Dear Tigersimmo and SJP,
Fantastic question!! You're right! And this is how we learn something new and useful.
But this only means that stroke plane trainers don't work correctly at the far margins of the stroke, as the vertical shoulder stroke really does stay out over the line, without any manipulation.
Let's put the question in two ways:
The first question is whether the sweetspot of the putter stays directly above the line in a vertical shoulder stroke. Yes, it does.
To see this, tie a short piece of string on the end of a straight stick or the butt end of a putter and hang a small weight off the string. Setup over a line on the floor with the end of the stick on the line and then make a vertical shoulder stroke. At the top of the backstroke, the end of the stick will still be directly above the line of the putt, as indicated by the "plumb bob" string hanging beneath the end of the stick hovering on the line.
The second question is whether the shaft or stick in the above stroke stays in constant contact with an infinitely long rail 5 inches above the ground. No. Only a vertical shaft would do that. Any tilt angle in the shaft will result in the shaft separating from the rail vertically going back, but this separation does not result from anything other than moving the shoulders vertically in plane.
So what should a stroke trainer look like for a vertical shoulder stroke? Apparently, the angle of the shaft causes each point along the shaft to describe a vertical arc offset from the putt line and parallel to it. Just as the putter head arcs vertically in a "smile" above the line at all times, so each and every point along the shaft of the putter describes a similar "smile" oriented vertically to the ground offset back from the putt line. This series of "smiles" stacked one after another from the handle to the putter head makes a cone shape altogether, as the twirling of any straight line in a circle to which is attached an angled line, by twirling the straight line at their apex, makes a cone. In this case, the "line" that is twirled at the apex is the horizontal line out from the pivot at the base of the neck, and the pivot is twirled about its horizontal axis as the shoulders move vertically. The angled line attached at the apex at the pivot is the line from the pivot to the putter head (not really the shaft, unless the arms shold the shaft stuck straight away from the chest with arms matching shaft exactly). The interesting point is that whatever spot on the shaft is initially in contact with the rail, this same spot remains vertically above the rail at all points in a straight stroke.
If the two lines (pivot out horizontally and pivot to putter head) make a 90-degree angle, the shaft is vertical and stays in constant touch with the rail as different spot on the shaft slide over the rail and the putter head rises up to the rail. The putter head rises more in this stroke for the same shoulder action than in any angle of the putter and the side-to-side sweep of this stroke is larger than a stroke at any other shaft tilt. (Unless the arms and hands can be bent out a little past the feet and down to get this shaft angle, this 90-degree shaft is not at all normal, and in any event would result in the heel being off the ground by at least 10 degrees.) As the angle becomes progressively more acute (less than 90), raising the shaft on a tilt incrementally until it is horizontal and matches the axis line out from the pivot (where the apex angle would be 0), the shape of the "smile" above the rail gets progressively flattened up and down and shortened side to side. Once the shaft angle matches the stroke axis horizontally out of the pivot, the "smile" is so short it disappears, and the up-down rising is so small it disappears as well, resulting in the putter head simply twisting in place.
In order for the spot on the shaft NOT to rise vertically above the straight rail when the shaft angle is tilted across the rail, but for the same spot (only) to remain in contact with the straight rail as the shaft glides back, the dipping down and back of the shoulder is not enough to keep the spot headed away from the feet along the rail, so some extra extension is required in just the right amount to make up the difference as the stroke gets further back. Without this just-right extension, the spot where the rail touches the shaft gets closer to the putter head, so the putter head and the sweetspot in this sort of stroke is coming inside on the backstroke.
Consequently, in a vertical shoulder stroke, the "rail" ought to look like a "smile" oriented vertically to the ground, parallel to the putt line but offset back a little. That's very interesting. The EXACT shape of the "smile-rail" would depend solely upon the offset distance from the putter head to where the shaft touches the "smile-rail" and not on the golfer's setup or even the angle of the putter shaft (which is really set by the putter head-rail offset horizontally). In this sense, creating a "smile-rail" shape tailored for any specific golfer and setup and putter is the same problem the Putting Arc people have in making the shape of the horizontally curved face of their training aid just right. That makes perfect sense.
Looking anew at what the "straight-rail" stroke trainers are teaching for a stroke pattern, even running the shaft straight along the rail teaches a stroke pattern in which the putter head comes inside off the line. This brings it down to whether you want to train a stroke "as if" running on an elevated rail or "in fact" that moves the putter head back and forth on a line.
I guess that's why I'm sceptical about training aids! It's not hard to move a putter straight back, so why make it tougher by trying to mold a stroke motion to a rail when that doesn't really help?
This is a very good day for putting, because we are really onto something here!
Contradictions and paradoxes are the soul of scientific advance!
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
http://puttingzone.com
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