This is from Tom Wishon (www.wishongolf.com) and his latest e-Tech newsletter which I believe is a must read for serious golfers (even if they don't own Wishon equipment).
One of the areas of interest to more accomplished players is the matter of whether a driver head can be designed to intentionally deliver a higher or lower amount of backspin on the shot. Thanks to the greater presence of launch monitors combined with the reports from the professional tours that top players all focus on hitting the driver with between 2,000 and 3,000 rpms of backspin, a very high number of players today seem to be obsessed with finding a driver that will deliver a spin measurement in this desired range.
The purpose of this article is to clearly state what is and is not possible with respect to designing a driver head to deliver a difference in backspin performance.
First off, when a golfer hits two drivers and notices a difference in spin between the two, it is very difficult to know precisely what brought about the spin difference. To really know requires a testing procedure that must isolate each possible variable between the two drivers that contributes to spin. Few golfers are in a position to be able to do that in a very controlled environment. Hence it is very common for a golfer to hit two drivers, see that one launches the ball lower or with less spin, and make the wrong conclusion about what caused those differences in the shot.
The elements that can bring about a difference in spin between two driver heads are:
1. Loft – obviously the lower the loft, the lower the spin with all other things being equal. The problem with assuming that a loft difference brought about a spin difference is the fact that the real loft of a driver head often times is not what is stated on the head. +/- tolerances as well as some companies’ intentionally making some of their driver heads with a different loft than what is stated makes this comparison more difficult. Hence the ONLY way to know the difference in loft between two driver heads is to have the lofts measured by someone who is very experienced in doing this, using the proper precision equipment. Robot tests we have done show a spin difference of 260 rpms for each 1° change in loft at a clubhead speed of 100mph, for the same center face impact point. (And of course, as clubhead speed increases so too does the spin and vice versa)
2. Vertical Roll Radius – All driver heads are made with a certain amount of radius up and down on the face. (there is also the hoizontal radius on the face, but that is not pertinent to this discussion) Not all driver heads have the same vertical roll radius. The more roll (meaning more curvature as indicated by a lower radius number in the measurement) the higher will be the loft above the center of the face, and the lower will be the loft below the face center. The less roll, the less the loft increases above the center of the face and the less the loft decreases below the center of the face.
3. Point of Impact on the Face– FOR THE SAME LOFT and FOR THE SAME VERTICAL ROLL RADIUS, impact above the center of the face reduces spin due to the phenomenon of VERTICAL GEAR EFFECT. Likewise due to VGE, impact below the center of the face increases spin. VGE can be quite significant. Our robot tests done with the same driver head at 100mph with the same ball show a spin reduction of 400rpms for impact 1/2? above the face center and a 400 rpm spin increase for impact 1/2? below the center of the face.
4. Face Angle of the Driver Head – For some golfers, even a 1° change in the face angle can bring about a slight change in the effective loft of the head at impact that can result in little differences in the launch angle and spin of the shot.
5. Face to Back Center of Gravity Position in the Driver Head - By varying the wall thickness of the head body in the front vs rear of the head it is possible to create a little more forward CG vs little more rear CG position inside the driver head. It is also possible to vary the CG position by using materials of different density (graphite, tungsten, etc) within the head construction which are placed/oriented more to the front or back of the head body. The closer the CG is to the face, the lower the launch angle and lower the spin can be, and the farther back the CG is from the face, the higher the LA and spin can be.
This is a CAN BE because and NOT a “for sure” result for all golfers. The reason is because the effect of face to back CG depends entirely on its effect on the amount of forward bending of the shaft coming into impact. A more forward CG can cause the shaft to bend forward LESS which means the dynamic loft at impact will not be increased as much, which means slightly lower launch angle and spin. A more rear CG can cause the shaft to bend forward MORE which means the dynamic loft at impact will be increased a little more, which means slightly higher launch angle and spin.
But not all golfers can make the shaft bend forward coming into impact. Only golfers with a later to very late unhinging of the wrist cock angle on the downswing are able to make the shaft come into impact in some amount of forward bend position. The earlier the golfer unhinges the wrist cock angle, the sooner the forward bending action on the shaft occurs. So for early to midway release golfers, the shaft goes into its forward bend position before impact so by the time the clubhead gets to the ball, the shaft has had the time to rebound back from being bent forward. Thus only for later to very late release golfers will the more forward CG position in the driver head bring about a slightly lower launch and slightly lower spin.
The other variable affecting the face to back CG position on spin is the shaft’s stiffness design vs the golfer’s clubhead speed, transition force, downswing tempo and point of release. The stiffer the shaft overall, and/or the stiffer the tip section of the shaft all in relation to the golfer’s swing characteristics, the less the shaft can bend forward to have an effect on the launch angle and spin of the shot. And vice versa, the more flexible the shaft overall, and/or the more flexible the tip section of the shaft all in relation to the golfer’s swing characteristics, the more the shaft can bend forward to have an effect on the launch angle and spin of the shot.
6. Vertical Center of Gravity of the Driver Head – While everyone has been taught that a low CG in any clubhead means higher shot and higher spin and a higher CG means vice versa, the vertical CG in a driver head can be a very confusing factor for ball flight because 99%+ of the time we hit the ball with the driver off a tee.
Due to the use of a tee the position of the CG of the ball to the vertical CG of the driver head can be all over the map. Golfers tee the ball in all manner of different heights relative to the driver head. Because of this, the point of impact on the face relative to the CG of the ball can vary tremendously. This is not nearly as much the case with the other clubheads which are far more often hit with the ball sitting on the ground.
In addition, because so many driver heads are designed so close to the same 450-460cc volume “size”, driver heads today simply do not offer much in terms of a variation in the vertical CG position as can all other clubheads. When you design a driver to be close to the limit for volume, owing to the fact the head cannot weigh much more than 200g, the wall thickness of the body cannot be varied by more than a small amount to try to have a different effect on the vertical CG position.
As a result, theoretically the vertical CG position can have an effect on launch angle and spin, but in practice because of the tremendous range in tee height, point of impact and angle of attack into the ball coupled with the narrow range in driver size, the vertical CG of a driver simply does not account for much of the difference in launch angle and spin on the shot.
Bottom Line regarding spin characteristics of different driver head models – While there can be slight differences in launch angle and spin from different driver head designs, if the golfer wishes to hit the driver lower or higher in height and spin, the number one way to do that is by using a driver head with a lower or higher loft than the golfer currently plays. For each 1 degree change in loft, the launch angle changes by 0.8°. And based on a driver clubhead speed of 100mph, for each 1° change in driver head loft, the height of the shot changes by 10 to 12 feet and the spin changes by around 250 rpms. These factors apply to ALL golfers regardless of swing characteristics. The effect of a change in the face to back CG location on launch angle, shot height and spin only happen for golfers with a later to very late release.