WHAT YOU NEED TO KNOW: Mizuno’s woods lineup for 2021 is led by two drivers aimed at vastly different player types with very subtle differences in their internal weighting. The ST-Z uses a more centred centre of gravity that emphasises low spin with a dose of forgiveness for good measure. Just millimetres different, the ST-X pushes the CG closer to the hosel to encourage more of a draw ball flight by making the face easier to close at impact. Meanwhile, the new ST-Z fairway wood again uses a unique high-strength steel alloy insert for the thinnest variable thickness face in a fairway wood in the company’s history.
PRICES: ST Z driver $799, ST X driver $799, ST Z fairway woods $499.
THE DEEP DIVE: The most powerful part of a driver might be the part you can’t see or touch. At least that’s what the team at Mizuno is finding out about how different golfers are maximising performance with different kinds of drivers, like the latest ST-Z and ST-X models. Now, of course, you have to have the structure, size and shaping pleasing and functional. And by all means, the face better be hotter than a car missing a licence plate (more on that later). But what might make a driver most functional for a specific player is the location of its centre of gravity.
Those overwhelmed by the mere whiff of science, the centre of gravity is basically the imaginary point in space (usually, but not alway, within a clubhead) around which the entire weight of the head is balanced. In terms of how a club performance, if a shot is struck in direct line with the CG, that provides the most direct energy transfer without any instability and energy loss at impact.
But that’s really about how the club and golf ball interact at impact. Mizuno thinks there’s two kinds of players: those who want optimal ball-club impact and those who need help getting the club to the ball. The former is the ST-Z, the latter is the ST-X.
The ST-Z features a CG more in line with the centre of the face, while on the ST-X, the CG is designed closer to the shaft, making it easier for players to control the face for a built-in draw bias. In each case, it’s about the push and pull between maximum skill enhancement and maximum flaw correction, said Mizuno’s David Llewellyn, director of research and development. The farther back within the head that the CG is located, the more forgiving it can be for off-centre hits for a higher moment of inertia (MOI). Of course, the farther back the CG, the harder it is to also keep it low. And then, as Llewellyn explains, there’s another MOI involved, one that refers to how hard or easy it is to control the face because of how far or close the CG is to the shaft.
“With the ST-Z, we’re very focused on keeping the centre of gravity as close to the “Z axis,” or neutral axis, and as deep as possible for the MOI advantage, but we don’t want to sacrifice spin performance,” he said. “That’s the tradeoff we’re battling with ST-Z is having that deeper centre of gravity but not allowing the sweet spot to project too high on the face. Low-mid spin performance with a higher MOI.
“With the ST-X, our design focus is moving the centre of gravity closer to the shaft while maintaining MOI of the head. But we’re also trying to lower the MOI around the shaft to help with closing the face.”
All these efforts towards building different CG locations within the two heads of the ST-Z and ST-X is a direct result of how the two head utilise carbon composite panels in their designs. While both use weight saving carbon composite crowns to help keep the extra weight low and deep for better launch, the ST-Z uses carbon fibre panels on the heel and toe sections of the sole to better balance the CG in line with the centre of the face.
Conversely, the ST-X has a large composite panel on the toe section of the sole. That leaves heavier titanium in the heel region of the sole plus an 11-gram heel weight-screw, shifting the CG closer to the heel. That helps to produce a more draw-biased flight for some players, but that doesn’t mean it’s a head exclusively aimed at chronic slicers.
“We’re actually seeing tour players that can use that head, that actually enjoy that little bit of draw bias, guys that one to take the right side out of play and always see the ball turn over,” Llewellyn said.
Of course, all that talk of CG and MOI is lovely stuff for the science fair or a biomechanics seminar. A driver’s still got to bring the heat, too, and Mizuno again is making inroads in that area with its beta titanium alloy face insert. Called a “super alloy titanium,” the SAT2041 debuted on last year’s ST200 drivers. Its advantage, Llewellyn said, is that once it deflects at impact it returns back to its starting shape quicker to produce better on-centre ball speed and a larger area of the face with the hottest speeds. This year, the SAT2041 face features a new variable thickness design because the alloy is even better understood this go-round.
“It gives us more flexibility to be more aggressive in terms of our multi-thickness face,” Llewellyn said. “Compared to the ST200, we’re not only getting more overall flexure on the face but particularly low on the face.”
Helping with the impacts low on the face also is a new design for the wave structure on the sole. The new design thins out the area between the face and the wave structure and changes the height, angles and thicknesses within the wave.
That wave structure is again part of the design of the new ST-Z fairway woods, too. A thinner wall at the front of wave feature helps to increase lower face flex. Further increasing the way the face flexes is how Mizuno again is using a high-strength maraging MAS1C steel in the face insert. According to LLewellyn, the new variable-thickness insert is the thinnest such design in a fairway wood in company history.
Like the drivers, the fairway woods feature an adjustable hosel that tweaks loft by plus/minus 2 degrees. That allows the two heads (15 and 18 degrees) to accommodate a loft range from 13 to 20 degrees.
The ST-Z comes in 9- and 10.5-degree lofts, while the ST-X is offered in 10.5- and 12-degree lofts. It’s also offered in a longer, lightweight version that uses a 10-gram lighter head to give players more control (called “J-Spec,” a reference to what is the standard setup for drivers in the Japan market). It includes a 39-gram MFusion shaft. The Mizuno ST-Z, ST-X drivers and ST-Z fairway woods will be at retail February 18.
“We’ve made it a priority to really push the limit of COR [coefficient of restitution, or spring-like effect] in a fairway wood,” he said.
