How Club Path and Face Angle Determines Ball Flight

[Big Lex]

Initial direction a ball is going to travel is decided by the swing path. Which way the ball is going to curve is decided by face angle relative to swing path. Iacas is right, the diagram is incorrect, no matter what way you look at it.

I'm afraid I have to disagree.

I was taught that the path determined the starting direction, and the face the curvature, just as you say. But this is incorrect, at least partially. Before I go on as if I've deduced this in my laboratory, let me say that I learned all of this from reading several scientific books on golf, the most purely scientific of which is "The Physics of Golf" by Ted Jorgensen, an emeritus physics professor from the University of Nebraska. Other books that explain the ballistics and aerodynamics of impact and initial ball flight are "The Search for the Perfect Swing" by Cochran and Stobbs, and Homer Kelly's "The Golfing Machine." But the Jorgensen book is really the best source. Anyway, both swing path and face angle influence the starting direction of the ball, but face angle has a greater influence. Basically, the golf ball compresses to some degree on the face, and then rebounds off the face on a vector "normal" (perpendicular is not exactly the correct word, but the same concept, in 2 dimensions) to the face of the club. For the brief period that the ball is "stuck" to the face, obviously the path of the head will carry the ball on its line for a very short distance, but the rebound vector from the face exerts a stronger influence on ball flight when the ball finally separates. The final direction is somewhere in between what would be determined purely by the path and face angle, and overall closer to that of the face angle. I'll check my sources tonight and attempt to get a quote or quotes for you guys tomorrow.

[Big Lex]

By the way--the second part of bwinger79's statement - that the curvature is determined by the face angle relative to the swing path - is correct. But the starting direction is not purely due to club head path, in fact it's more to do with face angle...As I said, tomorrow I will try to get some quotes for you.

One more thing--there are lots of pseudoscientific things written about golf all over the web, and I have no doubt many could find a "scientific" explanation of golf impact somewhere on the web that states initial direction is determined by club head path...I still disagree. I'm going with the university physics professor, and the guys who've studied this with high tech gear.

Dave Pelz, incidentally, has studied and measured the same things in putting. Putts don't hook or slice in the air obviously, but for putts the starting direction of the ball is dependent on both the path and face angle, again with the face angle exerting greater influence. I'll find that reference, too...I think it's in his first book "Putt Like the Pros."

[Big Lex]

Disclaimer: I am not a physicist (nor did I stay at a Holiday Inn Express last night,) though I do have a degree in engineering and am generally a physical sciences geek.

Great post...I hadn't read it before I posted my reply. Again, I will consult the Jorgensen book tonight, but I believe the reason your explanation is incorrect has to do with the elasticity of the ball. As you say, your model assumes a rigid ball, which it is not. I believe that much of the force of the swinging club head is stored as potential energy in the ball. According to the coefficient of restitution of the ball/face system, the ball rebounds with some of this energy, but the direction of rebound is normal to the club face, not in line with the club head path. In the end, more energy is imparted to the ball along the line normal to the face than along the line parallel to the club head path. As you say, both forces influence the starting direction, but the face angle is believed to have more effect. If I'm wrong, I'll 'fess up tomorrow. But either way, I'll give you the research answers in the AM...

[bwinger79]

Hey.... you guys can get into all the physics you want to, but I can go out on the course and show you that (other than extreme cases) initial path is determined by swing path and curve is determined by face angle. Of course there are exceptions to the rule. I can open up a 6 iron and still hit a high draw. There are far too many factors to take in to account to make this an apples and oranges conversation. Theory is great, but unless you can put it to practical use on the course, it really doesnt matter.

[Big Lex]

Hey.... you guys can get into all the physics you want to, but I can go out on the course and show you that (other than extreme cases) initial path is determined by swing path and curve is determined by face angle. Of course there are exceptions to the rule. I can open up a 6 iron and still hit a high draw. There are far too many factors to take in to account to make this an apples and oranges conversation. Theory is great, but unless you can put it to practical use on the course, it really doesnt matter.

bwinger79: If you're going to discuss something technical like this, you gotta be precise. First of all--I don't disagree that sidespin is determined by the relationship between the face angle and the club head path. What I'm debating is what determines the initial flight vector of the ball.

Your idea that you can "prove" it on the golf course doesn't really apply here, because what you think you're doing with the club and what it is actually doing at impact may be 2 very different things. Until Spiderman takes up golf and can see impact in ultra-slow motion, we have to rely on physics, experiments, etc., if we really want to answer this kind of question. All you've proven in your example of "opening it up and hitting a high draw" is that you can address the ball in this manner and produce a high draw. It tells us nothing about what happened at impact, unless you were in a lab somewhere having the swing path and impact conditions recorded. Yes, it has been written many times that the club head path determines the face angle. If you believe this, you can interpret anything about the flight of the ball with the assumption that it's correct. But that doesn't prove anything. This much I'm sure I remember from the physics book: Initial starting direction is determined by both face angle and club head path. I am relatively sure that face angle has a greater effect, and I am unsure as to how much greater.

[bwinger79]

bwinger79: If you're going to discuss something technical like this, you gotta be precise. First of all--I don't disagree that sidespin is determined by the relationship between the face angle and the club head path. What I'm debating is what determines the initial flight vector of the ball.

I practice on a launch monitor 3 times a week. I am very familiar with whats going on at impact. Dont need to be spiderman if you have science and slow motion cameras. Unless we have accredited physicists on this forum, which I am sure we dont, there are far too many variables happening at impact for anyone here to wrap their mind around. If you guys feel like continuing an argument that is speculation at best on all sides, feel free. Practical application seems to be what really matters in the end though.

[Ringer]

You don't have to be spiderman when you have SwingVision..

1:06 on this video...

[iacas]

I think you also need to define "initial direction".

It doesn't mean the first 1/100 of a second. It means just what you would mean by common sense: the direction a ball travels before it beings to curve noticeably. That's about one or two seconds.

Prime example is a flop shot, how come the ball doesn't go with your club, if you had a ton of sidespin you'd see it when it hit the green.

That's not what we're talking about here. We're talking about low-loft situations. In a flop shot (well above 45 degrees of loft - sometimes 70 or 80), the vector in the x axis is much larger than in the y axis.

Besides, bunker shots do tend to spin to the right. Same with a flop shot. They're just not travelling forward with enough velocity for you to see a curve. But it's right there on bunker shots - they spin right because you cut across the ball a little.

What I'm debating is what determines the initial flight vector of the ball.

I don't think you're right either. Again, we're talking about very limited angles here. A square driver face with a path outside-in about four degrees will start nearly three degrees left of the target line and cut back about three degrees to the center line.

Just as I was reading the current Golf Digest an hour ago, I saw this (page 204): "How to hit a Draw: Here my stance is closed, so the ball will start right, but my clubface is square promoting a draw back to the target." Again, I'm limiting this to very small angles. Once you get outside of 8 degrees or so, the angle of the clubface begins to take precedence.

Yes, it has been written many times that the club head path determines the face angle.

It's been written by teachers of some of the best golfers - including the best golfer - in the world. It's not just random schmucks writing this stuff.

Putting is a different thing altogether, so I don't really care what Dave Pelz says. Your putter clubhead isn't moving 90+ miles an hour. That's another reason this discussion changes once you get past your 6-iron or so: clubhead speed drops and the ball doesn't compress anywhere near as much. The Golfing Machine is a fairly old text. Launch monitors are fairly new. I know what I saw in watching thousands of golf balls hit with pretty accurate launch monitors when I did the Titleist Science Van article. Once you get outside about eight degrees, I agree the angle of the clubface takes over. Once you get below a certain speed or above a certain loft it can take over too. I'm talking about less than about a 6-iron's loft (at most up to an 8-iron or so, which pros deloft quite a bit), swung quite fast, and with a difference of clubface versus swing path of +/- 8 degrees (of perpendicular). In other words, drivers and mid- to long-irons. Just the kinds of things you'd want to cut and draw.

[iacas]

You don't have to be spiderman when you have SwingVision..

We're not discussing the launch due to clubface loft, and I don't think that video adds anything to the discussion.

[Ringer]

We're not discussing the launch due to clubface loft, and I don't think that video adds anything to the discussion.

Which again begs the question, why is loft angle so different from face angle? The physics do not change. What makes the ball go up is also what makes the ball go left or right.

[iacas]

Which again begs the question, why is loft angle so different from face angle? The physics do not change. What makes the ball go up is also what makes the ball go left or right.

Okay, then here you go: Tiger's dynamic loft is about 7 degrees, yet his ball launches at about 11 or 12. Why? Because he's swinging ever so slightly up at the ball as well. Plus, these are additive forces - the equivalent of swinging inside-out with an open (relative to path) clubface. And we're still seeing clubhead path affect his launch. Just not as much (4/11 or 5/12 versus 7/11 or 7/12).

Outside of that, the physics of loft are very different than the physics of slidespin. Not only is the spin always in one direction (backspin), but there's a force directly opposing it (gravity). Several other factors ranging from the location of the center of gravity to the location and behavior of the clubshaft also affect these types of things in different ways than they do the other things we're discussing. P.S. Hit down with a wedge and you get a lower ballflight and more spin than you do with the same dynamic loft but a flatter swing. All that's

[Big Lex]

I don't think you're right either. Again, we're talking about very limited angles here. A square driver face with a path outside-in about four degrees will start nearly three degrees left of the target line and cut back about three degrees to the center line.

You may be right--you're definitely right about some of what you say, but I still don't think you've got it 100%. Again, my memory may be failing me here, but I'm fairly certain that at least for the conditions tested in the books I'm referencing, face angle affects starting direction to a greater degree. As for some of the other points you make:

Remember that the various devices known as launch monitors produce a description of a golf shot that is not 100% measured data. The Vector monitor measures spin and launch angle...everything else about the shot is calculated based on formulae...other devices measure certain aspects of the impact and initial flight conditions...anyway, the point is that just because a launch monitor may say "face angle 0 degrees, path 4 degrees out/in, starting vector 3 degrees left" doesn't mean all of those numbers are measured data. It may have measured one or two things and calculated the rest from assumptions. As I said, I'm pretty sure that in one of the science texts, the initial flight characteristics were all measured - with driver shots - and the answer to this question about the role of face angle and path was clearly explained. The fact that reputable teachers and players believe that the path determines the starting direction means nothing to me. As I said, path does have some role, and who knows, maybe these guys are so consistent with their face angle that, for them, path is the only thing that makes any difference in what they see as the starting vector. Or maybe when they produce a draw, for example, the face is actually pointing open to the target, and not square to it, and the path is just slightly more rightward (for a righty) relative to it, producing the 'start right draw back' shot. But someone's opinion judging by their golf shots doesn't answer the question. It's a conceptually simple physics question. It isn't easy to collect data to answer it, but I think we have had a credible answer to the question. I'll try to provide this source's answer tonight or tomorrow.

[bwinger79]

But someone's opinion judging by their golf shots doesn't answer the question. It's a conceptually simple physics question. It isn't easy to collect data to answer it, but I think we have had a credible answer to the question.

I wasnt basing my opinion on what I see on my golf shots. My opinions are based on the technical data collected from about 6 hours a week on a launch monitor, for the past year or so. But I digress.... all I really care about is the end result. I'll leave the physics to the scientists and let them make cool GUI tools that lets me understand it.

[iacas]

The Vector monitor measures spin and launch angle...everything else about the shot is calculated based on formulae

Let's refrain from assuming I've only ever seen results on launch monitors that measure the direction and spin of a golf ball. They make 'em a good bit better and more complex than that these days.

The fact that reputable teachers and players believe that the path determines the starting direction means nothing to me.

Suffice to say I feel the same about an old text in light of my first-hand accounts of watching rather expensive launch monitors do their work.

I'd also like to think I'm a good enough golfer that I can reproduce a swing fairly consistently - consistently enough that nothing I've experienced has done anything to contradict what I've been saying in this thread. I used to believe it was the opposite - that what you're saying was true. When I came to realize that was wrong my understanding of my golf swing and how to fix certain problems improved dramatically. Furthermore, again, let's be clear that this is a fairly small sample size. You've got to swing fairly quickly to compress the ball enough for it to be "carried" on the clubface. Not everyone can do that, and it was a lot easier to do 10 years ago with wound balatas, but the parameters here are still fairly tight: high clubhead speed, small angular differential, low loft.

But someone's opinion judging by their golf shots doesn't answer the question.

I'm not using my own golf shots as anything but experiential evidence, and again, it's done nothing but back up what I've witnessed on some very expensive launch monitors.

Even the (relatively) cheap launch monitor Titleist used in their driver fittings could show you precisely where on the clubface the ball was struck, what angle the clubface was, which direction the clubhead was travelling (in both axes), etc.

It's a conceptually simple physics question. It isn't easy to collect data to answer it, but I think we have had a credible answer to the question. I'll try to provide this source's answer tonight or tomorrow.

High school physics is simple. This ain't, and I'm not sure I or the others here are going to agree with your source when we have first-hand knowledge that it's not true.

And with that, I'm out. I used to believe the opposite was true, I've been convinced otherwise, my golf game has improved because of it, and nothing I've seen since has in any way convinced me I was wrong the second time.

[bwinger79]

Just to add to Iacas's post, Hot Stix Golf doesn't use "cheap launch monitors" either. I am very fortunate to live right around the corner from one of their facilities. There is no club fitting company in the world that understands golf swing dynamics better than Hot Stix in my opinion, but as Iacas alluded to, I am sure the club manufacturers like Titleist use top of the line stuff. Its a lot more than loft, backspin and swing speed. Technology is a beautiful thing......I know its helped my game.

[Big Lex]

I'm going to do more research...I am not an expert on launch monitors, although I know what they do, have hit balls on them, and have read quite a bit of information about tour players' shot characteristics when they hit on monitors.

You guys have far more experience on these things than I do, and you may be right. Perhaps what I've read is now proved wrong by these devices. But I'm not convinced...I want to be sure you understand what I am saying.

What I'm trying to get across to you guys is that launch monitors produce comprehensive reports of golf shots, but not necessarily everything in that report is something directly measured by the device. Obviously, the 3-D rendering of the flight of the ball must be calculated, since the shots are hit into a net. Similarly, some of the basic info--whether it's face angle, path of club, etc.--may also be calculated and not directly measured.

Any time you calculate something, you need a formula to deduce the variable. If the formula is wrong, then the output is garbage.

For instance, let's assume a fictional launch monitor with certain, limited abilities. It has a certain number of lasers/strobes/cameras, etc., and is able to photgraph the ball's initial flight and spin, measuring spin rate and the path of the ball in three vectors, and a few inches of club head path. That's it, nothing more. It's unable to measure club face angle, but the manufacturer of the device realizes people need to know the club face angle. So, they come up with a formula to calculate face angle based on the other things they measured. If the formula they choose assumes, for example, that the side vector of the ball is determined 80% by the club head path, then the face angle calculation it makes will obviously lead you to believe that, in fact, club head path determines 80% of the side vector.

I'm not saying any launch monitor does this, or that someone would even go about solving the hypothetical problem in this specific way. I'm just pointing out that just because a machine spits out a number, it doesn't mean the number is a true reflection of reality.

All other points made are excellent--yes, putting and driving are two different animals, and the ballistics of impact are related but different. And yes, there are probably golfers with slow enough swing speed that the relative contributions of face angle and path change. But I think the relationships are probably fairly linear over a range that would capture most male golfers, but I have no way of knowing this. Iacas's point that the relationship may change with different lofted clubs and with marked differentials in face angle v. path also sounds logical, but again I haven't read anything about this aspect of the question.

Anyway, I'll be back later with something for you guys to chew on.

[iacas]

Similarly, some of the basic info--whether it's face angle, path of club, etc.--may also be calculated and not directly measured.

I said I wouldn't add more to this conversation, but I feel compelled to say this: the launch monitors I've seen don't calculate the club's position. They

measure it. The only thing they calculate is distance, really. Let's not assume some fictional launch monitor with limited abilities. The ones I've seen used aren't very limited.

[$2 Nassau]

Wow. I'm confused. I've always seen it the way Iacas has described it. Swing path has the greatest influence then face angle. Wouldn't that explain when I hit a draw the ball starts a little to the right then draws back in? My clubface is slightly closed there, with the other theory I would be hitting the ball dead left, but I'm not.

conversely, when I cut across the ball with an OTT swing and open clubface, the ball starts left then slices back to the right. With the other theory that ball would just go dead right...but it doesn't.

???

:confused