Measuring
Swingweight
Measuring swingweight is more accurate than estimating it
from tables or
equations. So why did I wait so long to touch on it? Well, so far I've
been dealing with the early design process, the part where you're
selecting
the components to order from a catalog. Once you have the components in
hand, actual measurement is superior to (or at least a valued adjunct
to)
calculation or estimation.
There are two common ways to measure swingweight:
 With a swingweight scale. We covered this early
in the chapter on heft.
 With a gram scale and
other generalpurpose shop tools.
Of course the swingweight scale is the fastest and easiest
way to
measure swingweight. A typical swingweight scale costs about $100, but
they are available for as little $50 for an economy model (similar to
the Ping scale I've used for years), and as much as $300 for a deluxe
digital version. Every professional clubmaker has a swingweight scale,
but most beginners and even some serious amateurs can't justify the
purchase. For them, here is a way to measure swingweight with
more generalpurpose tools. It is less convenient, and
requires computation, but uses tools you should have anyway if you're
going to build clubs.
You will need a good postal scale that
measures in grams or
small fractions of ounces, and a ruler that measures to over 40 inches.
You have to take pains to be precise. Remember:
 A 0.1 ounce (3 gram) error in weight will
result in more than
one swingweight
point of error in the result.

A 1/4" error in position of the CG will result in more than one
swingweight
point of error in the result.
This should give you some idea of just how good your scale
has to be.
Having covered that caveat, here's the method. (My
thanks to Rich
G. Ciccotti for pointing out this method from Ralph Maltby's book.)
 Measure the distance of the balance point of the
club from the end of the grip (in inches).

Subtract 14" from the result, and multiply it by the club's total
weight
in ounces or grams.
 The result is the torque (in
inchgrams or inchounces)
about an axis 14" from the butt, the base definition of swingweight.
 Use the following table to convert to the swingweight point
scale:
Swingweight 
Inchounces 
Inchgrams 

C0 
196.00 
5550 
C1 
197.75 
5600 
C2 
199.50 
5650 
C3 
201.25 
5700 
C4 
203.00 
5750 
 


C5 
204.75 
5800 
C6 
206.50 
5850 
C7 
208.25 
5900 
C8 
210.00 
5950 
C9 
211.75 
6000 
 


D0 
213.50 
6050 
D1 
215.25 
6100 
D2 
217.00 
6150 
D3 
218.75 
6200 
D4 
220.50 
6250 
 


D5 
222.25 
6300 
D6 
224.00 
6350 
D7 
225.75 
6400 
D8 
227.50 
6450 
D9 
229.25 
6500 
E0 
231.00 
6550 
Measuring
Moment of Inertia
Like swingweight, measuring moment of inertia is more accurate than
computing it from the component specifications. And it can be done. The
instrumentation to directly measure MOI is considerably more expensive
and complex than swingweight, because MOI is a dynamic quantity while
swingweight can be measured statically.
...With a Pendulum
The
simplest way to measure the moment of inertia of a golf club is to
mount the club the club by the butt in a frictionless bearing, and let
it swing like a pendulum. The period of the pendulum's swing is
partially determined by the moment of inertia of the club. We can find
the equation
for the period (T, a time in seconds) in most college physics textbooks:
Where:
T 
=  The period of a full cycle of the pendulum, in
seconds.  I 
=  Moment of inertia. (Scientists use "I", not
"MOI")  mg 
=  Total weight of the club (mass times the
acceleration of gravity) 
L 
=  The distance from the axis to the balance point
of the club. 
Note that everything here except the moment of
inertia can be measured
with tools on hand:  Period of the
pendulum with a stopwatch.
 Weight with a scale.
 Position of the balance point with a ruler.
That means that you can measure everything else and solve the
equation for I, the moment of inertia of the club. The
solution turns out to be: I
= const * T^{2} m L
where the constant depends on the units you're using
for mass (grams,
Kg, or ounces), length (inches or cm), and moment of inertia.
If you are so inclined, you can do it all yourself.
 You measure length and weight all the time. This part is not
a problem.
 The computation is easy, just multiplication on
a calculator.
 Measuring the period accurately
enough is harder, but you
can do it. You can probably click your stopwatch accurately to within a
fifth of a second (0.2 sec). The period for most clubs is in the
vicinity of 2 seconds. So you are only measuring to an accuracy of 10%,
not nearly good enough. You will have to let the pendulum swing for a
counted number of cycles (somewhere in the 20 cycle range will be OK),
and divide the total time by the number of cycles.
If you do this all the time, it might be worth the money to get one of
the moment of inertia meters on the market. Tom
Wishon's was probably the first, and seems to by typical of the breed.
It consists of:  A pivot that grips the
club at the butt and allows it to swing like a pendulum.
 An
electronic timer that measures the period of a pendulum
swing. The technology is much like that of an opticallysensed
frequency meter.
 A computer program that prompts you for measured period, m,
and L  you still have to measure mass and balance
point  and gives you back a moment of inertia.
There are convenience features and new variations, but
basically that
is how they work. And you can use them like a swingweight scale to
match a set of clubs. But you're now matching them according to MOI
rather than swingweight. 
As of 2008, there is another, better MOI meter on the
market. GolfMechanix makes a device they call the Speed Match. Think of
it as a horizontal pendulum, so gravity does not play a part. Instead,
the club vibrates back and forth against a spring. So the moment of
inertia is a function of only the spring constant (which is built into
the instrument) and the period (which is measured by the instrument).
This gives a more precise MOI measurement than the pendulum (no errors
due to weight or length measurements), and possibly more accurate as
well (this depends on how carefully the spring is calibrated).
...With
a
Swingweight Scale
But wait! Usually you are matching a set to a club
that you have
determined is the right heft for the golfer. It isn't important to know
the numerical value of the MOI, just that all the clubs have the same
MOI. If that is the problem you are trying to solve, you can do it with
your swingweight scale.
Let me start with the rules of thumb for matching MOI with a
swingweight scale, and how to use them to MOImatch a set of clubs.
Then we'll discuss why the procedure works, with lots of numerical
examples.
First,
the simplest possible guideline:
The swingweight should go down by
one point for every inch the club's overall length increases.
That's a simple slope of one point per inch.
Now, here's a somewhat more precise and general guideline:
 The swingweight should go down
by 1.31.4 points for every inch the club's overall length increases.
That's about one and a third points per inch. This is a little more
precise about the slope.
 If, at some
point in the set being matched, you change shaft models so that one is
substantially lighter than the other, then add one
swingweight point to the target swingweight for each 20 grams the raw,
uncut shaft gets lighter.
So what does this say about how to build an MOImatched set
using your
swingweight scale? A lot of the process is the same as
swingweightmatching a set, using adjustments controlled by what you
see on the swingweight scale. Here are the differences:
 You are not trying to match all the
clubs to the same swingweight.
Measure the swingweight of the club whose heft you're trying to
duplicate  that's the same as swingweight matching  but then make
a list of target swingweights for each of the clubs. You will
use the guidelines above to come up with the target swingweights.
 Make sure you are measuring swingweight with the
same grips on all the clubs. Better yet, match them before
you put the grips on.
That's because we know that, while grips will affect the reading on the
swingweight scale, they have very little effect on MOI. So, if you're
matching MOI using a swingweight scale, you have to take the grip
weight out of the picture. (Actually, that's also a great idea with
swingweight matching as well  but the logic for it isn't as obvious
as for MOI matching.)
 You can adjust the
swingweight to get the target weight using either clubhead weight or
the length interval between the clubs.
Again, you can do this when adjusting swingweight as well. But you're
more likely to need length interval adjustment when trying to MOImatch
a set. That's because the clubheads you buy as components have a
weight interval built in that assumes swingweight matching. You're
going to need heavier short iron heads. Even more of a problem, you may
need lighter heads for your longer clubs. While you can generally add
weight, it's very hard to remove it from a clubhead.
 Tom
Wishon, who has more experience with MOImatched clubs than almost
anybody, says that you should not use the same MOI for the
woods that you do for the irons.
He's probably right; it's a good idea to fit woods and irons
separately, for reasons I laid out at the beginning of this
chapter.
Pretty simple, huh? Well, if you don't think so, then use a
pendulum and measure the MOI itself.
The
rest of this page is a bunch of numerical examples intended to
convince you that MOImatching with a swingweight scale works. I used
an Excel spreadsheet
to design a number of MOImatched sets, and looked at the swingweights
that resulted
 again using the spreadsheet. Carey Winquist built the spreadsheet
using the formulas from earlier in this chapter.
Remember that I cautioned about putting to much faith in the
accuracy
of those equations. So why should I believe them now? For one thing,
Carey was very careful to use as much information about the components
as possible  much more than my simplest equation, and closer to the
exact equation. But there is a more important reason.
The sort of thing
that fools the equation for swingweight also fools the equation for MOI
 and by a similar amount in the same direction. For instance, a shaft
with a high balance point will reduce the swingweight; but it will also
similarly reduce the MOI. This sort of thing is true for all the
clubhead and shaft properties that might fool the equation. So, if you
build to a measured swingweight and not just the length and weight on
the spreadsheet, it is very likely that you will get the expected MOI.
That means you need to be working with a real swingweight
scale to get
the MOI match, not just using the trim and weight from the spreadsheet.
Any errors the components induce in the equations will affect the
swingweight and MOI similarly, so a sloped swingweight according to the
guidelines will give a pretty darn good MOI match. You may have noticed
that I never mentioned an 8gram head weight interval nor a 0.4inch
length interval, as I did in my earlier
recommendations. You will get a much more accurate MOI match
if you work from swingweight rather than
computed dimensions.
Anyway, here is the raw data pasted from the spreadsheet, so
you can see for yourself what is happening...
First I did a simple match, MOImatching across a whole set
with
graphite shafts. I started with predetermined lengths, and chose the
head weights to give an MOI match. The graph is a plot of swingweight
vs club length, for the MOImatched clubs. It is very close to a
straight line, with a slope of 1.35 points per inch. That means that
for this combination of shaft weight and starting head weight, you can
MOImatch a set of clubs by building them so the swingweight decreases
1.35 points for each inch longer the club is.
Club 
Length
(inches)  Head Weight
(grams)  Swingweight
(points above/below D0) 
MOI
(1000s
gram
inches²) 
Slope
= 1.35 points per inch
 D
 44
 197.5
 4.3  420
 3W
 43
 208.5
 2.7  420 
5W 
42 
220 
1.3
 420  7W  41  232  0.0
 420  2I  39.5  246 
1.9
 420  3I  39.0  253 
2.5
 420  4I  38.5  260.5 
3.2
 420  5I  38.0  268.5  4.0
 420  6I  37.5  276.5 
4.6
 420  7I  37.0  285 
5.3
 420  8I  36.5  293.5 
5.9
 420  9I  36.0  302.5 
6.5
 420  Note:
All the runs have in common:
 52gram grips on all clubs.
 Graphite
shafts weigh 75 grams raw uncut.
 Steel
shafts weigh 118 grams raw uncut.

But is the straight line and 1.35 slope a general property of
swingweightmatched clubs? Or is it just a property of the particular
set I matched? I designed a few more sets with different
constraints to find out.
First I repeated the exercise for a set of steelshafted
clubs, built to the same lengths and MOI
as the graphiteshafted set above. Again, the swingweight is a nearly
straight line, this time with a slope of 1.3 points per inch. So far at
least, we have a consistent slope.
But the swingweight plot is at a considerably lower
swingweight for
each club. On average, the steel clubs have 2.2 points less swingweight
than the graphite clubs, for the same MOI. That means that shaft weight
matters. When we divide the 43 grams difference in shaft weight by the
2.2 points of swingweight, we get 20 grams per point. That means that,
for every 20 grams of shaft weight increase, we have to decrease the
target swingweight by a point.
Club 
Length
(inches)  Head Weight
(grams)  Swingweight
(points above/below D0) 
MOI
(1000s
gram
inches²) 
Slope
= 1.3 points per inch
Swingweight about 2.2 points less than graphite
( 20 grams per point )
 D
 44
 181
 5.9  420
 3W
 43
 192
 4.6  420 
5W 
42 
204 
3.1
 420  7W  41  216.5  1.8
 420  2I  39.5  232.5 
0.5
 420  3I  39.0  239.5 
0.1
 420  4I  38.5  247.5 
0.8
 420  5I  38.0  255.5 
1.4
 420  6I  37.5  264 
2.2
 420  7I  37.0  272.5 
2.8
 420  8I  36.5  281.5 
3.5
 420  9I  36.0  290.5 
4.0
 420 
For my next trick, I matched a set where the
irons had steel shafts and
the woods graphite. If the rules of thumb are valid, we would expect a
pair of straight lines, each with a slope of 1.3 to 1.4, and offset
from each other by several swingweight points. With a 43gram
difference between the raw shafts, the offset should be 43/20 = 2.2
swingweight points.
And that is pretty much what the data shows. The two segments
have
slightly different slopes of 1.3 and 1.4, but both well within the
guidelines. The offset between the two segments is about 2.5 points 
close enough to the 2.2 points from the guidelines.
Club 
Length
(inches)  Head Weight
(grams)  Swingweight
(points above/below D0) 
MOI
(1000s
gram
inches²) 
Slope (irons) = 1.3
Slope (woods) = 1.4
Swingweight about 2.5 points less in steel
( 17 grams per point )
 D
 44
 197.5
 4.3  420
 3W
 43
 208.5
 2.7  420 
5W 
42 
220 
1.3
 420  7W  41  232  0.0
 420  2I  39.5  232.5 
0.5
 420  3I  39.0  239.5 
0.1
 420  4I  38.5  247.5 
0.8
 420  5I  38.0  255.5 
1.4
 420  6I  37.5  264 
2.2
 420  7I  37.0  272.5 
2.8
 420  8I  36.5  281.5 
3.5
 420  9I  36.0  290.5 
4.0
 420 
So far, we have achieved the swingweight match
by adding or subtracting
weight to the clubheads. But we know (from the graphs on the previous
page) that swingweight and MOI don't track exactly the same way if you
vary head weight or vary length. So suppose we just took a standard set
of clubheads, and achieved the MOI match by messing with the length
interval between the clubs instead of the clubhead weights. Would we
see the same result?
I did it. Here is the result. The range of club lengths over
the set is
a little compressed, as we would expect. But the compression is not all
that much: 7.4" instead of 8.0" with the usual increments.
And the guidelines are still pretty good. Again, the two
segments
(graphite woods and steel irons) have slightly different slopes: 1.2
and 1.4. But those are still pretty consistent with the guidelines. And
the offset is again about 2.5 swingweight points between graphite and
steel, giving a shaft weight allowance of 17 grams per swingweight
point.
Club 
Length
(inches)  Head Weight
(grams)  Swingweight
(points above/below D0) 
MOI
(1000s
gram
inches²) 
Slope (irons) = 1.2
Slope (woods) = 1.4
Swingweight about 2.5 points less in steel
( 17 grams per point )
 D
 43.80
 200
 3.8
 421 
3W  42.90
 210
 2.3
 421  5W  42.05
 220
 1.0
 421  7W  41.20
 230
 0.0
 421  2I  39.35
 235
 0.2
 421  3I  38.90
 242
 0.6
 421  4I  38.45
 249
 1.2
 421  5I  38.00
 256
 1.7
 421  6I  37.60
 263
 2.4
 421  7I  37.20
 270
 3.0
 421  8I  36.80
 277
 3.5
 421  9I  36.40
 284
 3.9
 421 
Another confirmation of this approach
is anecdotal and purely
empirical. But it has happened more than once now.
In
September of 2009, I got a call from Charles Homes, a
professional auto mechanic and serious amateur golfer and clubmaker.
Without having ever heard of MOI matching, he spent months tinkering
with the swingweight of his irons until they all felt alike when he
swung them. The result was the best set of irons he had ever used. He
subsequently ran across my web site, and wanted to ask me what I made
of his
experience. He gave me the measurements of his irons, 6iron through
gap wedge. Here is how they
plot, swingweight vs length.
The yellow dotted line is the bestfit linear regression line, the best straightline fit to the data. Things to note:
 The
fit is really good, both by eye and according to the .94 correlation
measure Rsquared. So the best fit for Charles probably is indeed a
straight line for swingweight vs length.
 The slope of the yellow line is 1.28 points per inch. That is almost incredibly close the the 1.3 slope proposed above.
Then, on 13 July 2010, a GolfWRX member called DieselG produced the following post:I just
built my "new" set of clubs. 3L PING BeNi ISIs. I wanted to try
something different this time  MOI matching.
I
remember reading in the past about MOI matching clubs, as opposed to a
standard swingweighting (example D1 for all clubs.). I found a great
link at Tutelman website about this principle, that if you increase SW
about 1.3 for every inch the club gets shorter you will be pretty close
to MOI matched.
I
took my old Eye2s out to the range and hit balls with the 3 iron,
changing the swingweight until it felt "right". I then did the same
with my Pitching Wedge until it felt right.
To
temporarily adjust SW, I used pennies, and/or nickels that I "molded"
into a rounded shape (the same radius as the hosel), and temporarily
attached them to the hosel with tape. Each penny (2.5 grams) gave me
approx 1.25+ Swingweight, and each nickel (5 grams) was about 2.5+
swingwight. I had cut one penny in half to "fine tune" as well.
Once
I discovered what felt the best, I took them home and measured and
found that my 3 iron felt best around D0 and my PW Felt best around
D4.5.
I
then built my BeNi ISI irons using this slope, where the 3 iron = D0, 4
iron = D0.7, 5 iron = D1.5, 6 iron = D2.2, 7 iron = D2.7, 8 iron =
D3.5, 9 iron = D4.2, PW = D4.5. SW and LW were both progressively
heavier as well. I have a spreadsheet I made that allows me to
calculate SW to the tenth, using weight and balance point.
. . . . .
The
results were fantastic. I love the way these irons feel. I feel much
more in control than I ever have on all irons.
If
you want to test this you might try the "taping pennies or nickels" to
your clubs until you determine what feels best. One swingweight for all
clubs may not work best for you. Assuming
DieselG's clubs had a halfinch length increment (a highly likely
assumption), this comes out to a slope of 1.29 points per inch 
again, incredibly close to 1.3.
This
admittedly limited and anecdotal evidence implies that matching clubs
on a 1.3 swingweight slope is a very good way to make them play the
same. 
My conclusion is that you
can MOImatch a set of
clubs quite
well using a swingweight scale and some fairly simple guidelines. I
have built sets of irons like this and corresponded with others who
have, and the results are uniformly good. The clubs are playable and
the subjective reaction of golfers who try them is that they feel very
similar.
My older guidelines were based on matching sets of irons
only. But
these new rules of thumb, based on a perlength slope rather than a
perclub slope, seem to work well for woods as well.
Last modified Oct 27, 2011
