Tech Story 5
Where on the club head TrackMan measures its data points.
As the club head moves through impact on an arc while simultaneously rotating, different parts of the club head, therefore, travel at different speeds and in different directions.
For a driver, the difference between the path of the club head’s center of gravity (CoG) and the path of the center of the club face is approximately 3 degrees (center of club face path being more outside-in). This is because the CoG is located approximately 25-50 mm behind the club face. The same counts for Attack Angle of CoG and the center face, where the center of the face moves more upwards compared to CoG for a driver (typically 1 degree).
For non-drivers the physical distance between CoG and the center of the club face is much smaller, which makes the parameters being less sensitive to which point on the club face is being used. TrackMan reports the Club Speed, Attack Angle and Club Path relative to the CoG, or more precisely at the geometric center of the club head.
TrackMan directly measures the 3D silhouette of the club head and uses this to determine the geometric center of the club head, a point which is typically located inside the club head, except for cavity back type of irons/wedges. The geometric center of the club head is located very close to the CoG of the club head, typically less than 6 mm apart. Optical based launch monitors almost always measure Club Speed, Attack Angle and Club Path relative to the center of the club face, since it is where the reflective markers are positioned.
TrackMan’s motivation for selecting Center of Gravity or CoG as reference points is twofold:
1) The mass of the club head is what the golfer is swinging and what collides with the ball and makes it move.
2) This is closely related to what TrackMan directly measures (3D silhouette), meaning no assumptions of club-head design is required.
To conclude, it is very important to be aware and to understand the differences in measurement methodologies applied (Center of Gravity vs Center of club face, for example) when tracking the collision interval between the club and the ball. More often than not, the numbers measured (or calculated) aren’t comparable.
Club face orientation
For Face Angle and Dynamic Loft, TrackMan uses the orientation of the club face at the center point of where the ball is in contact with the club face, meaning “at the impact location.” For irons and wedges where the club face is completely flat (no presence of bulge or roll, as found on drivers), there is no difference between the orientation of the impact point and the center of the club face.
On woods and Drivers, however, there are significant differences between the club face orientation of the center of the club face and other locations on the club face. This is due to roll and bulge of the club face. For standard drivers, a 10mm impact towards the toe will cause the Face Angle to be 2 degrees more open at the impact location when compared to the center of the club face. Similar an impact 10 mm low on the club face will cause the Dynamic Loft to be 2 degrees lower at the impact location when compared to the center of the club face.
TrackMan has chosen to use the Face Angle and Dynamic Loft at the impact location, since this is the only orientation of club face the ball is influenced by.
When in time TrackMan measures club delivery
Concerning when during the collision interval the club delivery should be measured, the question is which approach should be used: 1) at first contact between the club face and the ball, 2) at maximum compression of the ball, or 3) when the ball separates from the club face.
When considering what method should be used to capture club data, it is important to distinguish between player controlled movement of the club-head and ball collision induced movement of the club-head. When the club-head collides with ball at least two things happen: 1) the club-head is slowed down since it is transferring energy to the ball, and 2), the club-head is deflected downwards since the ball is launched more upwards than the attack angle.
If the club path is not aligned with the ball launch direction, the club-head is also deflected horizontally as well. If we also consider that the ball launch is not aligned perfectly with the CoG of the club-head (meaning off-center impact), then the club head will be rotating around its CoG. This rotation can happen both horizontally and vertically.
The magnitude and direction of the collision-induced movement of the club head depend on several things, most importantly the club delivery (club speed, spin loft, and face-to-path), clubhead parameters (weight, MOI, etc.) and impact location. Collision-induced movement can be quite big. For a toe impact with an iron, for example, the club face can open 1-2 degrees during the collision interval from first touch to ball separation from thenclub face.
For a driver with a typical spin loft of 10-15 degrees, the attack angle can become 4-6 degrees steeper at ball separation compared to first touch. Note that both the collision-induced rotation and deflection will quickly vanish shortly after the ball has left the club face since the shaft will equalize the twisting that happens during the collision.
TrackMan’s Club Speed, Attack Angle, and Club Path data solely reflects the player controlled movement, meaning reporting on pre-impact data only. This means there are no deceleration or deflection of the club-head due to collision induced club head movement taken into account, also turf interaction while impacting the ball (or just before impact “fat shot”) are excluded.
As a result, TrackMan’s Club Speed, Attack Angle, and Club Path data exclusively reflects how the player swings the club. The time used for these three parameters is at maximum compression, but only using pre-impact data. Note that the actual influence on the data when selecting maximum compression instead of, for example, first touch, is very minimal, as long as only pre-impact data is used (less than 0.05 mph and 0.4 degrees).
The reason for selecting maximum compression as the time reference and not first touch, which might have been more intuitive, is to align time wise with the definition of Face Angle and Dynamic Loft and thereby to make the usage of D-plane easier. As mentioned, Face Angle and Dynamic Loft can change 1-2 degrees during the impact interval.
TrackMan determines what is the average orientation of the club face at the impact location in that brief time span. This closely aligns with the orientation of the club face at maximum compression. TrackMan applies this impact moment to define Face Angle and Dynamic Loft as it most accurately explains why the ball launches in a given three-dimensional direction.
TrackMan has chosen the club parameter definitions above with the clear goal to:
A) Keep it simple and have as few parameters as possible. The fewer parameters one has to relate to while still telling the full story, the easier the system is to use – this is particularly true for coaches and players.
B) Deliver actionable data, meaning when a golfer needs to change a club parameter; it must be clear what needs to be done swing or equipment wise.
C) Have a club and ball flight model that links club delivery with resulting ball flight, making it possible for coaches and players to connect all data points in an easy and understandable manner.
What definitions are most useful for explaining ball flight
By using TrackMan’s definitions of club delivery parameters and understanding the concepts of the D-plane, gear effect and friction, it is possible to explain the connection between club delivery and resulting ball flight. Important: One cannot use the same concepts if other definitions of club delivery parameters are being used.
|Parameter||When (Time)||Where (Location)|
|Club Speed||At maximum compression*||CoG of club head|
|Attack Angle||At maximum compression*||CoG of club head|
|Club Path||At maximum compression*||CoG of club head|
|Swing Plane||Forward swing arc from knee to knee height||CoG of club head|
|Swing Direction||Forward swing arc from knee to knee height||CoG of club head|
|Low Point||Bottom of forward swing arc||CoG of club head|
|Face Angle||At maximum compression||At point of impact on the club face|
|Dynamic Loft||At maximum compression||At point of impact on the club face|
|Ball Speed||Immediately after ball leaving club face||Center of the ball|
|Launch Angle||Immediately after ball leaving club face||Center of the ball|
|Launch Direction||Immediately after ball leaving club face||Center of the ball|
|Spin Rate||Immediately after ball leaving club face||Around center of the ball|
|Spin Axis||Immediately after ball leaving club face||Relative to the horizon|
Table 1 is an overview on where and when TrackMan measures its data parameters.
*Only using pre-impact data