Dave and Tom: So it looks like we have a bit of differing opinions here. Dave, you would suggest that the COR, for a given clubhead/ball combo, is close to a constant for all clubhead speeds. Tom seems to indicate that the COR is non linear enough to make a significant difference.
It seems a lot of the COR would depend on the construction of the ball, maybe to an extent that the flexing of the face is almost irrelavant. I don't know. Interesting discussion. Max... Tom Wishon wrote: > Guys: > Been reading your comments on spring face with interest. Lots and lots > of things to comment on but don't want to make this too long, so I will > try to hit the high points about which I think mine and my engineering > consultant's knowledge is pretty solid. > > 1. Perhaps the worst piece of misinformation in all this is the actual > term trampoline effect, for the reason it gives you the mental image of > the face acting like a 'slingshot'. What happens to increase ball > velocity off a so-called spring face is that when the head is allowed to > lose more energy by the face deflecting more, then the ball loses less > energy - and then can leave the face at a higher velocity relative to > the head's velocity. If you have less face deflection for the speed of > the head, then the ball is deforming (squashing) more, the ball then is > losing more energy and it cannot leave the face as fast. > > 2. Remember - it is the BALL that is the source of almost all of the > energy loss at impact. The greater the squashing of the ball, the more > the ball energy loss. This matter of loss VS ball squashing is NOT > linear. If it were, then twice the squashing of the ball would produce > twice the energy loss. What happens is as the deformation of the ball > doubles, the losses more than double. > > 3. Yes, it was me and my engineer who came up with the idea of > different swing speed limits for the different head designs when I was > with Golfsmith. Reasoning for this was based on #1 and 2 above. Higher > swing speeds cause more face deflection for any particular face > thickness. No matter what, a slower swing speed will deflect a face > less. And the less the face deflection, the more the ball loses energy > for that particular golfer. If you make ONE face thickness to fit all > golfers, the more the golfer's swing speed decreases from the highest > swing speed the face is built to withstand, the less ball velocity to > swing speed improvement will be realized. To maximize the potential for > the slower swinger, you have to reduce the face thickness to allow their > slower swing to deflect the face more - but the downside is if that club > is used by a golfer with a much higher swing speed, they will > "over-flex" the face past its stress limit and cause permanent damage. > Hence the reason for the WARNING stickers on the faces of those drivers > we designed for GS. > > 4. If you plot a curve of the COR VS the Spring Rate of the face, the > COR starts out at a low value with a face that is very flexible and > increases as the face assumes a higher spring rate until the peak is > reached at about a 0.86 COR value. Continued increase of the spring > rate results in a LOWER COR until the idea levels out to a point where > the face would be totally rigid, at a COR of about 0.76. > > 5. The keys to higher COR drivers that can maximize the ratio of swing > speed to ball speed, are, a) larger faces that are more TALL than wide. > Our studies indicate that face height is 3 times more influential on the > ability of the face to deflect than face width. B) face materials with > a high yield strength + low modulus of elasticity AT THE SAME TIME > TOGETHER. This is why Beta Ti alloys are so good to use. But do not > forget, super high strength steels that allow less face thickness than > the beta ti alloys are good too - we were able to make an Aermet face > driver that had the same 0.845 COR as the Beta Ti back in 2000. Their > only drawback is their higher density - as the face area increases the > weight does too - which puts limits on how large you can make a steel > head and still have the desired headweight for swingweighting purposes. > > 6. It is possible for a slow swinger to get more ball velocity from a > thin face big head than if that same slow swinger was using a much > thicker faced driver. But do not even think they are getting close to > what the faster swinger is getting in terms of face deflection/ball > energy loss reduction. Use common sense - a 50 mph swing is still going > to deflect a thin face more than it will a thick face. It won't get the > MOST out of it like the 100mph swing will, but it will still get some > improvement over what happens with the thicker face. > > TOM W
