ShaftLab - 4
from the data
It turns out that the ShaftLab graph of a golfer's swing is not
exactly what the article, nor the ShaftLab literature, implies. Let's
see what this means, and use it to better understand when and how the
golfer is bending the shaft.
trace covers a lot more than the downswing
This "lesson" isn't about shafts so much as it is about ShaftLab. The
trace isn't exactly what it purports itself to be.
to the ShaftLab literature and the article, a ShaftLab trace represents
the downswing. In some of their writing, they are more careful
with their wording. The trace is slightly more than the downswing; they
describe it as "from the beginning of load (just before the start of
the downswing) to impact." But most of their writing just describes it
as the downswing.
In fact, only
the last 42%-57% of the trace is downswing! Only about a half of the trace is actually downswing. The rest is
backswing; the trace contains almost a third of the backswing.
did I come to that conclusion? I guess I was primed by the fact that
the durations of the trace didn't make much sense. A downswing duration
is well known to be in the 200-350 millisecond range, with most
professionals in the lower half of that range. But the trace durations
for the pros in the ShaftLab manual were listed at 400-600
milliseconds. Something was wrong here. But I didn't bother to dig deeper to find
out what it was until September 2017.
was when George Hodgetts got into a discussion with me about kick
velocity. We still haven't agreed on that issue, but he sent me some
very interesting traces as part of the discussion. George has built his
own version of ShaftLab, attaching four strain gauges to a shaft in a
quadrature configuration -- just like ShaftLab. They are lower on the
shaft -- the midpoint rather than just below the handle -- but the idea
is the same. George's traces looked nothing like any of the traces I
have seen nor measured myself with ShaftLab. How do we explain the
I wrestled with it for a bit, I found a way to look at George's traces
in a comparable way to one from ShaftLab. In the picture at the right,
we have a single-hump ShaftLab graph on top, and George's graph on the
bottom. Here are the things I had to do to get some measure of
they still don't look quite right. Or they didn't, until George
reminded me that his graph was an entire backswing-downswing set; he
even sent me a version of the graph where he marked the start of the
downswing. Once I saw that:
- Combine separate traces on one set of axes. Red for
lead/lag and dark for toe up/down.
them to look more like the ShaftLab traces. George's had both curves
with the same maximum deflection of 3 inches. I don't know why it did
(doesn't make sense to me), but I halved it for the graph to a maximum
of 1.5 inches. That gets it part way to what ShaftLab looks like. While
I did that, I also inverted it to the same direction as the ShaftLab
curve (that is, lead deflection is positive).
- I also changed his axis labeling from sample count
to seconds, for easy comparison to ShaftLab.
But we still have a problem
comparing George's traces to ShaftLab's. If a ShaftLab trace
corresponds to the downswing, then "Start of Downswing" should be very
close to the left edge of the green-shaded area. It isn't. Not even
close! Only 57% of the ShaftLab trace consists of
downswing and transition.
- I marked off a section and
shaded it in green, corresponding to the ShaftLab trace. If you look at
the green-shaded area, it has very much the same shape as the ShaftLab
trace. Yes, there are significant differences, but there are
substantive differences from trace to trace out of ShaftLab. That is
its effectiveness as a fitting tool; it captures differences as well as
similarities in swings.
How can that be? One possible explanation might be that a ShaftLab
trace starts way before the downswing. That would also be
consistent with my earlier observation about the duration of ShaftLab
traces: 400-600ms, compared with known downswing durations of
200-350ms. How can we test this hypothesis?
have several measures of my own swing, including a ShaftLab trace and a
video that I
can step through frame by frame. So I am in a position to compare them
in detail. I just never bothered to before. The discussion
motivated me to. Here is a comparison of my ShaftLab trace
frame snapshots of the actual video at those moments in
swing is closer to a "ramp" rather than a "one-hump" swing,
conclusion is even more extreme than George's. Only 42% of my ShaftLab
trace is downswing; that would be the yellow-shaded portion of the
ShaftLab graph. This is pretty strong confirmation of our inference
from George's graph.
is nothing particularly odd about either George's swing nor mine. Since
we're talking about time durations, we could do a sanity check by
comparing both swings against Novosel's "Tour Tempo"
hypothesis: ideal swings have a ratio of 3:1 for backswing time vs
downswing time. George and I have monitored this for our own swings
over the years. He has been stable at 2.7:1 and I have been stable at
2.9:1. So both our swings have timing that is consistent with good golf
swings. The issue has to be with ShaftLab's algorithm to determine the
start of the backswing; it
doesn't work very well.
have gone back and looked at the rest of my conclusions in this
article. Only one depends on this flaw in the ShaftLab algorithms: the
statement about the time and direction of maximum bending. I have
that part to conform to the new information.
(10) When and how
does maximum bend occur?
This is the assertion in Weathers' article. He says,
nearly all players, toe-up bend is far greater than any other bend
during the swing. It peaks for most players somewhere near the middle
of the downswing."
would imply that the peak bend occurs very close to the peak
toe-up bend, and a consequence of that is that the direction of maximum
bend is mostly toe-up.
close examination of the actual
data shows no
such thing. Fortunately, ShaftLab traces explicitly show the point of
maximum bend, as well as its amount. That bend is broken down
toe-up and lag bend, which can be read off the graph. So it is easy to
calculate just how far from toe-up the maximum bend is. Here is a plot
of the bend components at maximum bend. There are 18 points: the nine
tour golfers, each with a driver and a 5-iron. If Weathers' implication
is correct, most of the points should be close to the toe-up axis. The
graph shows a completely different result.
there is a toe-up tendency, but only rather slight -- definitely not
the dominance that Weathers' statement implies. Consider:
about the difference between the driver and the iron? Can we say
anything about that? Not really. I calculated the mean and standard
deviation for each, and they were closer than you would expect even for
pure random chance.
- If the maximum bend were dominated by toe-up bend,
then the data points should be clustered next to the "Toe Up" axis of
the graph, perhaps no more than 10°-15° from it.
- Such is not the case.
The average over all swings has the maximum bend 37°
from toe-up. That's a long way from toe-up; a 50-50 split between
toe-up and lag would have been 45°, only slightly larger than what we
- A quarter of the swings are closer to lag bend than
toe-up bend. (That is, below the 45° line.)
are two points that actually show a lead bend component. Let's ignore
those -- the red-circled points -- for now, along with the third
red-circled point. We'll see later where they come from.
|So there is
nothing consistent we can say about what direction the
shaft bends from golfer to golfer. How about when the shaft
is a distribution of the time maximum bend occurs, measured in
hundredths of a second before impact. There is certainly some
So the time
of maximum bend has a very different sort of result from the very scattered direction of
The timing has a strong trend to about 0.1 second before impact, and
even the data outliers have an explanation related to the shape of the
swing's trace. In the next section, we'll try to understand the
different swing shapes and how they influence the peak bend.
- All but 5 swings see peak bend at .08-.12
seconds before impact, with seven of them (more than a third) smack-dab
at 0.10 seconds.
- Of the other five, two of them (at .14 and .20) come
from the only single-peak swinger in the sample.
remaining three (at .23, .26, and .36) come from a few of the
double-peak swings. Specifically, they come from swings where the
maximum bend occurs at the earlier
the way, now we know where those red-circled angles of maximum bend
(including two with lead bend) come from. If the maximum bend is in the
first peak, it is probably in the backswing. We'll see more about this
Interpretation of swing profiles
I originally referred to this section as "speculation" because I don't have complete
data about things like where the transition is for each of the swings
in the ShaftLab documentation library. I am also speculating about the cause of the trace
shapes. This is the start of a discussion with new information: the
evidence that about half the ShaftLab trace shows the backswing, not
the downswing. So feel free to take issue with my interpretations. But...
if you are challenging my starting premise (that transition is roughly
halfway through the trace), you will need better counter-evidence than I have
First, how many different swing profiles are
there, for fitting purposes. The ShaftLab orthodoxy says three: Ramp,
Single Peak, and Double Peak. As I went through the traces, there
seemed to be a significant number of golfers with a fourth shape, a mix
of Ramp and Double Peak. (I'm especially interested in this profile
because it describes my own swing.)
But once we correctly identify the transition point, there
are only two shapes of
Early Peak and Late Peak. The other differences in the profile are due
to differences in the backswing, not the downswing at all. I don't know how much sense it makes
to fit a shaft to the backswing; we should be focusing on the downswing
Let's look at a few traces to see what I mean. I'm
using traces from the "library" that is part of the ShaftLab manual.
I am identifying transition arbitrarily around the middle of the trace
for several reasons:
- George and I have actual swings where we can tag the
transition, and that is where it lies.
- It gives reasonable downswing durations, unlike the full
trace that ShaftLab purports to be the downswing.
lends itself to reasonable descriptions of what is actually bending the
shaft during the downswing. (The double-peak downswing doesn't lend
itself to any reasonable interpretation that I have ever seen, but I
can explain it if only the second peak is during the downswing.)
start with the Single Peak swing. This graph was made by lifting the
trace's pixels from the library graph and copying them directly to this
graph. The first half of the trace shows not downswing, but the last
third or so of the backswing. The trace is a graph of bend in the toe-up/toe-down
plane, not the total bend. I haven't shown the lead-lag trace, and it
does affect the total bend. But it doesn't dominate either the size or
the position of the maximum bend; the toe-plane bend either dominates
or shares determining those properties.
So the Single Peak swing
has the maximum toe-plane bend near the transition, rather early in the
downswing. Even if it isn't exactly at the transition itself, the lead-lag bend
can't move it too far away from transition.
What is a good
physical explanation of shaft bend in the Single Peak swing? It seems
to me that the golfer exerts most of the bending moment on the grip
while executing the transition from backswing to through-swing. There
is no lightening of the grip to pull the club along the shaft axis.
Just set the lag, the plane, and the grip pressure, and turn back then
have chosen to plot the other three swing shapes together on one graph.
They look rather different before the transition. But after transition,
they rise (some some need more rise than others) to a late peak, then
fall sharply towards impact. This raises the question of what is
happening in the swing to make these traces happen.
The thing to realize is the unshaded (backswing) portion of
all the traces -- even the Single Peak trace -- consist of toe-up
to stopping the backswing in preparation for the downswing.
- The Ramp has a smooth stopping of the backswing, which
gradually loads the shaft in a way that increases through the
transition and into the downswing.
- The Double Peak stops the backswing more abruptly, bending
the shaft a lot. (I'm inclined to think it was a faster backswing to
begin with, but have no data to support that guess.) Then the golfer
eases off on the bending moment from the hands, in preparation for the
downswing. There are a few ways this might be happening, and I would
not be surprised if they all contribute to the double peak for some
golfer or other:
- A pause in the backswing. Easiest explanation, but
probably not the most common in practice.
- Dropping the club "into the slot" -- changing the plane
of the swing. I suspect this is common with better golfers.
- Changing from a hand couple to an axial pull. Also likely
with better golfers.
- Ungrip-and-regrip at the transition. Perhaps common with
less-skilled golfers, where the ungrip-regrip is a well-known flaw.
- The Combination is, as the name suggests, a hybrid of the
Ramp and the Double Peak. It starts smoothly like the ramp, then eases
off even more. The reasons are probably similar to those for the Double
Peak, but from a much lower starting bend.
this in mind, let's revisit the distribution of time between peak bend
and impact. Here is the same chart, but color-coded with the swing type
for each measurement. I see three distinct clusters of data:
I should note that Pavin's swings are reflected twice in this data. He
has two red data points, swings where the maximum bend was in the
backswing. But I also found his maximum bend during the downswing,
and added those two (one driver and one iron swing) into the data.
Those were .10 and .11, smack dab in the middle of the late-peak
- The red cluster reflects a peak bend during the
backswing. It says nothing about the downswing at all.
- The black cluster comes from the only golfer in the
sample with a Single Peak swing. These are early peaks; there is more
time left between the peak bend and impact. While this is based on only
one sample golfer (two swings) from the library, George Hodgetts'
swing, not in the library, is also single-peak, and it
has even more pronounced early peak behavior. (George's swing peaks
0.3sec before impact, at the start of the downswing.) In addition, a
good look at the nature of the Single Peak swing says
that it should
produce an earlier maximum bend.
- All the rest are late peaks, tightly centered around
0.1 seconds before impact. They reflect all the other swing shapes:
Ramp, Double, and Combination. In fact, we don't have nearly enough
data to see whether there is any tendency of each of them individually.
To the level of detail we have, they behave essentially the same.
If you believe that shaft fitting should be based on the downswing and
not the backswing, then the fact of a double peak isn't much of an
issue; the first peak occurs in the backswing, and isn't very
interesting. From the data I have, the second peak occurs where the
Ramp and Combination swings do.
for fitting purposes, you have the Single Peak swing (late peak), and
you have everything else (early peak around .10 seconds before
impact).. It isn't clear that one would fit an
early-peak and a late-peak swing the same; maybe so, maybe not. But
even if they are different, that only separates out the Single Peak
swingers from everybody else.
Application to shaft fitting
Let's be even more speculative, and imagine what this might mean in
terms of shaft fitting. Suppose we start with Wishon's model
of shaft fitting. He identifies three criteria -- properties
of the swing -- that determine the flex profile you need in a shaft:
he has identified an effective fitting heuristic (and I'll just assume
that for now, I won't defend it), our new way of looking at ShaftLab
traces might be a very good input to the process, especially if we can
guess even somewhat well when the transition occurs in the trace. We
know it's somewhere in the middle. With the following interpretation,
the exact placement would not be too critical.
- Transition: smooth, average, or forceful.
- Tempo: smooth, average, or fast.
- Release: early, midway, or late.
We have learned a lot about shaft fitting since Wishon's book was
published in 2006, but over the years a lot of successful fittings have been done with
this method. It likely gives a very good starting point for fine-tuning
the exact shaft selection. And I suspect that the measurement
proxies on the ShaftLab trace will give as good a measure as the
informal "by eye" descriptions in Wishon's book.
shaft bend at transition should be a very good proxy for Wishon's
"transition" measurement. Note that the slope of the trace is fairly
small in the middle of the duration, so any error in estimating the
time of transition won't introduce much error in the "transition"
- The time from peak bend to impact should be a very good
proxy for Wishon's "release" measurement.
"tempo" measurement is a little more subtle. But reading between the
lines, it is a measure of acceleration that feeds into the overall
stiffness required of the shaft. So it would be worth trying the
maximum bend as a proxy for tempo. In fact, TrueTemper's own
experience using ShaftLab as a fitting tool seems to do just that for
overall stiffness. They didn't pay as much attention to specific butt
and tip stiffness, but our application of the ShaftLab trace to
Wishon's process might provide the additional detail required.
The TrueTemper ShaftLab may be no more, but there were many lessons
that can be learned from it and the results it provided to both
researchers and fitters. Some of those lessons still need learning by
too much of the golf population.
- These are not
instrumentation of the same swing, nor were they
even taken in the same year. But I have videos and ShaftLab traces
taken more years apart than this, and they were substantially the same
in shape and duration. So my conclusions here are valid to a first
approximation. And they are dramatic enough that a first approximation
is all we need.
you counted the points. You noticed there are 20, not 18. There are two
sets of points for Pavin's swing. If you read ahead, you'll see that
his peak bend occurs in the backswing. I also added the points for his
swing's peak bend during the downswing.
- The traces
I'm using for each of the shapes are the most
representative of each shape, according to my eye.
- Single Peak: Russ Cochran
- Double Peak: Davis Love III
- Ramp: John Cook
- Combination Ramp-Double: Bob Estes