically rigorous statement of Theory #1. It says
why it might be a good idea to have the most stable direction of the
shaft in the target line.
The basis of the theory is
that feel-finding may not be all wet. And it isn't. Feel-finding just has the
problem that directional stiffness isn't the only thing that can cause
a shaft to snap to one position and stay there; residual bend affects
the position, too.
The shaft in the picture is flexed in the direction of the red arrow.
That is not the same direction as the NBP. We know from intuition and
experience that the shaft wants to snap so that the NBP is
aligned to the direction of the bend. The word "wants" implies a
torque on the shaft that rotates it into a position so the NBP is
aligned with the bend.
What does this have to do with spine alignment? Well, the usual advice
derived from this observation is that you want to align the NBP in the
9-3 plane, where it will be down the target line at impact. The
motivation is so that the torque on the shaft is in a direction to get
the clubface square, perpendicular to the target line. It guides the
club rotation during release, so that the clubface is square at impact.
Arguments for and against this theory:
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In order for this theory to work, the shaft bend must
be in the target
plane (the swing plane) during the period coming into impact -- say,
the last 20 milliseconds before impact, as the hands are squaring the
clubface. But we know
that this is not true; toe droop is at least as important as
lead-lag bend during this period. In fact, the shaft bend at impact is
between 25º and 65º away from the target line, depending on the golfer
and the golf club. And that is the closest
it gets during those last 20 milliseconds before impact.
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If this theory is correct, then the flight differences
due to a misaligned shaft would be the problems of an un-square
clubface. But the
Butler study found that the problems of a misaligned shaft
are more consistent with a larger dispersion of impact between the ball
and the clubface -- the golfer misses the sweet spot by more with a
misaligned shaft. This is not the problem we would expect from an
un-square clubface.
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If this theory is true, then
alignment with the spine at 9-3 (rather than the NBP) should be a very bad
position. In order to square the clubface, you'd have to rotate the
shaft "uphill" -- in opposition to the torque. But most experience
indicates that either the NBP or spine in
the target line gives pretty good results. There are camps that argue
one or the other is better, but all agree that either alignment is much
better than, say, a 45º alignment.
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It could validate the myths about
"supershafts",
since an aligned club with a large spine could be more self-squaring
than a shaft without a spine. (But so far the supershaft is a myth. I
know of no study, careful or otherwise, that has validated the myth.)
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Theory 3: Bend causes out-of-plane motion of the shaft
This is the one I feel has the biggest likelihood of actually affecting
the shot.
If, during the downswing, the shaft bends in a direction other
than the spine or NBP, the spring force
caused by
shaft bending will not be in the plane of the bend. Because the spring
constant (the stiffness) of the shaft is greater at the spine than the
NBP, the force will be pointing a little to the spine side of the bend.
Out-of-plane forces
will accelerate the head to out-of-plane motion. The result is that the
clubhead is out of position at impact. This sort of effect is accentuated by a few things:
- More bend. The larger the bend, the greater the spring force, thus the greater the potential for out-of-plane force.
- Bend further away from the spine and NBP.
The further the bend is from the principal planes of the shaft, the
more out-of-plane the the spring force is. There is a curve showing
this effect in the article on FLO physics.
- Earlier bend.
The effect depends on force, which creates acceleration. But the
performance difference depends on how far out of plane the clubhead is
at impact. Distance from the sweet spot is affected by acceleration,
but it isn't the same thing as acceleration. Acceleration needs time to
create velocity, and
velocity needs time to create displacement. So there is a strong
dependence -- actually a square-law dependence -- on time for a force
to create a displacement. That means that the earlier in the downswing
the bend occurs, the more out-of-plane the eventual displacement of the
head at impact.
According to this theory, the best alignment
strategy is to place either the NBP or the spine in the plane where the
large, early bend occurs. That can be made to work because the large bend is early in the downswing, and pretty close to the heel-toe plane of the club.
And it can work with either the NBP or the spine because those are the
directions of bend that generate no out-of-plane spring forces. Arguments for and against this theory:
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It is quantifiable physics, not
hand-waving.
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It would explain the results of the Butler study, the
major study so
far, that misaligned spines can cause a bigger impact dispersion area
on the clubface. An out-of-position clubhead results in an off-center impact, which is the result found by the Butler study.
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It would explain why there are
those advocating NBP
to target and those advocating spine to target. Under this theory, they both
work pretty well, so advocates of either are right. (Though it isn't
"to target" that matters for this theory; it's the heel-toe plane. But
now we know that if the NBP is at the target then the spine is
heel-toe, and vice versa.)
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Some initial analytical studies
(including my own and another by Werner and Greig) show
that it takes a lot
of spine to give the sort of dispersion the Butler study reported. Since Butler said
nothing about how big the spines were, there's no way of telling
whether this theory is a match to their observations.
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Theory 4: NBP aligned with the club's CG
Tom Wishon has come up with another interesting theory of what spine
does and consequently how to align it. His theory goes like this:
- In the vicinity of impact, the only force creating shaft
bend is
centrifugal force, which acts on the center of gravity (CG) of the
clubhead. Therefore, the shaft is bending in the direction of CG.
That is, the plane of bend
is the plane defined by the shaft and the CG of the clubhead. So the
shaft bends in the direction of the yellow arrow in the diagram.
- If that is true, then the most stable plane of the shaft
(the
NBP) should be in the bending plane. So the NBP should be oriented
toward the CG of the clubhead.
- If you choose a different alignment, the bending forces
might
generate some torque on the clubhead which would interfere with the squaring the clubhead at impact.
Arguments for and against this theory:
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It is basically like theory #2, except that it recognizes that the
torque is controlled by the direction of bend -- and that direction is
not in the target line at impact.
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Unfortunately, it adopts another erroneous assumption about the direction of bend at impact. As we know from ShaftLab,
the bend at impact is not aligned with the CG. It shows more lead than
CG alone would explain, and it varies considerably from golfer to
golfer. In more detail:
- For a driver, we would expect the bend to be 16º-18º from heel-toe, based on CG.
Actually, it is at 37º-66º for the golfers measured, with a median of 44º.
- For a 5-iron, we would expect the bend to be 10º-14º from heel-toe,
based on CG.
Actually, it is at 24º-59º for the golfers measured, with a median of 41º
So alignment with the CG does not align the NBP to the bend.You would
need something like ShaftLab to measure each golfer for spine
alignment, if a theory like #2 or #4 is valid.
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Wishon has done some computer
analyses that suggest this might be a problem. And there is some
limited anecdotal experience (Bernie Baymiller has reported some
success with irons) in support of the theory. But there have been no
actual measurments nor comparisons with other approaches.
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Theory 5: It's all feel at impact and after impact
One possible explanation is that the difference between well-aligned
and poorly-aligned shafts is just the feel of the
club as a result of impact. When the club hits the golf ball, the shaft
bends backwards very sharply and suddenly. This translates into the
feel of impact one gets through the hands. There are at least
two possible reasons that a poorly-aligned shaft might give a bad feel
at impact:
- The
shaft suddenly bending back has an out-of-plane force on it, unless
either the NBP or the spine is in the direction that the bend was just
applied.
- The shaft suddently bending back applies a torque that
wants to twist the grip away from the current position, unless a stable
direction of the shaft is aligned with the bend just applied.
Arguments for and against this theory:
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This is consistent with the general wisdom that either the NBP or
the spine must be in the target line. While most say NBP, a substantial
minority favor the spine. Either one is consistent with reason #1
above. The NBP alignment is also consistent with #2 above.
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This theory does not explain the clubface impact results from the SST/Butler study,
nor other studies where actual performance differences were observed. It just addresses feel.
Is this serious to dismiss the theory? Maybe not. It is well-established that
golf performance stems in part from expectations and confidence. There
is no doubt that a good-feeling club breeds confidence, and a club that
feels lousy at impact raises negative expectations. So a bad-feeling
club, if that feel problem were due to misaligned spine, could account
for performance problems as well as feel problems.
Unfortunately,
none of the studies did anything to distinguish feel problems from
genuine performance problems. In order to test for performance issues
unrelated to feel, you have to remove expectations. That means not only
double-blind studies, but only one hit per "turn" with each club,
before the test subject moves on to another (blind-random) club. This
removes any expectation about how the club will feel when you hit it.
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than the SST/Butler study, most studies and anecdotal reports don't
distinguish clearly between feel and performance. Some, in fact,
explicitly talk about feel rather than measured performance. So a
theory that explains a difference in feel explains most of the reported
results. |
Theory 6: Just get it consistentEd
Reeder did a critique of a draft of this article. He suggested
that I had missed a theory. The theory he proposed was, "We have no
clue
what's really going on. The best we can do is to minimize the possible
effect of spine by aligning all the clubs the same way -- so at least
they're consistent."
I don't agree with it, but it does make
sense to include it for completeness. So let's give it the same
scrutiny we gave the other theories.
Arguments for and against this theory:
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It doesn't answer John Kaufman's original question. It doesn't
address, "What fault are we trying to fix with spine alignment?"
Inconsistency by itself isn't a fault. If we took a set and painted
some shafts black and some white, they would be inconsistent. But, if
there were any difference in performance, we'd have to admit that the
differences were between the ears of the golfer using them.
(Incidentally, this test has been done -- well over a decade ago, when
graphite shafts were beginning to have a market presence. And yes,
there were differences in performance, and they were between the ears
of the golfers.)
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If this theory is correct, then why do some alignments (NBP at 9-3,
spine at 9-3, spine at 6-12) get consistently better reviews than
either NBP or spine on the diagonal. If consistency were all there were
to it, then a set consistently at 4:30-10:30 would be as good as a set consistently at 9-3.
And almost all the data, both studies and anecdotes, disagree with that
assertion.
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Similarly, if it is just consistency, then all tests of drivers
would be inconclusive. There would be no way to say any orientation is
better than any other orientation, because consistency doesn't mean
anything for a single club. But again almost all our data says there is
a substantial difference in alignment orientations from one driver to
another.
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It is worth noting that, in designing a test for theories
#5 or #6, robots are of no help whatsoever. The other theories might be
tested either with human or robot swings, but the last two are
completely dependent upon human reactions to the shaft spine. Here's an
important corollary to this observation. Those (e.g.- shaft
manufacturers) who argue that spine means nothing because robot testing
has not shown it to mean anything do not have complete evidence for
their position.
Bottom lineThe only theories that might be viable answers to John's question, based on all the data I have seen, are:
- #3, out-of-plane forces cause impact farther from the sweet spot, and
- #5, feel at impact and after impact are significantly affected by spine alignment.
Last modified -- 11/30/2008
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