Spin Loft: Part I - Smash Factor and Ball Speed

Have you ever been told to hit up more to make your drives fly further?  Ideas like these are fine, but are only correct if you understand what else is going on at the same time.  Hopefully I'll make things a little more clearer in this blog post regarding 3D Spin Loft, Smash Factor, and Ball Speed.

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

There are a lot of directions that you could go when talking about Spin Loft: how it affects the Smash Factor and Ball Speed, how it affects the Spin Rate of the shot, or how it affects the Spin Axis and the curvature of the shot.  Before we go there, I'd like to define Spin Loft.

Trackman defines Spin Loft as the difference between dynamic loft and the attack angle (Pg. 7, Trackman's Glossary).  Unfortunately, this is incomplete as they are only looking at the vertical components (dynamic loft = Vertical Face, attack angle = Vertical Path), and disregarding the horizontal aspects of the shot.  Spin Loft needs to be measured/considered in 3D, combining both the Vertical and Horizontal Differentials (think: perpendicular to the Spin Axis, instead of just an arbitrary direction like up), in order to properly account for its effects on ball flight.  Spin Loft is actually the angle between the 3D Path and 3D Face, which is different than just looking at the vertical components, as if from a face-on view.

As an example of 3D Spin Loft, let's take a shot with a downward VPath of 5.0° and a VFace of 25.0°.  The Vertical Differential, or Trackman's Spin Loft, is 30.0 degrees.  However, if this same shot had an HFace of 5.0°R (Face is aimed 5 degrees right of the target line) and an HPath of 10.0°L (Path is going 10 degrees left of the target line), we have to take the 15.0° Horizontal Differential into account.  The difference between the horizontal components stretches the Spin Loft and, in this case, combines with the Vertical Differential to result in a 3D Spin Loft of 33.5°.  While it's not a monumental difference, we've still discovered more than 10% extra to the measurement that only took the vertical components into account; this higher number will more closely correlate with other measurements such as Spin Rate and Smash Factor.

Another definition: Smash Factor is easy to figure out; it's simply a ratio of the resulting ball speed to the shot's club head speed (Smash Factor = Ball Speed / Clubhead speed, eg. 145mph Ball Speed / 100mph Clubhead speed = 1.45 Smash Factor).  Newer drivers have been maxed out in terms of their COR (trampoline effect), so you won't see too many Smash Factors over 1.5, with legal golf clubs, anyways.

Lower spin lofts create more compression, so we get higher resulting ball speeds (and, therefore smash factors). Higher Spin Lofts are more of a glancing strike, so less energy goes into propelling the ball, and you get slower ball speeds.  While there are a few factors in play (shorter shafts resulting in slower clubhead speeds, for one) causing wedge ball speeds to be significantly lower than wood ball speeds, Spin Loft is the major one.

In terms of maximizing our driving distance with a given swing speed, lower Spin Lofts have the double benefit of increasing Ball Speed and decreasing the Spin Rate; both of which will increase driving distance for almost anybody out there. I'll discuss how the Spin Rate comes into play, as well as talk about the downside of lower Spin Lofted shots in the next Blog post, Spin Loft: Part II - Spin Rate and Spin Axis.

Summary:
As Spin Loft decreases, effective compression increases = Smash Factor and Ball Speed increase.
As Spin Loft increases, effective compression decreases = Smash Factor and Ball Speed decrease.

Thanks for reading, feel free to ask any questions.

-Mark
@StrongerGolf

Nov. 27th Follow Up:

Here's a chart showing the relationship between Smash Factor and Spin Loft.  Since the data is from Trackman, it isn't 3D Spin Loft being measured, but Vertical Differential.  I have no doubt that, if the data for these shots' horizontal components were available, the correlation would be even stronger between Smash Factor and 3D Spin Loft.