Flex-face irons can be more forgiving

than rigid-face irons

• My point: while drivers and 3-woods are about maximum distance, irons need to be about repeatable, dependable distance. It's OK if I can't hit my 7-iron as far as you can hit yours, but it's a disaster if I don't know how far my 7-iron shot is going to go.
• Tom's point: there are lots of golfers who would love to hit their irons longer. Distance sells. Older golfers want to get back the distance they lost. Etc, etc, etc.
• My point: There is going to be some falloff of COR as the strike moves away from the center of the clubface. With good design there may not be much, but there will be some. So that adds to the loss already there for an off-center hit. Yes there is loss even for a rigid clubface, due to clubhead rotation because the moment of inertia is not infinite. Adding to this loss with COR falloff further increases the uncertainty of how far the ball will travel.
• Richard's point: based on his experience, his customers' experience, and Tom's assurances, there is no more loss of distance with the 770cfe than with rigid face irons. In fact, they were even more forgiving of an off-center strike.

I was skeptical of that last claim. This was my reasoning at the time... The COR of a rigid face club and a modern ball is about 0.77. COR increases with the face flex. The COR boost due to spring effect is roughly proportional to the amount the face deflects where the ball strikes it. Let us look at the face deflection for a few possible clubface designs. The green curve is the rigid clubface. It is just a flat line at zero -- obviously, because "rigid" means the clubface does not deflect. So there is no COR boost for this face; the COR is stuck on 0.77. The dark blue curve is a uniformly thin face. It flexes a lot in the center, but the COR boost falls off fairly quickly as the hit misses the center. This would not send the ball a reliable distance, because the COR would drop off towards 0.77 quickly if the strike was not on-center. The red curve depicts what I felt could reasonably be done by selectively thickening the face and changing it flex characteristics.This retains most of the high COR for some degree of off-center hit, but falls off if you get a bit farther from the center.

Remember that the goal is to make the club as forgiving as possible, in the sense that the distance does not change much in the event of a mishit. Of the three curves shown, the rigid face is the best at meeting this criterion. No, it does not give maximum distance, but it does give the least loss of distance for an off-center hit. The red curve is also pretty good, as long as you don't miss the center by too much, but still loses a lot of distance for a significantly off-center hit. And it was my belief that the red curve was about the best you could do in this regard. Let's remember that there is another reason we lose distance when we miss the center of the clubface, even with a perfectly rigid clubface -- clubhead rotation. A high clubhead moment of inertia can minimize this loss, but even a high-MOI clubhead will show significant loss if the strike is significantly off-center. So whatever "rolloff" of ball speed we get from off-center reduced deflection is on top of the loss from clubhead rotation.

1. The MOI of the clubhead is greater than the normal iron. This was certainly possible, because the thin portions of the face provide a bit of "discretionary mass" to be moved to the periphery. But Tom insisted that the bulk of the difference was in COR. So we are left with...
2. The face is designed such that the COR increases (not decreases) as the impact point moves away from the center. That is, the deflection of the face at the point of impact must be more away from the center than at the center. This is not just a constant COR (like a rigid clubface), but a COR that curves upwards away from the center -- totally unlike the red or dark blue curves above.

• If the force is dead center (x=0), then each spring gets to resist half the force.
• If the force is at either end (x= ±1), then the entire force is resisted by just the spring at that end. So the bar -- where the force is applied -- experiences twice the deflection.
• Between the center and the ends, we would expect some sort of smooth variation. This is not a for-sure given, but there is no reason to expect otherwise. And the analysis will bear it out.

deflection  =

F 2K

(x2 + 1)

The next few diagrams show the evolution of the concept from physics textbook to the Wishon catalog. We start with the original version of the system. Then we remember that coil springs are not the only kind of spring available. The second figure has replaced the coil springs with leaf springs, like those that allow the wheels of a car to soak up bumps in the road. But what are leaf springs? They are just thinner, more flexible pieces of steel. If they were the same steel and the same thickness as the rigid bar in the diagram, then they would be more of a rigid bar, and not springs at all. So we could spring the bar by simply narrowing it to a thin strip of the same material at the ends. Of course, the bar isn't perfectly rigid; there will be some small amount of flex in it. But compared to the thin section at the end the thick section looks rigid -- and the thin section looks like a spring. It is still the same as the original model with the coil springs and the rigid bar, to the tolerances that matter in a golf club.

And that is essentially what Tom Wishon has done. The third figure is a picture adapted from the 2009 TWGT catalog. The face is a separately fabricated insert, shown in cross-section and color-coded in blue. It has a thick (read that as rather rigid) portion in the middle, and thinner flanges that connect it to the conventional rigid steel frame. But the thin flanges are much wider than they need to be just to connect to the frame. There is enough thin area there to be a spring edge for the rigid center. So the thick center acts as the rigid bar, and the thin edge acts as the leaf spring. As long as you hit somewhere on the thick part, you will see the spring behavior where increasing COR makes up for the loss from clubhead rotation. If the hit is so far off-center that it is on the thin portion, you will still get some "save" out of it -- though the COR must necessarily fall as the strike approaches the conventional, rigid frame supporting the periphery of the face.

Analysis

Deflection of uniform beam

deflection  =	Fab 6EIL	(L2 - a2 - b2)
=	Fab 6EIL	([a+b]2 - a2 - b2)
=	Fab 6EIL	(a2 + 2ab + b2 - a2 - b2)
=	Fab 6EIL	(2ab)
=	Fa2b2 3EIL

deflection =

F (1+x)2 (1-x)2 6EI

=

F (1-x2)2 6EI

Deflection of rigid beam with springs

slope  =

aF - (1-a)F 2K

=

F (2a - 1) 2K

deflection  =	F 2K (2a - 1)t  + F 2K
=	F 2K {(2a - 1)t  +  1}

(1+x)F  =  2aF

a  =

1+x 2

deflection  =

F 2K

{(2

1+x 2

-1) t  +  1}  =

F 2K

(xt + 1)

deflection  =

F 2K

(x2 + 1)