SPIN MEASUREMENTS

It’s not difficult to measure the spin imparted by a putter. The measurement must be made just after impact but before the ball hits the putting surface – that way you don’t confuse ball spin due to skidding with spin contributed by the putter.

Normally it takes just a fraction of a second for the ball to hit the ground after flying off the putter face. So, to measure impact spin in the conventional way, you need a high-speed video camera to record the ball coming off the putter face. A high-performance camcorder capable of a thousand frames per second could do the job. You then need a computer with video analysis software to detect and analyse very small changes in ball rotation to measure the imparted spin…

The secret is to be unconventional and remove the putting surface. This change of scenario in no way changes the initial ball spin and velocity, but it does enable you to make precise measurements with very simple and readily available means.

Here’s how it’s done…

The picture on the left is a double-flash photo of a ball being putted off the edge of a tabletop. The tabletop could be covered with an artificial putting surface, but this refinement makes no noticeable difference to the ball spin.

The left side of the photo shows the ball being hit by the putter. To create this part of the image, a microphone picks up the sound of the impact and triggers the first flash.

The second flash is timed to show the ball in mid-air exactly one tenth of a second later.

The reference mark on the ball reveals any backspin or topspin rotation. A short straw is attached to the golf ball to form an extension of the reference mark. This has negligible mass and negligible effect on the ball flight, but it makes it possible to observe the direction and approximate rate of spin just by eye.

In the photo, the angle between the two images of the reference mark shows that this impact produced backspin of 0.5 rev/sec. The scale behind the ball is in centimetres (with the actual distance between the scale lines adjusted to compensate for the camera’s angle of view). From this, the launch velocity is found to be 2.2 metres/sec or 7.2 ft/sec.

Doubling the putter swing speed and halving the time between flashes would give almost exactly the same picture with the ball and putter in the same positions. The backspin would be 1 rev/sec and the launch velocity 4.4 metres/sec or 14.4 ft/sec. Since spin and launch velocity change together, we can quantify spin by its ‘roll ratio’, which is more directly related to the putter design.

The roll ratio (R) equals the ball peripheral speed due to spin divided by its linear or translational speed. (A golf ball has a circumference of 5.3 inches, so when it spins at one revolution per second its peripheral speed is 5.3 inches per second.) With R = +1 the ball travels with pure rolling motion. When R is negative, the ball has backspin.

Referring back to the double-flash photo, the roll rate (expressed as a percentage) is –3%. Provided the ball is struck on exactly the same point on the putter face, the roll rate will always be close to –3% whatever the strength of putt.

This second photo of the Odyssey 2-Ball putter in action shows considerably more backspin, with roll ratio of -10%. With this roll ratio, the ball loses 34% of its initial kinetic energy in the first few inches of a putt through skidding.

What makes the difference is that here the sole of the putter swings above the ‘ground’ by over half-an-inch, compared to less than one-eighth of an inch in the first photo. The higher the putter head at impact, the lower the impact point on the putter face and the greater the backspin. This shows that putting the ball on the upswing does not induce topspin (as might be expected) but actually increases backspin since an upswing stroke tends to contact the ball near the bottom of the putter face.

The double-flash or time elapsed photography technique is an excellent way of making certain that the spin measurement is valid and accurate. However, in addition to a camera, it requires two external flash units and special sensing and timing circuitry. An even simpler method is shown in the two photos below:

         

According to the manufacturer, special grooves on the face of the putter in these two photos are supposed to impart exceptionally high topspin. However, the photos prove that it has just the same spin characteristics as any other blade putter of the same general shape and head weight. It might be easier to get the ball to start skidding earlier with this putter. (For example having less loft angle on the face.) The ball would have almost the same amount of skid as a putter with slightly more loft but it would lose initial velocity earlier and so start rolling earlier.

In the two side-by-side photos, the one on the right provides the actual measurement. The photo on the left, showing the ball in its pre-impact position, is not normally required but is shown here for completeness. A ‘stop plate’ is placed in the path of the ball flight at some fixed distance away (e.g. 20 centimetres).

The stop plate has two electrodes, which are normally electrically isolated or ‘open circuit’. One electrode is a solid metal sheet and the other is an aluminium foil placed slightly in front of the metal sheet. When the ball reaches the stop plate it pushes the aluminium foil against the metal sheet to make contact and trigger the flash. (In practice, the foil and metal sheet are placed much closer together and only require a tiny force to make contact.) This is crude but effective, and any flash camera with a hot-shoe or other flash trigger connection can be used. With this technique, the ball is momentarily stationary when the flash fires, so even long duration flash exposures give sharp images.

Finally, by replacing the aluminium foil on the stop plate with old-fashioned ‘carbon paper’ (ink side facing the ball) you get what must be the simplest way possible of measuring imparted spin from a putter. As the ball strikes the vertical stop plate, the carbon paper transfers a mark to the ball on its (mid-air) horizontal equator. This shows up any angular difference between the mid-air horizontal equator and the pre-impact horizontal equator and so gives a quick and fairly accurate measure of spin.


WHAT WE LEARN FROM SPIN MEASUREMENTS
The two putters featured above illustrate the difference in spin performance between blade style and mallet style putters.

Blade putters are similar to iron clubs where the centre of gravity (CG) is quite close to the impact face. As a result, blades don’t usually have much gear effect and therefore spin is more constant at different impact heights. The C-Groove putter was found to have roll ratios of about +1% topspin for impacts near the top of the face reducing to about -2% backspin for impacts near the bottom of the face. (The corresponding figures for the 2-Ball were –3% and –10%.)

A mallet style putter has its CG placed well back from the face and is in this respect similar to a driver or fairway wood. Deeper CG results in a much more marked change in spin with change in impact point due to gear effect.

The CG in the 2-ball is exceptionally deep but it is also very high off the sole, so the sweet spot is high on the putter face. In the first double-flash photo the impact point is slightly below the sweet spot. To actually hit the sweet spot on a 2-Ball, the putter must be practically touching the ground, or the face must be severely hooded. The soft insert material and fairly massive head design helps to smooth the impact ‘feel’, so the high occurrence of impacts below the sweet spot is less noticeable.

Hitting below the sweet spot affects the length ‘forgiveness’ of a putter. As the impact height varies, impacts furthest from the sweet spot suffer greatest loss of launch velocity and highest backspin. These two factors combine to reduce putt length. Lindsay putters are designed to reverse this process by having the CG both deep and exceptionally low. This means that putts are on average made above the sweet spot, resulting in topspin gear effect. The higher the impact point above the sweet spot, the higher the topspin and this compensates (beautifully) for loss of launch velocity.

Yet another ‘side effect’ of deep CG in mallets relates to sidespin and directional forgiveness. It’s well known that hitting off the heel ‘pulls’ the putt and hitting of the toe ‘pushes’ the putt. The usual explanation is that the offset impact closes or opens the putter face so the ball deviates accordingly. This theory is correct for high-speed impact clubs like drivers but Dr Norman Lindsay has proved that the directional errors in a putter are almost entirely due to gear effect. As a result, mallet putters tend to have worse directional forgiveness than blades. (Line errors for heel-toe offset impacts with the 2-Ball are more than three times greater than the errors for the same offsets using the C-Groove putter.) This topic will be discussed in more detail in future editions of the Lindsay Golf website.