Wednesday, April 3, 2013

The Road to Golf Club Fitting Nirvana - Part II

In part I, I discussed the basics of MOI matching/fitting and MOI Balance Index. For this part, I will examine the spreadsheet created by Monte Doherty with regards to determine the MOI Balance Index of the club. The spreadsheet is available for a free download here:

The spreadsheet took Doherty over 3 years to create in order to tweak the formulas and the formatting of the spreadsheet. On the bottom of the spreadsheet you will see what they call ‘tabs’ in MS Excel. This is like going to a new page in Excel. We want to read the first tab called ‘Instructions.’ Then ‘AppNotes’ and ‘Tools.’ You can go to the ‘Theory’ tab if you’re looking for a little more detail on MOI matching and MOI Balance Index, but it’s not mandatory to read. I would then turn my attention to the tab called MBICalc. This tab is the very basic form the Calculator to determine the MOI Balance Index.

(Click to ENLARGE)

There are 7 different calculations that the calculator uses to determine MBI

Shaft + Grip MOI
Shaft + Grip Weight
Total Club MOI
Club Weight
Club Balance Point
Head Self MOI
Low MOI Correction (Relatively Unimportant)

In this case, the numbers inputted by Doherty are for his Driver. And the MBI comes out to 49.85. But one thing we have to note before I move on. The head design of a club influences the balance point.

This is called ‘Head Self MOI’.

As we can see, a driver will have a different Head Self MOI than a fairway wood/hybrid which have different Head Self MOI’s than a Super Game Improvement iron which is different from a regular cavity back which is different from a muscleback blade style of iron.

Since the MBI calculator has Doherty’s Driver calculations in there; let’s go to the tab on the left that is called ‘919_Driver’.

(Click to ENLARGE)

You have a choice of simply allowing this spreadsheet determine the MBI for you or using the data and inputting that into the MBICalc spreadsheet.

You will notice that the MBI is not the same as the MBICalc spreadsheet show. The difference is 49.88 vs. 49.85. That’s only a difference of 0.03 and that is more than acceptable for margin of error.

Any of the cells in spreadsheet that are white (no color) are cells that we will input the data into. If the cell is colored, that is a cell we leave alone because it has formulas involved with it. If you want, you can simply change the numbers in one of the white cells and you will see how that changes the other numbers in cells where there is a formula.

For example, if you go to the right where it says ‘Loft & Lie’, click on the box that says ‘Adjust Loft/Lie.’

In the cell that says ‘Current Lie’ and has a lie angle of 58.0°; change that to 56°. Notice how the MBI goes from 49.88 to 49.83. And the club’s total MOI in cell D23 goes from 2,829.45 to 2,836.25. Just in case you’re wondering, if the lie angle changes that will alter the total club MOI. In this case, by flattening the lie angle by 2°, the club’s heft increases slightly. Conversely, if the lie angle was to get more upright, that makes the club’s heft a bit lighter.

So, if you’re using forged irons, one of the reasons to make sure the lie angles are in tact over the course of time is that it will affect the club’s heft.

But as we can see, we have a section for some basic Club Head information in the top left corner.

Then we have a section for the shaft information. This is a bit more complex because it wants the balance point of the stand-alone shaft and the balance point of the shaft with the head on.

And then we have a section for the grip which not only includes the weight, but the cap thickness and the balance point of the grip (no shaft) as well. This information is recorded because it affects the clubs MBI. Theoretically, you could have 2 grips that weigh exactly the same, but one is more ‘cap heavy’ and that alters the MBI to some degree.

Also, if you go thru the tabs for the irons, you will see that with his 9-iron, PW, and AW he ended up changing out the shaft, going to a similar model shaft, but heavier.

Look at tab labeled ‘PW’. You will notice in cell H11 that the MBI is 58.71. That leaves a difference from the optimal MOI of 7.68. This means that the MBI is a bit too much towards the head from what the target MBI has.

The issue here is that we need to keep the Total Club MOI in tact while being able to move the MBI closer to the target MBI. That’s where Doherty uses a heavier shaft model, going from a SK Fiber 95 gram model to a SK Fiber 100 gram model shaft.

From what I’ve been told, you can expect that with balancing a set of clubs. The short irons will require a bit heavier shaft and the 3 or 4-iron may require a bit lighter shaft.

To finish this part off, you can go to the ‘Summary’ tab and see that each club is automatically updated. And if you look at Doherty’s notes (which you can put your own notes in), he discusses how the clubs that that have a negative balance (more shaft/grip heavy) tend to produce more of an inside-to-out path while the clubs that have a positive balance (more head heavy) tend to produce a more outside-to-in path


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