String movement facts/myths

[BreakPoint]

That's what I was trying to say, the strings are more elastic so they snap back far sooner than the ball, so the ball leaves the strings because the strings have snap back not because the ball snaps back, when the ball leaves the strings it is still deformed. That's why Breakpoint theory that the ball leaves the strings before they snap back is nonsense.

A cement wall is completely inelastic. When you slam a ball against a cement wall, why does the ball come flying back at tremendous speed if the wall doesn't snap back at all? According to your theory, shouldn't the ball just stick to the wall since the ball doesn't leave because the ball doesn't snap back unless the wall snaps back first? If you hit a ball at the same velocity against a cement wall and then against the stringbed of a stationary racquet, why does the ball come back from the wall at a greater velocity than from the stringbed if the stringbed is more elastic than the wall? I know why, but you obviously don't.

Once he realized that, he came with a second BS, he claims that the strings snap back in the horizontal plane first and then they snap back in the verticall plane which is another nonsense since the high speed camera shows that the two are simultaneous, which makes perfect sense since the elasticity is the same in both planes.

"Once I realized...."??? I've NEVER changed my assertion. Snapping back in the same plane as the stringbed has nothing to do with adding spin to the ball. If it did, you'd see pros spraying WD-40 on their stringbeds, but you don't, do you? Oh, and I've never said anything about snapping back in two planes. The only snap back that matters in this discussion is the snap back in the same plane as the stringbed (side to side).

Oh, and the effective elasticity is not the same in both planes. In the normal plane, the middle main strings are being supported by 60 to 18 cross strings which prevent the main strings from stretching freely. The cross strings create a barrier to the main strings elasticity. In the same plane as the stringbed, only frictional forces are limiting the main string's inherent elasticity. Take a racquet and use your thumbs to push back the center of the stringbed. Notice how much force it takes to push back the stringbed. Now measure how much in distance you're able to stretch the middle of the stringbed. Then use your fingers to pull aside a middle main string to the side. Notice how much less force it takes and how much greater distance you can stretch it. Then with the palm of your hand, hit the middle of your stringbed and time how long it takes to snap back. Then do the same with the middle main string by pulling it to one side. Notice how much longer it takes the main string snap back after you pulled it to the side. The ball has left the stringbed by then. The elastic force of one or two main strings as the ball rolls across the stringbed is not enough to overcome the tremendous force of the ball pushing that one or two main string in the opposite direction while you're swinging vertically upwards at high speed and with great force. Thus, it can't snap back until that downward force had mitigated which happens right after the ball leaves those main strings.

 

[BreakPoint]

I think we are confused with fully snap back with partially snap back. You could be right, before the string return fully to its old position, the ball might have already left the string, this results energy loss, which is no good.

Let us image the moment of when both the string and ball reach their full potential energy state. In this state, the ball stops being deformed, and the string stops stretching because it reaches this max.

After that moment, the ball starts to bounce off the string bed and string starts to move back (snap back) at the same time. If you look up the Elastic modulus of the tennis ball and strings, you will find that all strings have a much higher Elastic modulus than the tennis ball, which means the string recovers and moves faster than the ball when the same pressure is released (even if the friction between string is high, it will snap back fast initially). Therefore, all strings start to snap back (partially or fully) before the ball leaves the string bed. The difference is that which string recovers to its old position faster before the ball leaves the strings. And we all know that poly is stiffer, so it will move back faster or more before the ball leaves. Just to point out, increasing the friction between the string and ball will make string move more if the friction between the strings is not increased as much, but it is another topic for another time.

The energy is not lost. The stored energy is used to snap the strings back into place. But this can only happen after the ball has left those main strings in question due to the tremendous force the ball exerts on those main strings in the opposite direction to the snap back.

 

[JediMindTrick]

A cement wall is completely inelastic. When you slam a ball against a cement wall, why does the ball come flying back at tremendous speed if the wall doesn't snap back at all? According to your theory, shouldn't the ball just stick to the wall since the ball doesn't leave because the ball doesn't snap back unless the wall snaps back first? If you hit a ball at the same velocity against a cement wall and then against the stringbed of a stationary racquet, why does the ball come back from the wall at a greater velocity than from the stringbed if the stringbed is more elastic than the wall? I know why, but you obviously don't.

My theory is fine, you need to brush up on your physics. The strings snap back first because they are more elastic than the ball. Since the wall is inelastic it doesn't snap back, so the ball leaves the wall because/when it snaps back, it's perfectly consistent. Are you actually saying that the strings don't snap back before the ball?

 

[BreakPoint]

My theory is fine, you need to brush up on your physics. The strings snap back first because they are more elastic than the ball. Since the wall is inelastic it doesn't snap back, so the ball leaves the wall because/when it snaps back, it's perfectly consistent. Are you actually saying that the strings don't snap back before the ball?

Looks like you never learned your physics at all. You said that the ball only leaves the strings because the strings snap back and not because the ball snaps back. So why does the ball leave the wall at an even greater velocity when the wall doesn't snap back at all? Why would the ball leave the wall but not leave the stringbed (unless the strings snapped back)?

.... so the ball leaves the strings because the strings have snap back not because the ball snaps back,.....

 

[sureshs]

A cement wall is completely inelastic. When you slam a ball against a cement wall, why does the ball come flying back at tremendous speed if the wall doesn't snap back at all?

Guys be very careful here about use of elastic. The collision is very elastic, meaning kinetic energy is conserved. In a completely inelastic collision, objects stick together, and the kinetic energy is stored in the "compressed spring" that exists virtually between the bodies. Breaky is not talking about elastic vs inelastic collisions when he uses the term elastic here, but rather about material properties.

 

[pvaudio]

Guys be very careful here about use of elastic. The collision is very elastic, meaning kinetic energy is conserved. In a completely inelastic collision, objects stick together, and the kinetic energy is stored in the "compressed spring" that exists virtually between the bodies. Breaky is not talking about elastic vs inelastic collisions when he uses the term elastic here, but rather about material properties.

I had said pages ago that this conversation is getting too "technical" for its own good. Talking about elastic and inelastic collisions when discussing tennis ball spin is just a waste of time. It is, with all due respect to those involved, a ******* contest to see who has his or her science down the strongest. Nothing is going to be decided here.

 

[JediMindTrick]

You seem very confused.

Looks like you never learned your physics at all. You said that the ball only leaves the strings because the strings snap back and not because the ball snaps back.

Yes, that's what I'm saying. Do you agree or not? I feel like I'm wasting my time here.

So why does the ball leave the wall at an even greater velocity when the wall doesn't snap back at all?

Because in the ball-strings collision scenario, the energy goes in the strings deformation and the ball deformation. Because the ball leaves the strings when the strings snap back the energy stored in the strings deformation is recovered. The ball snaps back after it leaves the strings so the energy that went in the ball deformation is lost.

In the ball-wall collision all the energy goes in the ball deformation and it is recovered because the ball leaves the wall when it snaps back.

Why would the ball leave the wall but not leave the stringbed (unless the strings snapped back)?

What do you mean "unless the strings snapped back"? That's why it leaves the stringbed, because the strings snapped back.

 

[sureshs]

I had said pages ago that this conversation is getting too "technical" for its own good. Talking about elastic and inelastic collisions when discussing tennis ball spin is just a waste of time. It is, with all due respect to those involved, a ******* contest to see who has his or her science down the strongest. Nothing is going to be decided here.

I think even use of the term elastic in the materials sense may not be correct here. For example, as textbooks point out, steel is more elastic than rubber. It is the stress/strain ratio, which does not necessarily correlate with our limited physical experience.

 

[sureshs]

That's why it leaves the stringbed, because the strings snapped back.

It is not a one-sided thing, as if the ball was sitting there and the string decided to kick it out.

Collisons like this are modeled by an imaginary spring that exists between the bodies on contact. At first the spring is compressed from both sides and gains potential energy. Then, in elastic collisions, the spring, well, springs back, and converts the potential energy into kinetic energy for both parties. In completely inelastic collisions, the spring stays deformed (the potential energy dissipates into heat) and the bodies stick together.

Both sides get deformed, and both regain their shape after collision. Third law forces act on both bodies. One is not passive and the other active, or vice versa.

 

[olliess]

I had said pages ago that this conversation is getting too "technical" for its own good. Talking about elastic and inelastic collisions when discussing tennis ball spin is just a waste of time.

On the other hand, the difference between a nearly-ideal elastic rebound (stretching strings) and a not-very-ideal rebound (tennis ball squashing and rebounding) is pretty central to almost everything that happens between the racquet and ball. So I'd have to say it's at least interesting to me as a reader.

 

[olliess]

In the ball-wall collision all the energy goes in the ball deformation and it is recovered because the ball leaves the wall when it snaps back.

Of course it is not nearly recovered, or a dropped tennis ball would bounce back up to almost the same height it was dropped from.

 

[share1law]

I like the fact that Breakpoint challenged the widely accepted spin generation theory - the main string snaps back and generates spin, and brought up a few good questions such as if we glue the strings together, does it mean we generate less spin or cannot even generate spin at all? We know it is not true. And why some rough strings generate more spin but the main strings slide less and are harder to slide.

Maybe main strings slide and snap back is not the reason of why spin is generated, but of why poly generates more spin?

I think we will have the answers if we understand which of the following string set generates more spin. Please do not ask how rough or how smooth of the interfaces are, it is just a general question, everything else is the same except for the friction of interfaces.

Set 1: Smooth string/ball interface, smooth string/string interface
Set 2: Rough string/ball interface, rough string/string interface
Set 3: Rough string/ball interface, smooth string/string interface

 

[jester911]

This is one more thread this article posted in another thread is pertinent.

http://www.theatlantic.com/magazine/archive/2011/01/the-new-physics-of-tennis/8339/

 

[Consolation]

I like the fact that Breakpoint challenged the widely accepted spin generation theory - the main string snaps back and generates spin, and brought up a few good questions such as if we glue the strings together, does it mean we generate less spin or cannot even generate spin at all? We know it is not true. And why some rough strings generate more spin but the main strings slide less and are harder to slide.

Maybe main strings slide and snap back is not the reason of why spin is generated, but of why poly generates more spin?

I think we will have the answers if we understand which of the following string set generates more spin. Please do not ask how rough or how smooth of the interfaces are, it is just a general question, everything else is the same except for the friction of interfaces.

Set 1: Smooth string/ball interface, smooth string/string interface
Set 2: Rough string/ball interface, rough string/string interface
Set 3: Rough string/ball interface, smooth string/string interface

Keeping all the other variables the same means that the answer to your question is only relevent to you, if those other variables match your swing. I haven't seen any data one way or the other, but it seems reasonable to theorize that the answer to your questions may vary depending on impact conditions (force, angle, etc).

It's not a simple a=b equation.

If you read the article linked to above, you'd think that all that matter is interstring friction. Yet clearly there's a lot more going on than that. Just comparing the spin data from this article:

http://twu.tennis-warehouse.com/learning_center/spinexperiment.php

with TWU's friction data:

http://twu.tennis-warehouse.com/learning_center/COFreporter.php

In the article, spin for polys:

TCS 138 +/-14
ALU Power 162 +/-8
ALU Rough 165 +/-10

And friction data

TCS 0.098
ALU Power 0.102
ALU Rough 0.127

And Stiffness data:

TCS 205
ALU Power 248
ALU Rough 225

If surface roughness matters, why are ALU power and Rough indistinguishable? If slipperyness is important why did TCS have lower spin than both? If it's stiffness, well, there's no pattern again.

I think the answer is that it's not that simple. I suspect the interactions that lead to spin generation are a lot more complicated and dependent on more than just the strings characteristics. I think, from the tests that have been done, that it is incorrect to think that there is a consistent pattern accorss impact conditions. That's what leads me to the conclusion I mentioned earlier, that it is very hard to make any generalizations about spin and string expecially when trying to apply those to an individual.

 

[dozu]

^^^ isn't this music to ears for the racket/string industry... so all racketolics and stringholics can demo all the combinations out of the Ying Yang and generate KaChing for the retailers.

if everything can be calculated by plugging in some numbers into a formula.... hm, wouldn't that be boring.

 

[share1law]

If you consider the friction between the ball and string good friction for the spin, and the friction between the strings bad friction for the spin, it is not difficult to explain these data.

Do you have data for String Set 3? Which has more good friction and less bad friction?

 

[Consolation]

If you consider the friction between the ball and string good friction for the spin, and the friction between the strings bad friction for the spin, it is not difficult to explain these data.

Do you have data for String Set 3? Which has more good friction and less bad friction?

And how do you explain the difference between TCS and ALU Power?

I'm not sure how you would even have a set 3. Perhaps a very rough string with teflon string savers? Would be interesting to test. I still think there's more going on here than just the friction relationships though.

 

[share1law]

And how do you explain the difference between TCS and ALU Power?

I'm not sure how you would even have a set 3. Perhaps a very rough string with teflon string savers? Would be interesting to test. I still think there's more going on here than just the friction relationships though.

Their string/string friction is very close. Assuming the string/ball friction is very close too, they should have the same good or bad friction.

But Alu power is stiffer than TCS, so Alu power will deform the ball more and get more string/ball contact area, which will get more good friction. In addition, Alu power moves less and snaps back faster under the same good and bad friction condition, therefore more snap back power for Alu power is imparted to the ball before it leaves the string bed.

Set #3 is an ideal string set. You can put Teflon as you suggest or get the Set #3 data from the profiled strings. The profiled string basically increases the friction between the ball and string with Hex shape, while it keeps the string/string friction the same.

 

[Consolation]

Their string/string friction is very close. Assuming the string/ball friction is very close too, they should have the same good or bad friction.

But Alu power is stiffer than TCS, so Alu power will deform the ball more and get more string/ball contact area, which will get more good friction. In addition, Alu power moves less and snaps back faster under the same good and bad friction condition, therefore more snap back power for Alu power is imparted to the ball before it leaves the string bed.

Set #3 is an ideal string set. You can put Teflon as you suggest or get the Set #3 data from the profiled strings. The profiled string basically increases the friction between the ball and string with Hex shape, while it keeps the string/string friction the same.

Your second sentence makes several assumptions. First, that the ball will deform more. It doesn't take a lot of force to fully deform the ball. Put a tennis ball under 100 and 200# loads, the net deformation is almost indistinguishable (though one ball will be a lot warmer...). Second that the sliding friction will match the static values. Third that ALU snaps back faster. Stiffness is a measure of how much energy the string requires to deform, it is not a measurement of how much energy a string will return when the deformation force is removed, or how quickly it will do so. These properties can be, and often are, independent. And finally that even if ALUP does snap back faster, that means it returns more spin inducing energy to the ball as opposed to returning it a little later.

Your third sentence also assumes that ball-string friction is an important factor. For ball string friction to matter, you must argue that how quickly the ball stops rolling and/or what happens during the initial and final stages of impact are important in spin production. The problem here is that what happens in both cases is highly dependent on impact conditions. Impact takes pretty much the same time, no matter how hard the ball is hit. The harder you hit, the quicker the ball stops rolling. You can make the ball stop rolling very quickly swinging a polished metal plate, as long as you swing it hard enough. The variability in how long it takes to 'imbed' goes down as impact force goes up. At fairly modest impact forces, the difference becomes inconsequential.

It also matters as to when the 'spin enhancing' energy is being returned. If it is applied as the ball is leaving, when pressures are much lower (and differences in ball-string friction would be most apparent), than friction may very well be important. If instead it's applying a rotational torque to the elastic ball while it is still fully or partially compressed against the strings, then maybe it doesn't matter as much.

I think that this gets to the heart of the issue. That while people writing articles like to talk about 'polys', there is great diversity within the class. I have no doubt there are strings that a given set of impact conditions, result in more spin. Anecdotal evidence would certainly suggest that it is likely that the combination of characteristics that produce that result are more commonly found in polys. But even the anecdotal evidence is often conflicted. There are people that absolutely feel a certain string gives tremendous spin, while others think it's terrible in that regard. Is this a case of one being wrong, or could they both be right? I would even go so far as to theorize that there is no one formula by which a string can help generate spin. That different combinations of factors can produce similar results under different impact conditions.


EDIT: And to show why this sort of discussion is relevant, look the thread where they linked the article on poly strings and spin...it's all about low friction...

 

[BreakPoint]

In the article, spin for polys:

TCS 138 +/-14
ALU Power 162 +/-8
ALU Rough 165 +/-10

And friction data

TCS 0.098
ALU Power 0.102
ALU Rough 0.127

And Stiffness data:

TCS 205
ALU Power 248
ALU Rough 225

This data supports my assertion. Stiffness is what produces the spin. TCS had the least amount of spin because TCS is the least stiff of the three strings tested despite the fact it is the most slippery and probably slides back and forth the fastest. Again, the snap back is not what produces the spin.

 

[share1law]

Ball/string friction is important for spin generation. Swinging harder will press the ball harder, as the pressure increases between the string and ball, string/ball friction increases (for both static and slide friction if you care), in addition to the increased contact area. Same reason applied to swinging metal plate analogy.

Ff = µFn

µ = coefficient of friction (static or kinetic)
Fn = magnitude of normal force (pressure between the ball and strings)

String manufactures know it too but it not easy to make. Making the string rough to increase string/ball friction will also increase the string/string friction, it is why some rough strings do not necessarily generate more spin.

High ball/string friction and low string/string friction is the way to go. Profiled string with Hex shape coated with Teflon is the closest one to achieve this goal. But the Hex shape gets smooth after 1-2 hours of hitting, but it is another problem to solve.

You really do not need the formulas posted in the TWU articles to figure it out.

 

[Consolation]

Ball/string friction is important for spin generation. Swinging harder will press the ball harder, as the pressure increases between the string and ball, string/ball friction increases (for both static and slide friction if you care), in addition to the increased contact area. Same reason applied to swinging metal plate analogy.

Ff = µFn

µ = coefficient of friction (static or kinetic)
Fn = magnitude of normal force (pressure between the ball and strings)

String manufactures know it too but it not easy to make. Making the string rough to increase string/ball friction will also increase the string/string friction, it is why some rough strings do not necessarily generate more spin.

High ball/string friction and low string/string friction is the way to go. Profiled string with Hex shape coated with Teflon is the closest one to achieve this goal. But the Hex shape gets smooth after 1-2 hours of hitting, but it is another problem to solve.

You really do not need the formulas posted in the TWU articles to figure it out.

Your absolutely right, but my point with the plate analogy was, at a certain point the ball/string friction becomes so great (because the pressures are so high), that the variation due to different string characteristics becomes irrelevent. An asphalt road is a lot stickier than a polished wood floor. But if you put a 1 ton steel block down on either, they are equally difficult for a single person to move.

And for the record, it's not clear that a sharp edge is better. The only test I've seen that looked at shaped strings showed that the sharp edged strings did bite the ball well, so well that they rotated axially, allowing the ball to roll longer. I mentioned previously that Babolat apparently has done quite a few tests not only on string composition, but also shape, but they aren't sharing. I can say that they shape they use for PHT and RPM deforms in interesting ways when under pressure...

 

[BreakPoint]

Ball/string friction is important for spin generation. Swinging harder will press the ball harder, as the pressure increases between the string and ball, string/ball friction increases (for both static and slide friction if you care), in addition to the increased contact area. Same reason applied to swinging metal plate analogy.

Ff = µFn

µ = coefficient of friction (static or kinetic)
Fn = magnitude of normal force (pressure between the ball and strings)

String manufactures know it too but it not easy to make. Making the string rough to increase string/ball friction will also increase the string/string friction, it is why some rough strings do not necessarily generate more spin.

High ball/string friction and low string/string friction is the way to go. Profiled string with Hex shape coated with Teflon is the closest one to achieve this goal. But the Hex shape gets smooth after 1-2 hours of hitting, but it is another problem to solve.

You really do not need the formulas posted in the TWU articles to figure it out.

I believe most rough strings do generate more spin, but I suspect that ones that don't are because they are not as stiff as the ones that do. ALU Power Rough is both rough and stiff. That's why it produces so much spin, not because it's slippery and snap back fast.

 

[smileykj]

Just to add fuel to the fire. Here is a great article looking into which strings produce the most spin and why from actual scientific data. Definately worth the read. Published in The Atlantic-"The New Physics of Tennis: Unlocking the mysteries of Rafael Nadal’s killer topspin" by Joshua M. Speckman

Will definately provide evidence that challenges some peoples notion of what they "believe" to occur.

http://www.theatlantic.com/magazine/archive/2011/01/the-new-physics-of-tennis/8339/#

 

[Netspirit]

My personal view is that high friction between the ball and the strings allows for a longer contact between them, less slipping of the ball, and more topspin.

On the other hand, as the TWU and video above show, low friction between mains and crosses allows them to bend and snap back easier, with more energy accumulation/transmission. Such a stringjob is very elastic in the contact point, losing/dissipating little energy.

So the ideal stringbed has zero friction between strings, but infinite friction between the strings and the ball. This is impossible to achieve, so the best string combination would have to provide some compromise between the two - relatively good gripping, and relatively easy snapping.

As the TWU research shows, the least amount of mains-to-crosses friction happens in hybrids with natural gut & polyester.

I cannot claim it, but I can speculate: having some textured/gripping/biting string in your mains and oily/slippery one your your crosses should generate more topspin for groundstrokes than the opposite combination.

 

[BreakPoint]

Just to add fuel to the fire. Here is a great article looking into which strings produce the most spin and why from actual scientific data. Definately worth the read. Published in The Atlantic-"The New Physics of Tennis: Unlocking the mysteries of Rafael Nadal’s killer topspin" by Joshua M. Speckman

Will definately provide evidence that challenges some peoples notion of what they "believe" to occur.

http://www.theatlantic.com/magazine/archive/2011/01/the-new-physics-of-tennis/8339/#

That article was already posted above in this thread.

I watched the videos and what I saw was that each individual lubricated main string would snap back as the ball rolls away from that string. I did not see any evidence of any string snapping back while the ball was flattened against that string and while the downward force of the ball was still pushing the string down. I also did not see any evidence of the video showing the snapping back of the strings adding any additional spin to the ball. Besides, the stringbed in these videos were purposely heavily lubricated for the purpose of this experiment so I'm sure they were much more slippery than your average retail poly string. I'd bet if you measured the coefficient of friction of these lubricated strings it would be lower than for the average poly string.

 

[Consolation]

I believe most rough strings do generate more spin, but I suspect that ones that don't are because they are not as stiff as the ones that do. ALU Power Rough is both rough and stiff. That's why it produces so much spin, not because it's slippery and snap back fast.

How do you explain gut, which is 1/3-1/2 as stiff as most polys, but results in similar spin results?

 

[Guardian]

I played 10 years with kevlar stings at high tension (mains and crosses), which are of course significantly stiffer than polyester strings. The swap to softer polyester definitely gave me better spin. There are probably many reasons for this but I do not believe stiffer stings equals more spin.

 

[BreakPoint]

How do you explain gut, which is 1/3-1/2 as stiff as most polys, but results in similar spin results?

Because gut is naturally rough and not slippery. That's why you need to wax the gut while you're stringing it so you can pull it through without burning it. Less string movement = more spin.

Besides, what do you mean "1/3-1/2 as stiff"? Those stiffness ratings are neither absolute nor relative. A string that measures less stiff by their methodology can actually play stiffer and vise versa. Also, tension has a lot to do with how much the ball compresses against the stringbed. Depending on the difference in tensions, you can probably get a gut stringbed to play as stiff as a poly stringbed.

Oh, and if gut really produced as much spin as poly, all the pros wouldn't have switched from gut to poly and Nadal would be using a full stringbed of gut. The pros want to avoid injury and save their arms as well.

 

[BreakPoint]

I played 10 years with kevlar stings at high tension (mains and crosses), which are of course significantly stiffer than polyester strings. The swap to softer polyester definitely gave me better spin. There are probably many reasons for this but I do not believe stiffer stings equals more spin.

If the stringbed is TOO stiff, there would be almost no pocketing of the ball whatsoever on the stringbed and thus makes it much harder for the stringbed to "grab" the ball to impart spin to it. You want to have the maximum compression of the ball when it impacts the stringbed, which you get with a stiffer stringbed, but you also want the stringbed to pocket the ball some so that it can "grab" the ball and impart spin to it as you're swinging up. Poly strings give you a lot of ball compression because it is very stiff but not so stiff that it doesn't pocket the ball at all. Thus, you get the optimum combination for imparting spin to the ball.

Imagine using solid wooden paddle to hit the ball with. The paddle is stiff but it doesn't pocket the ball at all. It would be very hard to get much spin on the ball.

 

[Consolation]

Besides, what do you mean "1/3-1/2 as stiff"? Those stiffness ratings are neither absolute nor relative. A string that measures less stiff by their methodology can actually play stiffer and vise versa. Also, tension has a lot to do with how much the ball compresses against the stringbed. Depending on the difference in tensions, you can probably get a gut stringbed to play as stiff as a poly stringbed.

Oh, and if gut really produced as much spin as poly, all the pros wouldn't have switched from gut to poly and Nadal would be using a full stringbed of gut. The pros want to avoid injury and save their arms as well.

Well, stiffness is easily measured. It does vary with tension and string pattern of course, but it's still easily measured. Due to it's elasticity, gut has a remarkably consistent stiffness, regardless of tension/pattern. You can get polys to really low stiffness measurements also (still higher than guts) by stringing them really low (usually sub 40#). But, that doesn't take into account what happens during the collision. During a hit, the pressures and tension go up dramatically, resulting in even a loosly strung poly becoming very stiff. Gut on the other hand stays pretty soft, even during the hit. Tension determines how much the ball compresses against the stringbed only insomuch as it effects stiffness. There is a certain amount of energy in any collision that has to be accounted for. How much energy goes into the ball (leading to ball deformation) is dependent on the relative stiffness of the stringbed (or whatever surface it's colliding with) compared to the string.

And in regards to guts ability to produce spin, I'm just going by the published data from the article above. The 2 ALU strings strung at 52# resulted in more spin than the one gut they tested. In all other cases (including the tighter strung ALUs, and all the other polys regardless of tension), gut produced at least as much spin. Which further illustrates that the answer is more complicated than just material, friction, or stiffness alone can account for.

EDIT: also forgot to mention that in that study they did measure string deflection also. Their data shows without a doubt that the amount of string movement has zero predictive value on spin production.

 

[BreakPoint]

EDIT: also forgot to mention that in that study they did measure string deflection also. Their data shows without a doubt that the amount of string movement has zero predictive value on spin production.

String movement in which direction?

 

[Consolation]

String movement in which direction?

They measured movement in the plane of the string bed. Movement outside the plane of the stringbed has also been measured, and varies with dynamic stringbed stiffness, as it should since that measurement is how string stiffness is defined.

There isn't a simple answer...

 

[Netspirit]

I wonder if it is possible to produce a string that is very, extremely slippery (relative to itself).

Then, after the stringjob is done, some process could be used to "sand"/"texture" the outer half of the strings - the one that faces the ball, getting the best from both worlds.

For recreational/club players it would be too much of a hassle, but there is nothing pros would not do to get a slight edge here and there.

 

[BreakPoint]

They measured movement in the plane of the string bed. Movement outside the plane of the stringbed has also been measured, and varies with dynamic stringbed stiffness, as it should since that measurement is how string stiffness is defined.

There isn't a simple answer...

When they said "the amount of string movement", did they mean how quickly or easily the string moves or the total distance the string is displaced or stretched? Poly should move less in total displacement distance because it is stiffer so it moves and stretches less in distance than more resilient strings do.

 

[Netspirit]

^ the distance does not matter, elasticity does.

The string can move just 1mm, but if the contact point is absolutely elastic, they return all the energy they accumulated when they snap back, so you get the spin. But if they get stuck like multis, some of that energy dissipates into heat and never gets to the ball.

If you see that your strings have moved, it means the ball moved them and therefore lost some rotational energy.

 

[Consolation]

When they said "the amount of string movement", did they mean how quickly or easily the string moves or the total distance the string is displaced or stretched? Poly should move less in total displacement distance because it is stiffer so it moves and stretches less in distance than more resilient strings do.

They measured the amount of deflection, read the article yourself. Doesn't correlate.

 

[Consolation]

^ the distance does not matter, elasticity does.

The string can move just 1mm, but if the contact point is absolutely elastic, they return all the energy they accumulated when they snap back, so you get the spin. But if they get stuck like multis, some of that energy dissipates into heat and never gets to the ball.

If you see that your strings have moved, it means the ball moved them and therefore lost some rotational energy.

Just because they moved back, doesn't mean the energy was returned to the ball.

 

[BreakPoint]

^ the distance does not matter, elasticity does.

The string can move just 1mm, but if the contact point is absolutely elastic, they return all the energy they accumulated when they snap back, so you get the spin. But if they get stuck like multis, some of that energy dissipates into heat and never gets to the ball.

If you see that your strings have moved, it means the ball moved them and therefore lost some rotational energy.

That assumes that the "snap back" has something to do with the spin, which I totally disagree with.

Poly moves less in total distance so more of the energy is used to impart spin to the ball compared to a nylon in which more of the energy is used to move the string. The string has to stop moving before it can become a solid barrier from which to impart spin to the ball while you're brushing upwards on the back of the ball. Imagine a string that never stops moving. The ball would just keep on rolling right off of the face of the racquet and drop to your feet while you're brushing up on the back of the ball. OTOH, a solid barrier created by a firm, non-moving barrier like a string that doesn't move or has stopped moving can thus move the ball by rotating it as you brush upwards on the back of the ball.

 

[Netspirit]

Just because they moved back, doesn't mean the energy was returned to the ball.

It means the energy was returned to the ball. Just like when the flat ball deforms the stringbed and it rebounds, it returns the energy to the ball.

The only difference here is that there should be good friction between the strings and the ball in order for snapping to do anything useful. Otherwise they will just slide back behind the ball without gripping it.

 

[BreakPoint]

It means the energy was returned to the ball. Just like when the flat ball deforms the stringbed and it rebounds, it returns the energy to the ball.

Minus the energy that was used to deform the stringbed in the first place.

The only difference here is that there should be good friction between the strings and the ball in order for snapping to do anything useful. Otherwise they will just slide back behind the ball without gripping it.

There's no evidence that the strings slide back while the ball is still compressed on those strings. Not even on the non-retail, heavily lubricated test strings that were used just for a non-real world experiment.

 

[Netspirit]

That assumes that the "snap back" has something to do with the spin, which I totally disagree with.

"Snapping back" is just "throwing up". It means releasing accumulated tension back to the ball during the contact time. Less stiff strings bend more and their contact time with the ball is longer. But both stiff and non-stiff strings will accumulate and return the same amount of energy given the same conditions (no free energy anywhere), so the contact time only affects feel and control, not spin.

What affects spin is the quality of contact between mains and crosses, and between mains and the ball.

Insufficient mains-ball friction causes ball's slippage. Excessive mains-crosses friction causes non-elastic energy dissipation. This is all there is to it.

 

[Netspirit]

Minus the energy that was used to deform the stringbed in the first place.

No. If the contact is elastic, it is all stored and returned. Non-elastic collisions (think multis) dissipate energy.

There's no evidence that the strings slide back while the ball is still compressed on those strings.

Gawd, it does not matter. There is some contact time during which strings push the surface of the ball up. This is what I call "snapping back". Not too stiff strings cup/wrap around the ball more, so the ball gets in contact with more strings, which should increases friction and reduce slippage.

 

[Netspirit]

http://en.wikipedia.org/wiki/Elastic_collision
http://en.wikipedia.org/wiki/Inelastic_collision

 

[BreakPoint]

No. If the contact is elastic, it is all stored and returned. Non-elastic collisions (think multis) dissipate energy.

The collision between a tennis ball and the stringbed of a tennis racquet is never totally elastic. That's why if you clamped a racquet and fired balls at the stringbed, the ball will never return with the same amount of energy as it went in. Energy is lost in many forms: e.g., shock and vibration transmitted along the strings and to the frame, frictional forces between the strings, breaking down of the string material which eventually causes them to break, etc.

Gawd, it does not matter. There is some contact time during which strings push the surface of the ball up. This is what I call "snapping back". Not too stiff strings cup/wrap around the ball more, so the ball gets in contact with more strings, which should increases friction and reduce slippage.

The strings push the surface of the ball up when strings finally stop moving or stretching. The downward force of the ball prevents the string from "snapping back" while the ball is still compressed against that string. Don't forget that when you're swinging upwards at a high rate of speed, you are continuously exerting downward force on that string for as long as the ball is in contact with the string. Once the ball rolls away or leaves that string, only then is the string free to "snap back". But by then that string is no longer in any meaningful contact with the ball so therefore the "snap back" cannot be adding any spin to the ball.

 

[BreakPoint]

I played 10 years with kevlar stings at high tension (mains and crosses), which are of course significantly stiffer than polyester strings. The swap to softer polyester definitely gave me better spin. There are probably many reasons for this but I do not believe stiffer stings equals more spin.

Another poster in another thread would disagree with you about kevlar strings:

I have found that kevlar gives the most spin, but less power. The kevlar can be all worn out+moving all over the place, but not moving back in place. But still produce huge spin, so this theory does not make sense.

http://tt.tennis-warehouse.com/showthread.php?t=363079&p=5309571

 

[Consolation]

It means the energy was returned to the ball. Just like when the flat ball deforms the stringbed and it rebounds, it returns the energy to the ball.

The only difference here is that there should be good friction between the strings and the ball in order for snapping to do anything useful. Otherwise they will just slide back behind the ball without gripping it.

There's a narrow window during which the string returning can transfer energy back to the ball. As you said, the friction between the ball and string has to be high enough such that when the pressure from the collision drops below the threshold for the string to break free, there is still sufficient 'bite' to transfer a rotational force. There are multiple permuations of that scenario where the 2 variables would not match, and the strings could snap back either behind the ball as you mention, or after the ball has left.

The magnitude of the 'snap-back' force is also important. Keep in mind that the primary force imparting spin on the ball is the massive force of the racquet/body complex striking the ball in a low to high swing path. Even in the case of a perfectly elastic string, much of the energy expended to deform the string laterally is lost due to friction. Even the lowest friction string will still have quite high friction under impact conditions. There is also friction to overcome during 'snapping-back'. While the speed of the racquet and ball are providing the energy to deform the string, the string's inherent properties are the sole source of the energy available to snap it back. Some of the stored energy will be needed to break the string free (static friction will be considerably higher than sliding) and slide it back into position. Quantifying 'some' is very important. It's possible to imagine a string where x joules of energy are 'stored', and it requires x energy to snap back. In that case the strings will indeed return completely, but zero energy will be returned to the ball. It's also possible to imagine a string where 10x is stored and x is required to return (which doesn't mean that 9x is transferred to the ball, the timing is still critical, but at least the potential to return energy exists). Visually these strings may be indistinguishable, even under extremely high frame rates.

If snapping back is indeed a contributory to spin generation, there are a lot of variables, both string based and swing based, that need to fall into place pretty precisely for the energy to be transfered at the correct moment. I personally think the phenomenon is important, but that the complexity of the interaction goes beyond simple generalizations.

 

[share1law]

T
The strings push the surface of the ball up when strings finally stop moving or stretching. The downward force of the ball prevents the string from "snapping back" while the ball is still compressed against that string. Don't forget that when you're swinging upwards at a high rate of speed, you are continuously exerting downward force on that string for as long as the ball is in contact with the ball. Once the ball rolls away or leaves that string, only then is it free to "snap back". But by then that string is no longer in any meaningful contact with the ball so therefore the "snap back" cannot be adding any spin to the ball.

1. String will start to snap back when the snap back force is higher than the downward force. It means that while the racquet is still being swung but slows down, the snap back has already occurred.

2. Snap back does not only happen when the main moves over the cross and then snaps back. If you glue the main and cross together perfectly, snap back still happens. It happens on the main between crosses. It is the most efficient way to snap back.

3. The ball might have already left the string bed before the main is moving back to it original position, but it does not mean snap back does not happen, it happens in the first 1/2 or 1/3 snap back movement, depends on what kinds of strings we are using.

4. Lubricating the strings so they can move easier, it only improves the efficiency. It is not the reason of why the spin is generated but of why it generates more spin.

 

[BreakPoint]

1. String will start to snap back when the snap back force is higher than the downward force. It means that while the racquet is still being swung but slows down, the snap back has already occurred.

That's not possible while you're swinging upwards at a rate of speed high enough the generate sufficient topspin because the force of the snap back is so tiny relative to the massive downward force on the string by the ball while it is compressed against it that the snap back force is, for all intents and purposes, negligible.

2. Snap back does not only happen when the main moves over the cross and then snaps back. If you glue the main and cross together perfectly, snap back still happens. It happens on the main between crosses. It is the most efficient way to snap back.

We are talking about the lateral "snap back" of the string in the same plane as the stringbed, which is the one that some people claim adds spin to the ball. If you glued the mains to the crosses, the mains wouldn't slide or move laterally in the first place, and if it doesn't move, there can't possible be any snap back.

3. The ball might have already left the string bed before the main is moving back to it original position, but it does not mean snap back does not happen, it happens in the first 1/2 or 1/3 snap back movement, depends on what kinds of strings we are using.

The snap back of each string happens only after the ball is no longer in any meaningful contact with the particular string in question.

4. Lubricating the strings so they can move easier, it only improves the efficiency. It is not the reason of why the spin is generated but of why it generates more spin.

If this were true, you'd see pros spraying WD-40 on their stringbeds. But I've never seen that.

In fact, TW rates Nadal's old Duralast string (which is less slippery) as being more spin friendly than Nadal's new RPM Blast string (which is more slippery). This makes sense as Nadal has been flattening his forehand and serves more this past year, which helped him win his first US Open. If you saw the two recent exhibitions Nadal had with Federer, it's amazing how much flatter Nadal is hitting his forehands.

 

[sureshs]

Besides, the stringbed in these videos were purposely heavily lubricated for the purpose of this experiment so I'm sure they were much more slippery than your average retail poly string.

"In technical studies published in 2006 and 2007, International Tennis Federation researchers reported that the same movement that Kawazoe observed with lubricated strings occurs with copoly as well."