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New Ball Flight Laws

For almost the entire history of golf instruction, and certainly since the publication of John Jacobs’ Practical Golf in 1972, the ball flight laws have been set in stone. The PGA manual of golf, and generations of golf professionals have believed that, a well struck golf shot will start on the path that the club was swinging along, then change direction towards where the clubface was looking at impact. Many top teachers still believe that the swing path controls the initial direction of the ball flight. This assumption is fundamental to the teaching theory of all of the major teaching organisations, as well as being built into the software of most golf simulators. There is now clear evidence that this theory is outdated and needs to change. Recently some teachers have put forward the theory that the ball will leave the clubface in exactly the direction it is looking, then bend because of the sidespin applied by the club path. If golf ball development were to continue unchecked, this theory may eventually be correct. At present the correct ball flight law should be that:

 A well-struck ball will always leave the clubface at close to the direction it was facing, then change direction relative to the differential between the clubface and club path at impact.

 I want to be clear, the original theory was (probably) correct and this is supported by many years of successful teaching. It is outdated now because of the changes in modern equipment - in particular “hotter” golf balls and clubfaces. The modern golf ball flies much further and straighter than the 3-piece soft balata balls, common in the 1950’s to the 80’s. But technology has changed the game, and has also altered the ball flight laws. There is a lot of complicated physics effecting the collision between the club and ball, but the most important element is easiest to understand and that is the change in the elasticity of the ball.

 If you drop a golf ball at arms length onto a hard surface, it will rebound, but to a slightly lower height; this is because some of the energy is absorbed through the elastic properties of the ball. The amount of the rebound is at a constant rate over a large range of speeds and is known as the coefficient of restitution, or COR. So a ball dropped from 10 feet that rebounds to 7 feet has a COR of 0.7 and will rebound to 14 feet if dropped from 20 feet. A perfectly inelastic ball (if one were ever to exist) would have a COR of one, and a ball of putty would have a COR of almost zero, not rebounding at all. You could describe the effect of a lower COR as allowing the ball to stay in contact with the colliding surface for longer; this rule also applies to angled collisions. It is a fact that the COR of golf balls has changed over the last 30 years. There is even a considerable variation between the balls available today. I recently tested two golf balls to try and illustrate this change in elasticity over the years. A modern ball dropped from 60 inches rebounded consistently to 44 inches, giving a COR of 0.73. Whereas a balata ball from 1990 dropped from the same height, only rebounded to 36 inches, giving a COR of 0.60 – a substantial difference! 

 It is easy then, to visualise a high-speed video of a clubface striking objects where the face is (for example) 10 degrees open to the path. If you were to hit a steel ball it would leave the clubface almost instantly at 10 degrees to the path, but strike a ball of putty and it will remain on the clubface and travel along the path, at zero degrees. We may also observe that the cushioning effect of the sand during a bunker shot, will reduce the COR of the golf ball, allowing it to follow the club path for longer. A ball from the 1950’s may have had a COR of less than 0.5, and probably would have followed the club path for some time, before changing direction quite violently, because of the spin created during the impact. The modern ball, however, when struck solidly, will compress on the clubface then rebound at slightly less than the angle of the clubface, but it will not follow the path of the club.

The New Ball Flight Law

 A well-struck ball will always leave the clubface at close to the direction it was facing, then change direction relative to the differential between the clubface and club path at impact.

 Or, if you prefer:

 “The clubface sends it, and the differential bends it.”

 This change simplifies the teaching and analysis of ball flight considerably, because there is now only one club path (the actual club path relative to the clubface) and we no longer need to refer to the club path as being in to out, or out to in, of the imaginary target line. The clubface aim is now our primary concern, and the swing path is either square to the face or not, with the differential creating the sidespin and therefore movement, left or right. One of the most important changes this brings about, is that the ball will never cross the club path (unless it is moved by the wind). For a right-handed player, a ball struck with the face closed to the swing path, will always move to the left of the swing path immediately, and visa-versa. A ball struck with the face closed to the swing path, can never start to the right of the path (or even on it) and then move left, it can only moved left immediately.

 

 If we were to reintroduce the imaginary target line, then to hit a ball that starts to the right of the target line and draws back to the target, you would need the clubface to be open to the target line, but closed to the swing path.

 

Conclusions

 

It is now clear that the golf balls used many years ago, had a low COR and when combined with the equipment available, would probably create impact conditions that allowed the ball to follow the swing path, then move in the direction of the clubface. This created the current (flawed) ball flight laws. With the advent of modern balls and clubs that have high COR properties, the ball now immediately travels close to the direction the clubface was looking, then changes direction only if there is a differential between the clubface and the club path.

 “A well-struck ball will always leave the clubface at close to the direction it was facing, then change direction relative to the differential between the clubface and club path at impact”.

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