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MOI of a golf club, where did it go?


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Hi guys, Great thread and discussion! I am just an amateur golfer and club builder, but a structural engineer by trade. I started trying to find a better way to build a set of irons because constant

The spreadsheet will account for both "aux weight" and "backweight", but it only provides a place to enter the weight and the CG. To determine the CG of the weight alone you need to use the same meth

Thanks for that reminder. I did read those at the time. Here is the link to the first of the series.https://3jack.blogspot.com/2013/04/the-road-to-golf-club-fitting-nirvana.html

M4 Driver
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So the KISS version of this is,

1) Find a shaft fitting that gives a great balance feel.

2) Vary the weight of the shafts throughout the set in order to get that same balance.

3) MBI shafts will get heavier as the clubs get shorter.

4) Assuming MOI matching, (But even without it the set should be somewhat close to matching the balance feel.)

Is this essentially right?

 

If so, I can forsee a chart of the kind that Howard Jones makes. Essentially, lines for various 6 iron shaft weights, each column shows the shaft weights needed for the rest of the set. I know this kind of simplification will probably offend the MOI advocates because it would skip a lot of the fine detail that goes into matching. But, it's an idea.

M4 Driver
3, 5, & 7 Woods

Hybrids have been banished
4, 6-GW basic GI irons, weak lofts and +1/2" for more speed and launch
56º, 62º Wedges
Odyssey Jailbird Mini
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Sounds like your theory about whats works the best and why is wrong. (your idea about why we should MOI match is wrong)

If you actually swing the clubs with the same use of power, MOI will always be the best

If you swing the clubs using progressive more power the longer the club is, progressive resistance is the best (Flat SW or another progressive resistance slope)

So its your way of swinging the clubs that make one system to work better than the other.

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I've read that and it's pretty much useless from the standpoint of presenting any validation from the standpoint of the physics behind it. It's really no more than a marketing pitch followed by instructions.

Tutelman's post was a bit more useful - but mainly to point out that there is really is very little concrete support for Monte's forumulation, especially outside the scope of a zero wrist torque swing - which we know is rarely the case in a real golfer.

Now I don't doubt that the first moment (or really the c.g.) might be important and might play a role in the feel. But from the dynamics standpoint that contribution is extremely complex and highly dependent on the specific mechanics,

But all we really have here is someone who is playing around with a quasi arbitrary forumula through trial and error to find out what seems to work or what doesn't based on player feedback. That should trigger off a sense of deja-vu - because that's exactly how the swing weight concept came to be developed.

Now that's not necessarily a bad thing. The swing weight concept may not have been perfect, and it does have quite a few limitations - but it did turn out to be useful for quite a long time. So who knows how this will turn out.

 

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I'd like to thank just about everyone on this thread for their posts, questions & replies. I'm not a poster but have been an avid reader of this forum, Dave Tutelman's site, Tom Wishon's books, Richie3Jack's posts, and several other sources in my quest to understand the golf swing and why certain clubs seem to work for me and others don't. I believe MOI is a helpful measure to characterize the feel/release of the club but I'm not fully convinced the butt end of the club is the appropriate axis of rotation to measure it (i.e. the theory is based on physics principals but the implementation may be golfer specific and I'm not currently aware of any device which measures an individual golfers pivot axis/wrist cock location between the arms & the club). Howard alluded earlier to 4", Theodore Jorgensen (The Physics of Golf) uses an arbitrary distance (his words) of 5" to calculate both the first moment and second moment of a golf club about the wrist cock, and others have postulated other locations. I also am of the opinion (i.e. personal belief) that MOI matching is the most accurate proxy for the release of a range of clubs (e.g. irons) only if one's ball position, foot position, backswing length and level of effort are consistent regardless of club and distance required (Dave Tutelman discussed this during his initial investigation of MOI matching and why he originally didn't find it worked for him as he changed the location of the ball in his stance based upon the club required for the shot...it was only later during his second foray into his MOI investigation when he held the ball position constant that he was happy with the results).

Howard's method of fitting golfers using the longest & shortest irons to get consistent impact location and launch parameters and then building out the set based upon the slope makes a ton of sense to me and eliminates building a set of clubs to either a swing weight (lorythmic 14", official 12") or a butt end MOI but tailors it to an individual golfer. Obviously if the golfer changes his swing (either as the result of lessons, aging, or some physical injury/disability) an adjustment may become necessary. My current set of irons is MOI matched using lead tape and is working for me but I may experiment with another set of heads and shafts that I have.

 

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In the end, I really don't think finding the "why" in the physics matters if you consider the goal for the end result. Using MBI matching accomplishes the original goal of achieving the same "balance" feel across the set. It's as simple as that. MOI matching alone leaves the longer irons feeling "head light", while MBI matching does not. Take it or leave it, but I know that it absolutely provides what I was looking for.

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Yes, you have the essentials down. I have recently begun re-experimenting with counterbalancing in irons, but this time from the perspective of MBI matching. The question I'm investigating is whether butt-end counterbalancing can be used to take a constant weight set of shafts and change them into an ascending weight set by using progressively higher amounts of counterbalance weighting. Golfworks sells these weights and they're easily modified to get the exact amount of weight needed for the progression.

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Note that the "assumed" axis of rotation really doesn't matter if you consider how the MOI is measured. The MOI of a set matched using the machine (which uses the end of the very end of the butt as the axis) will still be very close to being matched if it were measured 4" from the butt across all the clubs. Will it be exactly matched? No, but it would be close enough that you should not get hung up on where you believe the axis of rotation truly sits for your swing. MOI matching will still produce what you're looking for if that's getting the dynamic effort equalized across the set.

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Noodler...I do have an Auditor MOI device, use it for my clubs and I'm a believer in the theory. What I'm considering doing is modifying the butt securing piece to allow a variation in axis location (within reason while not exceeding balance point requirements of the machine) to experiment and see if I can achieve a "better" match for my less than perfect swing. If the actual axis is indeed under the hands, theoretically there is also a way to match the heft of the clubs without impacting the first moment or second moment of the club (i.e. add weight at the axis point).

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I would definitely be interested in a modification for the device. The method to secure clubs in the arm definitely has its shortcomings. The main challenge I see in any modification is for the calibration of the device. I believe the calibration rods provided with each device are custom weighted for each device (since they're labeled individually with a specific weight). You might want to confer with Golf Mechanix about any modification to better understand the implications.

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It's not essential, evidence based formulations can work but leave a big question of how applicable it might be to the wide range of players and wide range of swings that exist. But historically they don't have a lot of longevity. On top of that if it really does work, there should be valid reason so there is no real reason not to look for it. That's how we learn and advance our understanding. Random trial and error doesn't advance our knowledge of whatever phenomena were working with. It also requires a lot more work to validate the method. The amount of actual experience and testing MBI has gone through is in its infancy compared to what's really required to validate it as a universally useful method.

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But I think many people feel that there is still something lacking in terms of the predominant heft matching systems. So thanks for explaining MBI. I appreciate it.

 

M4 Driver
3, 5, & 7 Woods

Hybrids have been banished
4, 6-GW basic GI irons, weak lofts and +1/2" for more speed and launch
56º, 62º Wedges
Odyssey Jailbird Mini
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Noodler...I'll contact Golfmechanix and see what is in the realm of possible regarding modifications. I've already got an account with them but have never corresponded. I did a quick check of balance points on my irons and they range from ~ 27 3/8" to 29 3/8" from the butt end so there is a limitation with hypothetical pivot point/wrist cock locations before no longer meeting the 25" minimum radius of gyration for the device.

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  • 2 weeks later...

Noodler...I've emailed GolfMechanix (Mondher Latiri) with questions about modifying the MSM. I received ed an out-of-office response from his account but will hopefully hear back from him in the next week or two. In the meantime I'm going to experiment a little and see how direct measurements agree with parallel axis theorem calculations by using a spacer to extend the club 2" (i.e. axis of rotation outside the club). At a minimum I'm at least brushing up on my physics knowledge (or lack thereof).

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Just saw this thread and wanted to chime in. The only link I found to the Monte D. MBI spreadsheet in 3Jack's blog didn't work, so thanks @Noodler for the old forum posts. Still have to read up. I'm sure it's impressive work but concerned that Stuart is saying it's not showing all the physics behind it.
Tutelman showed origins of Robert Adams's original 14" fulcrum scale: https://www.tutelman.com/golf/design/swingwt1.php#swScale. It says early 1900's club builders did the math to get their version of MOI matching - just a simple head mass * club length squared, which is where the 7 gram per 1/2" increments came from. After experimenting Adams settled on a 14" fulcrum. I kind of stumbled on this when testing my SW calculator, though using the BP instead of overall length. After entering my clubs, which are all over the place, here's what I found. What I call the "simplified to BP" MOI calculations were actually a pretty close indicator for matching flat SW at 14". What I think Tutelman meant about early 1900 guys' MOI hand calcs (last column using length of club not BP) didn't really match but must've been "good enough". *Neither of these are meant to mimic an Auditor, just simple calcs to observe*. See below. I marked my 6 to 9 irons as they haven't been modified since I got them from the factory, at flat D1 I believe, except for switching grips.
[img]https://s3.amazonaws.com/golfwrxforums/uploads/UOLZRKVUFU5X/sw-calc-test-png.png[/img]To the OP, like Howard says, probably most pros and companies don't care. I'd imagine most pros have been pretty good since they were kids with whatever clubs they could've gotten their hands on, and got so comfortable with a likely flat SW set. So that may still be their preference. They're so good they could probably adjust and still be +5 cappers with a set that's all over the place, even if they feel off.
I'm kinda shocked to see that the Cobra SL irons have different shafts and lie angles. Really?! Thought they would all be the same 7 iron shaft and lie, and remove all those variables. Maybe they did it for draw bias in the long irons. Or maybe the engineers lost an argument.

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@joostin
Tutleman is a good source of information, but the page you get to about SW and MOI has some weakness compared to what we know today about Total weight and a issue not mentioned, todays norm for shaft weight progression in the bag. Tutleman seems to treat it like we was still playing descending weight shafts of the same type all the way, and in that scenario, we get a very different total weight progression, and since the shaft is part of both MOI and SW, his numbers looks weird compared to what we are used to. As example a 43 long club today is most likely a 3W, and in Tutlemans chart where he compare the #9 of 35.50" to a #1W/driver at 43, he use a head weight of 195 grams for the driver as SW matched and all the way down to 180 grams as MOI matched.
A 3W of 43.00" today has a head weight of 208 grams, and thats a huge difference from 195, so there is something he dont really mention, and thats shaft weight in this clubs back then and what it looks like now. We know that if we go down on shaft weight, we will have to go up on head weight to get to the same SW or MOI value, so i think thats where the "error" is if i can call it that.
Anyway, without me knowing much about actual shaft weight back then, or actual head weight, i did some numbers for todays WOODS in Excel, and stumbled on something that match good with Tutlemans writing about the simplified math for MOI he meant they was using in the early days of matching. il just cut and paste the post i wrote on WRX about that.
"Im playing around with some numbers to better be able to see how MOI vs SW moves depending on head weight and play length, and make a few observations that makes me ask....If head weight of real Wood heads is the same we use for metal woods, did they actually try to MOI match their woods?
Today we typical see 10 grams up from 3W to 5W and another 10 grams to 7W, and normally we play this clubs with a play length difference of 1.0"....looking at the numbers for head weight MOI as contribution to the club got me thinking.... it does NOT look like designed to be SW matched...
In my Excel app, using 43.00" for 3W and a GOG 5/8" above the sole, the others 10 grams plus and 1 inch shorter, returns this MOI values for the head alone.
3W = 2401 - 208 gram head
4W = 2401 (half inch longer plus 5 grams from 3W ) - 213 gram head
5W = 2401 - 218 gram head
7W = 2389 - 228 gram head
Its not much head weight correction needed on that 41.00" long 7W (1.2 grams only), so if we play them with shafts that has the same BP. 3-5 grams pr inch up pr. inch shorter (depending on shaft weight, LOW = less progression, HIGH higher progression), this clubs ends dead on a perfect MOI match.
Back in the older days, it was Hickory shafts, i can only assume they became descending WGT, but not enough to make up for the needed progression to become SW matched when the head MOI is equal, so to me, it actually looks like they tried to MOI match woods, but i have not been around for long enough to know why head weight on woods is like it is, so i might be wrong, but those numbers got me thinking."
END NOTE:
Total weight and total weight progression is overlooked, and is more important than MOI or SW, so if we forget to fit total weight correct, we will not be able to make that club feel or work right no matter how good we match it on MOI or SW, and Tutleman seems to have run into that problem himself in his first attempts on MOI matching where he lost distance on his short clubs. My explanation for that is that he came over and beyond his own limits for Total weight in the short end, since he used the short cut to MOI and just added more head weight as clubs goes shorter, so his starting point was "not the best", so i think its important to find our limits for Total weight first, then we can move on to what resistance from head weight the club or set should have.
Thats why my DIY MOI match sets ups starts in the short end of the set, using the #8 or #9 as starting point. If those clubs works good as they are, we want run into total weight issues like both Tutleman and myself did the first time i tried MOI matching. the other reason for starting from the short end, is because its in that end play length is critical, we must prevent going to short, while in the other direction we can manipulate play length down from standard to get MOI right without any risks.

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Here is what im talking about (the "Error" in Tutlemans charts).

The RED frame is how a set CONTANT wgt shafts will look like as contribution to MOI from #3 to #9 in set of irons. Even Constant wgt has a higher contribution pr club longer, and its in my example about 50 points from #9 to #3. IF Hickory shafts can be directly compared to todays Descending wgt shafts, we will see a loss of shaft weight of about 10 grams from #9 up to #3, and thats the BLUE arrow. That makes a contribution to moi that has a progression of 50% higher than Constant wgt, or a difference from 50 points as constant wgt, to about 75 points as Descending weight.

That seems to be the reason for why Tutlemans head weight specs of only 195 grams for a 43" long clubs is so much lighter than what we see today, i think it is the shafts contribution to total MOI. (very heavy wood shafts compared to irons)

image.png

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Howard...appreciate the link, ended up going to the site (https://www.rational-golf.com) to see if it offered more then what was in the youtube infomercial. While they may be on to something, its a little lacking in details about what exactly they are doing wrt to biomechanics and physics. It may simply be an attempt to operationalize Jorgensen's (The Physics of Golf, Technical Appendix - Section 2, The Matching of Clubs) mass (M) and 2nd moment (I) parameters to a wrist cock axis, but it doesn't seem to be concerned at all about matching the 1st moment (S) about the same axis.

BTW...thank you very much for your time and sharing of knowledge on this platform. I find individuals like you (e.g. Tutelman, Wishon, Maltby, the late John Kaufman, etc) have greatly challenged my understanding of this great but perplexing sport.

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Just for the record, i really have no idea about what Rational golf is up to. The man behind it all was a member on this board a few years ago, but for a very short time. I asked deeper into what he was up to, but instead of a reply, he deleted everything he had written, so it seems like he disliked to be challenged on his ideas he say comes form a NASA study. I mentioned him, since he is using insert weights to the grip end to get "his idea of total weight" to match, and it seems to have some familiarity to MBI, but can be totally different.

My approach is, we can not, and never will be able to make a fitting trough numbers on the paper alone, actual testing has to be done, and just like Tutleman have written, the players way of swinging the clubs (more or less use of power), his ball position etc, both can and will make a difference for what type of "matching" that will work the best.

My own bag is MOI matched in groups of clubs, and with a progression of only 50 MOI points from my irons to my driver. that difference is the same a what Constant weight shafts alone in a set of irons adds to the clubs total MOI value. I also use the same ball position for all clubs from driver to wedge, but i use more power when i swing my driver than i ever will on my wedges or irons. i also have a shaft weight progression from club to club, who IMO is a very important part of it all, not only for the sake if total weight itself, but to get a "balanced feel" of head weight vs shaft weight on each club.

My advice is quite simple, take care of one parameter at the time, starting with play lengths, then Total weight, then find the head weight or resistance slope that works the best. Dont ever think that MOI or SW matching alone can over rule total weight or play lengths, so never focus on MOI or SW before you have settled for play lengths and found the limits for Total weight. For resistance/heft, measured as MOI or SW i advocate to forget both the SW scale and the MOi scale until a practical test and tuning of the longest and shortest club in the set is done. Then its time for those tools to get a short cut to build the others. if you follow that advice, the clubs will be match to a resistance slope that fits YOU, no matter how it looks like on the paper, the SW scale or the MOI scale.

In the end, all clubs in the bag is a fitting object of its own, and no matter how hard we try to make short cuts by charts or tools to build the others, we want make it right or "perfect" if we mess with the order of priority, (#1. Length - #2 Total weight - #3 Balance/heft/SW or MOI) , or use a system we dont know or understand the limitations for. The SW scale is the most misunderstood and misused tools of them all, but its a good instrument if its used the way it was meant to be used, and for players who fits to play clubs matched with that system.

PS! i dont belong to the group of people you mention...im just a club fitter and club maker, or more correct, i WAS, so now im just a amateur who writes on this board, and do my best to help others with club tech question, but im pleased to know you have found my writings useful.

If you wants to know where my knowledge comes from, Tom Wishon has been my largest source of influence, and my hands on mentor in Club making was Ed Mitchell, but without years of studies, and players who came to my test studio for the most crazy experiments, i would never have been able to develop the fitting system i did, or to understand club tech as i do. If you never saw that fitting concept, its post #8 in my DIY driver tune up, the system is called VISUAL FITTING, and its nothing like anything you have seen before, and it can be used by "anyone", and all you need to know is there.https://forums.golfwrx.com/discussion/909991/diy-driver-tune-up-diy-fitting/p1

 

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@Howard_Jones Great info as always. I hadn't looked too deep at Tutelman's MOI or SW matching, but what you say about his low head weights up top makes sense. Per your look at the wood heads, those 10g to 1" progressions definitely must have come from MOI numbers for the heads alone (though probably in imperial units like oz-in^2). I ran the same head contribution numbers in your example and got 2410, 2410, 2408, and 2398 for 3W to 7W, a little different but same story.
No arguments about total weight. A while back I tried going 3/8" progressions at some SW without regard to overall weight. I.e. lead tape, long steel shaft extensions even to get a measly 1/8". On top of that I heard a recommendation I think from Maltby to go heavy on wedges, and lead taped those suckers into the E's. Well that was before I read up more (esp. top dogs on golfwrx like yourself) and really started looking at weights and torques. It was a short lived experiment, and it's been by feel & performance since, even if it doesn't make sense on paper...

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How did you calculate those MOI numbers for the heads? im interested in the formula you have used, we might have been using the same short cut, only a difference to the heads BP or the way we judge play lengths, since both will make a difference, but like you, i get the same Values for 3W, 4W and 5W, and a slight difference to the 7W, just like your numbers.

Haha, total weight is a strange fellow, my first MOI set back in 2009 was using DG R300 Sensicore, and the standard L #5 iron as starting point. My PW became so heavy i could not swing it if my life depended on it, i did not see that coming. I did it that way since adding weight is so easy compared to removing weight, and i tried to keep the 4/8" between clubs so it all could be done by lead tape, but i learned fast that this way of doing it, WANT work, since we dont take Total weight into consideration at all. Thats why i turned it upside down, and starts from the short end, then it will work, and we can even modify a set strait from the racks by a progressive butt cut of 2.0 mm for each club longer, and get a set thats close to actual MOI, and without any need for head weight adjustments. (1.1 SWP pr inch progression vs 1.33)

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Swingweight_Torque_Calculator.xlsxIt's just MOI = mass * radius^2. So for the 3W, it is 208g (converted to 0.208 kg) * 42.375" (converts to 107.6325 cm)^2 = 2410 kg-cm^2.
Calculator attached.. if it saved I plugged the numbers toward the bottom. You might see up top I tried playing with matching butt end torques and SW.

D Adams XTD, 70M4X 3W TEE CBX, HZBk75S 5W Titleist 915F, D+80X

3I Srixon Z 545 4I Mizuno MP-H4 6I-PW Mizuno MP-54, C Taper Lite X

GW Nike VPC, DGP S300 54 60 Cleveland CBX, DG S300 Cure CX3

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Ok that seems to be the same short cut ive been using, its called the #2 MOI or "area MOI" and its not the actual MOI we get when its dynamically tested like on a auditor MOI scale but close. As i understand it Area MOi is used for calcs on construction beams to buildings and bridges. (they dont move much, like a golf club does)

( i might have made a typo when my numbers came out as 2401 instead of 2410 when i did those numbers a year ago)

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@Howard_Jones said Ok that seems to be the same short cut ive been using, its called the #2 MOI or "area MOI"
Just an FYI, it's appears to be the MOI of the club head, leaving out the shaft/grip contribution. Area MOI, or 2nd MOI, is something completely different. More commonly used in bending properties, not rotational dynamics.

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