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Topspin not Hopspin

Rollin', Rollin', Rollin', Keep them Golf Balls Rollin' ...

This is another one of those geeky pages that explains things at a detailed technical level. Read on if you go for that sort of thing....

When a golf ball is rolling smoothly across the green, its motion is more stable and predictable than when it is bouncing, hopping or skidding.  We'll talk about technical explanations for that later, but first we'll describe some tricks that putters use to try and generate "topspin", and why some of them backfire.  Honu putters give pure roll in a straightforward manner that avoids the problems.

Tennis players know that you must hit a hard ground stroke with topspin, otherwise the ball will fly out the back.  To get topspin on a tennis ball, you "hit up" on the ball, starting with the racket below the strike point and moving the racket upward in addition to forward.  It works great in tennis, so maybe it's good for putting too?


Topspin on putts is good, but you just can't "hit up" on a small ball that's sitting on the ground.  There is no way to get the putter below the ball because the ground is in the way. If you can't start below the ball, the putter cannot be moving upward at impact.  (Well, if your putter face were really shallow maybe it could come from ever-so-slightly below the impact point at the equator of the ball, but not enough to do any good.) Okay, then, suppose that we start moving upward right at impact?

That doesn't work either.

A golf ball stays in contact with a putter head for something like 1/4000 of a second. There is no way you would be able move the club upward at any useful speed between the initial impact time and when the ball has already "left the building".

Here's a really cool video showing golf ball impacts, from the biomechanics department at Manchester Metropolitan University.

The putter is at the end, about time 1:00.  The ball touches the putter face for at most 3 frames.  At the 12000 frames per second of this video, that's 1/4000 of a second. In this video, the putter is moving horizontally at impact.  Even if it were moving slightly upward, it would not have time to move up to any significant degree during the brief contact period.

Some putter faces have sharp grooves with the intention of increasing friction to impart topspin. It doesn't work.  Increased friction only causes topspin if the putter is moving upward at impact, thus applying the frictional force in a direction that would make the ball spin.  As we've seen, there is no reasonable way to get upward motion.  The overwhelming majority of putter motion is horizontal.

So what makes golf balls roll?  It happens after the ball has already left the putter face. The ball moves mostly horizontally after leaving the face (more on that later), gliding across the green. Eventually (in a few thousands of a second) the bottom of the ball touches the green and friction "holds back" the bottom of the ball while the center continues forward.  The ball starts to spin a little, and also "hops" ever so slightly as the ball bounces off the ground. Each time the ball touches the ground it gets a little more spin.  Eventually it spins at just the right speed to make the ball roll instead of "glide and hop".  "Just the right speed" means that the forward speed exactly matches the roll speed - in the same way that your car tires have to spin 6 times faster at 60 MPH than at 10 MPH.

If you were to try putting on ice, where there is very little friction, it would be next to impossible to get any roll.  The ball would just skid and skid.

Hop on Pop

Now let's talk about "mostly horizontal".  If a putter contacts a golf ball exactly on the equator, the ball will move horizontally.  Draw a line from the contact point through the center of the ball and that tells you which direction the ball will try to move.

If the contact point is below the equator, the ball "flies up" a little because the line from the contact point through the center is tilted upward.  The ball will "launch" and fly a little before gravity brings it down to earth.  The higher the launch angle, the longer before the ball touches down and begins the rolling process.  So you don't want much "launch" under most circumstances (the exception is if the green surface is really awful so your ball is sitting in a hole, then it might be helpful to have a little bit of "launch").

If the contact point is above the equator, the ball will try to go a little downward - except that the ground is in the way.  So instead of "flying downward", the putter pushes the ball into the ground, which compresses, pushes back, and the ball bounces off, possibly at a weird angle.  This is the dreaded "hopper".  They go all over the place.

For any kind of reasonable putting stroke, the thing that controls the contact point (at, below, or above the equator) is the effective loft of the putter face.  "Loft" is the amount that the putter face is tilted away from vertical.  If it is tilted backward (in the same tilt direction as an iron club) - which is called positive loft - the contact point will be below that equator and the ball will fly.  Forward tilt - negative loft - causes an above-the-equator contact so the ball hops.  If the face is exactly vertical - neutral or zero loft - the contact is at the equator, so the ball moves horizontally and rolls as soon as possible.

Earlier I said "effective loft".  That's the actual face angle relative to the ground.  The putter itself has an "inherent loft" which is the face angle relative to the sole of the putter itself.  If the sole is flat to the ground at impact, the inherent and effective lofts are the same.  But that's not always the case.  You can tilt the shaft forward - so the sole is not flat - thus making the effective loft less than the inherent loft. Or you can tilt the shaft backward to increase the effective loft.

To make it even more confusing, a putter shaft can be installed or bent with some amount of forward tilt, so the sole could be flat on the ground even with the shaft tilted forward - and then the golfer can tilt more or less from that position, perhaps with a "forward press" move.

What matters is the angle of the putter face relative to the ground at impact. Horizontal initial flight gives the best roll.  That comes from zero effective loft, regardless of which combination of putter head inherent loft, shaft tilt, and hand position is used to achieve it.

On the Roll Again

Honu putters achieve optimum roll with a geometry that encourages horizontal "ball flight" when the ball leaves the putter face.  The face is exactly perpendicular to the sole, so when the sole is flat on the ground, the face is vertical (zero loft). The sole is large so it's easier to tell when it's flat on the ground. The shaft is tilted forward by an amount that fits the individual golfer, so the sole stays flat in that golfer's preferred hand position.  The Honu putter head's center of mass in the vertical direction is aligned with the equator of a golf ball, so the "kickback" as the putter strikes the ball is horizontal.

Why Roll is Stable

When the ball is rolling, its motion is more stable than when hopping or skidding.  Both speed and direction are more consistent.  The speed is stabilized because, in order to slow down, the forward momentum and the spin momentum ("angular momentum" to be precise) must both be reduced at the same time.  Each helps to "prop up" the other.  The direction is stabilized because of the gyroscope effect.  A spinning object resists changes in the direction its axis is pointing, just like a spinning top resists falling over. When the ball eventually does slow down, the gyroscope effect is reduced and any little bump around the hole can throw the ball off-line.

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