Effects of Clubhead Features:

Material and Manufacturing Process

There are quite a few dimensions to the choice of material for a clubhead:

• The "feel" and "playabiity" of the clubhead.
• The distance the clubhead can hit the ball.
• The weight of the clubhead, or the peripheral weighting (forgivingness) that can be achieved.
• Maintenance required for the clubhead.
• How much can the clubhead be adjusted to achieve different playing characteristics (e.g.- bending for loft and lie).

Playability, feel, performance

Forgivingness and workability

The few studies that played mix-and-match with these relationships (for instance, cast stainless musclebacks) came away concluding that the difference in "feel" was actually the difference between a cavity back (big sweet spot) and a muscle back (small, critical sweet spot). The material had nothing to do with it.

Feel

Still, a lot of golfers seem to believe that a softer clubhead material changes the feel of the club, and there are manufacturers out there encouraging this belief and producing clubheads to sell to it. So let's analyze whether there's anything at all to it...

The "softness" of feel is a combination of the softness of the clubhead and that of the ball. More precisely, the softness of feel is the inverse of the total stiffness of the collision between ball and clubhead. Stiffness is measured as a force divided by the deflection produced by the force, designated "force/deflection". The lower this ratio (either due to reduced force or a longer compression), the softer the collision feels.

During impact, both the ball and the clubhead deform and produce elastic forces. Consider the compression stiffness of each of the two materials:

• Force/deflection for the clubhead material is K_h.
• Force/deflection for the ball is K_b.

Of course, steel -- any kind of steel -- is much stiffer in compression than a golf ball. That is, K_h is greater than K_b, by a factor somewhere between a hundred and a thousand. A little algebra will show us where this is going. The overall stiffness of the collision is completely dominated by the stiffness of the ball; the metal of the clubhead has very little to do with it.

For K_h many times larger than K_b,   K_b+K_h ≈ K_h , so:

How about a few examples to show us that the stiffness of the collision is dominated by the ball's construction, not the clubhead's. Let's assume the ratio of K_h to K_b is 100, just to give the greatest possible chance to show that clubhead material does matter.

• From the softest steel to the hardest, the range of K_h is only a 10% range. Suppose forged carbon steel were a full 10% softer than cast stainless. Then wouldn't the collision feel 10% softer? No, it wouldn't. From the equation, a 10% reduction in K_h results in only a 0.2% reduction in overall stiffness of the collision.
• Well then, let's choose a really soft metal like copper, and use it as an insert. (As I originally wrote this in the 1990s, such a set of irons had appeared on the market.) Even copper, at only half the stiffness of steel, results in only a 1.0% reduction in the overall stiffness of the collision. Not even the most practiced pro could feel this; even if he/she could, ball-to-ball variation is considerably greater than this.
• Finally, suppose we reduced the ball's compression by 10%. This time, the collision is in fact 10% softer. So the compressibility of the ball dominates the feel of the collision.

Feel again, but really sound

Let's look at another rationale for believing that clubhead material can affect feel. And this one, as counterintuitive as it is, has the best chance at being real. More than one study has concluded that sound is a major component of what golfers call "feel". These studies typically used earplugs with various sound "shapers", from a low-pass filter (removes the high frequencies) to complete blockers of all sound. A couple of interesting results are:

• Clubs tested with low-pass earplugs were reported by golfers to have a softer feel than clubs tested without earplugs.
• Clubs tested with high-pass earplugs were reported to feel harsh or hard.
• With complete sound-blocking earplugs, the golfers were unable to consistently distinguish the feel of the various clubs.

Can this play back into cast vs forged steel clubheads? Very possibly. The forged clubheads are "softer" in the sense that they are more malleable. That is, they can be made to take a permanent bend more easily. (More on that below.) Consequently, it might be expected that they would damp out the higher frequency sound components. Taking sound into account as a major component of feel, the perception would be that the clubs feel softer.

Time on clubface

In the late 1990s, golf club companies (probably Callaway first) started increasing the coefficient of restitution (COR) of their drivers by making the clubface thinner so it would flex more. The result was an increase in distance.This was in violation of the rules, but it wasn't discovered until there were many thousands of the non-conforming clubs in the hands of golfers. By 2003, the ruling bodies (USGA and R&A) agreed to regulate COR to a maximum of 0.83. Measuring COR directly is an expensive proposition; it needs a precise-velocity cannon for golf balls and the ability to measure the golf ball's velocity. So a few years later, the ruling bodies agreed on an indirect measurement that was strongly correlated to COR. They bounced a pendulum bob off the clubface and measured the time the pendulum was in contact with the face. The longer the pendulum was on the clubface, the higher the COR.

Since the inception of this "characteristic time" test (CT), the belief has grown that keeping the ball (not a steel pendulum bob, but a golf ball) on the clubface longer imparts some mystical, magical properties to the golf shot. I have seen claims of more distance, more energy transferred to the ball, more spin, and better control. Of course, some club manufacturers and even some instructors are selling all sorts of snake oil purporting to keep the ball on the clubface longer.

It is time to debunk this. Note that it takes two leaps of reasoning to sell the improvement: (1) a longer time on the clubface imparts these magical properties, and (2) my product/technique will keep the ball on the clubface longer. Do away with either #1 or #2, and you debunk it.

• No extra distance nor energy transfer occur just because the ball is on the clubface longer. The myth comes from the correlation with the COR test. But, as everybody is saying the last couple of years (as science has come more publicly into golf), correlation is not the same as causation. And it is particularly silly when it is in fact cause and effect -- but the proponents have gotten it backwards. And they have in this case. Something that does in fact increase energy transfer and distance -- thinning the clubface for a spring effect -- will as a by-product increase the time the ball stays on the clubface. That hardly means that keeping the ball on the clubface will in general improve energy transfer. Let's look at clubhead material, for instance. Consider two cases:
• Titanium can increase energy transfer, but not just because it's titanium. Titanium allows the construction of thinner faces than steel, which flex more and thus add to COR. The increase in contact time is a by-product, not the cause of the higher ball speed.
• Forged irons are more malleable than cast irons. Let's suppose they are enough softer to make a noticeable difference in the time on the clubface. (They are not that much softer! But let's just assume, and see where it takes us.) They are softer because they are more malleable. To that extent, they absorb energy during impact; they do not give back all the energy of their spring deformation. So any increased time-on-clubface due to soft, malleable iron faces would result in lower COR, not higher.
• OK then, how about spin? For a full shot, spin is almost entirely a function of the ball. In fact, high spin balls might actually lose spin if you could keep them on the clubface significantly longer. (As we did above, we will assume a longer contact time to see what would happen, even though it is contrary to fact.) According to an article by Bill Gobush of Titleist, high-spin balls are "tuned" to give maximum spin for a particular time in contact with the clubface. The internal shear stresses between and within the layers of the ball get relieved, ideally, as the ball leaves the clubface. If we could keep it on the clubface significantly longer, the stresses would build in the opposite direction and retard spin.

Distance

Weight

The whole clubhead can be made of graphite or titanium. We have seen quite a bit of this in metalwood clubheads since the mid-'90s. There is the opportunity to make performance improvements with the lighter material:

• Overall lighter clubhead, for lower swingweight or longer clubs. Theoretically this should give more distance. There are now oversize driver heads on the market with weights as low as 190 grams, and even lower is feasible with titanium.
• Bigger clubhead, for more forgiveness. This seems to be where everyone is putting their titanium, and some are putting their graphite.
• Thicker cross sections, for a stronger clubhead. This is where some of the titanium is going, because it has the strength of steel at half the weight.

• Titanium clubfaces in otherwise steel clubs have several advantages. They allow more peripheral weighting for forgivingness. And they allow a springier face for a higher COR. In the 1990s, when titanium clubheads were very expensive, we saw some steel metalwoods with titanium faces for these reasons. And today a few irons have faces of either titanium or specially treated steel to give a higher COR than we normally think of for an iron.
• Most clubs perform better with a lower center of gravity. Part of this can be done by designing the shape. But the last two decades have seen designers get really creative in the use of materials to lower the CG of a clubhead. Cases in point:
• Historically, I date this trend to the TaylorMade Rescue club from the late 1990s. Most people consider the originality of this innovative club to be the hybrid shape. (Even today, I hear a lot of non-TaylorMade hybrids incorrectly referred to as "Rescues".) But one of the reasons it was such a superior design was the use of materials. The top half of the body was made of titanium (the lightest of the clubhead metals) and the bottom half of tungsten (the heaviest). The result was a standard clubhead weight, but an extremely low CG; it could get the ball airborne much more easily than its competitors. Incidentally, the Orlimar metalwood of about the same vintage advertised tungsten sole weights; they were just thin tungsten foil, and were but a fraction of the weight needed to make a significant lowering of the CG.
• We have seen "composite" driver heads -- mostly-titanium bodies with carbon fiber crowns -- intended to lower the CG. Carbon fiber is not as strong nor stiff as titanium, but considerably lighter. As long as the shock of impact is absorbed by the titanium, the thin carbon crown keeps the club together and streamlined while minimizing weight high in the clubhead.
• Finally, let's look at the Acer XS titanium 3-wood from Hireko Golf. In 2013 designer Jeff Summitt asked himself, "What if I built a low-profile 3-wood out of titanium and gave it a spring face?" With a much smaller head than a driver, it it turned out not to need much material for strength. You could easily build it to a weight under 130 grams. But, since a properly-performing 3-wood head is about 210 grams, that sounds pretty useless. It isn't! Jeff used the 80g difference for discretionary weight (probably stainless steel) to be put in the sole, as low as it could be placed. The result is in my own bag, and it is by far the best 3-wood I have ever hit.

Maintenance

The most obvious of these is rust. The carbon steel of a forged head needs to be chrome plated to prevent the head from being destroyed by rust. Scratches in the chrome show oxidation fairly quickly. The other materials (stainless steel, graphite, and titanium) are quite inert and don't rust.

This brings up a potential problem: some kinds of maintenance of forged irons will just invite rust. For instance, a regrooving tool, used to restore worn grooves, is likely to remove the plating and the grooves will quickly rust. And I do mean quickly; it is hard to clean and dry the grooves, so they stay moist when you put the clubs away. Even worse is grinding the sole, which is a common modification to wedges; better golfers want their wedge soles shaped to facilitate shots they like to play. If you're going to do this, you should probably have the clubhead re-chromed afterwards.

Wood and fiber composites scratch and abrade more easily than titanium or steel. Wood is pretty straightforward to maintain, for anyone who got through high school woodworking shop, but graphite (carbon fiber) is more specialized. And when very thin carbon fiber is used to reduce weight, even the approved, specialized repair methods are likely to degrade performance.

Of the stainless steels, 18-8 and 304 are the softest (most "scratchable"), and 17-4 the hardest (most scratch-resistant).

I have never tried to refinish a metalwood head, but that's only cosmetic anyway. I'm less concerned about the club's looks than its playability or integrity. But for those who do worry about cosmetic issues, metalwood refinishing kits are available.

Adjustability

As for the various kinds of metal used in golf clubs:

• Carbon steel (as used in forged irons) is very bendable.
• 18-8 and 304 stainless steel (used in a small number of cast iron heads) is similarly bendable.
• 431 stainless steel (the most common material for casting iron heads) can be bent a limited amount. The usual rule of thumb is no more than 2 degrees of bend, though I have done 4° with no ill effects. But I have also seen 431 clubheads break catastrophically when attempting a 4° bend.
• 17-4 stainless steel (used in almost all metalwoods and a few iron heads) shouldn't be bent without "special equipment", according to the 1995 Golfsmith catalog. (I have been successful myself on one occasion at bending a 17-4 clubhead 2.5 degrees. I have also seen broken 17-4 clubheads after an attempted bend of only 1 degree.)
• 15-5 stainless steel I have only seen in metalwood heads, and have never heard of its being successfully bent.
• Titanium in metalwood heads may be bendable. According to Mitchell Golf (which makes the best-known bending machines), forged titanium has a softer structure than cast. BTW, there is lots of good information about bending in the Mitchell article.

The reason for this is the hardness of the material. The less sure you are of your loft/lie, the higher on this list you want to be. By the same token, harder clubheads will not nick as easily and will probably last longer; if you ARE sure of your loft/lie, move toward the bottom of this list for a club that will look better longer.