Design Notes - Golf Physics p4

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Common myths debunked

Now we're in a position to think about some of the things I see all the time in articles and forum discussion -- things said with the ring of truth that are not true at all. For instance...

A harder clubface gives more distance. This has been advertised so often that many people seem to believe it and repeat it as the truth. It simply isn't! I have written a whole article debunking this one. But here's the short form of the real story.

Almost all the loss of energy at impact occurs in the ball. Its material, when compressed as much as it is, will give up energy to its own internal friction. The ball manufacturers are not in a position to improve things here, because USGA rules insist that a certain amount of energy is dissipated by the ball.

But what about the other participant in impact, the clubface? If we let the clubface flex a little, that's a little compression that the ball does not have to do. And springy metals like steel and titanium can flex a little and spring back with almost no loss of energy; they have very little internal friction. That suggests that you get better energy transfer (less loss) if you let the face flex.

And in fact it works that way! You often hear the terms "spring face" and "trampoline effect" to describe the hot-face drivers available today. This refers to increasing the coefficient of restitution (COR) of a driver by using a flexible face. In practice, it is indeed possible to absorb some of the flex in the low-loss clubface and relieve it from the higher-loss ball -- with distinctly measurable gains in distance.

Bottom line: Rigid, hard faces don't give more distance, flexible faces do.

On a full shot you want to hit the ball as hard as you can ... with both hands. This is advice from Ben Hogan's classic book, "Five Lessons: The Modern Fundamentals of Golf". I agree with almost everything in the book. But not this. And now we know the problem with this particular piece of advice. Any addition of hit with the hands has to be timed exactly right in order to help more than it hurts. I have done a study on this and found that, no matter how you try to hit the ball hard at impact, you can't. At least you can't if you have held the lag instead of "casting". If you have a decent golf swing, the clubhead is already moving so fast through the impact zone, you can't move your hands fast enough to even keep up with it -- much less exceed it enough to further accelerate the head.

That does not mean Hogan's advice is bad instruction, just bad physics. There can be other benefits to the move, if it acts as a "trigger" to hold the lag until the impact zone. If it serves that purpose, then any later attempt at forcing a release is pretty harmless. But that is psycho-biological reasoning, not physics.

Keep the club accelerating through impact for more power. Like the previous point, this is good instruction but bad physics. There are lots of things that will go wrong with your swing if you don't accelerate into the ball, but loss of potential distance is not among them. Let's do the math.

We'll consider a typical strong adult male with a 100mph clubhead speed and a downswing that takes 0.3 seconds from transition to impact.
So the clubhead accelerates from 0 to 100mph in 0.3 seconds. That's an average of 333mph per second. (Yes, "miles per hour per second" is a legitimate unit of acceleration. You won't see it often in physics texts, but you'll hear it a lot in auto sports. "Zero to sixty in five seconds." The implied unit of acceleration is miles per hour per second.)
Suppose you could keep that average acceleration going right through impact. (We all know you can't, but suppose you could. Let's be really optimistic here!) Let's compute how much of an increase in clubhead speed that would give from the beginning to the end of impact. We compute that number, because it is the absolute maximum additional contribution to clubhead speed you could use when you figure ball speed and distance. In fact, it would be no more than half of that.
333mph per second times 0.0005 seconds (the duration of impact) gives 0.17mph. That's only one sixth of a mile per hour. Good for less than half a yard of distance, and that is probably not even measurable among the effects of normal shot-to-shot variation.

So you're not going to get any measurable additional ball speed during impact by accelerating through the ball. And that means that you won't get any measurable additional distance.

So why don't I dismiss the advice as bunk. Instead, I follow it myself. It's because golfers are brain and muscle as well as a source of momentum transfer. If the brain tells the muscles it's OK to ease off at impact, all sorts of other things go wrong. The muscles relax a bit too early, and you lose a lot of speed before impact. Or you stop turning, resulting in wrist cupping, and the ball goes dead left. As I have said before and undoubtedly will say again, just because it's bad physics doesn't mean it's bad instruction.

The ball starts in the direction of the clubhead path and spins in the direction the clubface is pointing. This is often presented as a recipe for hitting an intentional hook or slice. "Line up with your swing path (shoulders, hips, feet) pointing where you want the ball to take off, and the clubface pointing where you want the ball to end up."

Now we know that the ball starts out between the clubface direction and the swing path -- and considerably closer to the clubface direction than the swing path. So this recipe, simple though it is, has no validity in fact. It will produce a hook or slice as advertised, but taking off much closer to where the clubface is pointing than to the swing path.

How come many pros still win tournaments while giving this advice in televised tips? It's because they themselves are not doing what they think they are doing. They practice the shot enough that they know what it feels like and how to do it. If you actually took enough slow-motion video, you would see that the clubhead is coming into impact differently from the way they visualize it -- but exactly as they have learned to do it in practice.

Hook/slice is caused by the clubface rotating closed/open during impact. Another recipe I've seen espoused for an intentional hook or slice. "Rotate the hands closed at impact for a hook; rotate them open for a slice."

My opinion is that these instructions may actually be effective for some golfers -- but not because the ball's spin is affected by the clubhead's rotation. It isn't! But the golfer focusing on rotating the face closed is likely to come into the ball with the face already closed. And that will produce a hook.

Another way to look at it is to see just how much rotation you can accomplish during impact. Numbers!!!

In a good swing, the wrists stay cocked until about 50 millisec before impact. So the club "lag" goes from 90° to 0° in 50 milliseconds. This is an average of 1.8° per millisecond.
Without even trying to rotate the club, the structure of the body rotates the clubface in synch with the uncocking wrists. So the clubface angle goes from 90° to 0° in 50 milliseconds.
Since impact lasts only 0.4 milliseconds, the clubface rotates 1.8° times 0.4 = 0.72°, during impact. That's less than three quarters of a degree.
It is very unlikely that deliberately rotating the club (with hands or forearms) will more than double the rotational speed at impact. But let's be generous and say that it could add another 3/4° of rotation at impact. How much hook do you think this tiny rotation will produce?

Basically, any value from this recipe has to do with the by-products of trying to rotate the club, and nothing to do with the actual rotation during impact.

Square grooves work because the sharp edges dig into the ball. I'm amazed how widely this is believed. It makes a lot of intuitive sense if you assume that the force between ball and clubface is a couple of pounds; the sharp grooves will increase the friction. But now we realize the ball is compressed on the clubface by a couple of thousand pounds, and clubface roughness really isn't needed at all to create maximum spin.

In case you still have any doubt, consider the great square groove war of the late 1980s. It was precipitated when Ping (the only club manufacturer using square grooves at the time) rounded the edges of the grooves a bit so they wouldn't shred ball covers. It accomplished that goal (therefore, it must have radically reduced the "dig" effect of the edges) without affecting the spin performance at all. So something else must be at work here. And now we know that it's the volume of the grooves, and the ability of that volume to channel away the slime that builds in grassy contact.

Gear effect is caused by face bulge. This is repeated by people who should know better -- folks who write instruction about clubmaking. Most of them are no longer making this mistake any more, but I still see it in some places. It is true that gear effect and face bulge are intimately related -- but this myth gets cause and effect mixed up. In an important sense, face bulge is caused by gear effect.

Face bulge is the horizontal curvature of the clubface. You see it in woods, especially drivers, but never in irons. That's because it is intended to compensate for the gear effect on a shot. If there were no gear effect, then face bulge would not be necessary nor helpful. In that sense, bulge is caused by gear effect.

Look at the diagrams. The one on the left has a flat-faced driver with three shots: center-face impact, toe impact, and heel impact. We know now that the toe-impact shot will hook and the heel-impact shot will slice. This is a result of gear effect, and has nothing to do with face bulge.

The picture on the left is not pretty. Depending on where on the clubface you strike the ball, you may hook into the left rough or slice into the woods on the right. But there is a way to make the driver a little more forgiving: curve the face like the driver on the right. In the right-hand picture, the hook from the toe starts out to the right because of face curvature -- bulge. The hook takes the ball back to the center. Similarly with the heel slice; bulge starts it left, and it slices back to the middle.

The shaft is a string at impact. This one is true! This is not a myth at all, but a very reasonable metaphor. What it means is, "If you think you can affect ball flight by force you are exerting on the handle at impact, think again. You have no more control on the head during impact than if the club were on a string."

This is seriously counter-intuitive, so let's take a good look. We've already covered this from the point of view of frequency and time. The time constant of the shaft is orders of magnitude longer than the half millisecond of impact. But now we're ready to look at the forces involved rather than the more abstract time considerations.

Clamp a shaft horizontally by its butt, and hang a one-pound weight from the tip. How much did the tip deflect when you added the weight? Even if you used a stiff shaft, the deflection was close to half an inch. So the shaft deflects at least an inch for every 2 pounds.
Now, how much force does the head exert on the ball during impact? We figured it out in this section, and it's about 2000 pounds.
So how much would the shaft have to deflect for the force it exerts to make a significant difference in what happens to the ball. Well, a one percent change in force is just about at the threshold of a detectable change in performance. That is, if the force change is less than one percent, any difference in the result is going to be too small to notice and very difficult to even measure. One percent of 2000 pounds is 20 pounds.
To finish the story, how much would the shaft have to bend for you to apply at least 20 pounds to the clubhead by something you do at the grip? If one pound bends the shaft a half inch, it will take ten inches of shaft bend to exert a force of 20 pounds. So the shaft would have to bend at least ten inches during impact to make a detectable difference in the ball flight. You know that's not going to happen.

Bottom line: The shaft won't exert any usable force on the head during impact. If your strength and weight don't get the job done while the clubhead is accelerating toward the ball, anything you do at impact won't have any more effect than if the shaft were a string.

A draw rolls more than a fade because it has topspin. It's amazing how often I hear this. But it's obviously not true. If it had topspin, it wouldn't carry 100 yards. That describes a duck hook, which may indeed have topspin, but not a desirable draw.

How did this myth start, and how close to true is it? Well, some draws reduce the backspin and some fades increase it. So, although a draw doesn't have topspin, it may have less backspin than a fade -- thus it will roll farther.

Why should this be? And why don't I say it's true for all draws and fades? Draws and fades can be caused by changing the swing path while keeping the clubface pointing at the target, or they can be caused by opening/closing the clubface. Only really good golfers have enough control of their swing path to determine ball flight via swing path; most golfers will try to open or close the face. This can take many forms: rolling the forearms through impact to close the face, using a stronger or weaker grip, setting up with face open or closed, etc. But they all have one thing in common; they involve closing the face by rotating the shaft around its axis.

Consider the figure at the right. Diagram (a) shows a perfectly upright 90º lie. If clubs were actually built like that, then rotating the shaft would open or close the clubface, and nothing else.

But real clubs are built more like diagram (b), with a lie in the vicinity of 60º. Rotation around the shaft's axis occurs at an angle. So it does not simply open or close the face.

Rotating the shaft to close the face also hoods the face, reducing the loft.
Rotating the shaft to lay the face open also increases the loft.

So you would expect a draw created this way to fly lower and have less backspin, because it was struck with less loft.

The ball goes farther at high altitude because of the thinner air. This is sometimes true, but sometimes not. The prevalence of this belief is undoubtedly due to television announcers saying it repeatedly during coverage of The International, a tournament that takes place at Castle Pines, CO, almost 7000 feet above sea level.

Let's take a closer look at why it isn't true in all cases. It is plausible that the ball goes farther in thinner air, because there will be less drag. But there has to be a limit where thinner air means the ball does not go as far. That's because distance depends on lift, not just on reduced drag, and thinner air means less lift. If we hit a ball in a vacuum (the ultimate in thinner air), we lose a lot of distance from what we're use to at sea level; as we have seen, lift makes a big difference.

But 7000 feet is far from a complete vacuum, and the pros do indeed hit their shots farther at The International. So is the "myth" true for all practical purposes? No, but we have to look at a variety of golfers to see why. By now, we are used to seeing ball speed, launch angle, and spin make a difference in distance.

Let's look at three golfers with very different ball speeds, and vary the loft of their drivers (thus varying launch angle and spin).

First we'll look at the typical Tour pro, with a clubhead speed of 115mph. (The big hitters have a higher clubhead speed; 115mph is modest for a Tour pro.) This golfer does get more distance at altitude. If we look at the graph, we see that his usual driver (assumed to be optimized for sea level performance) has a 10º loft. Even at this loft, 7000 feet gives him an extra 7 yards of carry.

But he has yet another opportunity. The reduced drag and lift suggest that a higher launch angle should help, and the additional spin wouldn't hurt. By going to a 14º driver for The International, he can get better than a 20-yard advantage over his normal driver performing at sea level, and about 15 yards more than his normal driver at 7000 feet. Given the prize money at this tournament, it would be very reasonable for him to have a special driver just for this week (especially since he probably gets the clubs he endorses for free).

Now let's turn to the typical male golfer you'll find on US courses. His clubhead speed is probably around 85mph. (No, he won't say so -- and he'll tell you he typically hits his driver 270 yards. He is most likely delusional, and this is a very common delusion.) At sea level, his ideal driver is lofted at 16º-17º. At 7000 feet, he can get the same carry distance from the same driver. If he adds on a degree or two, he can wring out an additional 3 yards at 7000 feet -- not a lot, but something.

Reality check time! How many typical male golfers do you know who play with a 16º driver? He is much more likely to have a 10.5º driver, but let's give him credit for going to higher loft -- say 12º or maybe even 14º. At these lofts, he is still getting noticeably more distance at sea level than at altitude.

Finally, we turn our attention to the typical senior female golfer, with a clubhead speed of about 60mph. As we see from the graph, she gets more distance at sea level than at altitude for any loft. That's because, at that clubhead speed, she needs all the lift she can get.

The moral of the story is:

The TV announcers tell the truth for the touring pros they are covering. Those guys get significant increases in distance at Castle Pines.
If you're a typical male golfer (not a genuine big hitter), you'll probably get better performance at sea level, unless you get a really high-loft driver (in the vicinity of 18º).
If you're a slow-swinging senior, altitude isn't going to get you any extra distance.

A good driver design has a high launch angle and low spin. That's a qualified "yes", but heavily qualified. True, a properly fitted driver has more loft than you probably would have guessed. But it doesn't have to be anywhere near the mathematical optimum, which is generally much higher launch and lower spin than you can readily build into a driver.

If all you do is get the loft right so that you are on the "ridge" of maximum distance for your golfer's ball speed, then you are fairly close to the maximum distance that can be theoretically attained. Admittedly, that driver will have considerably lower launch angle and higher spin than the optimum driver. But you would have to make large differences in launch angle and spin to get even small differences in distance.

But wait! It's even worse than that. Suppose your driver is designed to be "on the ridge", and you are tinkering with it to get higher launch and/or lower spin. You must be very careful what you tweak; you are more likely to make things worse than better. Consider:

Most things you can change about the club itself will change launch angle in the same way as spin. That is, almost anything you can do to increase the launch angle will also increase the spin. This is guaranteed to reduce your distance.
If you manage to increase the launch angle by itself, without decreasing spin, you will not help distance at all. If you do very much of this, you will hurt your distance.
Same goes for decreasing spin without increasing launch angle. Doesn't help, and enough of it can hurt.

The only reliable ways to increase launch angle at the same time as decreasing spin are to increase your angle of attack and catch the ball high on the clubface. And the way to do that is practice, not purchases. The only ways you can accomplish that through equipment is longer tees (and the skill to catch the ball sitting way up off the ground) and a lower CG in the clubhead.

Last modified Jan 23, 2011