Heft of the club: Swingweight & MOI

What Is It, And Why Do We Care?

Feel: specifically, how head-heavy is the club?
Performance: specifically, how does the placement of mass affect the timing of the release?

Q. Now that doesn't sound so hard. Why not just make all the clubs the same weight?
A. Well, you can do that and still have major differences in the balance point. Total weight alone doesn't reflect what the golfer feels nor what the club does.
Q. OK, then. Make all the clubs with the same head weight, and the same shaft weight, and the same grip weight. That should do it.
A. Well, it would if all clubs were the same length. The real problem is that there is a length progression across the set. As you make the club longer, it will feel more head-heavy and will release differently.
Q. Oh! Well then, how about making the head lighter as the club gets longer.
A. Exactly! But how much lighter? That is the subject of this section...

Swingweight is an attempt to quantify the heft of a golf club, as it affects both the feel and the physics. Let me emphasize the word attempt!  Swingweight is not a magic quantity that can be shown by physical laws to have anything whatsoever to do with the things it would like to measure. It is, rather, an empirical approximation with an interesting history over a century old. Because it is so important to understand that swingweight doesn't represent anything really fundamental, let's start by reviewing that history.

Once upon a time...

History

By the early 1900s, clubmakers for professional golfers were already using mathematical formulas for matching their pros' clubs for heft across the set.  The formula they used was to match the product of the head weight and the square of the length.  Thus the longer the club, the lighter the head had to be, by about twice the percentage increase in length.

Let's take an example. Consider a 5-iron of the time: 37.5" long with a 255g head. If we made the club 1" longer (that's 2.7% longer), we would need to make the head 14g lighter (that's twice 2.7% of 255g). So, for each inch longer or shorter in the vicinity of a 5-iron, we'd need to vary the head weight by 14g. Since, for a normal set of irons, the club spacing is a half inch, the progression of head weight from each club to the next is a half of 14g, or 7 grams. Does that sound familiar? Now you know where it comes from.

At that time, clubs were made with hickory shafts and wound leather grips. There wasn't much you could do to change their weights, once the length and stiffness were determined. So none of the matching formulas included shaft or grip weight, because for all practical purposes they were fixed.

Now, think about the physics of such matching. Back in the chapter on Physics: moment of inertia, we saw that "the moment of inertia of each grain of mass is its mass times the square of the distance to the axis."  If the variation in shafts and grips could be neglected, then this formula made sure that the set matched for all clubs' moments of inertia about their butts.  The clubmakers of a century ago were building moment-of-inertia matched sets. (Well, almost. See note [1] below.)

Now, at that point, there was no such thing as swingweight; neither the measure nor the word was invented yet. But something was needed because the math was a little tedious. Remember, no electronic calculators or computers back then.

In the early 1930s, a clubmaker named Robert Adams invented the swingweight scale. It was a balance that measured the amount of torque the weight of the club exerted about a pivoting fulcrum.  The diagram shows a modern swingweight scale taken from the 2006 Golfsmith catalog, but it is basically the same instrument that Adams used alnost 80 years earlier. The weight of the club exerts a counterclockwise torque on the beam, because the center of gravity of the club is to the left of the fulcrum. The clubfitter moves the sliding weight until its clockwise torque balances the torque from the weight of the club. The position of the sliding weight then gives the "swingweight" of the club. Notice from the picture below (a scan from Adams' original patent), how little the design has changed over three quarters of a century.

After much experimenting, Adams concluded that a fulcrum 14" from the butt seemed to give the "best" match, in a subjective sense, for the pros he worked for. Why 14"? Did that correspond to some sort of "pivot point" in the golfer's swing? No, it was just a number that seemed to work; it yielded a set of clubs that Adams' clients felt were well matched.  (As we shall see later, this is not a perfect match to moment of inertia, but it's not a bad match at all.  So Adams-matched clubs would be a little different from MOI-matched clubs, but not hugely so.)  Adams' scale was used to match Francis Ouimet's and Bobby Jones' clubs, with obvious success.

Adams used an arbitrary letter-number scale (e.g.- "D-1") to measure swingweight.  That scale, which he called the "Lorythmic" scale, remains the most popular swingweight measure right up to the present.

Around 1945, Kenneth Smith bought Adams' rights to the swingweight scale, and began experimenting with it himself.  He came to the conclusion that the 14" fulcrum gave a good match for professional golfers, but a 12" fulcrum would produce a better set for the average amateur, which he called the "Official" scale -- even though the industry has never adopted it as official anything.  He was soon producing both kinds of scales.^[2]

So, by the mid-1900s, we have three approaches to heft-matching a set of golf clubs:

• The 12" so-called Official scale.
• The 14" Lorythmic scale (still the most popular).
• Moment of inertia (not much used by then, because it was so tedious compared with a swingweight scale).

The major difference among them is the amount by which the clubhead gets lighter as the club gets longer. Smith believed that the average golfer couldn't handle light long irons and woods, hence his proposed (and never really accepted) change in fulcrum placement.

For example, consider a heft-matched set in each of the systems, using the standard club lengths from the late 1900s (35.5" for a 9-iron and 43" for a driver).  Let's choose a common weight for the 9-iron head, and see what the driver head would weigh in a matched set.
 

System of Measure	9-iron head	Driver head
12" swingweight	284 grams	201 grams
14" swingweight	284 grams	195 grams
MOI	284 grams	180 grams

Heft and Release

I'd like to thank Bernie Baymiller for the strobe pictures of Bobby Jones. Bernie's father was the director of R&D for Spalding Golf in the 1940s, which is where and when the pictures were taken.

Here is a strobe picture of Bobby Jones swinging a driver. It was taken by Dr. Harold Edgerton of MIT, inventor of the strobe flash, and captures Jones and his club's position at intervals of about 0.007 second. I have taken the liberty of marking three positions of the swing with the "double pendulum", as follows: The red position, about 70 milliseconds before impact. Note that the wrist is still fully cocked at an acute angle. This is about where Jorgensen has identified the beginning of release, and Jones' swing corroborates that. The green position, about 20 milliseconds before impact. The wrist angle is much smaller, indicating that the wrists are well into the uncocking process. The blue position -- impact. The wrist is almost straight here, with just a little wrist cock left. This is a very good impact position; the club is just about fully released. So "release" is the process of the loss of wrist cock angle, from fully cocked during the downswing to almost straight at impact. And a club that is well-fit to the golfer will release so that the two arms of the pendulum are aligned at impact, with only a degree or two of wrist cock left.

In order to understand how to design the club for proper release, let's review the physics of the club's release. "Release" means rotation of the club about the wrist hinge. According to Newtonian mechanics, such rotation can only occur by the imposition of a torque on the club. The torque comes from two sources: Centrifugal force rotates the club about the wrist hinge. In addition, the golfer may apply some torque via through the wrist hinge, using the muscles of the wrists, hands, and forearms. This is a relatively small component of the release torque for most golfers, especially good golfers. Resisting this torque -- retarding the club from turning -- is the club's own moment of inertia around an axis at the wrist hinge. So, if we assume that the golfer makes the same swing -- applies all the same forces at the same times -- regardless of which club he is swinging, then it would appear that the way to match a set of golf clubs is to match their moment of inertia. That way, identical swings would result in identical release. So, if you find the correct moment of inertia for the golfer for some favorite club, you should build every club in the set to that same MOI. This is idealistic rather than ideal. Or, as my science teachers used to tell me, "The difference between theory and practice is bigger in practice than in theory." Here are some reasons that our argument for MOI matching may be too simple: Some golfers don't need or want identical release on all clubs. Golfers may not apply the same swing to all their clubs. The analytical model isn't quite right; the centrifugal force working to release the club depends not just on the golfer's swing, but also on some of the same properties of the club that affect the MOI. Let's address each of these points:

The need for identical release

Most instruction today teaches to place the ball in the same place in the stance, regardless of what club is being used. Usually, that recommended ball position is just inside the heel of the front foot (the left heel for right-handed golfers). For instance, see Butch Harmon's lesson article endorsing constant ball position. (Note: the links here worked at the time this was written. If it doesn't work for you, please contact me so I can find another page.) With the ball in a constant position with respect to the golfer's stance, an MOI-matched set of clubs should be ideal. All other things being equal, it will result in complete release occurring at the same position in the swing. If you can find an MOI such that the release position corresponds to the ball position, build all the clubs for that golfer to that MOI. Simple!

But constant ball position was not always the way golf was taught. In fact, it's a fairly recent development. Only a few decades ago, most golfers were taught to play the short clubs back in the middle of the stance, and move the ball forward as the clubs get longer. How do I know? I was taught that way in the early 1950s. And, since it works for me, I have not bothered to change. I'm sure there are many old fogeys like me, who still play that way. Not only that, there are a few instructors who still teach that even today. So what does that say about heft matching. The first thing we should notice is that we want an earlier release in the short clubs and a later release in the long clubs. We can accomplish this by making the MOI progress across the set, so it is lower in the short clubs and higher in the long ones. Another way of saying this is: the heads still get lighter as the clubs get longer, but they don't get lighter as fast. Now look at the table above, where we compared the three ways of measuring heft. Swingweight has the property we just described: as the club gets longer, the heads don't get as light for swingweight as they do for MOI. So swingweight matching may be good for a golfer who uses a variable ball position. Indeed, history also seems to support this. The popularity of swingweight scales with clubfitters dates back to the middle 1900s. And, at that time, variable ball position was the way almost all golfers were taught. So swingweight might have been exactly the right way to match clubs at the time. And it might still be the right way to match clubs for dinosaurs (like me) who still play that way. (Actually, I discovered this the hard way in my early experiments with MOI-matching in 1995. I am continuing to experiment with MOI matching and constant ball position; some day that may be my usual game.)

Different swings for different clubs

• A lot of modern instruction says to "hit down" through the irons, but to "sweep" the woods. This necessarily produces slightly different swings for irons and woods. In particular, release should not be complete for the irons; there should be more wrist cock remaining at impact than for the woods. Remedy: Match the irons to one MOI and the woods to another. Logic would say that the irons' MOI should be greater than the woods' (to retard the irons' release so there is still significant wrist cock at impact), but you have to determine this by experiment with the individual golfer.
  
• Different clubs obviously have different lengths. This necessarily results in different swing planes. This may result in different application of the muscles to produce the forces. Remedy: This is likely to be a smooth progression across the set, because the swing plane itself is a smooth progression. If so, some sort of "slope" on the swingweight or MOI may solve the problem.
• If the clubs don't feel the same (what ever that means, for that particular golfer), then he may change his swing a little in response. (We'll get back to this later in the section, when we talk about backweighting.) Remedy: The simplest way to deal with this is to work hard at making the clubs feel the same. Experience shows that MOI matching provides the most "same feel" across the set for most golfers. But not all; some golfers will be problems in this regard, and will have to be fitted on a club-by-club basis -- or perhaps a more drastic solution like a constant-length set.
  
• Golfers are inclined to think of different clubs in different ways, and swing them differently. Many will apply a different swing to the driver, because they are trying to kill the ball. Many will try to lift the ball with the wedges, rather than hitting down through it. Remedy: If you are trying to band-aid this problem by clubfitting (rather than fixing it properly, with lessons and lots of practice), you'll have to recognize it first, then experiment with clubs to find what works. But let me suggest the first experiment be increasing the MOI for the trouble clubs -- for both these faults.
  

Centrifugal force and MOI

• Making the club longer will increase the MOI. It will also move the balance point further from the wrist hinge, increasing the "moment arm" of the centrifugal force -- so the torque will increase.
• Making the head heavier will increase the MOI. It will also increase the centrifugal force, as well as move the balance point closer to the clubhead (hence further from the wrist hinge). Both of these effects increase the torque on the club.

• To the extent that the golfer "hits" (applies torque via the hands, wrists, and forearms), using MOI to match the clubs is the right answer for heft matching.
• To the extent that the golfer "swings" (depends on centrifugal force to produce clubhead speed), MOI matching does not hurt -- but the golfer is relatively immune to heft errors in the set anyway.

1. Just because it is fictitious does not mean it gives wrong answers. There are plenty of fictitious constructs in physics, that we use because they behave analytically as if they were real, and centrifugal force is one of them.
2. From a tutorial point of view, it is much easier for the non-physicist, non-engineer reader to understand. Here is a good explanation of the more classical physics. Not many people -- actually not even a majority of physicists and engineers -- would be able to visualize how release works from the diagrams and equations in the reference. You have to be able to mentally step through the Digital Differential Analyzer and see what the output would be.

Notes:

1. In 2008, André Cantin pointed out that the shafts are different lengths, and so will have different weights. As we can imagine, this increases the moment of inertia of the longer clubs relative to the shorter ones, because there is additional shaft weight at the tip in the longer clubs. I didn't think it would matter much, but did the calculations to see. As it turns out, there is enough of a difference to measure. In fact, it appears that the match is just about halfway between a moment-of-inertia match and a 14" swingweight match. Basic information for the calculations:
• The specific gravity of hickory is about 0.7. (That's a density of 0.7 grams per cc.)
• The hickory shafts of the time had a diameter of about 0.45" near the tip. (See Wishon & Summitt.)
2. Among the information that Dave Miko shared with me was some interesting correspondence between Kenneth Smith and Lloyd Rittenhouse, an engineer who got sucked into a discussion of how to convert between Smith's "Official" swingweight and the more common Lorythmic swingweight. The discussion, which went on for a year in the 1976-77 period, centered on Rittenhouse's unpopular assertion that there is no one-to-one correspondence; it is a function of the swingweight and the total weight. Eventually Smith came to see it the same way as Rittenhouse, and Lloyd published a technical paper (I don't know where it was published -- might have been just a private correspondence) on the conversion. Here is the bottom line, the conversion chart from Rittenhouse's paper.