Debunking the myth of spin in putting

A while back I saw an Instagram post which consisted of a short video clip of a bi-colored golf ball spinning, top-like, around its vertical axis, on an artificial putting surface—but not otherwise moving. The text accompanying the clip says, “This ball has side spin and yet it stays on the spot... but I thought side spin makes the ball go off line. 🙂”

The post, by a UK-based putting coach named James Jankowski, was obviously trying to make some sort of point, in an amusing way, about the movement of a putted ball, but the video clip was both pointless and useless from the standpoint of presenting useful information. “Why?”, you ask—well, the spinning motion couldn’t make the ball go off line because there was no “line”—the ball was not moving along the putting surface. Since the ball was just rotating in place about its vertical axis, the frictional and inertial forces generated by this motion were balanced and did not produce a resultant force that would displace the ball from its position.

A certain amount of back-and-forth discussion ensued when I commented on the pointlessness of the video and its accompanying narration; in this discussion there was talk of skidding and sidespin as the ball comes off of the club face—which was not mentioned or demonstrated by the original video clip—and an unfortunate descent into the ridiculous notion of “rifle spin”. I decided, then, that it was pointless to argue, or even attempt to discuss the complexities of the motion of a putted golf ball on Instagram, especially with a self-defined expert who has obviously had no education or training in the physics of objects in motion. What I decided to do, instead, was to take the matter long form, as it deserves, and write an article about it.

***************

Before we get into a discussion about the movement of a putted golf ball, there are some important terms which we must define: rotation, translation, rolling, spin, and skid.

The first three terms are very closely related. Roll occurs when the ball is moving across the surface, or translating, and frictional contact causes the ball to rotate about an axis that is parallel to that surface. When a ball is rolling the translational and circumferential speeds are equal; that is, the ball is moving across the surface with a one-to-one relationship of surface contact to forward motion. A rolling ball travels a distance equal to its circumference—about 5.28 inches—for every full turn.

A ball that is both rotating—with one-to-one contact with the surface—and translating, is rolling.

Spin, on the other hand, means that the circumferential speed is higher than the translational speed; there is slipping, rather than one-to-one contact at the point where the ball touches the putting surface. If a ball is spinning while in contact with a surface it travels a shorter distance along the surface than the length that the circumference of the ball has turned.

Skidding is the opposite of spinning; the translational ball speed is higher than the circumferential speed. When the ball is skidding it travels farther along the surface than the length that the circumference of the ball has turned.

In short:

• Roll – the ball rotates at the same speed at which it translates.
• Spin – the ball rotates faster than it translates.
• Skid – the ball translates faster than it rotates.

Be aware that many of the coaches and self-appointed “golf gurus” whose lessons you will find online use the term “spin” incorrectly when talking about putting. They refer to putting “topspin” on the ball—that is, hitting the ball such that the top of the ball is moving forward in the same direction in which the ball itself is moving. What they are referring to is roll, and as I will explain in the next article in this series, it is not an action by the golfer that induces the forward rotation that constitutes roll.

The truth is that referring to spin at all when talking about putting is misleading and incorrect. For spin to occur when a ball is putted, with the ball in contact with the putting surface, the tangential force applied to the ball by the club face—the force that would cause the ball to rotate—must be greater than the frictional force applied to the ball by the putting surface in the opposite direction. Even if the ball leaves the surface momentarily, whether because of a high-lofted putter or a poor stroke (with backwards shaft lean adding loft to the club face), the near-vertical attitude of the club face and the generally low force applied to the ball (compared to full-swing shots) will not produce spin; i.e., rotational speed that is higher than translational speed.

Think of an approach shot to the green with a well-hit wedge, then compare, in your mind, the club head speed of that shot, and the loft of the wedge, to the speed and loft of the putter—and consider that the approach shot is traveling through the air, and not dealing with the friction of the ball-to-turf interface.

Trust me, you are NOT spinning the ball when you putt.

Now that we have (hopefully) put the idea of spinning a putted ball behind us, let us now address the concept of “side spin”. While it is physically impossible to spin a putted ball, this term is also used to describe the movement of a ball hit through the air—but it is a misnomer.

The proponents of “side spin” would have you believe that the golf ball is simultaneously spinning backwards, about a horizontal axis, and either clockwise or counter-clockwise, about a vertical axis (like the spinning ball in the Instagram video.). The truth is that a sphere can only rotate about one axis. (If you don’t believe me, read up on Euler’s Rotation Theorem and argue with Leonhard about it – (https://en.wikipedia.org/wiki/Euler%27s_rotation_theorem.) The golf ball cannot simultaneously have back spin, which all balls hit with a lofted club have, to some extent, and “side spin”. Even if a way could be contrived to simultaneously induce spin in these two different modes, the inertial forces acting on the ball would resolve themselves into rotation about one axis that is neither purely horizontal nor purely vertical.

What has happened when an airborne ball does not have ideal, pure back spin (as in the case of that slinging banana hook that just took your Pro-V1 on a one-way trip to O-B Land) is that the characteristics of the contact between the club face and the ball—a combination of club head path and face angle relative to path—has produced spin about a single axis that is tilted with respect to the horizontal. When that happens, the Magnus Effect, an aerodynamic phenomenon that produces an imbalance in the forces acting on a ball in flight, will cause it to curve to the left or the right, depending upon the direction of the tilt of the rotational axis.

On the other hand, a ball that is in rolling contact with the putting surface is rotating about an axis that is parallel to that surface. It will react to the angle of tilt of that surface (the break), if any, by moving right or left, but that movement is not induced by spin; it is a combination of the effects of momentum and gravity that makes a rolling ball “take the break”. Depending upon the loft angle and the force of the strike when the putter contacts the ball, and the relative frictional characteristics of the club-to-ball and the ball-to-putting-surface interfaces, the ball will skid for some distance before settling into rolling contact, but that is not only not spin, it is the opposite of spin—it is skid.

Since I have already gone on for well over 1,000 words debunking spin in putting, I will break here so you can catch your breath before we move on to the next subject—and the real bugaboo of putting—skid.