Tuesday, 21 March 2017

Efficiency & Spin Axis

As someone who finds the maths and physics of Golf almost as interesting as playing I read and watch a lot in an attempt to improve my understanding. This led me to dig a bit deeper into my driver data, specifically efficiency and spin axis.

ST does not measure clubspeed it is calculated (with good accuracy). It measures ball speed with high accuracy. I therefore extracted my data and calculated bsCarryEfficiency, bsTotalEfficiency, csCarryEfficiency and csTotalEfficiency.

The PGA tour measures carry and total efficiency comparing clubspeed to carry and total distance.
For carry efficiency the 2017 range is 1.918-2.616, my average across my ST data is 2.4
For total efficiency the 2017 range is 2.391-2.936, my average across my ST data is 2.63
Fig 1
Figure 1 shows my bsCarryEfficiency distribution. The mean is 1.64, Q1 is < 1.6 and Q4 is > 1.69. It therefore seems reasonable to conclude anything at or above 1.69 is an efficient drive.

Having watched numerous youtube videos of various professionals I noticed a number of them were posting driver data with significantly greater carry distances than myself despite a similar or lower ballspeed. Spin rates and launch angle were similar so the distances should have been comparable. I therefore looked at other launch parameters and once again found myself looking into Spin Axis:
Fig 2
Figure 2 compares spin axis with ball speed carry efficiency for my driver data. PGA tour style carry efficiency would be better but I didnt want to rely on a calculation and using ball speed reduces (not eliminates) the effect of strike on the data. Within this data there is a curve (albeit with a low r2 value of .3397); increasing spin axis (left or right) generally reduces efficiency which makes sense. Using this data an efficienct drive requires a spin axis of +- 20deg (+-700rpm of side spin). My normal dynamic loft is 15.5deg so I need the face and path to be within 4.5deg of each other to hit an efficient drive (data taken from trackman university). The low r2 value will be the effect of strike and delivery, so you can have a low efficiency straight drive but you cant have a high efficiency crooked one (from the perspective of carry).

Skytrak, Trackman, GC2/GCQ
Going back to my youtube comment I decided to add some extra data; I added my trackman data from my recent Skytrak vs Trackman test, and manually captured data from youtube to see how the data compared. All of this data was produced on GC2/GCQ from various pros and also some amateurs:
Fig 3
Figure 3 shows the bsCarryEfficiency for Skytrak, Trackman and GC2/GCQ. Trackman shows a wider range which makes sense, it is measuring actual ball flight and there are lots of variables at play (open data). However, I think the GC2/GCQ data proves my observation that the youtube carry distances posted are significantly higher than ST and Trackman. This means for every mph of ball speed you put in a GC2/GCQ will likely calculate a more generous carry distance than a ST or a TM would observe.

Returning to Spin Axis
I then decided to reproduce the spin axis chart with the additional data and see how it compares:
Fig 4

Figure 4 shows the efficiency vs spin axis plot for the 3 data sets and I this this illustrates a few things:
1. The TM efficiency data is comparable to ST except the spin axis values are generally smaller. This was already noted in previous testing.
2. The range of spin axis measurements for ST and GC2/GCQ are comparable (more extreme than TM) so perhaps the ST isn't exaggerated compared to GC2/GCQ? Much more data would be required to prove this.
3. The GC2/GCQ is significantly more generous in its carry calculation, to the tune of 10 yards at my average ball speed of 155mph.

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