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Effect of front-back driver weighting on MOI/spin?


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I can't seem to find solid info on this but do we have a good thread somewhere outlining general guidelines for this? Theres quite a few woods out now especially drivers with the ability to switch a heavier and lighter front and rear weight. Obviously heavy weight rear is most forgiving and higher spin - and vice-versa - but is there a general rule similar to how much -1 of loft on an adjustable adapter changes the MOI in hundreds of points or how much spin is changed with each 2-4g increments for example? 

 

Example; if a driver comes with a 14g weight and a 2g weight; changing them around equates to a 250RPM change in spin and 2050MOI on a robot all things equal? What if that 16g was split into 8g and 8g weights making them equal? What if we pit 20g rear and 2g front is that ++200MOI/RPM? (making these numbers up)

Edited by third-times-a-charm

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I have the Mizuno STG-220 and it has two 7g weights. The difference between them being all the way forward vs all the way back is 100 rpm spin based on my Trackman sessions in the spring.

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I am interested in the data/feedback with this topic regarding 3 woods, but without the loft change. I have an Epic Speed Max that has weights like described in the original post, but does not have an adjustable hosel. This head comes stock with a 14g in the front and a 2g in the back. Putting the 14g in the back would not provide the change I am looking for. I am thinking 11/12g front weight and 5/4g back weight maybe the winner for me.
 

So, I ordered additional 3 weights sets in different variation’s for this head.  I was looking for how the weights would change the forgiveness and spin to help it perform a bit better for me. I ordered 2 pairs that reflect the stock weights (14g+2g=16g total), but in different gram combos (one example was 11g+5g). The 3rd weight set is heavier for a total of 17g or 18g since lighter would not help with my tinkering. I figured this test is not a budget buster if more combo’s are need to be purchased once dialed in. 
 

I was trying to avoid a shaft change if possible, but that maybe the final outcome. 

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46 minutes ago, getitdaily said:

I'm willing to bet @Howard_Jones has a thread on this...or the data.


While it would be cool to see the data, and if anyone would have it or know how to get it it would likely be Howard (maybe @joostin?), I wouldn't be surprised if he didn't bother attempting to go into that level of detail because it starts to get a little too academic IMO. What we're talking about is a sort of MOI to gear effect conversion formula where the impact of a mishit could be calculated with said MOI value. I think the problem is how many other variables would need to be normalized in order for such a formula to be accurate, and those variables are never that controlled in the actual golf swing so the more of those you remove for the sake of accurate math the less relevant the results actually are. I hope smarter folks than I can weigh in (@Howard_Jones, @Stuart_G, @joostin), but off the top of my head you would have to account for and normalize all of the following to actually do this:

1) Exact sweet spot location and strike location in relation to it. This is the most obvious and could be isolated with robot testing, however it would need to be accurate to less an 1mm (likely even less) to be considered accurate enough. 
2) Dynamic loft. Changes in MOI would result in changes in dynamic loft at impact, so these variations would need to be measured and accounted for with each MOI change so the same dynamic loft was delivered each time. 
3) Club path. Easier to control with a robot I think. Howard has shown some diagrams in the past detailing how club path shifts the location of the functional sweet spot on the head, so this would need to be tightly controlled as well. 

I think what you'd end up finding is that the meaningful variations exist more in the extremes e.g. SLDR vs. Ping G400 Max kind of thing, with shifting weights backwards/forwards within the same head producing comparatively minor result when it comes to spin outside of massive changes. That is just my gut feeling though and i'd be curious what others think and how much of this has either already been tested or enough similar tests have been done by folks like Howard that a conclusion can be drawn from their separate results. 
 

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For a measly $5K, you can buy the gauge from golf mechanix and run some tests.  🙂
 

https://www.golfmechanix.com/co/items.aspx?Pdts=07

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50 minutes ago, getitdaily said:

I'm willing to bet @Howard_Jones has a thread on this...or the data.

Haha, i dont, but this can be done with mathematics alone, but then we need to know how much we are moving RCOG, since thats the distance we change and convert to forward shaft bending. if we can move RCOG 3 mm backwards, the head will return 3 mm "earlier" (force itself 3 mm more forward at impact). Then the question is, how much will 1 mm more forward be as "loft" ? i dont know that, i never made any numbers for that. Dont forget that the heads i was working with, at the best had a changeable wgt, they hardy ever had options where that wgt could be moved to a different position. During fitting head wgt alone is focus, added wgt location becomes secondary. We simply tried a few locations following the "rules" for "expected direction of change", but i never isolated numbers for how much changes we found when a certain amount of lead tape was moved to another position.

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8 hours ago, third-times-a-charm said:

I can't seem to find solid info on this but do we have a good thread somewhere outlining general guidelines for this? Theres quite a few woods out now especially drivers with the ability to switch a heavier and lighter front and rear weight. Obviously heavy weight rear is most forgiving and higher spin - and vice-versa - but is there a general rule similar to how much -1 of loft on an adjustable adapter changes the MOI in hundreds of points or how much spin is changed with each 2-4g increments for example? 

 

Example; if a driver comes with a 14g weight and a 2g weight; changing them around equates to a 250RPM change in spin and 2050MOI on a robot all things equal? What if that 16g was split into 8g and 8g weights making them equal? What if we pit 20g rear and 2g front is that ++200MOI/RPM? (making these numbers up)

 

 

It's possible to compute - but you need to know the c.g. location and more importantly the MOI of the head about the heel-toe axis that goes through the c.g.   That's where the biggest problem is going to be.  Not even MGS has that MOI - they only measured the MOI relevant to horizontal gear effect, not the MOI relevant to vertical gear effect.

 

But if you ever get that number - or just want to see what it is for test values.    You can find an outline or the vertical gear effect calcs here.    Use basic c.g. calcs or MGS c.g. data for figuring out how the c.g. moved.   And use that movement (assuming same impact location) to see how the MOI changes and the vertical gear effect influence on spin might change.

 

https://www.tutelman.com/golf/ballflight/gearEffect2.php

 

 

 

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6 hours ago, Valtiel said:


While it would be cool to see the data, and if anyone would have it or know how to get it it would likely be Howard (maybe @joostin?), I wouldn't be surprised if he didn't bother attempting to go into that level of detail because it starts to get a little too academic IMO. What we're talking about is a sort of MOI to gear effect conversion formula where the impact of a mishit could be calculated with said MOI value. I think the problem is how many other variables would need to be normalized in order for such a formula to be accurate, and those variables are never that controlled in the actual golf swing so the more of those you remove for the sake of accurate math the less relevant the results actually are. I hope smarter folks than I can weigh in (@Howard_Jones, @Stuart_G, @joostin), but off the top of my head you would have to account for and normalize all of the following to actually do this:

1) Exact sweet spot location and strike location in relation to it. This is the most obvious and could be isolated with robot testing, however it would need to be accurate to less an 1mm (likely even less) to be considered accurate enough. 
2) Dynamic loft. Changes in MOI would result in changes in dynamic loft at impact, so these variations would need to be measured and accounted for with each MOI change so the same dynamic loft was delivered each time. 
3) Club path. Easier to control with a robot I think. Howard has shown some diagrams in the past detailing how club path shifts the location of the functional sweet spot on the head, so this would need to be tightly controlled as well. 

I think what you'd end up finding is that the meaningful variations exist more in the extremes e.g. SLDR vs. Ping G400 Max kind of thing, with shifting weights backwards/forwards within the same head producing comparatively minor result when it comes to spin outside of massive changes. That is just my gut feeling though and i'd be curious what others think and how much of this has either already been tested or enough similar tests have been done by folks like Howard that a conclusion can be drawn from their separate results. 
 

 

6 hours ago, Howard_Jones said:

Haha, i dont, but this can be done with mathematics alone, but then we need to know how much we are moving RCOG, since thats the distance we change and convert to forward shaft bending. if we can move RCOG 3 mm backwards, the head will return 3 mm "earlier" (force itself 3 mm more forward at impact). Then the question is, how much will 1 mm more forward be as "loft" ? i dont know that, i never made any numbers for that. Dont forget that the heads i was working with, at the best had a changeable wgt, they hardy ever had options where that wgt could be moved to a different position. During fitting head wgt alone is focus, added wgt location becomes secondary. We simply tried a few locations following the "rules" for "expected direction of change", but i never isolated numbers for how much changes we found when a certain amount of lead tape was moved to another position.

 

7 minutes ago, Stuart_G said:

 

 

It's possible to compute - but you need to know the c.g. location and more importantly the MOI of the head about the heel-toe axis that goes through the c.g.   That's where the biggest problem is going to be.  Not even MGS has that MOI - they only measured the MOI relevant to horizontal gear effect, not the MOI relevant to vertical gear effect.

 

But if you ever get that number - or just want to see what it is for test values.    You can find an outline or the vertical gear effect calcs here.    Use basic c.g. calcs or MGS c.g. data for figuring out how the c.g. moved.   And use that movement (assuming same impact location) to see how the MOI changes and the vertical gear effect influence on spin might change.

 

https://www.tutelman.com/golf/ballflight/gearEffect2.php

 

 

 

See @third-times-a-charm

 

You have all the tools you need.  Now you just need a physics degree. Tag us all when you publish your data 🤣

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This is one of the reasons I want to design a decent driver head in CAD - to be able to find and manipulate CG and MOI about any axis while moving mass around.  It doesn't seem to be of huge interest to anyone, myself included tbh, other than a few posts like these because, like @Valtiel said, it's more academia than anything.  (We are lucky to have Mr. Tutelman's website as he dives deepest into those mathematical waters, but he's not posting here.)

 

That said I will get around to finishing a driver model that I've started in CAD.  I'm just on a little putter design kick as putters are easier to design!

 

Another MOI of interest to me is taking it from an axis parallel to the face, perpendicular to neutral axis, instead of vertically through the CG.  It would be an increasingly small difference as loft goes down, but I think that could be more relevant to collision physics than the vertical axis MOI numbers we see from MGS, Maltby, and OEMs.

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20 minutes ago, third-times-a-charm said:

Okay so forget MOI 🤣 is there a rule Of thumb for spin?

 

No.  Too many details that can vary from head to head.

 

But with that type of movement, not a lot.  Since the rear weight is higher relative to the front weight.  Look at the MGS c.g./MOI charts to see how much motion a particular head might have.  Then look at Tutelman's page for the general effect of vertical motion of the impact on the spin.  Moving c.g. is the same as moving the impact location as far as gear effect goes.  That by itself will likely not be a whole lot.   However, the bigger effect will come from the change in MOI that moving the weight forward and backward causes (for both MOI's).  Forward will have much less MOI and therefore more gear effect.

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3 hours ago, third-times-a-charm said:

Okay so forget MOI 🤣 is there a rule Of thumb for spin?


I found a "rule" for how much spin and launch angle changes when we move impact position on the face, but i guess thats not really what you ask for, but here it goes anyway.
 

- As average we can say 1/8" UP makes a 240 rpm spin reduction, and changes launch angle with + 0.35*

Example, if we move impact 3/8" higher, we should se a difference to launch angle of about 1.0" and a spin cut off in the area of 720 rpm.

Ive found some old charts made by Hot Stics (via Tutlemans site), that shows a different ratio for launch and spin changes, but i have no idea how their numbers is made, so its hard to comment on them, but they seems to have been made before 2009, so that could also mean a sligthly different mix of heads or head models (VCOG- RCOG- MOI - FACE ROLL). HSs chart shows a larger change of launch and smaller changes of spin vs my charts. Launch in general is very low in their chart.

image.png.875d09cd658ca41acf0202c063c3a4ea.png
  

 

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