Clubhead Geometry in Pictures (Drivers, Fairways and Hybrids)

This chapter will study the geometry of hollow-bodied or thin-walled club heads which comprise of drivers, fairway woods and the hybrid categories. For golfers that are new to the game, whenever they first pick up a driver head component by itself, the first thing they notice is the head is so lightweight compared to its size. Now, this is not always the case when we look back when woods were once made of wood or the first generation of “metal” woods that were foam-filled. To maintain the larger volume and same weight, the modern heads became hollow-bodied with their outer shells being extremely thin. We will explain exactly why later in this chapter.

Before we do, the first thing we need to do is explain the anatomy of a wood clubhead. A better term would be a wooden-shaped clubhead, which takes on the form of clubs that were once made from wood. Here is a look of a driver from all different views followed by a brief description of each part. These could very well be a fairway wood or hybrid pictured as the designations will be all the same. This will serve as a foundation for the remainder of the text.

The circled numbers show the relative positions of the club as they are referenced.

Anatomy of a Wood Golf Club

The sole of club (#3) is the part of the club that rests on the ground. On the modern wood-shaped clubs there is a radius from heel to toe to allow the club to be hit from various lies off the ground. Even though a driver is intended to be hit off a tee, a heel-to-toe radius still exists.

The heel of the club (#2) is the juncture between the sole and the hosel (#4), or the part of the club the shaft is inserted into. Traditionally and most commonly, you will experience a bonded club or one where the shaft is epoxied into a hosel. The other option is an adjustable adapter in which the shaft is epoxied into and the whole assembly is then inserted into a socket in the head and fastened with a screw from the sole.

The crotch of the neck (#6) is the juncture where the hosel and the crown (#5) meet. The crown is the part of the head you see from the address position and angled with the apex of the crown positioned forward of the center of the face. The crown will possess a radius not only from front-to-back, but from face-to-rear as well. The furthest portion of the club from the hosel is called the toe (#1).

The striking portion of the club intended to contact the ball is the face (#7). It is customary to see score lines (#8) on almost all wood-shaped clubs, although they may not extend the full length of the face.  They are present to help frame the ball at address as well as channel moisture between the ball and face, but there is no rule that there must be score lines present.

The face height is the distance from the ground to the highest point where the face and crown meet. Because the face is asymmetric, the highest point may be @ 0.5” toward the toe. The crown height is the measurement from the top of the crown to the bottom of the sole. Again, the highest point will be biased toward the toe. We will see the significance once we show the center of gravity locations.

Looking at the sole view we can see both the toe (#5) and hosel (#4) from another perspective. There are two edges of the sole. The edge adjoining the face of the club is called the leading edge (#1), while the part at the rear edge of the club is called the trailing edge (#2). On a wood-shaped clubhead, there is a horizontal radius called the bulge along the leading edge which we shall explain the purpose later in this chapter.

It is common that the rear portion of the head is not symmetrical with more material out near the toe to offset the weight of the hosel. Therefore, the distance from the leading and trailing edges is not measured from the exact center of the sole. The distance from the leading and trailing edges of the club is called breadth.

The area where the leading edge and the sole meet is called the sole (#3) of the club. This is the portion of the club that contacts the ground.

Our next view is from the heel. The reason to show this angle is to see that not only do we have radius from the heel of the club to the toe, but we have a slight sole radius from the leading edge (#1) to the trailing edge (#2). This is what allows the ball to be struck cleanly from a variety of lies. This will not be as pronounced as you find on irons and wedges because the width is so much greater.

On a wood-shaped clubhead, not only is there a horizontal radius called the bulge, but there is a vertical curvature on the face (#4) which is termed face roll or simply roll. This extends from the leading edge up to where the crown meets. An explanation with the purpose will ensue later in this chapter.

The area that wraps around the rear of the club below the crown (#3) and above the sole is named the skirt (#5). The rate that the crown tapers dictate how high, low or whether a skirt exists at all. You will also need to understand the leading edge of the hosel (#6) when we talk about the differences between face progression and onset.

Unlike an iron or wedge with a narrow sole where the club is set to a square position to reference the loft, a wider shaped clubhead like a driver will want to naturally rest on the ground into a certain position. This is what we will call its “normal soled position.”  As the modern club has a radius sole, there is a little “wiggle room” as to what the normal soled position is compared to the days when the soles were nearly perfectly flat.  That is, have two people measure a club in the same apparatus and you might get two slightly different readings.

With the shaft perpendicular to the spec gauge’s base and the club sitting in the normal soled positioned a protractor is used to measure the loft. The angle created by the club face (the dotted line in the diagram) is the loft. Because most wood-shaped clubs have a curvature on the face from top to bottom (called roll), it is imperative that the loft is measured at the center of the club face with the loft protractor. Any readings below the center of the face will result in a lower loft reading while above produces a higher number.

The importance of having various loft angles is of major importance for trajectory control. Simply, the higher the loft (given all the same parameters elsewhere) the higher the ball will leave the clubface. Golfers who need assistance hitting the ball higher will require a more lofted clubhead, while those hit a naturally high ball will need a model with reduced loft.

Contrary to some manufacturer’s claims, the actual loft cannot be changed on say a driver by reinstalling the position of an interchangeable adapter (at least at the time of writing this). The club will still sit in its “normal soled position” unless the manufacturer addresses this to work in concert with a change in the adapter setting. The only specifications that often change with adjustable adapters are the lie and/or face angle. 

The lie published by most manufacturers use an industrial specification gauge to make sure the club is properly registered on the base. However, most clubmakers do not have access to one of these and will often use a lie/loft bending machine instead where the clubmaker will use the score lines as reference. Often the score lines on a wood (whether a driver, fairway, or hybrid) may not be parallel to the ground but running uphill or tilted toward the toe by 1.5 – 2° (as shown here). By using a lie/loft machine and setting the score line parallel to the base, the clubmaker could get an erroneous reading where it will be more upright than the manufacturer intended the clubs to be. 

Face Angle

The face angle is the position of the clubface relative to the intended line of ball flight. There are three different terms that can describe face angle. The simplest is a square face angle or where the clubface is aligned directly toward the target. A clubhead that is pointing right of the target (assuming a right-handed clubhead) is considered to have an open face angle. Conversely, a clubhead that is pointing left of the target (assuming a right-handed clubhead) is considered to have a closed (sometimes called hooked) face angle. The angles are expressed in degrees and the differences are very subtle, thus the reason precision gauges are required to measure them accurately.

This is the reason for various face angles. Golfers who push, fade, or slice the ball would benefit from using the same head but with a more closed clubface to what they are already using to reduce the severity of the missing the ball to the right of the target (again assuming a right-handed golfer). For those who pull, draw or hook are better off with the same head but with a more open face angle if they wanted to hit more toward the intended target line. To understand the ball flight pattern relationships, review Chapter Four: The Basics of the Swing.

By doing these things it becomes difficult to remember and duplicate exactly what was done in the previous swing to build up a reliance on exactly where the ball might go on future shots. This is the reason for various face angles or the ability to change them with an adjustable hosel adapter. Remember a person should not have to adjust to their equipment; the equipment should be fitted and/or adjusted to their natural swing.

Effect Loft - the relationship between loft and face angle

There is a phenomenon called “effective loft,” which is a relationship between the club’s loft and its face angle. With the introduction of interchangeable adapters to not only change out shaft shafts, but the face angle and lie angle too, it will be important for club fitter to understand the relationship between loft and face angle.

Effective Loft: The Clinical Viewpoint

 I want to first address how effective loft has been taught in many previous club fitting books. We explained how face angle refers to the direction of the club face at address in a spec gauge, but it can also be referred to at impact we referred to previously as face attitude. Effective loft is better defined as the wood’s loft when the club face is placed in a square (0 degree) position, whether the actual head is square or not. Perhaps the best way to review effective loft is through a series of examples of face angle and loft combinations.

How much? We say approximately the same corresponding amount to the number of degrees it was closed to move the face to the 0-degree square position. In actuality, it is slightly less than 1 degree, but for simplicity’s sake we will use a 1:1 ratio. You will need to look very carefully but note how the leading edge is now closer to the ground while the trailing edge is higher because of hooding the clubhead. At this point, the club is no longer in its normal soled position.

Again, note how the leading edge is now raised off of the ground while the trailing edge is lower as the club is no longer in its normal soled position. No better example can be found when we look at a wedge. We do not talk about the face angle on an iron or wedge, but we assume them to be square. By rolling open the face of a wedge it in effect has greater loft than in the square position.

In the following chart, we have a modern driver with an adjustable hosel or sleeve system with a 2° off bore angle. You will see a more on this in the Driver Fitting chapter, but this will at least demonstrate effective loft and the possible 8 positions this particular adapter can fit inside of the driver head.

While the loft is the same, the effective loft changes due to the face angle differentiation and the necessity to close or open the face to get the club to the square position. Often the manufacturer of these type of systems will offer their driver in multiple lofts even though each loft already creates multiple “effective lofts.” 

If three different loft options are available in the eight possible positions the hosel adapter can be placed in each lofted head, then there are a total of 24 combinations as we have here.

As you can see there is only a 2.5° difference between the lowest and highest loft available, but when you factor in the face angle into the equation, in this example the effective loft now has a range of 6.5°.

Now you are more aware of the effective loft phenomenon caused by the relationship between loft and face angle. The clinical definition of effective loft is the measured loft when the face is in the square position only and the shaft perpendicular to the ground, whether the actual face angle is square or if the clubhead must be manipulated open or closed to set it into the square position.

Effective Loft: The Reality Viewpoint

The clinical approach tells us that a 9° loft driver with a 3° closed face will have an effective loft of 12 degrees, which would have the same effective loft as a 13° loft driver with a 1° open face. This leads a person to believe the ball trajectory would be the same since we are saying they have the same effective loft. Let’s face it, the average player is not going to take a club that is 3° closed and hit the ball with the face square toward the target and then pick up the exact same club with all identical specifications other than it is 1° open and 13° loft and again hit it with the face angle square to the target on the very next shot.  And I almost guarantee that the ball trajectory coming off the face of these two clubs will not be the same either.

Effective loft assumes that the clubface is square and not what happens at impact!

It is important to realize that the loft and face angle at address will not be the same as at impact. This has to do with several factors such as angle of attack, swing path, wrist rotation, etc. compounded by the bending and twisting of the shaft as it comes into impact. Therefore, it is important to understand the concept of effective loft, just not always in the square position. We should always be more concerned with effective loft at impact (or what we call face attitude) anyway.

Have you ever heard the phrase “high fade, low draw” when a player discusses shot shape? Let us say a more accomplished golfer tees off with these two clubs mentioned in the previous example. With the ball positioned in the same location relative in the stance, same tee height, etc. then with the 9 degree loft / 3 degree closed RH driver, the ball will be propelled off of the face both lower and left as compared to that of the 13 degree loft / 1 degree open RH driver.  It would take some manipulation on the part of the golfer to square both clubs at impact so the “effective loft” at impact was identical.

Understand that you adjust the face angle to control direction. A byproduct of this will be a change in ball height. No better way to illustrate this is with the clubs on the market as all the factors with adjustable shaft adapters, other than face angle and lie remain identical.

We are assuming that a player leaves the face open at impact when they slice the ball. So, in order for the ball not to slice quite as much, the face angle must start out more closed. By selecting a more closed face angle position, this will not only straighten out the shot but lower the trajectory too.

Clubhead Volume 

Volume is simply the numerical designation given to the size of a club head (normally a driver, fairway, or hybrid) as measured by liquid displacement. Measuring the volume of a clubhead can be made in one of two ways. The easy method is to take a beaker of water that has a wide enough opening for the head and fill it with water to a specific level, like 1000cc or 1500cc to make it easy to figure out the displacement of water. Make sure that once the clubhead is placed into the water that the head can be fully submerged and that the water will not overflow the beaker.

Next, submerge the head up to the base of the hosel (if one exists) as the hosel (according to the USGA) is not considered part of the volume of the head. The water will rise inside the beaker as the clubhead displaces the water. Take note of the new volume in the beaker as you will have to subtract the starting amount of water inside the beaker from that measurement. Let us say the original amount of water inside the beaker was originally 1500cc. If the club was submerged up to the base of the hosel and the new displacement became 1950cc, this means the volume of the head is 450cc.

The use of water to make a volumetric measurement makes it easy as one cubic centimeter (cc) equals one gram of water. Set the beaker of water on an accurate gram weight scale and do the exact same method as above by submerging the head. Instead of looking at the volume displacement by reading gradients on the beaker, now we will look at the change of weight on the scale. Many electronic scales have a tare button which will zero out the weight on the scale. All you have to do is submerge the head, steady it, then read the new gram weight on the scale.  If the weight is 453g, then the volume will be 453cc.

History of Golf Clubhead Volume

One of the more remarkable achievements in golf club design is the evolution in the size or volume of the metal driver heads in the brief time span they have been in existence. Most golfers that are new to golf may think that metal woods have been around a long time – and they would be partially correct. The first British patent on a metal wood was issued in 1891 (Currie Metalwood) and several early attempts to make heads out of metal, such as aluminum, had been tried but none really stuck. The very limited usage of metal for a wood was in driving range clubs for its durability.

Someday it may only be a small footnote in the history of golf, but it was in 1976, John Riley of the Pinseeker Golf Company designed the first stainless steel metal wood called the Bombshell, although it looked more like a modern-day hybrid than a metal wood. Short-lived because of its torpedo-like shape, it paved the wave for other manufacturers to produce “wood” heads made from metal.

The late Gary Adams is considered by many to be the “father of the metal wood.” But it was a fellow co-worker at TaylorMade, John Zebelean, a Yugoslavian nuclear physicist, who is credited with creating the modern shape of the metal wood. In 1979, their design that Adams would aptly name the Pittsburgh Persimmon would reshape the industry forever and within only a decade.

The early metal woods of the 1980s began to catch on because they were easier to hit due to their superior perimeter weighting, less labor-intensive to produce and were more durable than wood. Another advantage was that each club was nearly identical to one another. A golfer did not have to go through a barrel of clubs to find one just like the one they lost or broke. There were also more loft options available to golfers.

In the late 1980s, the aerospace industry in Southern California was declining rapidly. Engineers and scientists began seeking employment for local golf club companies who resided in the area. These companies, even a little company at the time named Callaway Golf, invested millions of dollars in research and development. By 1988, sales of metal woods surpassed those made of wood.

Metal wood size was slightly smaller (150cc) than that of the wooden drivers (195cc) at the time. The volume remained relatively unchanged until 1991, when Mid-size metal woods came into existence.  These “larger” sized heads were about the same size (185-190cc) as the wooden driver. Part of the change to the size might have been spurred on as golf clubs made of lightweight carbon fiber were increasing in popularity by their larger volume (up to 230cc) and increased playability.

A milestone of sorts came in Japan (1990) as Mizuno created the first ever titanium driver called the Ti-110. Titanium had been well known for its high strength, yet lighter weight compared to steel. The extremely expensive price tag made this an extremely limited product to consumers. Driver heads produced from titanium did not become popularized until 1995 when the Callaway Great Big Bertha and the TaylorMade Titanium Bubble were introduced. These enormous sized heads were among the first heads to crack the 250cc barrier in volume. It took just 4 more years for Callaway and the rest of golf industry to make drivers that were 300cc. 1999 was the same year manufacturers were able to make stainless steel drivers with a 250cc volume with lighter and stronger alloys than 17-4 stainless steel as had been used prior.

In golf, progress is usually measured by decades, but in the case of driver head size and the ability to produce these clubs out of lighter, yet stronger materials, size grew exponentially in only the next few years. As a timeline, the year 2000 saw the first 350cc driver, followed by 2001 with a 400cc driver and finally a 500cc driver was made in 2002. It was well known at this time that the larger driver would have a higher moment of inertia and subsequently makes it easier to hit the ball straighter even on off-center shots. At this point, the USGA stepped in and began to propose limits on drivers as they were potentially seeing technology threaten to diminish skill level. So, in October 2003, the USGA imposed a 460cc limit on clubhead size effective January 1, 2004.

Since 1980, the modern 460cc titanium driver has tripled in volume and increased in size by an average of 1” taller from the bottom of the sole to the top of the crown and 1” wider from front to back.  Not coincidentally, tee height has gone from 2 1/8” to 3 ¼”– nearly the same 1” difference. Yet at the same time, the fairway wood had gone relatively unchanged. Why? This fact may be due to the driver being facilitated by the use of a tee and the fairway wood’s primary use is off of the ground.

In comparison to size, the volume of a traditional wooden #3-wood is approximately 150cc while the first series for metal woods had the volume closer to 120cc. As mid-sized metal woods came into existence, the #3-wood grew to approximately 160cc. This is where on average the average volume (+/-30cc) has stayed ever since. 

The only exception was around 1998 when you saw clubheads that fell beyond these dimensions.  First, there were extremely shallow face height woods, following the lead by the Adams Tight Lies that the #3-wood was in the 115cc range. These extremely playable clubs had one downside and that was they were perhaps too shallow that a golfer could run the club underneath the ball in the deep rough during the swing.  To the other extreme, the late 1990s saw how popular oversize irons were that it expanded into oversized fairway woods too. Some titanium and stainless steel #3-fairways grew as large as 230cc. These were fine off the tee, but to the average player, nearly impossible to hit the ball off of the ground because of the extremely high center of gravity.  In years following, most #3-fairways gravitated back to the 130-190cc range.

Now that we have seen a cap on the volume of a driver become established at 460cc by the USGA and the R&A of St. Andrews, manufacturers are going to now concentrate on how best to utilize that volume to maximize performance. In much the same way that John Zebelean, with the assistance of Terry McCabe of TaylorMade came to create the modern “wood” head shape from that of a wooden wood, there will eventually be a new shape that will be accepted by golfers across the globe. 

Volume: Rules and Design

Now that we have established the different methods of measuring volume, we will look at the importance from a design standpoint. Since the USGA and the R&A of St. Andrews established a volume limit on the head, every manufacturer is making drivers that reach this size limit. There are also a few other restrictions in the Rules of Golf that apply to the size of a driver. These are the following from Appendix II. 4. Clubhead, b. Dimension and Size:

(a)    The distance from the heel to toe of the clubhead is greater than the distance from the face to the back;

(b)    The distance from the heel to the toe of the clubhead is not greater than 5 inches (127mm); and

(c)     The distance from the sole to the crown of the clubhead is not greater than 2.8 inches (71.12mm).

If you closely examine a clubhead, you will notice all the angles, contours, and round corners. These in effect limit the volume as illustrated above. To make this or any driver conform, a lot of material must be removed to achieve 460cc.

While the maximum 2.8” dimension from the ground to the top of the crown and the 5.00” breadth or front -to-back dimensions are self-explanatory, the other dimension is not. The USGA has a specific test to measure whether the clubhead falls within the 5” limit from heel-to-toe.

C.O.R / C.T: Rules Regarding Springlike Effect

Another advantage of titanium (or other specialty materials) is that it allows the face to be thinner resulting into what is known as “rebound” or “trampoline” effect. The more a face flexes upon impact, the ball compresses less resulting into less energy lose and a slightly higher ball speed coming off the face for increased distance. While this chapter is devoted to woods, the same principles apply for any clubhead in the bag.

C.O.R. (short for coefficient of restitution) is the ratio of the relative speed after the collision of two objects to the relative speed before the collision. In 1998, C.O.R. instantly became part of the lexicon in a golfer’s vocabulary and has been used ever since. The coefficient of restitution is a number between 0 (perfectly inelastic collision) and 1 (elastic collision). To give an example, imagine a golf ball being propelled against a rigid steel plate at 100 mph. If the ball bounces back at a rate of 80 miles, then the C.O.R. is 80/100 or 0.800.

One of golf’s ruling body, The United States Golf Association (USGA), set standards for how much a head can rebound and still conform to the Rules of Golf. In 1998, the USGA set that limit to 0.822 with a test tolerance of .008 effectively taking the limit up to 0.830. The R&A adopted a different set of rules and did not implement the 0.830 limit until 2003 for highly skilled players and later for everyone on Jan. 1, 2008.

Face Progression & Onset

In the previous chapter we discussed offset along with two other terms: face progression and onset. The principles will be the same but let us show them in pictures on a wood shaped object. 

There are cases that a golf club may not have a hosel (like an older style Callaway driver) or have a tapered or asymmetrical hosel like the “wooden” woods of the past. So, it may be too confusing (if at all possible) to use some part of the hosel as a reference point to measure offset or onset like we do irons and why face progression remains the more accurate reference point.

Wood Head Center of Gravity

To find the center of gravity on a wood shaped head is a matter of balancing (sometimes very patiently) onto a small object or a commercially available clubhead CG marker. Unlike what we saw in the previous chapter for irons and wedges, the surfaces of the wood are all curved. That is why I like to rely on CAD programs to tell me exactly where the CG is located. Here is an example of what it might look like in three different views.

There are generalizations we can make about the CG location of a driver and quite possibly a #3 fairway wood. Looking at the face and heel views you can see the CG is approximately half the crown height although it might be slightly higher.  You have a crown, which is wider than the sole, plus the additional weight of the hosel. This is why manufacturers are concentrating so much on reducing as much weight as they can by making the crown thinner or using carbon fiber inserts attached to the crown. But there are limitations to how thin it is and still make the clubhead durable as well as produce an acoustical sound at impact that is pleasing to most golfers rather than a high-pitched sound. As far as the other fairway woods and the hybrids, as the loft increases the soleplates are cast heavier and a greater share of the weight begins to shift lower.

The view from the heel and sole shows the CG is not located half-way from front to back. This is due to the fact we still have the weight of the hosel, plus the forward portion of the club is higher than the back. Furthermore, the face is much thicker (approximately 3mm) to withstand the forces at impact helping further shift that weight forward. In comparison, the crown of a driver might be approximately 0.9mm thick (possibly less in certain areas) and the sole and skirt 1.2mm as those areas aren’t designed to withstand an impact with a ball.

There is one other position the CG can be and that is located between the toe and the heel plane. We will discuss that a bit later when we address clubhead bias. To do so, we must cover a few other topics first.

Moment of Inertia

One of the buzzwords in golf is the term Moment of Inertia (MOI for short). This can be a very confusing subject to discuss, let alone when we are talking about something with a complicated shape like a golf club. Let us break MOI down to plain language in order to make it just a little easier to understand.

Moment of inertia is a measurement of the clubheads ability to resist twisting about a known axis with a higher MOI being a strong indicator to the forgiveness of a clubhead. At impact, the golf ball is not always struck in line with the center of gravity, resulting in the head twisting and consequently consuming energy that could be imparted to the ball. A club with a higher MOI will resist twisting more so than another head with a lower MOI value.

Drivers went from being solid and made from persimmon or laminated maple to metal in the early 1980’s. Even though the sizes of those heads were approximately the same or slightly smaller, the metal woods increased their MOI over their wooden predecessor by about 25%. This is because the metal heads were not solid, but a hollow shell with a very lightweight density foam injected inside to dampen sound. All the weight that would have been in the center of the head was redistributed to the perimeter, thus the term “perimeter weighting.” Since the advent of the petite size metal drivers of the early 1980’s, the MOI has increased nearly three-fold in modern drivers.

To cap technology and to not diminish the skills of the golfer, the USGA put a limit on the moment of inertia of a golf club and assigned a value to it (5900g-cm2 or 32.259 oz-in2). To obtain this type of measurement, precision instruments are available to accurately measure the MOI of a clubhead, usually taken at 6 or 9 measurements about different coordinates. Moment of inertia can be measured in several planes. To illustrate this, we will show an example of a cuboid or rectangular sphere as it is the closest shape to the boundaries of a golf clubhead.

Referring to the volume discussion for a moment, the maximum dimensions a clubhead (primarily a driver) could be is 5” (12.7cm) by 5” (12.7cm) at the base.  If we wanted to achieve a 460cc volume (28.05 in^3), then the maximum height could only be 1.12” (2.85cm) tall. Since most drivers are roughly 200g, we can input the variable into the equation and the moment of inertia about the Y-axis would be 5376g-cm^2 or approaching what the USGA limits are.

The weight of the golf club has tremendous effect on the moment of inertia. Heads that are heavier and at the same dimensions would automatically have an increased MOI. So why not simply add more weight? To reach the limit of 5900g-cm^2 then all you would have to do is increase the weight to approximately 219g using the dimensions above. The problem is that is about the same weight as a #5 wood, thus would be proportionate in length unless the person wanted to wield a club that was very head heavy at today’s driver lengths.

 The shape of our example was used because it was simple and one that it easy to measure the MOI. Obviously, clubheads are much more rounded and asymmetrical and would not apply exactly to the formula. However, there is the “Plain in Shape” rule by the USGA, and that is the reason most drivers (regardless of manufacturer) take on a similar shape, just the proportions in height, length, and width (breadth) are different. To increase the MOI the manufacturer needs to better utilize the shape, like in the direction of the “square-shaped” club heads that were first popularized by Nike around 2007.

The other method of increasing the MOI of a clubhead is reducing the weight as much as possible by making the walls as thin as possible without risk of breaking under normal conditions. The modern 460cc titanium driver already has paper-thin walls in the non-stress areas of the head; therefore, a foreign material lighter than titanium may be incorporated into the head. The most common example of this is carbon graphite, usually in the crown of the club. This weight savings is called “discretionary weight” and gives the designer more material to re-positioned in strategic locations within the head; most often lower and rearward in the modern clubhead, but more importantly away from the center of gravity.

The same applies to the fairway woods and hybrids and the reason you see club heads which have a much wider breadth than those same models a decade ago. Remember though, when changing one dimension, such as the breadth of the head, then there can be a compromise in another feature such as face height, face area or MOI in another plane. Regardless, manufacturers are attempting to eliminate as much weight possible and redistribute that weight were it can be utilized best in order to make your miss-hits even more playable.  But do not be disappointed that drivers today may only have a MOI of 4500 g-cm2 compared to the maximum allowable (5900g-cm2) as consumers are still receiving a very forgiving head.

Offset drivers, fairways, and hybrids will place the center of gravity further behind the centerline axis of the shaft and create the greatest gravity angles. Higher lofted hybrids and fairway wood with more face progression will show the least gravity angles and, in some cases, none at all. Luckily, both those are often much shorter clubs and easier for a golfer to square up at impact.

A Brief History of Face Curvature

It is important to understand that over the 500 or so years golf has existed, there have been numerous experiments conducted to create the perfect golf club. Look through golf history books and you can find plenty of clubs early on with concave or “hooked” faces. Yes, that’s right! This is just the opposite of what you find today.

Determining the Correct Amount of Face Curvature

After the “bulger” clubs became the norm, the amount of face curvature incorporated in the design was most likely selected by trial and error or simply how good the club looked at address. But in 1941, John Baymiller and Robert Vose of A.G. Spalding Bros. Inc. received a patent (2,395,837) on clubhead design to minimize the ball flight dispersion from hitting out near the heel or toe of the face of a golf club. Through their extensive testing, they found a correlation suggesting the proper amount of bulge on the face, depending upon how far back the center of gravity was within the head.

Vertical Roll

In much the same way bulge works in the heel-to-toe (horizontal) plane, roll works in the sole-to-crown (vertical) plane. The proper amount of roll helps to correct the launch angle for shots hit high or low on the face relative with the center of gravity factoring in the launch angle plane of the ball. Remember that the measured loft will vary on the face with any club with roll. Take a quick moment to examine the following diagram.

 The first thing to understand is back spin will occur because of the loft of the club. This occurs regardless if the ball is hit in the center or high or low on the face. Now, let us take a moment to explain each scenario. 

Unlike the bulge, where there is no clubhead sideways rotation when the ball is struck in line with the horizontal center of gravity, the clubhead will experience a slight downward force when struck in the same plane as the vertical center of gravity. This causes the clubhead to rotate in a clockwise motion (as pictured). This phenomenon called the vertical gear effect imparts an opposite spin (back spin in this case) to the ball, which is added on top of the back spin provided by the dynamic loft at impact. 

Bulge and Roll in Concert

Seldom will one strike the ball directly in line with the center of percussion in either the vertical or horizontal planes. Therefore, one will experience both a certain amount of backspin and side spin (or tilting of the spin axis). The amount will be determined by the quadrant on the face and how far impact is made in relationship to the center of gravity.

One word of note, the center of the face is not necessarily the location of the center of gravity as we shall see shortly when we discuss “draw biased” clubheads. Lastly, because of the manufacturing processes there will be a slight tolerance of +/-1” to face bulge and roll. For instance, if the specification is 10”, then radius could be anywhere from 9” to 11” and be within tolerance.  But this is well within the limits not to unduly influence the ball flight but could explain why one might like one club over another, even though they are supposed to be identical in all other aspects. Plus, it is not a fitting factor. For starters, manufacturers do not offer their models with various bulges and rolls the same they do with loft. It is up to the manufacturer to design the proper amount of bulge and roll into each of their designs.

Examining Driver CG Location

Have you ever wondered where the center of gravity of a driver really is? Well, take a close look at the driver (or fairway wood or hybrid too). What you will find is the clubhead shape is very asymmetrical. Upon close inspection you may find there is a little more area on the face out toward the toe or the highest point on the crown is not in the center, but also biased toward the toe. Lastly, the rear portion of the head may also be biased out toward the toe rather than the center.

Draw Biased Clubheads

Most golfers assume the best place to hit the ball is in the center of the face and that would be a valid assumption. In a neutral biased clubhead, the center of gravity will be in-line with the centerline of the face as shown in the diagram below.

When discussing the phenomenon called “gear effect,” we stated that a shot struck in-line with the horizontal center of gravity would not have any side spin. However, any ball struck out on the toe side of the center of gravity would create hook spin. Any ball struck on the heel side; slice spin will occur.

A draw biased clubhead is where the center of gravity is shifted toward the heel. This would be true for any clubhead where the center of gravity is far enough behind the face to produce a gear effect, like in the case with drivers, fairway woods and many hybrids. 

There is a good chance that many of the drivers that have been intentionally made to be draw enhancing will also be closed face, further reducing the likelihood the ball may be pushed, faded, or sliced since that is what the majority of golfers do. In some cases, the manufacturer may choose draw enhancing on a square faced clubhead to give the appearance many golfers prefer, but with the game improvement benefit of a slightly closed club face. Offset drivers can be draw enhancing by the nature of their design. Often the longer hosel length and the extra weight required to form the offset or gooseneck hosel naturally shifts weight toward the heel.

There can be a compelling argument that if impact is not made in line with the center of gravity, then some energy will be lost in the collision. True, especially if the golfer just struck the ball in the center of the face and they feel they should be rewarded for that. But if the ball is slicing and heading towards the deep rough, tree line or worse yet – OB, then that becomes a moot point. This is where the aid of internal weighting and the shift in the CG is another benefit. Plus remember that a draw will produce a lower launch angle and greater run when it hits the ground, which can lead to potentially greater distance.

Clubhead bias can also be a result of any screw weighting that might be an option by the manufacturer. We will go more in-depth when we discuss this in the Driver fitting chapter, which will have the most profound effect.