Fitting for Heft
Fitting a golfer for heft is still more an art than a science.
Finding the right swingweight is largely a matter of trial and error.
If you know what you're looking for, you can home in on the right
answer faster, but that's just reducing the trials by knowing some of
the errors in advance.
If you want to cut to the chase and see my advice on how to
fit for heft, look at how
I do the fitting. But if you want to know the considerations
for fitting, here are some commonly-used approaches and things to
consider:
The favorite
club
A surprising number of relatively inexperienced clubfitters subscribe
to the theory that the best clubfitting is to duplicate the golfer's
favorite club across the set.
This approach has flaws, but it also has enough adherents -- and enough
promise -- that we should probably start the discussion by exploring it.
Does the golfer have a markedly favorite club? If so, look at the specs
of all the clubs. (You did
measure his existing clubs before you started fitting him, didn't you!)
If that club differs from the rest of the set in heft (swingweight or
MOI or total weight) then that should be a clue about fitting for heft.
Unfortunately, the favorite-club approach to clubfitting usually
doesn't work that well for heft. The reason is that other club-to-club
differences are more likely to cause the favoritism. In order of
likelihood, they are:
- Length - this club gives the most comfortable swing plane.
- Loft - this club gives the most reliable combination of
trajectory and distance.
- Flex.
- Heft.
The first two necessarily vary from club to club; you can't
duplicate them across the set. And they are the most likely reasons for
a favorite club. Flex and heft -- the things you can
match across the set -- are considerably less likely to be the reason.
But you might get lucky here. The favorite club might have a
substantially different heft from the other clubs. You want to look for
differences in excess of one, and preferably more than two, swingweight
points. (Or the equivalent in moment of inertia. We'll see the
equivalents on the next page.) This is pretty unlikely, because:
- It's too easy to match swingweight to expect a lot of
variation within a set, and
- You're asking for all but one club of the set to be a bad
match for the golfer.
Fitting by
hitting balls
Here are some rules of thumb to use when having a golfer hit clubs to
find a fit:
- Have the trial clubs be pretty similar in all respects
except the
one being tested. For purposes of this chapter, that means that the
clubs should be the same except for heft. In particular, length and
loft should be identical. You can have a little variation in flex, but
the better the golfer the closer you should have the flexes on the test
clubs.
- Don't let a golfer take more than three swings in
succession with the same club. Golfers are very adaptable, and you
might see better hits -- or at least better compensations -- as the
subject gets used to a club. Remember, the point of clubfitting is to
adapt the club to the golfer, not the other way around.
- Likewise, don't have more than three clubs in the rotation
at any
time. At some point, you make a conscious decision to remove a club
from the rotation and replace it with another. But you should never be
comparing more than three clubs at once, and just two is even better.
There are two different figures of merit for choosing which
club is better. Neither of them is distance, though distance is
strongly related to one of them. They are:
- Quality of strike, and
- Consistency of strike.
Both are important. I consider quality of strike more important
for long-term future of the clubs being fit. Of course, if a quality
strike is also consistent -- which happens often enough -- then you
have found the right fit. But... If a quality strike isn't consistent
but occurs reasonably frequently, that indicates the right club, that
some practice could make consistent. On the other hand, if the quality
strike only happens occasionally, consider it just and accident, not
the product of fitting, and move on to another club.
Release
A clubfitter with a good eye -- or perhaps a high-speed video camera --
can tell something about whether the club is fully released at impact.
Reviewing, "fully released" means that the wrist-cock angle is down to
some rather small value. You don't want the left wrist to be cupped at
release (that is, a negative wrist-cock angle). Ideally, the club
should continue the line of the left arm. But a slight residual cock at
impact is not a bad thing, if it's only a few degrees.
Too high a heft, and the release will be too slow -- resulting in the
residual wrist cock being more than just a few degrees.
Too low a heft, and the release will be too fast -- resulting in a
cupped wrist at impact. The usual result is thin hits and pulls.
Matching across the
set
Even if you find a swingweight and/or MOI that seems to suit the golfer
at
some club length, you still don't know how to match the clubs across
the set. Look
at the ball position
as the golfer plays different-length
clubs. A constant ball position is a strong indicator that MOI matching
will work well for the golfer. A ball position that is back for shorter
clubs and forward for longer clubs suggests a "sloped" or "progressive"
moment of inertia; swingweight matching will often work well in this
case.
For
a while, I fitted like this. By 2009, I had a different way to
determine whether I should match by swingweight, MOI, or something
else. But you haven't seen enough of the principles to understand that
method yet. All will be revealed later in the section on heft fitting.
Computing Heft
Note: You can
skip most of this section if
you're not interested in the equations to compute swingweight and
moment of inertia of a club. I would suggest that you look at the
diagram and definition for swingweight, but even that isn't absolutely
essential. Things that follow later will depend on these equations --
but they will not depend on your knowing the equations. If you're
mathematically inclined then this section may help you understand what
follows, but it isn't essential to using those results.
That said, if you even think about using these equations, you'd
better read the words
that go along with them. The equations are approximate at best and
downright inaccurate at worst. They explain a lot of the behavior of a
golf club in terms of heft, but they don't replace a
swingweight scale when you're actually building a club.
Errors of a swingweight point or two are common, and errors of three
points are not very rare. So don't treat their answers as gospel.
Since my first design notes were published in 1993, people have wanted
to dive right in and use the equation for swingweight. Sometimes it
gave
good results, and sometimes it proved quite misleading; the latter can
be disturbing if you're spending green dollars on components for the
ideal
set of clubs.
Approximate equation for
swingweight
Let me start out by repeating the above warning. Maybe, if I
repeat it often enough, you'll get the idea.
This equation is only an approximation of
an approximation.
Use it at your own risk.
In the first place, swingweight itself is only an approximation
of the true "heft value" of a golf club. In the second place, the
equation
makes a number of simplifying assumptions, which I'll make explicit
below.
The warning out of the way, let's
start with an equation that's
as close as possible to the physical definition of swingweight.
- The weight of the head produces a CCW torque. The
force acts at the center of gravity of the head.
- The weight of the shaft produces a CCW torque. The
force acts at the center of gravity (balance point) of the shaft.
- The weight of the grip produces a CW torque. The
force acts at the center of gravity of the grip.
If we measure the torques in gram-inches (that is, we measure weight in
grams and distance in inches, and multiply them together to get a
torque), then:
- A "swingweight point" corresponds to 50 gram-inches.
- The base measurement of D-0 corresponds to 6050
gram-inches, or 121 points.
|
Given this basis, we can write the equation that sums all the
torques together. There's a little trigonometry involved, because the
weighing is done with the shaft horizontal, but the CG of the clubhead
is usually specified with the shaft at the lie angle. Here is the
equation we come up with, in a pretty straightforward way:
|
|
(Lc
+ X cos Lie - Y sin Lie - 14)*H + (a*Ls - 14)*S -
10*G |
|
SW = |
|
- 121 |
|
|
50 |
Where:
|
SW
|
=
|
Swingweight
with respect to D-0. That is:
if SW = 2, then swingweight is D-2.
if SW = -4, then swingweight is C-6. |
|
Lc |
= |
Nominal length of the club (inches). |
|
Ls |
= |
Length of the cut shaft (inches). |
|
H |
= |
Clubhead weight (grams). |
|
S |
= |
Trimmed shaft weight (grams). |
|
G |
= |
Grip weight (grams). Most grips
have a CG pretty close to 4" from the butt,
so 4" was assumed to be the placement of the grip weight.
|
|
Lie |
= |
Lie angle of clubhead. |
|
X |
= |
Distance on ground from shaft axis
to clubhead CG (inches). |
|
Y |
= |
Vertical distance from sole to
clubhead CG (inches). |
|
a |
= |
Shaft CG (also called "Balance
Point") distance from butt as fraction of shaft's length. |
This is quite precise, but completely useless for selecting components
from a catalog when designing a club. The problem is that there are
several
non-catalog specifications here:
- "X" and "Y", the coordinates of the clubhead's CG, are
never seen on a
spec sheet.
-
"a", the position of the shaft's CG, can be found in Dynacraft's
"Annual
Shaft Fitting Addendum", but not in their free catalog nor anyone
else's
spec sheet.
-
The difference between "Lc" and "Ls" depends on the shaft penetration
in
the hosel and the sole rocker, neither a spec sheet staple.
In order to be useful, we must necessarily be less precise. Let's
"assume
away" the non-catalog specs:
- Assume that "X", "Y", and the lie angle generally cancel
out so that the
distance from the butt to the CG of the clubhead is the same as the
nominal
length of the club.
-
Assume that the CG of the shaft is at its center. This is close to true
for most shafts; this gives an "a" of 0.5. But for tip-weighted and
some
tip-reinforced shafts, "a" can be greater; we'll ignore that in the
simplified
equation. (The recent Taylor-Made "Bubble" shaft also makes deliberate
changes in the shaft's CG, which we ignore in the equation below. For
that
reason, the equation will give wildly wrong swingweight predictions for
clubs made with Bubble shafts.)
-
Assume that the shaft length is the same as the club length.
-
Apply an empirical adjustment to the "121" in the precise equation.
Choose
this adjustment by "reverse engineering" the equation so that it best
fits
an arbitrary collection of clubs to which the equation was applied.
This gives us the equation:
| SW = |
Lc*(H + S/2) - 14*(H +
S) - 10G
50 |
-124 |
Where:
|
SW |
= |
Swingweight with respect to D-0. That is:
if SW = 2, then swingweight is D-2.
if SW = -4, then swingweight is C-6.
|
|
Lc |
= |
nominal club Length (inches) |
|
H |
= |
Head weight (grams) |
|
S |
= |
trimmed Shaft weight (grams) |
|
G |
= |
Grip weight (grams) |
Just a few more qualifications and caveats about this equation before I
give it a rest:
- To use the formula, you have to compute a trimmed length
from the shaft's
raw length and raw weight (which are in most catalogs). I usually make
the simplifying assumption that it's proportional to the trimmed
length.
That is:
|
cut shaft length |
| S = raw shaft weight
* |
|
|
raw shaft length |
Historically, this has been a decent approximation for most shafts.
However,
it doesn't accommodate tip-heavy shafts, which to be accurate must be
computed
by separately considering the tip-trimming and the butt-trimming.
- Most grips weigh within a swingweight point of 50 grams.
But not all grips. Some catalogs list weight among the grip specs.
- While we're on the subject of grips, The coefficient of
"10" for grip weight
is empirical, based on my measurement of the CG of a number of grips. I
haven't seen any that would depart from this by as much as a point, but
I can't swear they don't exist.
- I'm going through this in more technical detail than the
average clubmaker
should need to know. The reason is that, of all the things in the first
version of the notes, the biggest criticism from actual users came from
people who used the equation and found that the measured swingweight
was
different from the equation-predicted swingweight. This detail is here
mainly to convince you not to use the equation unless you understand
enough
of the assumptions to be confident that the equation applies to your
club.
So I repeat: now that you know the imprecisions, use
this formula at your
own risk. It's probably worth mentioning that virtually all the
"swingweight calculators" I've seen on the Internet use this equation.
Some reference these notes, others don't bother -- but they all use it.
Approximate
equation for Moment of Inertia
Here's a similarly approximate equation for the moment of inertia of
the club around an axis at the butt. MOI is the most physically sound
measure of heft, the quantity that resists turning the club from cocked
to released.
MOI = Lc2*(H
+ S/3) + 10*G
Where:
| MOI |
= |
The moment of inertia of the club
|
| Lc |
= |
nominal club Length (inches) |
| H |
= |
Head weight (grams) |
| S |
= |
trimmed Shaft weight (grams) |
| G |
= |
Grip weight (grams) |
Again, we have some notes about the approximation:
- The mass of the head is approximated to be a point mass at
the
length of the club. This is similar to what we did in the approximate
formula for swingweight.
- The
shaft is approximated as constant mass per unit length. This is similar
to what we did in the approximate formula for swingweight.
- The grip can be ignored. It contributes less that 0.2% to
the total
moment of inertia, and normal model-to-model variations in grip weight
will make a lot less difference than that. So usually we are
going to
simplify matters and leave off the grip weight term.
Last modified Oct 16, 2019
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