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Testing irons on a mechanical golfer hitting machine will usually show that the clubface does not rotate as quickly coming into impact when the horizontal center of gravity distance is increased from the hosel centerline compared to an iron that has a reduced or traditional dimension. This is basically true and is observed on the mechanical hitting machine as a ball going slightly to the right of the target (even though the clubface is initially set up square in the hitting machine).
The actual reason for this (80% at least) is the fact that the grip of the club is rigidly clamped to the mechanical golfer mounting arm which causes the shaft to bow downward more during the swing, which in turn causes the dynamic lie angle of the iron head to flatten more (due to the shaft flexing or bowing downward). This tilts the irons face plane more to the right when the lie is too flat for the golfer (or mechanical golfer). The farther the horizontal center of gravity is from the hosel centerline, the more shaft bowing down that occurs and consequently the farther to the right the ball will fly. Another way to explain how this occurs is that the center of gravity of the head is farther from the shaft centerline causing centrifugal force during the swing to create a greater leverage effect on the clubhead causing the shaft to bend more downward. When I design irons with this characteristic, I almost always increase the lie angle from ½º to 1º more upright to compensate for the additional lie flattening effect. Keep in mind that the lie flattening effect is greatest in a mechanical golfer and occurs to a lesser extent with actual golfers because real golfers have a much more supple and less rigid attachment to the golf clubs grip. The exact amount of lie flattening to consider in the clubheads specifications should always be determined by actual player testing.
The fact is that when the horizontal center of gravity is farther out from the hosel centerline it slightly reduces the clubhead rotation because of the iron’s higher moment of inertia. This makes it slightly more difficult for the golfer to manipulate the face angle during the swing which actually results in a more stable clubhead and ironically better directional control on both centered hits and off center hits for all golfer types. I say ironically because some manufacturers would like you to believe that working the ball is significantly more difficult with anything other than a traditional blade type iron with the horizontal center of gravity closer to the hosel centerline. This is simply not true, but promoting this myth helps to sell the older style more traditional and sometimes lower playability iron designs.
Real world playabilty needs to be determined by real golfers.
A key point where actual golfer testing differs from machine testing is that golfers will automatically adjust their setup to hit the ball toward the target so that the ball flies in the intended direction. They are basically compensating for the more stable (less clubhead rotation) and easier to play clubhead. The machine cannot do this as the clubhead is always initially set square to the target. This was proven by me in a structured test where a higher moment of inertia iron (an iron with a larger “C” Dimension defined as an iron with a horizontal center of gravity farther away from the hosel centerline) was tested by both a machine and actual golfers. Distance and directional control were plotted on every hit, along with any unsolicited golfer comments. With the face set up square the hitting machine pushed the ball slightly to the right and was very consistent in doing so. The actual golfers all hit the ball as they normally would which is toward the target. The same test was done for comparison with a lower moment of inertia (reduced “C” Dimension iron) where the machine hit the ball toward the target with a square face angle setup and once again, so did each of the golfers. However, all testers agreed that the reduced “C” Dimension irons were harder to hit consistently regarding solidness of hit, directional control and trajectory. Finally, actual touring professionals further tested both types of irons with no tendency whatsoever to hit either type iron head design to the right.
Point: Mechanical golfers can be very useful when acquiring certain apples-to-apples data, but they cannot replace actual golfer testing to find out real world playability differences. Making assumptions or gathering facts about different iron design types from machine testing is very dangerous. Mechanical golfers hitting balls and actual golfers hitting balls is strictly an apples to oranges comparison. Unfortunately too many companies rely on machine testing to prove out new designs when actual player testing is far more beneficial and directly relates to how real golfers hit the ball.
Regarding working the ball (easier or harder) with different “C” Dimension irons: Keep in mind that curving the ball right to left and left to right is done by manipulating the clubface angle in relation to the clubhead path. The amount of difference in degrees between the clubface angle and the clubhead path causes the ball to curve by varying amounts. These are the only factors that cause this if you assume that the lie angle at impact is properly fit to the golfer so as not to tilt the face plane and also that the wind is not a factor. The perpetuated myth that any golfer cannot work the ball with greater “C” Dimension iron designs is simply not true.
When an iron is designed with the horizontal center of gravity farther out from the hosel (greater “C” Dimension) the clubhead is actually more stable coming into and at impact as already discussed earlier. This is also one of the important characteristics of a higher category Maltby Playability Factor (MPF) iron head. The fact that the higher category MPF irons generally resist clubhead rotation more than the lower category MPF irons means a golfer will be more consistent in the following ways:
It is important to always keep in mind the 3 conditions of face angle to path and the nine ball flight possibilities that result. The list below defines this premise:There are only 3 conditions of face angle and path at impact.
Regarding the nine ball flight possibilities:
#1, #6 & #7 above, the clubface is square to the path
#2, #5 & #9 above, the clubface is open to the path
#3, #4 & #8 above, the clubface is closed to the path
Final note: All types of golfers are currently playing the new higher moment of inertia and higher playability drivers because they are much easier to play than traditional designs or even driver designs a few years old. Isn’t it strange that players almost universally accept these new easier to play driver designs even though they have much more stable clubhead rotation characteristics? Note also that better players and touring professionals can still hit draws, fades, higher and lower shots as desired with this technology. If the touring professionals could not do this, they would not play with these highest playability drivers. I have noticed with interest, that since their introduction, the very latest driver designs utilizing square, triangular and concave shapes have generated talk amongst golfers that these new drivers will only hit the ball straight because of the even higher moments of inertia. This is manufacturers’ embellished hype which the media has embraced and reported as fact and golfers have picked up on this. While it is true that these new drivers compared to last years drivers are marginally more stable and marginally easier to hit, they can still be snap hooked and severely sliced if the golfers’ clubface angle and clubhead path angle at impact are not correct.
Performance wise, we as golf club designers are currently limited in making giant strides in driver performance mostly because of current USGA rules governing the maximum driver head size, the maximum coefficient of restitution (spring face effect) and also the maximum moment of inertia. We can and are making small incremental strides in driver performance, but golfers will not see massive differences, rather the differences will be harder to detect because of these limiting rules. Every improvement does help but the biggest driver performance gains are in the advertising copy.
With all that said, the most pleasurable eye opening experience for any golfer playing with older technology driver designs (even drivers that are only 3 years older and especially those even older still) is to move up to the latest 460cc head size with a very high moment of inertia. These drivers compared to older drivers are simply easier to hit, go straighter and longer and make golf more fun. You really should be playing these drivers if you aren’t already.
So, if you like the results and ease of hitting the highest playability driver designs, why not move up to the highest Maltby Playability Factor (MPF) iron designs and enjoy the same results? Stop listening to the myth purveyors about not being able to work the ball with modern higher moment of inertia irons. Start applying knowledge and logic to your equipment and go out and simply play better golf.
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See charts that illustrate the relationship between MOI and putter sweetspots.