Dear James,
Basically, the collision or impact of the putter face and ball
transfers momentum from the moving mass of the putter to the stationary mass of the golf ball. Exactly how this transfer of momentum occurs depends on the relative masses (usually about 7 or 8 to 1 ratio of putter head to ball), specifically in terms of the presentation of the putter face to the spherical shape of the ball in terms of motion of the putter head center of gravity (COG) in relation to the ball's stationary COG and the intended line of the putt, the putter face orientation in space in relation to the ball's COG and the intended line of the putt, the cover material on the ball, and the material of the putter face. A "solid" putt is one in which the putter face is moving squarely thru the center of the ball with its sweetspot staying on the intended line of the putt thru the impact zone. The term squarely is usually thought of in terms of the top or bottom edge of the putter face being perpendicular to the line of the putt, and while that is true, there is another consideration -- the COG of the ball also moving straight thru the ball's COG during impact, and not above or below the ball's COG. This would be a pure transfer of momentum.
I don't think this sort of "solid" impact happens as often as people imagine. If you take the case of a level trajectory of the putter COG thru the ball's COG, with the path staying horizontal all the way thru impact and at the same height as the ball's COG, with a putter face presenting zero loft (a perpendicular, untilted face) to the ball's equator, the collision will transfer maximum momentum of the putter to the ball, but that sort of impact will also impart ONLY an initial "skidding" motion in the ball. After the ball skids along the green's surface for a while, the skidding will change some of the ball's siding to rolling motion. After a certain point, usually about 15% of the total distance of the putt, the skidding will be totally trasformed by green friction into ONLY rolling. The physics is that the ball will skid only at first and gain topspin forward rolling motion by virtue of the skid forces on the bottom of the ball, and once the ball's speed slows to 5/7th its initial linear velocity, the skid totally is replaced by forward rolling.
The person who appears to know the most accurate information about all this is Norman Lindsay in England. On his
Lindsay Putter website is an excellent discussion of the impact factors in the ball-putter collision that promote less skidding and earlier rolling. Putts that get the ball rolling sooner and that reduce skidding roll truer and farther than otherwise. According to Lindsay, the factors that promote earlier rolling are a low COG in the putter head, an impact high on the putter face in relation to the putter COG, a deep putter COG front to back, a center-shafted hosel, an "oblique" presentation of the putter face to the back of the ball, and a minimal front to back weight distribution. All of this promotes top spin and reduces backspin and therefore promotes earlier rolling. Effectively, the physics of ball-putter impact are managed so as to impart some sliding and some forward rolling initially, instead of only sliding and instead of some sliding and some backward rolling (backspin).
The two dynamics that Lindsay relies upon to reduce backspin and promotes topspin and earlier rolling / less skidding, are "vertical gear effect" and an "oblique blow." the vertical gear effect is how the putter face in a low-COG putter with impact high on the face "rolls" during impact in a gear fashion so that the putter COG "gears" upward during contact over the back of the ball, imparting some forward spin or at least reducing backspin, depending upon the loft at impact. The "oblique blow" is a presentation of the putter face to the back of the sphere of the ball with minimal loft on an upward-angled trajectory thru the ball. Lindsay writes:
"Some loft is necessary to lift the ball at impact, particularly on slow greens. But loft generates backspin and raises the sweet spot, so choose a putter with the minimum loft that suits you. As well as imparting topspin, vertical gear effect lifts the ball slightly so less loft is required."
The lower pictures show the vertical gear effect. The impact high on the face shows the topspin gear effect.
Lindsay's putter have a variable-loft face, where the high half of the face has minimal positive loft but in case the golfer delivers the face too high to the ball (lifts putter in stroke or plays ball too far forward of the stroke's bottom or just sets up poorly), the lower half of the face has an insurance policy of negative loft. Considering only the lower half of the putter face, Lindsay putters are similar to roll-face putters or cylinder putters except that his negative loft on the lower half is a planar surface.
Harold Swash in England also teaches an "oblique blow." He calls it a "tangential blow." Swash teaches that the hands need to be leading the putter head into impact and rising thru impact. Exactly how the putter face relates geometrically to the back of the ball's sphere at impact is a little unclear to me.
In billiards, a level blow of the cue stick high across the ball's sphere 7/10th the way up the ball's diameter from the ground imparts ALL roll and no skid.
The trouble in putting is that the putter face is not like the end of a cue stick and the "slab" of the putter face can only "fit" onto a spherical shape at a tangent to the point of contact on the ball. Wherever on the ball the putter face makes contact, the plane of the face is perpendicular to a radial line from that point on the ball to the ball's center. The positive lofting in putter designs means that the putter face, unless changed by the dynamics of how the stroke is made, will necessarily impact the ball slightly below the ball's equator, where the radial line from center of ball to equator is considered zero and the angle of the radial line from center of ball to point of contact on the ball's surface is the SAME as the loft of the putter face.
A previous
post on the Flatick Forum discusses these issues in some detail. In discussing the comparison of putting with a cue-stick blow, Dr Lindsay writes:
"However, simply elevating a putter’s COG above the ball equator to 7/10ths diameter does not achieve the same effect as the cue stick in billiards or pool. It’s impossible to contact a golf ball at a height of 7/10ths diameter with the face of a legal putter unless you are hitting down or otherwise de-lofting the face very severely. At 7/10ths diameter, the tangent (i.e. the slope of the ball’s surface) is -23.6 degrees to the vertical but the R&A and USGA have put a limit of –15 degrees on the permitted amount of negative loft in conforming putters.
Even if you did hit the ball with a -23.6 degrees negative loft putter face, you would not get the ball to roll without skidding. Initially, the ball would dig into the putting surface at an angle of about 20 degrees and, yes it would have some topspin, but probably less than a quarter of true rolling spin. It would then undergo a severe and abrupt skid, probably jump into the air, and it’s anyone’s guess what spin it would have after that."
So, what is "oblique impact"? On his website, Dr Lindsay writes that the gear effect and oblique impact are the key dynamics to topspin:
"Lindsay putters impart high topspin over the entire striking face, providing modern-age performance that would enthral bygone masters such as Bobby Locke. This unique achievement is the result of expertly combining the only two mechanisms that genuinely put spin on a golf ball, namely;
* Gear-effect, which relies on the putter-head weight distribution, but is also critically dependent on the way the shaft attaches to the head.
* Oblique impact - the workhorse of golf shots. This shapes flight trajectory and puts backspin on the ball. Backspin is essential for distance in long shots and control in approach shots. In putters, it can be used ‘in reverse’ to give topspin."
On the other hand, in explaining these aspects, Dr Lindsay on the Forum writes:
"It is impossible to achieve any useful degree of vertical oblique impact with a putter. A rising blow with just a few degree of upward attack angle will lift the putter-head to make contact on the lower half of the putter face. This in turn will negate any slight topspin created by the oblique impact."
Reconciling these apparently contradictory statements, I gather that what he means by "oblique impact" is delivering the putter face low into the back of the ball, perhaps level, and then the gear effect during impact transforms the delivery of the putter face from flush or simply vertical to "oblique." The gear effect, then, seems to take loft out of the presentation of the putter face to the back of the ball
during impact. There is no attempt to manipulate the loft into an oblique presentation, as in Swash's teaching. Or so it seems.
If the putter face has some "minimal" loft and the stroke dynamics do not add or subtract loft, then this sort of unmanipulated delivery of the putter face for impact high on the face will impact the ball wherever the loft matches the tangent on the back surface of the ball. A two-degree loft delivered without manipulation, for example, would impact the ball slightly below the equator where the tangent is also two degrees. This is not very much below the equator at all (about 3/100th of an inch below the equator).
Now, back to your question:
What happens is that the putter face impacts the ball at a certain point, probably below the equator just a little, and depending upon the putter face's design loft and whether the stroke dynamics add or subtract loft and upon how low the putter COG is in relation to the impact point, the putter face compresses the cover material of the ball a little and imparts a combination of "launch" off the ground plus some modest backspin or modest topspin. The impact typically lasts between 0.5 ms to 1.0 ms, depending upon ball cover hardness, putter head mass, putter face material, and stroke speed. During this time, the gear effect twists the putter face vertically so that the face "gears" up over the back of the ball imparting some topspin (or at least reducing backspin). The ball gets a slight launch off the ground into the air and lands with a mild bounce into the grass with its spin and then the forward linear motion (translational motion) comes in conflict with the friction of the green at the bottom of the ball. This friction both increases forward rolling and slows the translational linear speed. The greater the friction, the faster this process completes itself. once the linear speed of the ball falls to 5/7th its initial value, the ball ceases skidding and then rolls on the grass. The rolling is sort of "on top of the grass." There is still some "rolling friction" slowing the ball, but this friction is not as strong as the "sliding friction" in the early phase of the putt. Eventually, the ball slows sufficiently that the ball starts to sink back lower into the grass, and the friction increases for this "decay phase." Once the decay phase sets in, the ball stops in pretty much the same distance for any length of putt. That is, the decay phase always sets in at the same slow speed of the rolling balls, and so the stopping action always takes the same time (roughly) from the onset of the decay phase until the ball has stopped rolling.
Scotty Cameron says that putters need 3-4 degrees of loft to get the right launch out of the pocket of grass at that start. This does not mean the putter has to have 3-4 degrees of loft designed in, but that the stroke plus the design needs to deliver the face to the ball with 3-4 degrees of dynamic loft. Dr Lindsay's work emphasizes "minimal loft' to get the ball launched well, and this will depend upon your typical greens. A tight bent-grass green shaved to a Stimpmeter speed of 10-11 should not require much dynamic loft at all, probably less than 3-4 degrees in any event. A shaggy Bermuda-grass green would take at least 3-4 degrees, or more, of loft at impact for the correct launch conditions.
In all of this, everyone seems to agree that driving the ball down into the ground at impact is not a great idea. On the other hand, if it is possible to get the ball started out of its initial cup in the grass without launching it into the air, and instead to preserve good contact with the grass immediately, this should promote greater roll-inducing friction at the outset and thus a quicker end to skidding. At the least, you do not want to launch the ball any more than is required to start the ball out of whatever sort of depression it may sit in. Personally, I don't think the ball sits in much of a cup at all on modern greens, and so probably doesn't need much at all of a launch off the ground. This aspect needs some clarification by testing launch conditions for early rolling on different surface conditions. For an ealier discussion along these lines, see this
post on the Flatstick Forum.
The path of the putter face in relation to the intended line affects the direction the ball starts out. If the putter hesad moves on line, the path sends the ball online. If the path is across the intended line a certain angle (say 10 degrees), only about 17% of this error is transferred to the ball's direction, and the ball starts off about 1.7 degrees (not 17 degrees) offline. The putter face orientation at impact is a lot more important than path. A straight-path stroke with the putter face twisted open 45 degrees will send the ball starting offline about 83% of the 45 degree error, or 37 degrees offline. A more typical error in putter face of 2 degrees results in the ball starting offline about the same 1.7 degrees as does a 10-degree path error, so that is around 5 times more important.
Impacts off the sweetspot also impart a little sidespin at the start, but this sidespin is quickly washed out by the rolling over the grass.
Whether the hands should lead the putter face into impact or not, or whether the putter should have a goose-neck design to artificially put the hands ahead, is not really a question of hitting up or down onto the ball. These aspects of technique really just alter exactly where the stroke levels out in the vertical dimension by moving the bottom of the stroke a little further towards the target. This, plus ball position and loft in the putter, all conspire to determine whether you hit down or up on the ball. It's probably best not to add much loft during impact, so ball position and bottoming out the stroke probably ought to be fairly close to each other, in theory.
The final problem is really the question of how much technique needs to be learned to get this theoretical true-roll improvement balanced against the benefit in scoring with the putter. My sense is that a little extra launching of the ball is a decent trade for accuracy of delivering a square face thru impact in an online stroke path. There seems to be a need to play the ball a little forward of where the stroke bottoms out so that the golfer has some extra margin of error to get past the bottom and present the face square into impact. If the design of the putter can reduce the launch and maximize the topspin and early rolling, so much the better. but if I have a putter that is just a stock putter that does not do such a hot job of helping with true roll, I still think it is more important to worry about square and online impact, even if the launch and spin conditions are not optimal.
Bigger fish to fry.
Cheers!
Geoff Mangum
Putting Theorist and Instructor
Geoff Mangum's PuttingZone
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