Prior to my work in developing golf balls, I completely misunderstood how a golf ball traveled through the air – especially the golf balls I used. After hitting the ball with my driver, they would start nicely down the fairway, but then would, all of a sudden, move from right to left. It remains a wonder to me why my friends would tell me it was nice to have a ‘natural draw’. To this day I remain intrigued by this comment, as it has paid virtually no dividends with the exception of the rare 309 yard dog-leg left. I imagined the ball sort of tumbling through the air, with the backspin and sidespin produced from my ‘natural draw.’ For every rotation of backspin, the ball would rotate some fraction of a revolution of sidespin depending how much english I put on the ball.
When I started working on the development of a golf ball, I began by researching the topic in more detail. I learned that a golf ball does not tumble; it picks up a spin axis at the point of impact and spins around that axis the entire length of flight. This spin axis is an imaginary line that always goes through the center of the ball. For a straight drive that is hit perfectly, the spin axis resides on a line that is perpendicular to the plane of impact and ball travel. For this perfectly hit straight ball, there is only true backspin and therefore no aerodynamic force is acting to move the ball right or left. The only forces are the lift and associated drag produced by the backspin. For a typical golf ball that is miss-hit, the axis of rotation represents both of backspin and sidespin components, as shown in the illustration to the left. This axis of rotation is fixed, but now the lift vector is shifted to one side, which results in a portion of the total lift vector acting to the side – the reason the ball hooks or slices.
In the early days of developing the hollow metal core golf ball, we used a high speed camera to capture a series of images of golf balls at the point when they were struck by a club and the subsequent moments of flight. Our camera could capture 50,000 images every second with a small field-of-view at our chosen focal plane.
With this camera, the moment of impact was clearly visible; the ball and club remained in contact for only a fraction of a millisecond. This gave us a few dozen images of the ball from the moment of impact until it moved out of the field-of-view. We could observe about 1 to 2 full ball rotations in each image set.
Determining the axis of rotation was difficult as we had not done this before, but once a procedure was developed we could work out the numbers rather quickly. Our camera was located to observe the impact from the side – exactly 90 degrees from the launch vector. The technique we developed required a cross-hair mark on the side of the ball – so that the cross hair was directly in-line with the camera. As the ball was struck and flew downrange, we could observe the rotation of the cross-hair frame after frame. The Figure below shows one frame from this analysis. For a perfectly struck ball, the cross-hair remained in the center of the ball as it rotated, indicating the axis of rotation was located on a line perpendicular to the plane of impact. Since our camera is located perpendicular to this plane as well, we would expect to see the cross-hair simply rotating in the center.
For an imperfectly struck ball with sidespin, the axis would not be in the center of the ball as viewed by the camera located perpendicular to the launch vector. Depending on the direction of sidespin, the cross-hair marked on a typical golf ball would begin to rotate around a point generally on or near an imaginary vertical line that goes through the center of the ball. A line through this point and continues through the center of the ball would form the axis of rotation for that ball.
For shots with a significant amount of side spin (hook or slice), what we found through our analysis was quite remarkable. Our intent was to document the sidespin of typical balls in comparison with various designs of the hollow metal core ball. As noted above, the axis of spin for a typical golf ball was as predicted, and we recorded the amount of sidespin for a given shot with the Iron Byron. This means that the ball has a component of backspin and a component of sidespin. For that same swing on a hollow metal core ball, the location of the axis was significantly different. This axis of rotation begins at a point that lies away from that imaginary line drawn vertically on the ball, when viewed perpendicular to the plane of impact and flight. The axis was shifted in such a way that a simple combination of backspin and sidespin could not explain the spin characteristics.
The answer for the shifted axis found with the hollow metal core ball is the 3rd component of spin: rifle spin. Rifle spin is a transvers spin, and is the same type of spin experienced by a bullet emerging from a rifled gun barrel. As viewed in the direction of flight, this type of spin rotates around the line of flight. The Figure below shows the three components of spin.
Unlike backspin and sidespin, rifle spin does not have an aerodynamic impact on the spinning body. This is because any aerodynamic effects on one side of the ball are balanced by an opposite effect on the other side of the ball. Most importantly however, is that it does have a stabilizing effect on the ball – it is a gyroscopically stabilizing spin. This stability is the reason rifles include the spiral turns on the inside to produce this spin. It produces much more accurate bullet trajectories.
Our analysis showed that the hollow metal core balls have over 2x more rifle spin than any other ball on the market today. It is one of the reasons for the supreme accuracy of the OnCore ball.