Spin Loft: Part II - Spin Rate and the Spin Axis

While the Spin Loft of a shot is important, our interest usually lies in how it changes what the ball will do. This post will focus on the spin of the ball for standard, full shots (there are additional factors when talking about shorter, green side shots); which way the ball will spin and how quickly.

As with a lot of my posts, checking out this page of definitions might help with some of the technical terms used.

If you haven't already, you'l probably want to read Spin Loft: Part I - Smash Factor and Ball Speed, as it defines Spin Loft and has some other helpful in there.

A golf shot's Spin Rate is a function of Spin Loft, Clubhead Speed, and contact location. More Spin Loft will result in more spin, as will more Clubhead speed. Less Spin Loft will result in a lower Spin Rate, as will slower Clubhead Speeds. [For complete info on contact location, please refer to my blog post on miss hits.]

These two shots would have identical Spin Rates, all else being equal (assuming no Horizontal Differential):
1) VPath (Angle of Attack): -3.0°, VFace (Dynamic Loft): +9.0°, Spin Loft = 12.0°.
2) Vertical Path: 1.0°, Vertical Face: 13.0°, Spin Loft = 12.0°.
The golf ball doesn't know if it was hit down on or up on, it just knows that it was hit with a 12.0° glancing blow.

So, while changing your Angle of Attack may affect your Spin Rate, it won't if the Dynamic Loft changes by the same amount, since you'd end up with the same Spin Loft (although it would result in a higher launch, since both the Face and Path are more upwards). This means that just hitting more up with your driver will only help you reduce your Spin Rate if you can prevent the Vertical Face (Dynamic Loft) from increasing at the same time.

In simplest terms: the closer the 3D Face and 3D Path are to each other, the lower the Spin Rate will be (also, the higher the Smash Factor and Ball Speed).

Now, the Spin Axis tells us how tilted the D-Plane was, and how sideways the ball will be spinning.  A perfectly straight shot, with no sidespin, is spinning perfectly backwards, around a perfectly horizontal Spin Axis (0.0°).  A theoretical shot with a perfectly vertical, 90.0° Spin Axis (either Left or Right) would be spinning straight sideways.

The Spin Axis will tilt any time the Horizontal Path and Face are not going in the same horizontal direction.  How much it tilts depends on the Horizontal and Vertical Differentials.

Two different golf shots could have identical Horizontal Path and Face numbers, but experience very different Spin Axis numbers, depending on the difference between the vertical components (Vertical Differential).  Shots with very high Vertical Differentials (like wedge shots) stretch the D-Plane upwards resulting in flatter, less tilted Spin Axis numbers than low Vertical Differential shots (like drivers), which will tilt much more.

For example: two shots could share the same Horizontal Differential (difference between path and face, as if viewed from overhead), let's say 3.5°.  The lower Spin Lofted shot (say, with a VDiff of just 5.5°) will experience a tilting of its Spin Axis of 32.7°!  This would not be a useful golf shot at all.  However, the same Horizontal components could result in a Spin Axis tilt of only 6.6° (slight curvature) if its' VDiff was much higher (31.5°).  This sounds like a lot of specific numbers, but it explains why drivers spin sideways way more than wedges.

This is the trade-off between distance and accuracy; lower spin lofted shots will tend to carry further due to higher ball speeds and lower spin rates, but the same shot will tend to spin sideways more if the Path and Face get out of line much.  Given this information, one might decide to try and hit shots with lower Spin Lofts on open driving holes where distance is a priority and hit shots with higher Spin Lofts on holes where you'd want more control.  In fact, Bubba Watson (admittedly, one of the more creative players on Tour) has been measured varying his Vertical Path between -7.5° and +5.0° during competition (Trackman Newsletter #2, Pg. 12), suggesting that he employs this kind of strategy (whether he knows it or not).

So, in summary, a golf ball's spin (amount and direction) is largely controlled by its Spin Loft, which is simply the difference between the Face and Path.

-Mark
@StrongerGolf