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June 2016

Should everyone play single-length irons?
By Paul Wood

Recently, there’s been a lot of interest in a set of irons where each club is the same length. When a talented young player on the PGA Tour uses clubs this distinctive and different, it is going to generate some headlines. It has also sparked some interesting questions about matching clubs in a set in the golf equipment world. I have some experience in alternative ways to match a set of clubs. In fact, my first project at PING was to build an inertia-matched set of irons. I hope this article provides some useful information about matching an iron set.

The idea of a set of single-length irons sounds appealing: If every club has the same swing feel and can be swung on the same plane, it would seem easier to groove one swing for the majority of shots. However, current sets aren’t just different lengths — they’re also different weights, have different lie angles and generally optimized for length progression. They’re also optimized to achieve good distance gaps between clubs. So to begin this discussion, we need to pinpoint what makes up the feel of clubs.

There are many ways to describe the feeling of weight, especially in an object intended to be swung at high speed. For those wishing to explore this topic in detail, I recommend a book called “The Physics of Golf” by Theodore Jorgensen. It was one of the books I really valued during my first couple of years at PING. Jorgensen describes three ways to measure the feel of weight of a club. To understand, it helps to imagine the golf club as being made of a collection of little 1-gram weights, or masses, all stuck together, as shown in Figure 1. In the diagram, the black circles represent the grip, the grey circles the shaft and the blue circles the head (if it’s not abundantly clear, I’m not employed for my artistic abilities). Hopefully you get the general idea.

Figure 1: Diagram showing how a club can be viewed as a collection of 1-gram masses where the feel properties of the club are described by each mass and its distance from the pivot point.

Jorgensen’s 3 measures of feel are:

  1. Mass. You can feel this by picking up the club at the shaft and holding it. It is simply the sum of all the little 1-gram masses in the figure. Adding more mass, whether to the grip, shaft or head, will add to total club mass and make it feel heavier overall.
  2. The first moment (swing weight). You can feel this by holding the club at the grip and then pointing the head straight out in front of you. You can feel the “weight” of the club pushing down on your bottom hand, trying to rotate. This value is calculated by taking each little 1-gram mass and multiplying by the distance from the pivot point. It’s measured in mass-distance or inch-ounces on a standard swing weight scale. So if you add 1 gram to the head, you’ll feel the effect much more than if you added 1 gram to the shaft. Adding 1 gram to the butt of the grip can even make the club feel lighter by this measure. On an actual swing weight scale, the pivot point is 14 inches from the butt end of the club, for reasons no one is exactly sure about, other than it has worked for the last 20 to 30 years. If you were trying to match a set to a true “first moment,” you’d use a pivot point more like 5 inches from the end of the grip (between the hands).
  3. The second moment (club MOI). You can feel this better by waggling the club around. It is the moment of inertia (MOI) of the club around the golfer’s hands, and is often called angular mass in other engineering fields. This value is calculated by taking each little 1-gram mass and multiplying by the distance from the pivot point squared. It is really a measure of how spread out the mass is, and is often described as a resistance to twisting. This value is much more sensitive to even a small amount of mass added to the farthest location from the pivot point. You are used to hearing about the MOI of the club head around the center of the face — a measure of the “forgiveness” of the head. In this case, we’re talking about MOI of the whole club around the hands. Same physics principle, but different axis of rotation.

So why do we care about all this? Because there are two somewhat competing priorities: distance and accuracy. In theory, we would match clubs so that it’s easier for golfers to swing all of them accurately and consistently, but golfers also want more distance. Jorgensen starts his chapter on club matching this way: “I thought perfectly matched clubs should all swing the same and therefore… increase the precision of his game. I found, however, that most golfers were interested in clubs that would give them greater distance on the course…”

Our design intent is to find the optimal balance of distance and accuracy through a set of 14 clubs for any given golfer. One of the main levers to alter distance in particular is the length of the club. If a driver is 45 inches and the shortest wedge is 35 inches, that 10-inch differential plays a big role in achieving good distance gaps while maintaining stopping power with each club. For example, a 4-iron at 7-iron length makes it difficult to generate the same height or distance you’d produce with a standard 4-iron. Moreover, as we change length, it’s difficult to match all three feel measurements to get the clubs to truly feel the same. Table 1 shows some typical values of mass, swing weight and MOI for a few clubs in the G family. You can see that swing weight stays somewhat constant, but the mass increases from driver to PW, while the MOI decreases. This is a function of the design trade-offs made for each club in the set.

Table 1: Typical values of club mass, swing weight and MOI (around a pivot point 5 inches from the grip end) for selected G clubs in a set.

So, all that said, is there a benefit to having at least the majority of clubs in a set at the same length? It’s a tough question to answer, because the results can only really be built up over time using a single-length set on the course. The trade-off seems to be better consistency when switching from iron to iron in this set, but the driver and fairway woods will feel very different from the irons, and it might be a struggle to achieve good distance gaps in the set.

The fact that at least one player has had good results on the PGA Tour shows that a single-length set can be effective, but that does not mean that it would work for everyone. The most famous current exponent of the single-length iron set also plays extremely upright lie angles, is a dedicated disciple of the Golfing Machine instruction system, and has been working diligently at this for years. His single-length iron set is matched for mass, swing weight and MOI, and allows him to use the same swing plane for all of his irons. However, the metal woods are still longer, lighter and have higher MOI. It’s probably unrealistic to expect that just chopping down your shaft lengths will by itself make a big difference. You can see from Table 1 that to make a standard 5-iron at 7-iron length, we also need to add 20 grams to the mass of the club to make it match.

I suggest the best candidates for a single-length set of irons are higher swing-speed players (who don’t have trouble generating distance) who want to take the time to experiment with their game and determine objectively whether the pros outweigh the cons. I don’t recommend that anyone buy such a set on a whim. It takes a lot of effort to adapt a set designed for progressive lengths into a functional single-length set. If you are interested, at least go and talk to a master club-builder for advice.

In the future I could see this approach working for people just taking up the game. In that case, I foresee a set featuring just a few clubs, all the same length. Out of curiosity, I’m tinkering with some single-length irons and hybrids myself right now. As my scientific training taught me, I’ll remain skeptical until I can verify some measurable improvement in my results.

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