----- Original Message -----
From: Max Dupilka <[EMAIL PROTECTED]>
Sent: Tuesday, July 02, 2002 6:12 PM
> 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.
You're right about a difference of opinion. At least I think you are. You
have correctly stated my position -- I said there's a little variation in
COR, but not enough to matter. I also said that there is even less variation
in the COR improvement due to a flexible face. I'll let Tom say for himself
whether you correctly paraphrased him, but that is how I read his note.
To highlight our difference, consider two of TW's statements:
> > 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.
-- and --
> > 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.
These don't necessarily follow. With the same face thickness, the higher
swing speed will produce more deflection of BOTH face and ball. That means
that the lower swing speed will experience a higher ball velocity to swing
speed ratio. Now let's consider the IMPROVEMENT due to a flexible face,
which I think is what TW was trying to get at. Certainly the higher clubhead
speed NEEDS more improvement, because it squashes the ball more. But not
only do the energy losses increase faster than proportional (that is,
nonlinearly); so does the force required to do the squashing. So it's not
obvious that the improvement is greater for the higher clubhead speed.
I believe both TW and I believe most of the results of a paper by Alastair
Cochran ("Club Face Flexibility and Coefficient of Restitution", Science and
Golf III, 1998). He uses a nonlinear spring for the ball, resulting in the
sort of loss characteristics that Tom refers to. The reason I think Tom
believes this paper is that its results agree precisely (for an undamped
spring face) with his statement:
> > 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.
That paper is the only place I've ever seen that curve, so I suspect we're
both working from that as the source. Let's see what the paper has to say
about variation of clubhead speed -- because Cochran DOES look at that in
his paper.
It plots the curve for a lightly damped clubface, for two clubhead speeds:
30m/s and 50m/s. This corresponds to about 67mph and 112mph.
* For a rigid clubface, the lower clubhead speed gives a COR of .773 and
the higher a COR of .756.
* For the optimum-flexibility clubface (it isn't exactly the same
flexibility for the two speeds, but it's really close), the lower clubhead
speed gives a COR of .828 and the higher a COR of .818.
Hence my statement that the lower clubhead speed gets a somewhat better COR,
because it doesn't squash the ball as much. This is true for both the rigid
face and the optimum spring face. But let's look at the improvement, because
I think that's what was really being asked, and what Tom answered.
Going to an optimum spring-face (with the assumption of light damping) from
a rigid spring face gives:
* For the 112mph swinger, a COR improvement of 9.2%.
* For the 67mph swinger, a COR improvement of 9.3%.
Just a tiny bit better for the slow swinger, but not enough to make a
difference in ANYBODY's golf game.
I stand by my previous statements, unless somebody can show me an equally
convincing work on the other side. Cochran's paper is out there for anybody
to see and criticize.
> 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.
It would seem.
I really don't know, so I won't say anything.
Cheers!
DaveT
> 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
>
>
