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 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

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.

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.
 
Swingweight is the torque produced by the weight of the club, about an axis (or fulcrum) 14" from the butt of the club. The diagram shows the forces involved:
  • 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 Feb 26,2013