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HARDNESS AND DISTANCE
or
A MYTH IS NOT AS GOOD AS A MILE

Dave Tutelman -- November 25, 1999
modified from original of April 19, 1998

I increasingly see statements (mostly in companies' advertising claims) that a harder clubface hits the ball further. This has already achieved the status of "golf myth". But it's simply not true. Here's the truth, probably in more detail than you really wanted to know...

What is Hardness, and Why Do I Care?

Hardness is the degree to which a material resists penetration or scratching by another material. For metals, the test is penetration; other materials (including ceramics) may use scratching. [1, 3-6].

At least as interesting as this limited (but complete) definition is a list of what hardness is NOT:

It's not the opposite of compressibility or flexibility. Compression is an elastic measurement, that assumes that the material recovers when the force is removed. To measure hardness, you measure the deformation that is left after the force has been removed.
It's not the same as strength, though the two are related in metals. For any given metal (for instance, among steel alloys), there is a positive correlation between hardness and yield strength [4,6]. But going from one metal to a DIFFERENT harder one does not mean you gain strength; different metals have different curves of hardness vs strength.

While we're here, it's probably worth reviewing the difference between compressibility and strength.

If you apply a force to a material, it shrinks (compresses) or stretches.The ratio of the force to the change in size is the "elastic modulus", which we can treat as "compressibility". But this ratio works up to some limiting force, after which two things happen:

The ratio decreases; more size change for a given force change.
The material does not recover when the force is removed.

That force is the "elastic limit", which we can treat as the strength of the metal. Exceed that, and you get two things:

Loss of energy, if the force is impact with a golf ball.
Permanent deformation of the golf club.

But there isn't any necessary correlation between elasticity and strength.You can have a material that's stretchy but strong, and another that's rigid, but gives up at a much lower force.

So hardness is positively correlated with strength, and strength is a good thing. And hardness has nothing to do with compressibility.

How Does Hardness Relate To Distance?

There's a myth afoot that hardness of the clubface increases distance.This arises from a number of assertions, some true and some false. But ultimately, they fail the test of analysis.

First of all, we see that any argument based on compressibility must fail, because there is no relationship between hardness and compressibility. So you can discount, right off the bat, any argument that depends on "compression"to explain a hardness/distance relationship.

The argument that's closest to the truth runs:

Energy is lost in the collision between ball and clubface. (True)
That energy loss is directly related to loss of distance. (True)
If the clubface is harder, less energy is lost. (Let's examine this!)

In most metals, #3 has some element of truth. Let's even assume that there is strong truth in the case of steel. So there is in fact less energy lost in the clubhead. But anyone who has ever seen a stop-action photo of the collision knows that almost all the compression is in the ball; the clubhead hardly compresses at all.

But what is the fraction of energy lost in the clubhead. Counter to #3 above, most technical references on golf assume that all the energy loss is in the ball. But there must be SOME energy lost in the clubhead, however small. I have a private communications from people in the industry that it's about 99% in the ball and 1% in the clubhead.

Under this model, suppose you could reduce the energy absorption in the clubhead by a factor of two (wildly optimistic, but suppose).You'd get only 1/2% more energy converted to distance. This would amount to less than a yard and a half on a 300-yard drive. So even with the most unrealistically optimistic assumptions, there aren't any big gains to be had.

Blame The USGA!

So it would seem like the place to begin improving distance is the ball. Reduce the energy lost in the collision; in technical terms, increase the "cofficient of restitution". Certainly the ball makers can and should be doing this, right?

Sorry, but the USGA rules limit how good you can make the ball. Technology can make balls that travel much further than your Titleists or Pinnacles or TopFlites. But they wouldn't be legal golf balls. Those "balls too long to believe" that you see advertised are really very believable -- just not legal.

OK, another thought. Steel can be made strong but flexible, so it can "give" elastically, and return much of its energy. (Springs are made of steel for just this reason.) So turn the equation around. If we can't make the ball more efficient, maybe we can make the ball compress less (and lose less energy) by making the steel compress MORE in the collision. The steel can flex and still return the energy as it rebounds, so the ball will go further. [7]

The USGA is ahead of you on this one as well. The rules state that the clubface "shall not have the effect at impact of a spring." So you can't use that one either.

By the way, some recent comments from Tom Wishon lead me to believe that the distance claims of the new maraging steel drivers -- if true at all -- are based on "springy" faces. If that's the case, you can be sure they'll be outlawed by the USGA.

Hold the presses! News flash!
In late 1998, theUSGA caved in to threats of non-compliance and even lawsuits from the major club manufacturers. The gist of their comments is that they will adopt new standards that allow all existing clubs to comply. But some of the latest existing clubs have faces springy enough to significantly improve the coefficient of restitution. (For a completely rigid clubface, the coefficient is less than 0.7; some of the current drivers are in the range of 0.8.)

So it looks like hardness (well, strength actually -- buth they are correlated) might conceivably help with distance, but certainly not in the way advertisers have led you to believe. More about this below.

But Doesn't a Hardness Test See Which Ball Bearings Bounce Higher?

Brief digression: I've seen the argument made that a common machine-shop test for hardness is to drop steel ball bearings on a metal plate. The higher the bearings bounce, the harder the plate. [3] So wouldn't this mean that a golf ball hit from a harder clubface will go further?

Not at all. Again, remember where the energy is being lost. In a collision between a ball bearing and a steel plate, at least half the energy is lost in the plate. Otherwise, you'd never be able to measure the hardness differences between plates with this "scleroscope" test.

But in a collision between a golf ball and a steel plate (or clubhead),almost all the energy loss is from the ball. So any difference in distance is due to differences in the ball, not the clubface. Of course, if golf balls were ball bearings....

What About Spin Reduction from a Harder, Smoother Clubface?

I have seen claims for grooveless drivers and for smooth hardened faces,for increased distances due to reduced spin. 'Fraid not. Counterintuitive as it may be, grooves do little or nothing to increase spin if:

There is good contact between ball and clubface, no water or grass in between them, and
It is a full swing (fairly high clubhead speed), not a chip.

Lots of studies [e.g.- 2] have shown that even a flat, grooveless face gives almost as much spin as a grooved face if #1 and #2 above attain.The difference in spin may make a yard or two difference in carry or roll, not more. The differences increase -- just a tiny bit -- for lofted short irons. But there isn't much you can do to a driver's clubface to affect distance by changing the clubface surface to affect spin.

How Can I Use Hardness To Make A Club Hit Further?

Hey, there must be some way I can use superior hardness to my advantage in club design, right? (I mean, other than the fact that the club will wear better because it won't scratch as easily.) In fact, there are several ways:

In golf, sometimes psychology can be as effective as physics. If you think you'll hit a club further, there's an irrationally good chance that you will. So advertising may actually make its own claims come true.
We can take advantage of the fact that harder probably means stronger, to build heads that are larger and/or lighter than a less strong metal. Either larger or lighter can help make a longer driver: lighter to get longer without a swingweight increase, and larger to give more forgivingness,which you'll need with a longer club. And, for a nontrivial fraction of golfers, the longer club will hit the ball further.
Even without a longer club, a larger clubhead is more forgiving. That means less loss of distance when you hit it off center. And that means longer average drives, even if your longest drives aren't any longer. And, in fact, the increased margin for error may allow the golfer to make a better, freer swing that will hit the ball further. (Back to psychology.)
Thanks to the USGA's spinelessness, you can now build clubs with faces that flex during impact. Counter to intuition, the ball will go further off a flexible clubface, not a perfectly rigid one. That means that you want to build a driver with a large face (for maximum flexing) and then use as a face material the metal with the highest strength-to-stiffnessratio. Since most steels have the same stiffness, but strength varies greatly, use the strongest steel you can (which is probably also the hardest),and make the face as thin as the strength will let you.

But, once again, the increased distance has nothing to do with hardness per se. It's the strength that gets the distances, and strength is coincidentally correlated to hardness.

Can I Get This Advantage from Hardness Treatments for Existing Clubs?

You mean like "Black Ice" or case hardening or cryogenic hardening? Nope. Except for the pure psychological advantage, the advantages of hardness are really advantages of strength. And that strength has to be the entire depth of the face to be of significant help.

Surface treatments like "Black Ice" are much too thin for significant strength strength increase. Even treatments like case-hardening the head only affect the surface [5], thus are too thin to help.

But cryogenic hardening claims to strengthen the metallic structure throughout the clubhead. I'm skeptical, but let's assume it does. I must repeat that the advantage of stronger materials is only that you can make the clubhead lighter or bigger or more flexible. Any treatment of an existing clubhead, even if it could increase strength throughout the material, would have no effect on distance, because you're not changing the geometry of the clubhead.

BIBLIOGRAPHY

I'd like to point out that Dean Symonds, Dave Miko, and others on ShopTalk have already made many of these points. But I wanted to pull them together in one coherent explanation. In addition to the posts from these gentlemen, I did a bit of book research.

1. Brady, "Materials Handbook".

2. Cochran & Stobbs, "The Search for the Perfect Swing".

3. Patton, "Materials in Industry".

4. Potter, "Fundamentals of Engineering".

5. Subbarao, et al, "Experiments in Materials Science".

6. Van Vlack, "Elements of Materials Science".

7. Farrally & Cochran, "Science and Golf II", Chapters 61, 62, and63.

HARDNESS AND DISTANCE or A MYTH IS NOT AS GOOD AS A MILE