DWELL TIME

THEORY OF DWELL TIME
‘ Dwell time’ or contact duration between two colliding objects such as a golf club and ball is a well-understood topic in the science of contact mechanics. The founder of contact mechanics was Heinrich Hertz, a brilliant young German physicist.

Heinrich Hertz 1857 to 1894

In 1882, Hertz was only 24 years old and working as a research assistant in Berlin University when he published a paper describing his theory of impact. This theory predicts what happens when objects collide and bounce off each other - how much deformation occurs, how the impact force varies with time and the total duration of contact, or what some putter manufacturers call ‘dwell time’.

A recent paper in Science and Golf IV by Professor Ieuan Jones of Flinders University presents convincing evidence that when you hit a golf ball, the force and duration of the impact obey the Hertz theory very accurately. Jones studied ball impacts over speeds corresponding to a gentle ‘tap in’ with a putter and up to a full drive down the fairway. Over this range of speeds the impact duration varied from 0.85 milliseconds for a gentle tap-in to 0.37 milliseconds for a drive. So the rule is, the faster the swing speed, the shorter the dwell time.

The ‘dwell time’ for a drive is just less than half that for a gentle putt, even though the ball speed off a driver is about 50 times faster than a tap-in. On the putting green, the variation in dwell time for different putt strengths is very much less. For example, on a level green, with the same putter and the same ball, a 10-foot putt will have just less than 15% more dwell time than a 40-foot putt.

The Jones study focussed on how accurately the Hertz theory predicts impact dynamics of one type of golf ball. The ball-hitting implement used in his experiments was made of stainless steel but was very much heavier than a putter head. Replacing this with an average weight putter head would reduce the values he obtained by about 6%. However, different weight putter heads do not change dwell time by much. The dwell time for a 450 grams putter head is only 3% greater than for a head weight of 250 grams.

The property of balls and putters that makes the most difference to dwell time is their hardness. Balata covered balls and elastomer face inserts give longer dwell time than harder materials such as Surlyn or steel, but even these soft materials do not increase dwell time significantly. The Hertz equations use basic elastic constants (Young’s modulus and Poisson’s ratio) whereas golf balls and putter inserts are usually specified in ‘Shore Hardness’ scales measured by a hardness tester called a ‘durometer’. This makes it difficult to apply the Hertz equations directly.

What we do know is that with any flat metal-faced putter, the dwell time is almost entirely determined by the ball material so there is no measurable difference between the dwell time from ‘soft’ metals like aluminium or copper and ‘hard’ metals like stainless steel. This is because metal putter faces are much harder than golf balls and all the impact deformation occurs in the ball. The Hertz equations also tell us that replacing a metal putter face with an insert made of the same material as the golf ball cover increases the dwell time by only 32%.

Since The Rules of Golf prohibit inserts that are softer than a golf ball, it is very unlikely that ‘legal’ inserts could increase dwell time by more than 40% to 50% compared to the value obtained with a steel face and a balata covered golf ball. A way of getting round the rules would be to produce a very soft-covered golf ball - much softer than alata - but his would be almost unplayable. To prevent this anomaly, a specification that face inserts must be no less than 85 on a Shore A durometer scale is included in A Guide to the Rules on Clubs and Balls.


FACT AND FICTION
All the basic facts about impact duration have been known for 120 years or so, but putter marketers still love to indulge in their own quirky theories about ‘dwell time’ and how this magically generates topspin.

So we learn…

“… the ball would be held on the face a millisecond longer than on a smooth-faced putter. That millisecond would result in immediate and pure roll of the ball…” C-Groove Putters

“Advanced Polymer inserts “hold” the ball on the face of the putter twice as long as conventional face putters, “wrapping” the ball and delivering True Roll performance benefits” FisherGolf

“ (The White Hot putter) helps the golf ball dwell on the putter face for a fraction of a second longer than other putters we’ve tested, which helps get the ball rolling with overspin more quickly” Callaway

“… the putter’s proprietary nubs create a “mattress effect” that holds the ball on the face a fraction longer than many other putters, with the net result being less skid and truer roll” TaylorMade

These are some quotes from a few of the numerous companies who have jumped onto the “dwell time gives overspin” bandwagon. Not every putter designer has a PhD in physics or engineering, so getting the numbers wrong is understandable. But it’s worrying to see ‘technical experts’ in this field and leading brand companies buying into the gimmicky idea that dwell time is an antidote to backspin. Grooves and insert materials, no matter what their dwell time might be, simply do not create topspin or for that matter backspin or sidespin. This is obvious if you just remember one of the most basic of Isaac Newton’s Laws; Action and Reaction are Equal and Opposite.


DWELL TIME MEASURED
Measuring putter dwell time is again not too difficult. The experiments shown below need some specialist equipment, but they are not beyond the resources of a good school science lab or a keen electronic hobbyist’s workshop.

The photo on the right shows a C-Groove putter with a small accelerometer bolted onto the face, near the heel. The putter is swung by a mechanical jig and the golf ball is positioned to impact the putter face very close to its sweet spot. This way the putter head has very little vibration at impact and the deceleration experienced by the putter head at impact is a simple short duration pulse – as can be seen on the digital storage oscilloscope display. The dwell time is simply the duration of this pulse. (The accelerometer weighs only 12 grams and the Hertz equations show that this increases the measured dwell time by a negligible 0.2%.)

The two traces plotted on the left show the C-Groove accelerometer signals for two different types of ball. Both balls were putted very close to the sweet spot with putt strength of one Stimpmeter®. In other words, with this putt strength, the balls would roll about 10 feet if the ‘green speed’ were 10 feet.

The top trace is for a Surlyn-covered ball, which gives a short dwell time of about 0.6 milliseconds. The ‘ripple’ on this trace is caused by putter head vibration. This would disappear if you could get the impact perfectly on the sweet spot, but this is very difficult and the hard covered ball shows up any tiny offset.

The lower trace shows the C-Groove deceleration pulse when a balata-covered ball was used. Here the dwell time has increased to about 0.8 milliseconds.

These results show that the dwell time on the C-Groove is almost exactly the same as a flat-faced metal putter and certainly not “a millisecond longer”.

This photo shows another way of measuring dwell time. Here a TaylorMade ‘Nubbins’ putter is fitted with a thin conductive-plastic film over the impact area. The film is about 60 microns thick, very flexible and in intimate contact with the face insert. This set-up ensures that the film has very little mechanical influence on the impact.



The ball is sprayed with a thin film of conductive paint and wires connect the patch of conductive paint on the ball and the conductive film on the putter to a simple electrical circuit. The circuit generates a negative pulse (seen on the oscilloscope screen) when the ball and putter are in contact during impact. The length of the negative pulse is thus equal to the dwell time.

TaylorMade are correct to assume that the ‘nubs’ on the putter face increase dwell time slightly. It depends on the softness of the insert material and the additional compliance provided by the voids between the nubs. We found that the dwell time for the Nubbins putter was about 0.9 milliseconds, fractionally longer than for a balata ball off a flat steel putter and with the same strength putt (one Stimpmeter®). However, the claim that this improves ball roll is pure fiction.

The nubs in the Nubbins putter are unfortunately similar to dimples on a golf ball and cause line errors. A special groove configuration on Lindsay putters is designed to reduce these line errors, but the TaylorMade nubs will tend to slightly increase dimple error effect. (More details of this error effect will appear in future editions of this website.) As it happens, the Nubbins putter is probably less prone to dimple error effects than some other putters because its soft insert material helps to reduce dimple errors.

So what is the maximum dwell time you can get from a ‘legal’ putter? This is going to be from a putter with an exceptionally soft insert - probably softer than a standard balata-covered ball. One likely candidate is a Fisher putter. Fisher claim that their putters have more than twice the dwell time of a flat-faced steel putter and quote some quite amazing ‘scientifically proved’ figures. Undoubtedly, their putters have very soft inserts – in fact just within the limits set by The Rules of Golf – but their figures for dwell time are highly exaggerated and again, equating dwell time with overspin is simply marketing hype.

Top trace: The Nubbins putter - dwell time just less than one millisecond. Lower trace: Fisher F-6 putter. Dwell time marginally over one millisecond. (Both putters tested against a balata-covered ball at one Stimpmeter® putt strength.)

Measuring the hardness of the Fisher putter with a durometer. Its hardness measures about 87 Shore A. This is within the legal limits set by The Rules of Golf but quite a bit softer than most ‘soft-covered’ golf balls.