All About Spines
Other technical issues
What causes spine?
Actually, the name "spine" originally came from the belief that, since
shafts were welded tubes, the seam created an asymmetry in stiffness.
In the 1990s, Apollo was alone in making its steel shafts without a
seam, and they based an advertising campaign on this issue of
directional uniformity. Wishon and Summit
debunked this in their 1992 book; turns out the Apollo seamless shaft
had even more stiffness asymmetry than the TrueTemper welded shaft in
the study. So the welding seam isn't the only cause of spine, and not
even the major cause.
That said, let's look at the four major processes for manufacturing
golf shafts, and see what sort of spine we can expect from each:
steel tubing - Today, just about all steel shafts are made
by bending sheet steel into a tubular form and welding it. As noted
above, the major manufacturers of steel shafts seem to have figured out
how not to let the seam interfere with directional uniformity of
stiffness. While I have heard of some steel shaft models that have up
to 4cpm of spine, I have never seen a TrueTemper shaft with more than
2cpm, and the vast majority do not exceed 1cpm. And TrueTemper manufactures the vast majority of the steel shafts used today.
metal tubing - From time to time, we see shafts (usually
aluminum) extruded seamlessly. Theoretically, they should be possible
to be made more directionally uniform than welded steel tubing. In
practice, they don't do any better -- at least in part because steel is
already so good.
graphite - (This is also referred to as "flag-wrapped".)
This approach involves taking a sheet of woven carbon fibers
and wrapping it around a rod called a "mandrel". In more detail:
- These shafts don't use a pre-preg. Instead, the individual fibers
(filaments) are wound in a spiral onto a mandrel. Actually, they are
more woven than wound; spirals in both directions are applied
simultaneously and woven together. The process involves a pretty
intricate machine to weave the fibers. After they are wound in place,
they are resin-impregnated and cured. The result is generally
remarkably free of spine. There are other disadvantages of
filament-wound shafts, but consistency of directional stiffness (and
consistency of specs in general) is a definite advantage.
You can expect most brands and models of shaft to
have spines in the 0-2cpm range, except for sheet-wrapped composite
models. Of course, there are exceptions both ways to this rule; there
are low-spine sheet-wrapped shafts, and a few disappointingly "spiny"
shafts from the other processes. So it pays to know the brands and
models you work with.
- The fibers may be woven together, or may be
unidirectional -- all fibers parallel. In either case, they are held
together in a sheet by being impregnated with a resin. Since the fibers
are going to be resin-impregnated anyway once the shaft is made up, the
sheet is referred to as pre-impregnated, or "pre-preg".
- Several sheets of pre-preg, with the fibers in different
directions, are wrapped onto the mandrel in multiple layers. For
instance, fibers oriented down the length of the shaft are for
stiffness and spiral fibers 45º from the shaft axis are for torque. The
number of layers of each kind (and the properties of the fibers) will
determine the bending and torque profile of the shaft.
- Sheets that are wrapped onto the mandrel have a beginning
and an end. The discontinuity at the beginning and end mean that the
shaft is slightly stiffer where the extra layer is. Hence there is a
potential for a spine; some directions have more fiber layers than others. In fact, sheet-wrapped graphite shafts do tend
to have more spine than other types of shafts. But there are ways of
mitigating this, for example:
So, while the biggest spines are found in sheet-wrapped
shafts, they don't have to be that way. There are companies making
sheet-wrapped shafts that have as little spine as any other process (for instance, SK Fiber).
- Design the wrap so the beginning and the end occur at the same
position around the shaft. That way, no one direction has more nor less
material than any other. This is possible, but requires careful control to get it to line up just right.
- Use more, thinner layers instead of fewer, thicker
layers. That way, the difference represented by a single layer is smaller.
- Design the wrap so the edge of the pre-preg is spiraled around the
tube. That way, no one direction gets the whole impact of the
discontinuity; it is distributed around the shaft.
you're in the range of 1cpm spines, further improvement is somewhere
between quality control and the inherent limits of the manufacturing
process. But that's probably OK. I have seen no evidence that further
improvement buys any additional advantage, whether or not the spine is
Cut shaftsThe question sometimes comes
up, "Will the spine/NBP directions be the same in a trimmed shaft as
they were in the raw shaft?" The answer quite often is "no". This
raises a couple of questions: why does this happen, and what should you
do about it?
answer to "why" follows pretty closely the reasons for spine in the
first place. Think of why spine occurs, then ask why we would expect it
to be in the same direction -- or, for that matter, in a different
direction -- over the length of the shaft.
OK then, what to do about it? The answer is pretty
simple. Don't worry about actually marking the spine carefully until
the shaft is trimmed. Then mark it and align to that mark.
- The cause of the
biggest spines is discontinuities in sheet-wrapped shafts, typically
resulting in a difference in the number of layers of pre-preg as you go
around the shaft. If the pre-preg seams are straight up and down the
shaft, then the spines that they cause should be the same over the
length of the shaft. But often the seams spiral down the shaft, either
by design or inadvertently. When that happens, the resultant spine is a
composite of the spines over the length of the shaft. By trimming from
one end or the other, you remove some directional components of this
composite -- and change the direction of the overall resultant.
of the causes of smaller spines are related to quality control or
manufacturing processes. For such shafts, the answer depends on the
specific defect or cause involved. And it isn't a big deal; most such
shafts have small enough spines that alignment is not an important
there's a bit of a hitch here. If the size of the spine is more than
about 3cpm, then the frequency of the club will depend on the
orientation of the shaft. If you have any reason to believe that the
spine will be sizeable, it is worth finding the spine early so you know
what direction to test the flex as you trim. Then, once the shaft is
trimmed, do a careful spine location that you will use for alignment.
How much spine is "negligible"?There
are still a few clubmakers around who believe that there is no such
thing as a negligible spine. To them, if you can find a spine, it is
important to align it. And if you can't find the spine, then you're in
big trouble -- because you know it's there and you won't be able to
align it. I've watched these guys FLOing a shaft and muttering curses
when it refuses to wobble in any orientation.
their antics because they are treating a very good situation as if it
were very bad. But then I'm convinced that "negligible spine" is an
concept. By negligible spine, I mean a level of spine below which you
can't get a measurable improvement in performance by aligning the
spine. Solid, controlled experimental work has not yet been done to
find this level,
so there is still a remote possibility that there is no such thing as
neglible spine. But, based on the sort of engineering considerations
that got us to this point in the article, let me argue the position
that alignment doesn't matter for spines below a certain level.
first, let's look at the arguments (I originally wrote
"psychologies", and probably still believe that) supporting the "no
such thing" side:
Ultimately the effect of a misaligned spine is a less-than-ideal force
or motion. These are things that can be measured. For instance, SST and Butler found that a misaligned spine can cause off-center hits. Now please follow this reasoning:
- The perfectionist.
I know a lot of clubmakers who build to much tighter specs than
necessary "because I can". These are people who get a frequency meter
that measures to 0.1cpm so they can do a better job of frequency
matching. They get a digital swingweight scale not for the convenience,
but so they can match swingweight to a tenth of a point. And they
insist on aligning a spine if they can measure it -- and are often
troubled if they can't detect it. To them I say, "If your time to do
this is compensated, either by money or personal satisfaction, then it
is worth it to you. But don't confuse 'because I can' with 'it matters
in terms of tangible results'."
- The golf mystic.
I know and like a few people who believe that everything about golf is
a mystical, magical experience. To them, a misaligned spine is "bad
karma", and will be punished by bad shots. I have learned not to
discuss golf technology with those people; we will never agree. As a
scientist and engineer, I believe that spine effect is a physical
phenomenon. There is nothing magic about it. A misaligned spine causes
because of the forces on or motions of the shaft that it causes, not
because of some mystical karma associated with shaft alignment. If you
got this far in my article, you are at least somewhat interested in the
scientific point of view, so let me just dismiss the mystical argument
as not scientific.
- The bio-chemical analogy.
Ecological studies sometimes conclude that there is no acceptably low
level of the studied chemical that is environmentally safe. That is
cited as an existence proof that the "no acceptable level" conclusion
has a place in science. And it does! But all the no-acceptable-level
results I've heard of involve a cumulative
effect -- continued exposure over long periods of time. What is
cumulative about a golf club? This argument has no relevance here.
That meets my definition of negligible spine.
is some acceptable degree of off-center, some amount of "miss" that
will not produce any measurable effect on the shot or the feel. Some
might say 1/4". Almost everybody would agree that a 1/10" miss of the
sweet spot will not affect a drive.
- Let us assume for a moment
(the assumption turns out to be true) that the off-centeredness of the hit
is monotonically related to the size of the spine and the degree to
which it is misaligned. ("Monotonically" means that, any time you
increase the spine or the misalignment, the off-centeredness increases.)
that assumption, the worst possible alignment of a spine is still
acceptable if the spine is so small that its effect is less than 1/10"
off center for the hit.
Let's look again at what a misaligned spine does to the shot result. We already listed the possibilities under "theories". The physical phenomena involved in all the theories is either:
Both of these phenomena can be quantified, and we can set an acceptable level on the result. Let's try that.
- A force that is not in the same direction as the bend of the club, causing out-of-plane motion of the clubhead, or
- A torque tending to rotate the club, when the bend is not aligned with the spine.
Out-of-plane forces - I have done a ball-park calculation
of how far off-center the hit might be due to an out-of-plane force.
The assumptions were a driver with a fairly soft shaft (230cpm). The
golfer swings the club in a way to produce a maximum bend of 3" during
the downswing. (That's quite a lot of bend. Less bend would give less
off-centeredness for a given level of spine.) One assumption contrary
to fact is that the shaft is in the worst possible alignment for the
entire downswing, (That will tend to give a very "safe" estimate, since the shaft does not stay in one orientation during the downswing. The
level of negligible bend would be higher if we assumed something more
realistic.) Here is the result:
So, at least as far as out-of-plane forces are concerned, the threshold of negligible spine is probably 4cpm or higher.
10cpm spine results in a 1/4" out-of-plane deflection of the clubhead.
This is on the edge of measurable performance or feel for most golfers.
- A 4cpm spine results in a 1/10" out-of-plane deflection of the clubhead. It is unlikely that any golfer would detect this.
for the record, the calculations showed that the off-centeredness did
vary monotonically with the size of the spine at any given degree of
misalignment, and also varied monotonically with the degree of
misalignment for a give size of spine. So our earlier assumption is
- The theories that operate here hold that the NBP should be aligned to
some direction, so that bend occurring at impact (or perhaps slightly
before or after) does not create a torque that might rotate the head
away from a square clubface. To see how big a torque would be
significant, we want to compare (A) the torque due to a misaligned
spine with (B) the torque already being exerted by the golfer on the
grip to square up the clubface. If torque A is a sufficiently small percentage of torque B, then the spine is negligible for practical purposes.
small is "a sufficiently small percentage"? Well, a lot of the torque
exerted by the hands on the grip is predetermined by anatomy. As the
wrist cock releases from 90º to 0º, the club naturally rotates the
clubface -- from in the swing plane initially to towards the
target at impact. If you try to prevent this from happening -- if you
try to release the wrist cock without releasing the rotation -- you
could actually hurt yourself. Relatively little of that rotation is
"discretionary", something that the golfer can control, either
consciously or unconsciously. So it is unreasonable to demand a very
small percentage like 1%; we will use a higher percentage.
support of this decision, remember that we use pretty much the same
swing to square up all our clubs, driver through short iron. Where we have different moves or
thoughts for different clubs, the rotation of the hands to square the
clubface is not part of that difference. Yet the clubheads
themselves have vastly different moments of inertia. (MOI of a clubhead is its resistance to
being squared up.) There is a 2 to 1 difference between a driver's and a
5-iron's moment of inertia. So there is substantial evidence that even a 50%
variation in square-up torque isn't a problem.
Given these consderations, a figure like 10% is probably close to the
mark for "negligibility", but let's be conservative and pick 5%. That is, if the torque due
to a misaligned shaft is no more than 5% of the torque needed to square
the clubface, then the spine can be considered negligible
Again, I have done a ballpark calculation
of the torque needed to square up a driver, and the torque due
to a misaligned shaft in the vicinity of impact. The assumptions in
this case are:
The result is that a 4cpm spine (actually a little less, 3.6cpm), when
misaligned as badly as possible (whatever that means) will provide an
undesirable torque of 5% of the torque the golfer is already exerting. So the level of negligible spine for torque is
- The same soft driver shaft as before (230cpm).
- A modern 460cc 200g
- All the uncocking and rotation in the last 100msec before
impact (consistent with strobe photos of good swings, and reinforced by
- A bend at impact of
1.5" (that's a very large bend, according to ShaftLab data; smaller
bends show less effect from a misaligned spine, thus a bigger
It looks like a 4cpm spine is somewhere around the threshold of
"negligible", whether the problem created by spine is off-center hits
or torque due to a bending shaft. The calculations leading to this
conclusion are rough; they might be off by a factor of two either way,
but probably not that much. I'm pretty confident that the number for
each effect is somewhere between 2.5cpm and 6cpm. And most "consensus"
estimates are also in that range.
What I do
You may well ask, "OK then, what do you do about spine
alignment in clubs that you
build?" Here's what I do:
To begin with, I want to make very clear that I don't think spine
alignment is a fine tuning for performance or feel. It is damage control!
Spine is a shaft defect. The best answer, IMHO, is to minimize the
defect to begin with. Spine alignment is a last resort, IMHO, to
minimize the damage that the defect does.
I go out of my way to order shaft models that I know to have negligible
spine -- then I just don't worry about aligning. My personal belief is
that the threshold of negligibility is probably 3-5cpm, and I treat a
spine of less than 3cpm as something I can ignore. Some ways I use to
dealing with a shaft that I am not sure is spine-free (for practical
purposes), I determine the spine by FLO, because it's quicker than
Differential Deflection and, unlike feel finders, locates the true
spine. Then I determine the size of the spine by comparing the
frequency in the two FLO planes.
- Some manufacturers are a lot better
about minimizing the spine than others. I try to keep track of this,
both from my own experience and reports in the forums where I hang out.
steel shafts and filament wound graphite shafts have almost no spine at
all, certainly much less than most
sheet-wrapped graphite shafts. There are exceptions. A few [relatively
small market share] brands of steel shafts have noticeable spine. Some
sheet-wrapped graphite manufacturers have negligible
Sometimes I have to deal with a non-negligible spine. For instance, the
really want some shaft model and for whatever reason I don't say no.
When that happens, I
align the shaft with the spine in the heel-toe plane. Reasons for
is an interesting issue: if there is a significant spine, then the
frequency at the spine is significantly different from
the frequency at the NBP. So which of the two frequencies do I use for
matching? Unlike most clubmakers who align spines, I
trim to the spine
The reason is that's where most of the bending occurs, if you align the
spine in the heel-toe plane. You want the frequency (or stiffness, if
you match by deflection) to be effective during bending, and most of
the bending occurs in the heel-toe plane.
- It assures an in-plane restoral force during big bending
early in the downswing, because that large early bend is heel-toe.
- The stronger of the two forces is fighting toe droop at
that results in the NBP in the target plane at impact -- as recommended
by the usual "conventional wisdom". That's because of the 90º
relationship between spine and NBP. So, even if the first bullet item
is not a good reason, I am able to automatically hedge my bet.
Let me finish by indulging in a little rant about the Rules of golf.
Appendix II of the Rules (the part that details the rules of clubs) says
and Twisting Properties
At any point
along its length, the shaft must:
in such a way that the deflection is the same regardless of
shaft is rotated about its longitudinal axis; and
the same amount in both directions.
That rule has been there for a long time. I find almost identical
wording in my 1987 rulebook, and it was old then.
Think about it. The
rule says that shafts should not have spine.
If it is impossible to meet that rule precisely, then testable limits
to spine need to be set -- and enforced. The USGA has done this with
other specs (e.g. - coefficient of restitution).
the USGA has dropped the ball here. They have caved in to the
shaft manfacturers on one side (who don't want to be held to standards)
and Dick Weiss on the other (who threatened lawsuit unless the USGA
legitimized spine alignment). The decision they adopted is:
Manufacturers of clubs may orientate or
shafts which have spines for uniformity in assembling sets or in an
effort to make the shafts perform as if they were perfectly
symmetrical. However, a shaft which has been orientated for the purpose
of influencing the performance of a club, e.g., to
correct wayward shots, would be contrary to the intent of this Rule.
This is unenforceable, as it requires knowing the intent
of the clubmaker. It also allows "supershafts", assuming that the
supershaft effect is real. (As I noted earlier, there is no public
information supporting that it is. But it might be.)
time! The USGA needs to find its
spine. They should:
view this as "protecting the game", which they are supposed to do. It
would mean that golfers would not have to worry about buying a
spine-aligned club. It would reduce the influence of special equipment
in winning, as opposed to simple skill at the game. I expect such a
proposal to be opposed by:
- Conduct test to determine at what level spine alignment
and enforce a limit on manufactured shafts below this level, to assure
that spine alignment has no effect on the performance of a club.
come from the golfing public -- if they knew enough
about the issue to generate support. Unfortunately, they do not
understand spine alignment except as a buzzword, nor do they have any
sort of effective lobby. The USGA should
be their lobby, but it seems to have forgotten that over the last
couple of decades.
- Shaft manufacturers.
Those shaft manufacturers who don't meet the specs would see their
costs increase. Those who already manufacture low-spine
shafts would lose
their competitive advantage.
The OEMs operate by calling for competitive bids for shafts, and
manufacturers by price. This is yet another spec the would need to
control. Additionally -- and more bothersome -- it will probably
increase the price of the low bidders (who
likely achieve their current low cost in part by not controlling spine).
They currently offer spine alignment as value added, either as a
competitive advantage or an extra-price option. This will be gone.
Given the USGA's recent history, I expect the opposition to be
(If you want to take me to task based on some political view about
regulation vs free market, please first look at my article on the
role of rules in sports.)
- As of 2006, TrueTemper's share of the steel shaft market was 82%. The company's Chad Hall is quoted in GolfWrx Blog, "Retail
market share information is tough to substantiate in the golf industry,
but we estimate our worldwide steel shaft market share at approximately
82%. Tour usage in iron shafts is at 98%. The wood shaft category on
tour is much more highly fragmented with the leader averaging around
25% each week."
ball-park calculations involved using the FLO spreadsheet from another
article, and measuring the deviations in early oscillations.
- This time, the ball-park calculations are fairly extensive. You can find them in the appendix if you're interested.
are several different theories of spine effect that depend on the
torque resulting from bending a misaligned shaft. Those theories differ
as to what they consider properly aligned. One says NBP to target,
another says NBP to center of gravity. This calculation doesn't take
sides; it just computes the torque assuming an amount of bend in the
worst possible direction for the shaft -- however it may be oriented.
Last modified -- 3/2/2008