Clubhead Geometry in Pictures (Irons and Wedges)

While there is no substitute for precision gauges and instruments to measure clubheads accurately and quickly, the powers of observation can go a long way into understanding what the club head designer intended a club to do. They say a picture is worth a thousand words and that might be an understatement. What I want to do is show you what to look for if you are not equipped with all the latest measurement tools.

The first thing we need to do is explain the anatomy of a clubhead. Here is a look of an iron or a wedge from all different views followed by brief description of each part. This will provide the foundation for the remainder of the text. The circled numbers show the relative positions of the club are often referenced. 

Anatomy of a Golf Club Iron

The sole of club (#3) is the part of the club that rests on the ground. On the modern iron there is a radius from heel to toe to allow the golf club to be hit from various lies off the ground.

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. The crotch of the neck (#6) is the juncture where the hosel and the top line (#5) meet.

The top line is the part of the head you see from address and will be angled with the part furthest from the crotch or called the toe (#1) is the highest. In many cases the top line on the #3 through #7 irons will look straight (there is a slight radius), while the #8 and 9 iron as well as the wedges will be more rounded.  The rounding provides for a taller face height or face profile from which to hit the ball.

The toe height is the distance from the ground to the apex of the top line. The hosel length is the measurement from the top of the hosel to the heel where it intersects the ground line. The hosel length can vary from longer in wedges and clubheads designed for better players to shorter for game improvement models. We will see the significance once we show the center of gravity locations.

Blade length is the measurement from the heel where it intersects the ground line to the outermost portion of the toe. The blade length can vary as well and goes hand-in-hand with hosel length as one gets longer the other becomes shorter.  The reason for this is we are working with fixed weights. A typical #6-iron is 263g and you can only stretch one measurement without increasing its weight.

The score line area is the place that is designed to first frame the ball at address. You will see that it does not extend the full length of the blade length or even the entire flat area of the face. The center of the scoreline area is the focal point and most golfers would get the notion that the center of this area is the optimum position to make impact with the ball. However, this is not always the case. The score lines are present to channel off any water or grass juices between the face and the ball to reduce the ball skidding up the face.

The toe show is the area out on the toe of the face of the club that is devoid of score lines or decorative sandblasting. By adjusting the distance of the toe show, this can have an impact on where you are to aim the club. When the toe show increases, it actually moves the center of the scoreline area closer to the hosel. 

Looking at the sole view we can see both the toe (#5) and hosel (#4) from a different 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). It is common to see that the sole is tapered from heel to toe with the toe being the widest. This helps to offset the heavy weight of the hosel. Therefore, the sole width or distance from the leading and trailing edges is generally measured from the center of the sole.

The area where the leading edge and the sole meet is called the neck (#3) of the club. It is not unusual to see a gooseneck appearance as this is what forms the offset of the head. We will be discussing offset in more depth later in this chapter.

The next view shows the back of a typical modern cavity back iron. The hole in the center is an open cavity (#3). The shape will vary from iron to iron, but the basic principle is the same; to remove material from the center of the club and move it to the perimeter area - hence the term perimeter weighting. The area carved out the back of the iron is then displaced around the perimeter of the club or more specifically to the heel (#1) and toe (#2) regions of the lower portion of the sole. This is what creates greater stability for the club when impact is made other than directly in-line with the center of gravity of the head. 

There are two types of irons heads that do not possess an open cavity. One is the oldest form of iron construction, which is referred to as a muscle back or blade. Today, very few irons on the market are produced this way as they offer little in the way of game improvement. However, most wedges sold separately of iron sets will be a muscle back design. These will have narrow soles and a compact blade length.

The other type of non-cavity iron is the two-piece hollow-bodied construction. These clubs are often oversized and possess a bulbous shape and wide sole. These are some of the forerunners to today’s hybrid.

Cavity back irons may have more than one cavity or recessed area, but often that additional cavity may not be visible at first glance. The secondary cavity may be located behind the rear flange (#5) inside the cavity. This is called an undercut cavity (#4), and it is designed to remove additional material to be used elsewhere in the head. A cut-away view shows this in more detail. Expect an iron with an undercut cavity to be amongst the most forgiving designs.

The further rearward the club balances; the lower the center of the club becomes. We will see why this is important later. This diagram shows the CG locations in various views.

Iron Head Moment of Inertia

A golf club's center of gravity is no more than a mere spot. Therefore, it is very conceivable that two irons may possess the exact same center of gravity coordinates but play much differently. Most modern irons possess some sort of cavity as a means of re-distributing weight elsewhere in the head to enhance the performance in the form of perimeter weighting. Non-cavity back irons are called muscle back designs as all the mass forms a solid “muscle” behind the face. This next diagram shows both irons side by side.

The reason this is important is that a golfer will experience some energy lost during a collision with the ball when struck off-center. Increased MOI will result in more distance and a more solid feeling at impact than a head equal in weight and size where the mass is more centrally located.

Iron Offset, Onset and Face Progression

There are a few clubhead design parameters that directly affect the direction and trajectory one hits a golf ball. These terms are face progression, offset, and onset, which are either foreign to the average golfer or at least confusing at best. Let us help sort out these terms as they are all related to one another by first starting out by establishing a point of reference, which is the centerline axis of the shaft indicated by the dotted line.

Iron onset

The leading edge of the hosel of the iron is in front of the leading edge of the face, which is offset. If the opposite were to occur, as in rare instances, this would be called onset. Onset is a negative offset and in iron design may be called a face-forward design. We will talk about onset more when discussing woods and hybrids.

Manufacturers will vary in providing you with their offset measurements. In some cases, the units are in inches and others are in millimeters (mm). Inches are easily converted to millimeters by simply multiplying by 25.4 or to convert millimeters to inches, you divide by 25.4.  

Some Japanese manufacturers have eliminated the use of offset altogether and simply gone to face progression in their listed specifications. Therefore, it is important to understand these differences. The reason is offset is unfortunately directly related to the outside diameter of the hosel, unlike face progression, which has one standard reference point.  Not all irons will have the same outside hosel diameter due to the material they are made from and the diameters necessary to provide enough strength. 

The modern irons made from grades of either stainless or carbon steel will typically have an outside diameter (OD) of 0.535” (13.59mm) with a plus or minus tolerance 0.005” (0.13mm). There are few iron heads made of zinc (found primarily in starter sets) may have a hosel OD closer to 0.560” (14.22mm). An identical head made from zinc will make the club appear as it has additional offset, even though the face progression would be the same as that same iron with a conventional hosel OD.

Progressive Offset 

Listed in the following table are typical amounts of offset in a game improvement and a player's iron set. We also converted to face progression (F.P. for short). 

Let us first start by explaining offset with the game improvement set. In a normal set of irons, you might see the face progression starts with the leading edge of the face is rearward of the center line axis of the shaft and eventually moves forward. In this set, the #5 and 6 irons will have the leading edge appear almost directly in line with the shaft.

In an iron design for a more skilled golfer, the set is often designed with less offset as shown in the table on the right. Even there, you find that more offset is designed into the longer irons than the shorter ones.

Weight Distribution

While there are few standards in golf, surprisingly iron heads weight does not vary much from manufacturer to manufacturer. That is the modern #6-iron may weigh 263g plus or minus a few grams. The reason for this is that most steel-shafted irons are designed to be a specific length and swingweight so there is a formula in the head weight that will satisfy those conditions.

Therefore, it is important that if you make an iron longer or with a taller face height, or with a wider sole, material needs to be taken from somewhere else to maintain the acceptable head weight. That is why it is especially important to look closely at an iron as the weight distribution follows the geometry of the head.

Traditional Iron 

Let us start out with a muscle back iron to illustrate this point. Muscle back or blade irons were how clubs were made in the past. Their dimensions are what I will refer to as symmetrical. That is the blade length and hosel lengths are both approximately 3 inches (76mm) long.

You will notice two rectangles in this diagram. One is the outer perimeter of the toe, sole, hosel and top line. The other is the gray shaded area whose upper right-hand corner defines the CG position. With these two boxes affixed to one another, they can be stretched or squeezed to form the boundaries around the head. In most cases, the actual center of gravity of the head will overlap the upper right hand of the gray shades box because the CG is proportional to clubhead geometry.

A high concentration of head weight is accounted for in the hosel and the face of the iron. The only exception is when a designer can add a secondary (or more) material whose density is different. This is why manufacturers will opt to use a maraging steel or titanium face plate or add tungsten in strategic areas of the club. This will start adding to the cost of the club as well. The advantage, if re-distributed correctly, can transform a game-improvement design into an ultra-game improvement design.

We just saw as loft is increased the result is the CG is progressively shifted back from the center line axis of the shaft. Compensating for the rearward shift in CG, manufacturers starting building in progressive offsets.

Offset has more than one meaning. It can also mean to “place away from a center line; off-center.”

If we look closely at the diagram, we can see the importance of the offset in the rear CG location relative to the center line axis of the shaft. On the far left is an example of a reduced offset (1mm) iron. Remember the offset is the distance from the leading portion of the hosel to the leading edge of the face. We also show CG offset or how far the rear CG is away from the center line axis of the shaft.

This will become more apparent when we look at the moderate offset (4mm) iron in the middle. The darker shaded area illustrates the offset while the lighter shaded area represents the CG offset. The addition of offset (changing hosel position) directly influences the rear CG location.

Lastly, we show an iron with a heavy amount of offset. In this situation, the CG is offset the furthest from the center line axis of the shaft. This is important to understand. The actual CG in relationship to the leading edge is not much different in each case, but the CG in relationship to the center line axis of the shaft does. This has a considerable influence on how high you might hit the ball, the face angle at impact as well as on shaft selection.

The importance of this directly affects the gravity angle. By moving the CG further away from the hosel, it will decrease the gravity angle. The less the gravity angle is, the harder it will be to close the face at impact.

If we increase the offset with, we increase the CG offset. This is one reason compact blade length irons have a reduced amount of offset and those irons which are longer from heel-to-toe generally have more offset throughout the set. An iron with a long blade length and reduced offset produces a club with an exceptionally low gravity angle and ends up as a fade biased combination. This is fine for those who made draw or hook the ball, but remember most golfers hit the ball with a push, fade, or slice.

Then why not have an iron with a compact blade length and ample offset as this would create a high gravity angle? The shorter blade length would reduce the effective hitting area and confidence level of the golfer hitting it. Subsequently the moment of inertia reduces as well making it less forgiving on off-center shots.

Offset is built into the design to normalize the gravity angle and make it easier to square the face upon impact.

Instead of a spot, I like to think of it as a “sweet area.”  It is the area where maximum distance can still be obtained. A good goal is to have the cluster pattern of balls hit on the face approximately the size a quarter, while the touring pros might be able to contain that impact area to a dime. On an iron, the center of gravity is roughly located near the 5th score line and may be in the center of the scoreline area or slightly biased toward the heel as a result of an elongated hosel.

Sole Radius

One of the biggest clubmaking design changes occurred in the early 1990's when front-to-back sole radius was introduced to irons allowing the ball to be hit cleanly from a variety of lies. Before this, the sole in the front-to-back plane was flat.

In Chapter 6, we explained the difference between angle of attack and approach. This is often a byproduct of the ball position in the stance. Not only that, but we are also not hitting off flat even terrain every time. So, it is important we have some relief otherwise the leading or trailing edges of the club will dig into the ground causing a decrease in the distance and/or feel at impact.

Bounce Angle

Bounce angle is a term associated with wedges, but any club can have a bounce angle. Besides the golf shaft torque, bounce angle may be the next most misunderstood concept of a golf club design. Part of this lies in its definition. I have seen many places where the writer defines bounce as:

(Old definition) The measurement of the number of degrees from where the club rests on the ground and the club’s leading edge. 

While the definition above may have been true in the past, it is technically not correct anymore. Before I explain why, let us lead you gradually in this discussion by examining the anatomy of the sole. First, there are four factors that go together in understanding this design parameter of a golf club; sole radius (if at all), sole width, leading edge height and contact point on the sole.

Looking back at the anatomy of the sole, there are a couple important terms to remember. The outermost dimensions of the sole are the leading edge (positioned at the bottom of the face) and the trailing edge (positioned along the back edge of the head). The distance between the leading and trailing edges is the sole width. Note that the trailing edge of the sole may be tapered, so the sole width may vary along its’ length. Most manufacturers will reference the center point of the sole for this dimension. It is also important to realize that few irons are perfectly flat on the sole although it may look that way. In addition, head manufacturers will normally grind or radius the leading or trailing edge so that it is not a sharp point.

Here is an example of the same flat-soled iron with a 4° bounce angle with the shaft parallel to the ground and next to it, the same club at impact with a 4° angle of attack. By creating the bounce leaving the leading edge above the ground at address, avoided the club from digging into the ground at impact. As the angle of attack matched the bounce angle, the contact point of the sole on the ground is in the center of the sole. If the angle of attack had been any less than 4°, then the trailing edge would have contacted the ground, thus the ball would make contact on the face closer to the leading edge of the face. However, if the golfer struck the ball with an angle of attack greater than 4°, then the leading edge would have contacted the ground first.

The second model is the same segment of the circle but rotated 4° counter-clockwise from the center of the circle so that the leading edge is higher than the trailing edge. The dimension to the right of each segment is the dimension from the leading edge to the ground line. If this were a right-handed club viewed from the toe, this would be considered positive bounce because the contact point is now located rearward of the center of the sole. Again, it is not the contact point that determines the bounce it is the difference between leading and trailing edges in relationship to the ground line.

The third model in the diagram shows when the segment of the circle is rotated 4° clockwise from the center of the circle so that trailing edge is higher than the leading edge. This creates a negative sole angle, but due to the radius, the leading edge remains above the ground line (0.052”). The last model represents what happens more on a sand wedge where the bounce is much higher than typically the rest of the set (in this case 12°). The contact point is much closer to the trailing edge, which also raises the leading edge higher off the ground. A situation where the leading edge is too high can lead to shots that can be bladed or skulled in certain situations.

As mentioned earlier, the narrower the sole, the less height the leading edge is above the ground line. By narrowing the sole from 1.25” to 0.781” (closer to a normal sole width), the leading-edge lowers with the same 2” sole radius.  

We are going to use the same scenarios as above so you can see what happens when sole width is changed. The model on the far right illustrates just how bounce itself does not tell the whole story. There is a term called effective bounce, which is the bounce measurement, along with the leading-edge height and sole width. Even though the fourth model in the two diagrams have 12° bounce, the leading-edge height is a little over 0.10” difference. While this may not seem that great, it can make a significant difference in the playability from a tight lie versus a fluffy lie, with the former being better for tighter lies or firmer terrain.

Look at the difference between the first models in the two diagrams as both have 0° bounce. Where the difference really shows up is when the sole is rotated clockwise, the same as if the head was de-lofted due to a descending angle of attack, the leading edge is not as low to the ground and less likely to dig in.  This is one of the reasons why normally you find more bounce on narrower soled clubs as often the sole has more radius than a wider sole model.

A prime example of this (although it does not exist in any head that I am aware of) is if the radius was small (0.625” radius) and the width was extremely narrow (0.5” wide). Even if the club had 30° bounce, the leading edge would only be 0.25” above the ground line! 

Progressive Bounce

You might even see long irons with negative bounce as part of their specifications. Once considered that the head was inaccurately manufactured if the bounce was negative is no longer true. Often the #2 and even 3-irons are used off a tee. Thus, any time the ball is off the ground, then an upward or ascending angle of attack occurs which will add both loft and bounce to the club at impact. Even if clubs with negative bounce are hit off the ground with a level swing, the modern sole radius will prevent the chance of a “fat” shot as the leading edge will be above the ground line. 

Most manufacturers do not provide bounce specifications other than for the wedges, for good reason as it can be quite confusing to the customer. Even if they did, sole radius and sole width specifications will not be included. So, it is up to the manufacturer to understand these relationships when designing a particular model to make it playable.

Very few times you see the exact same head, but in different bounce option from a fitting situation. The only time multiple bounce options are available occurs with only a few name brand manufacturers who will sell enough to make it a worthwhile investment in tooling. The two leaders in the wedge category (Cleveland and Titleist) offer high bounce and even low bounce options for the different conditions and the golfer’s angle of attack. Otherwise, it will require a skilled clubmaker to grind the sole or alter the loft to customize the effective bounce. Additional information can be found on bounce in the Wedge chapter.

Iron Head Center of Gravity and Shaft Interaction

The CG location of a head can influence the type of shaft that might be selected. After all those two items (along with the grip) function as a system and do not work independently of one another. If you have been playing golf for a while and played a variety of heads and shafts, you may have experienced that certain heads and shafts seem to go together better, like peanut butter and jelly. Then you go to re-shaft an iron you have been playing with an entirely different type of shaft of the same flex and then all of the sudden they go together like oil and molasses.