There are a few clubhead design parameters that 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. Lets help sort out these terms as they are all related to one another by first starting out by established a point of reference which is the centerline axis of the shaft indicated by the red line in Figure 1.
Golf clubs are measured by foundries and golf club manufacturers using heavy duty industrial specification gauges. The base of a specs (for short) gauge has a series of lines engraved, one of which shows how the vertical centerline of the shaft translates to the horizontal plane as a means of referencing many things about a golf club. One of these is face progression which is simply the measurement from a shafts centerline to the leading edge of the club face. In Figure 2, the leading edge of the face is indicated by the green line. The difference between the red and green lines is the face progression.
Face progression is usually a term that is related to woods, but can be used for irons, wedges, hybrids and putters too. On a wood, the leading edge is forward of the centerline axis of the shaft. For irons, wedges and hybrids the leading edge may be in front of, behind or even with the centerline axis of the shaft, so there are positive and negative values associated with face progression. For our reference, when the leading edge is forward of the centerline axis of the shaft, this will be a positive value.
A term that is commonly used to describe irons and wedges is offset. Offset is somewhat similar to face progression as the leading of the face is used as a reference point, but instead of the centerline axis of the shaft being the second reference point, the forward most point of the hosel is. In Figure 3, you will see the most forward part of the hosel by the purple line and the difference between the purple and green lines is the offset.
As you can see, the most forward part of the hosel of the iron is in front of the leading edge of the face, which we call offset. It is the opposite in the wood, but instead of negative or positive values like face progression, we have a different term. Whenever the leading edge of the face is in front of the most forward part of the hosel, it is referred to as onset.
Offset is unfortunately directly related to the outside diameter of the hosel, unlike face progression which has one standard reference point. Not all clubs will have the same hosel diameter due to the material they are made from and the diameters necessary to provide enough strength. Many modern irons made of either stainless or carbon steel will have an outside diameter (OD) of 0.535 (13.59mm) with a plus or minus tolerance 0.005 (0.13mm). 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 out of 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.
Manufacturers of golf clubs do not provide onset or face progression measurements for woods as part of their specifications and maybe for good reason. First, this would probably only create confusion amongst consumers, Secondly, hosel diameters are more varied among drivers and fairways depending upon material. Average sizes for the outside diameter of a titanium hosel is closer to 0.500 (12.7mm), stainless steel hosels 0.480 (12.2mm) and aluminum hosels are approximately 0.512 (13mm) in diameter. However with the advent of hybrids, face progression or onset might become a common specification some day to further describe the heads.
There are cases that a golf club may not have a hosel (like an older Callaway driver), have a tapered or asymmetrical hosel, or the shaft may be designed to go over a post. So it may be too confusing (if at all possible) to use some part of the hosel as a reference point to measure offset and why face progression remains a more accurate reference point.
As face progression might be foreign to you, it is easy to convert from offset and visa versa. Since the centerline of the shaft bisects the hosel, all we need to know is the hosel diameter. As most iron hosels are 0.535 (13.6mm), the difference between offset and face progression is half the diameter of the hosel or 0.268 or 6.8mm. Listed in Table 1 is a typical amount of offset in a game improvement set which is also converted to face progression (F.P. for short). As a note, in order to convert mm to inches, divide by 25.4.
In a normal set of irons, you might see the face progression starts with the leading edge of the face is rearward of the centerline 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 Table 2.
Even a typical set of players clubs will have offset built into them. This should be considered one of the more modern design elements by incorporating additional offset into irons for more accomplished players, as it was not that long ago a players set did not possess any offset at all. A zero or non-offset iron or wedge will make the leading edge even with the front of the hosel and with any tolerance or if the club was bent for loft may create having the shaft appear forward of the front edge of the hosel. A set of irons were the set varies in offset throughout the set is called progressive offset. A set of irons could all have the same amount of or a constant offset (or none at all) throughout the set, made with little offset in the lower-lofted long irons and gradually increase as the loft becomes higher. But by far, the vast majority of irons are produced with a progressive offset that decreases as the loft increases.
On the other hand fairway woods are manufactured with progressive onset, where the leading edge of the club becomes more forward of the front of the hosel as the loft increases. This is not necessarily by design, but to keep the clubs from increasing in onset, an offset-type hosel would have to be part of the design, which will be talked about later.
Table 3 is an example of approximately the amount of onset of a line of popular, medium height fairway woods. One will notice just how much material there is in front of the hosel (onset) or the shaft (face progression) on a fairway wood. The term face forward design is sometimes used to describe a wood, hybrid, iron or wedge with onset.
To show the difference in the onset, we have the #3, 9 and 15 woods from this set shown in Figure 4. The green vertical line is the centerline axis of the shafts. One observation can be made and that is how one positions the ball in the stance using any of these clubs. Playing the #15 wood forward in your stance will create a high probability that the club may end up striking the ball with the leading edge. That is one reason we are taught as the loft is increased, the ball should be positioned further back in the stance than a lower lofted club. We can go step further than that and include irons into the discussion as well. As most irons have progressive offset, ball position should change slightly as well with the lower lofted clubs requiring the ball positioned further forward in the stance.
The size or volume of the head also influences the amount of onset or face progression of a metal wood. It should be noted that wooden woods possessed more face progression than metal woods for strength but will not be included in this discussion. Table 4 shows the approximate dimensions of metallic driver heads have started to increase in size. For your reference, these were all 10 degree drivers. As clubs became larger, so did the face heights to make them proportionate. So if we have two drivers with the same volume, the one with the deeper face will have more onset and face progression. In addition, a driver with more loft will add to face progression, while reduced loft decreases it. Vertical roll will not influence the face progression unless it is asymmetric where by the roll is reduced on the bottom half of the face but not the top half.
An offset feature is not only for irons, but woods, hybrids and putters. In Figure 5, the standard driver is on the left and the identical driver, but with an offset hosel is to the right. At this time, note the space between the leading edge (green line) of the face in relationship to the front edge of the hosel (purple line). An offset driver isnt offset by definition as the leading edge is still in front of the most forward part of the hosel. A better description is that the head on the right has reduced onset.
In Figure 6, we show two examples of offset in a putter. The putter on the left is a traditional style putter with a post extending above the putters body with a spur hosel which creates the offset. What the offset hosel design does in this case is move the centerline axis of the shaft very close to the leading edge of the face for alignment. The putter on the right is an example of a bent putter shaft creating the offset. In this case we must use the furthest most portion of the shaft as the reference point as a hosel doesnt exist. The shaft in this case has two centerline axes; as one is below the bend entering the head and the second top portion of the shaft above the bend. In this case, the leading edge of the putter (green line) and the centerline axis of the top portion of the shaft match doing exactly what the putter on the left does with a hosel.
Hopefully you have gained a better understanding of what face progression, offset and onset are. Of the three, offset is the one most people are familiar with, but these terms are all related. Offset is considered as a game-improvement feature. Why? There are many ideas behind the importance such as it delays the impact for a split second to help the clubface square up at impact or position the shaft forward so that the golfer will hit the ball with more of a descending angle. But one thing for sure that it does and that is it repositions the center of gravity further behind the centerline axis of the shaft. The importance of this phenomenon will be discussed in another topic.