# Moment of Inertia (MOI)

3 Jack; can you please explain MOI for us. Is this one different than the one commonly used to mean forgiveness. Also how do you measure MOI. What works best and Is there a way around.

MOI Matching is a replacement for swingweight matching. So, if you are Joe Schmoe who walks into a golf store and wants a new set of irons and gets fitted and they find a D-3 swingweight is good for you, your irons will all have a D-3 swingweight. 3-iron will have a D-3 swingweight, 5-iron will be at D-3, PW will be at D-3, etc.

Have you ever had a set of irons where for some reason you hit, say the 4-iron and 8-iron poorly compared to the rest of the set? Chances are that the MOI is off.

Part of the flaw of swingweight is…say I have 2 different Hogan Apex Redline 6-irons. I have made them the same length and the same swingweight. But, one of the 6-irons I have put in a shaft that is much heavier. I then backweight it enough so the swing weight is the same as the other 6-iron which has a lighter shaft. Same swingweight, but how the clubs swing is very different.

MOI…for these purposes…measure the amount of force required to swing the club. What is force? Force = Mass X Acceleration. And remember, weight is just the measurement of the gravitational pull an object has. So when we measure force, we are measuring mass (the amount of matter in an object) and acceleration required to swing the club.

So, in our example of the 2 Hogan Apex 6-irons with the same swingweight, but different shaft weights and a backweight…the mass is different and the acceleration required will be different. Let’s say you measure the swingweight at D-3 for those Apex 6-irons. But the MOI could be drastically different.

I’ll continue this on the next post…

The MOI measurement is actually in….kilograms / cm squared. When we MOI match, we first want to find our most optimal MOI measurement. There are 2 methods for finding our optimal MOI:

1. Take the iron that you hit best and measure the MOI and match the rest of the clubs to that MOI. This method has some flaws in it, but I will get into that later.

2. Hit shots with your 6-iron. Bring some lead tape or weighted magnets with you. Hit some shots with the 6-iron with no weight on the clubhead. Then add some weight, hit more shots. Fiddle around with the weight until you find something that feels the best to you and performs the best for you. One could use Trackman to help with readings such as spin rate, smash factor, clubhead speed, path, etc. But, what feels best to you is just as important so Trackman certainly is not required.

After you find the club that you want the MOI measured, the recommended method is to get a MOI machine and have it measure the MOI. Here’s a video of a MOI Machine in action.

The only problem is that the MOI machines are not cheap. They are about \$500. There is a way to do this cheaply with a cheap measuring scale and understanding mathematical formulas and a lot of diligent work. It would be a lot cheaper, but I believe it takes a lot more time and from what I’ve been told, on average it’s nowhere near as precise (although I believe getting the MOI close is better than nothing at all).

I’ll continue with more on the next post…

The measurements will be usually in the range of 2,550 to 2,900 (kg/cm squared). The lower the number, the lighter the club is. Higher the number, heavier the club is.

Let’s say you find the best feel and performance in a 6-iron for you and it is at 2,775. You will then want to get the rest of your irons at 2,775. This can be done by simply adding weight to the head. (some clubmakers do it with different methods, but the adding weight to the head is the most common way).

So yes…you can simply add lead tape to the head, measure until you get the MOI to match. What most MOI certified clubmakers do is use hosel weights. These are tiny weights that they can install into the tip end of the shaft. I think they prefer the hosel weights for cosmetic reasons and I’ve found that after a month or so, lead tape wears down and that does affect the MOI measurement. The MOI Machine is a super, duper sensitive machine. I’ll put it this way, I took ¼’ of an inch off the butt end of the shaft and it changed the MOI by 60 points.

Anyway. Let’s say you find the perfect 6-iron for you. You measure the MOI at 2,775. Let’s just say you measured the swingweight for kicks and it came out to D-6. Now…let’s say you MOI match the rest of the irons to 2,775. What will happen if you were to measure the swingweight of the rest of the irons is that the lower lofted clubs would be lighter in swingweight. You could wind up with something like this:

Club…………………MOI……………….Swingweight
3-iron………………2,775…………………D-3
6-iron………………2,775…………………D-6
9-iron………………2,775…………………D-8.5

But, they will all ‘swing the same.’

One more thing in the last post…

I’ve found that MOI matching has an uncanny accuracy. Although I think part of it is due to my years and years of playing golf that I’ve developed a good feel for when things don’t feel right with a club. The problem is that it’s difficult to tell exactly what doesn’t feel right and why.

For instance, I have a set of Wishon irons that I use. The 4-iron is the best club I hit in my bag. The 3-iron is the worst. Both have about the same exact swingweight (I have an old SW scale, but it looks like the 3-iron is at D-2 while the 4-iron is at D-3). I knew the 3-iron felt different, although I presumed it was too heavy. I also had struggles with the 9-iron as well while hitting the 7-iron probably the 2nd best out of the bag. Here’s where the MOI measurments of these irons:

3-iron: 2,625
4-iron: 2,702
7-iron: 2,700
9-iron: 2,639

So, I was right…there were issues with the 3-iron and 9-iron and there were reasons why I hit the 4-iron and 7-iron so much better….the MOI’s of the 3-iron and 9-iron were very low and the 4-iron and 7-iron were heavier and almost exactly the same. I added lead tape (for now) to the 3-iron and 9-iron and I hit them entirely better.

I also have a set of Mizuno TN-87’s that I hit the 3-iron thru 7-iron really well, but struggled with the 8 thru PW. Here were the measurements I got:

3-iron: 2740
4-iron: 2727
5-iron: 2738
6-iron: 2735
7-iron: 2728
8-iron: 2655
9-iron: 2655
PW: 2670

As you can see, the 3-7 irons had MOI is a pretty tight range. But the 8-PW were much lighter. I haven’t quite gotten around to fitting myself for MOI, but I think I’m around 2,730.

A couple of last things:

1. The woods and hybrids should have different MOI’s from the irons. The general rule of thumb is to add +150 to the driver, +100 to the fairway woods and +7 to the hybrids. Not sure if that would work with persimmon like that. Although it’s a general starting point and one should fiddle around with the weight until they find what works best for them.

2. It’s not recommending lighter weights. It’s basically allowing the golfer to figure out what weight they like the best and then making it so the each club ‘swings the same.’ You’ll see some golfers do sort of the same thing, with the long irons at a lighter swingweight than the shorter irons. So, they have the right idea….the amount of force required to swing the club is different if the swingweight is the same. But, this is more exacting.

3JACK

Thanks a lot 3Jack; I have my work out for next year.
I have always been skeptical about swing weights. I think swing weights kind of relate to your concious mind’s idea of sameness at rest or low speed and the total weight (which is related to MOI) is more accurately sensed the same by the unconcious mind at high speed. That will be a good neuroscience experiment to do (no I am not a neuroscientist).

You have to remember, what is weight? It’s the gravitation pull an object has.

What is mass? It is the amount of matter in an object. So…if a man weighs 200 pounds on earth, they will way much much less on the moon. Why? Gravitational pull is different. However, their mass stays the same.

So when we are analyzing the MOI, which measures the amount of force required, we are measuring mass and the acceleration. That alone means that there’s more to force than there is weight.

I haven’t found the optimal MOI for me yet. I just haven’t gotten around to taking out the lead tape and adding it to the 6-iron and fiddling around with it. But, I did match the MOI to 2,700 for each club (using lead tape) and I’ve seen a dramatic difference because matching the MOI alone, even though I haven’t fitted myself, is better than nothing.

It’s actually not a new concept. Tommy Armour had a set of EQL irons that were all the same length. That made the MOI match. Howevever, it ran into 2 issues. 1) the concept of 1 length of irons was too radical (we don’t make the irons the same length here, just add or subtract weight). 2) Even though the MOI matched, it may not be the best MOI for that golfer. So a golfer may hit clubs with a 2,750 MOI the best, but the EQL irons may have had a MOI of 2,675.

3JACK

3JACK,
Thank you for your very detailed and informative posts. I read your thread a few days ago and was hoping you would elaborate on MOI. Great job and thank you again.

Bob

Thanks for the posts…

interesting to see an analytical device supporting the idea of progressive swing weighting.

Here’s something kinda interesting. I was measuring my clubs last night. I’ve got them at 2,700 kg/cm2 for now. Except, I’ve never messed with my SW and LW. I measured the SW awhile ago and it had a Iomic grip on it and came out to 2,685. Later on, I put a Lamkin 3Gen grip on it. Measured it last night and came out to 2,655. So a 30 point difference just by changing the grip which is funny because I has been struggling a bit with the SW lately when I changed out the grip. It appears there is about a 2-6 gram difference in weight between the grips, which would explain some of the difference.

Here’s a look at the MOI of my clubs without any lead tape added to them.

3-iron 555C: 2625
4-iron 555C: 2702 (+77)
5-iron 555M: 2670 (-32)
6-iron 555M: 2680 (+10)
7-iron 555M: 2694 (+14)
8-iron 555M: 2658 (-36)
9-iron 555M: 2639 (-20)
PW 555M: 2653 (+14)
SW Miura K-Grind: 2688 (+35)
LW Ping Tour-S: 2655 (-33)

I’m guessing that the best MOI for me is around 2,725 - 2,735 or so. I just ordered a bunch of different hosel weights. The machine has a software program that you can plug in your numbers (length of club, MOI) and it will tell you how much weight to add to the head.

3JACK

I want to try to explain the concept of MOI, not as it specifically relates to golf, but as the generic physical term.

MOI is the analogue to mass in the equations of rotational motion.

We are all mostly familiar with the formula F=ma, right?
Mass is the property of matter that measures its resistance to being pushed.
Mass is ratio of force applied to resultant acceleration.
Increase the mass, and it takes more force to achieve the same acceleration.

Ok, MOI plays the role of the mass when you study rotational motion.
The corresponding formula is Torque = MOI * angular acceleration.
MOI is the property of a body that measures its resistance to being turned.
MOI is the ratio of the torque applied to the resultant angular acceleration.
Increase the MOI, and it takes more torque to achieve the same angular acceleration.

Now, MOI differs from mass in one important way.
It depends on the axis about which you are going to rotate it.
If you rotate a pencil around an axis defined by the lead (like spinning a drill bit),
the MOI is much smaller than if you rotate that same pencil around an axis
perpendicular to the lead (like twirling a baton or propeller).

So, this leads us to an important understanding.
Every physical body has more than one MOI. In fact, it has an infinite number.
This is because there are an infinite number of axes about which you could rotate the body.
The axis does not have to go through the body. (i.e. the Earth has an MOI for rotation about the sun)
The axis used for measuring golf clubs is an axis perpendicular to the shaft near the end of the grip.

Another important concept about MOI is that it depends not just on the mass of the body, but the arrangement of that mass.
Two golf clubs could have the same length and mass, but different MOIs.
The club that has the most mass located as far from the grip as possible will have the greater MOI.
Imagine a club with a bore-through head and an un-tapered shaft, so that we could slide the head up and down the shaft.
The maximum MOI is found when the clubhead is at the customary end of the shaft.
The minimum MOI is found when the clubhead is located as close to the grip as possible.
The MOI increases as the square of the distance you move the mass down the shaft.

Anyway, I hope that was not too dry/technical.
But MOI is an important physical concept and not uniquely important to golf.

Thanks photonn.

The simplest way to think about it is that generally clubs are ‘swingweight matched’ and MOI matching is a much more advanced way to measure and match the clubs so they ‘swing the same.’

I’ve become a big-time believe in this because:

1. I think MOI matching alone, even if you haven’t fit yourself for the best MOI, will help you play better.

2. I think MOI matching alone, even if you haven’t fit yourself for the best MOI, will help with your swing mechanics.

3. I think it can be superior to frequency matching as well.

On #3, I can hit some great shots with a shaft flex that is way too weak for me. The problem is being able to time the kick of the shaft. I can’t remember the exact data, but the research showed for every 1 cm of forward shaft deflection, it closes the face by 0.7* and increases the dynamic loft by a certain amount. That’s why I can hit some great shots with say a ladies flex shaft, but when I don’t time it correctly I’ll hit a hook and generally hit it high.

But, with MOI matching I can get the same amount of acceleration required to hit the club as I would in the best club I can hit. It can still result in times where I can’t time the kick of the shaft correctly, but the feel of the club is in line with the rest and the adjustment to the shaft flex is less.

3JACK

Time to post my experience.

After reading richie3jacks’ posts last week, I experimented a bit on the range. Took my favorite club (which just happens to be a 6 iron), with a swing weight of D3 as a standard. I had all my irons right at D3. Removed weight from my 3, 4, 5 irons. Then added some back until these clubs felt like my 6. With the 7, 8, 9, and E, I added weight until they felt like the 6. I had real good feel on Sunday, and hit all the irons much better than before, especially the 3, 9, and E wedge. Swing weighted them that afternoon. I didn’t record the results formally but the sw’s are something like:
3 D0
4 D1.5
5 D1.5
6 D3
7 D3
8 D4
9 D5
Ewedge D5

I really appreciate the insightfull posts. This “guess weighting” for moi matching did help my ball stiking immediately. Thank you r3j for the information.

There is a way to use mathematical formulas to MOI match your clubs. But you need to have a digital scale and such and be pretty good at the math. I think the Tutelman site has stuff on it.

I remember Lag got into ‘progressive swingweighting’ with his own equipment. Obviously, he has a very keen feel for his clubs. This is a good example as to why MOI matching has merit IMO. Essentially, Lag has developed such a keen feel over the years with his equipment that he could sense that his longer clubs were requiring more force to swing them than his shorter irons. So, instead of adjusting the force to the swing, he adjusted the club. With the MOI machine, it just gives a more accurate measurement.

Also, I’ve found that 2 grams of lead tape = 25 MOI points or so.

3JACK

Richie,

Do you know what would happen to the MOI reading if you were to add say 10gms to the grip and do nothing to the head?

R3J

Out of all my sets I think these Mac VIPs may be the most fit for MOI matching. I still believe swinging the heaviest clubs may ultimately help your ball striking. Also, although these are not pinned but taking the shafts out will be a nightmare.
Can you please suggest as to what needs to be done. Right of the bat, I think the 5 iron needs some tape.
The length is 5 iron at 37.5".

I’m not sure on what the MOI would be if 10 grams of weight were added from a different grip. I just put 10 grams of lead tape and wrapped it around the grip and the MOI went down about 10 points.

Macs - if your best club you hit is your 6-iron, then the 2-5 irons have to be a little lighter in swingweight and the 7-PW have to be a little heavier.

If you’re happy with D-6 with your 6-iron, then the 2-iron probably needs to be at about D-2 and the PW needs to be about D-9

A very big ballpark would be to add or subtract 0.7 swingweight points from the best swingweight.

3JACK

It’s important to remember that your body will adjust over time to heavier gear, not just in feel, but strength also. The biggest advantage I have experienced is much better distance control… and since short irons are easier to swing, they can be much heavier than longer clubs. Distance control is not quite as important as with the short irons, because in all practicality you should be zeroing in on the pin with your short irons, while the long irons you are generally just trying to get the ball safely onto the green in a reasonable fashion unless of course you happen to be really on that day.

How do I take weight away from those irons?? There is no tape on those. May be if i try to take the shafts out, they may have added some powder; but that will be a heck of a labour. I just finished swing weight matching all my iron sets and thought I was done tinkering for a long time.

How about grinding them? Where will be a good place to grind??

Thanks Richie,

So presumably it would be possible to add weight to both ends and for the MOI not to change much.?

Or could you say add 28 gms - some to the head, some to the grip and actually get the MOI to go down?

Cheers.