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Old 11-01-2012, 08:19 PM   #41
ChicagoJack
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Hi Wiley,

I was watching the thread develop through out my work day, had no time to respond and was kinda waiting for the dust to settle anyways. Apologies for the long read. Hard to do a complex subject like this justice with a few sentences. Just three main topics for me here. 1 : Measuring Dwell and Deflection. 2. Dwell and Spin. 3. Dwell and Control.

Issue 1: Objectively Measuring Dwell Times & String Bed Deflection :

1a. One of the early statements in the thread that took the conversation into a very argumentative place was that dwell time could not be measured, is strictly a feeling, nothing more, therefore the term should be banished to the realm of the subjective, and dealt with entirely from that perspective alone. The data reveals ridiculously minor differences, but both Dwell time and string bed deflection are measurable. See screen grabs below. If you want to make a case that the differences are so minor that they are likely not perceivable to the player, or that dwell time isn't perceivable at all, then I just put $10.00 in to the basket on that sermon. If one's thesis is that dwell time has got very little to do with spin, amen to that my brother! But that's a whole 'nuther can of worms. Plenty of reasons to poo poo the concepts of pocketing and dwell time if one had the inclination, (many of which I agree with) but lack of objective measurements, isn't one of them. Not calling out PvAudio here, he's largely been the voice of reason and good information through out the whole thread, just pointing out this one dark corner of the TW University String Database that might have been overlooked.

1b. The dwell times read about 5-6 times longer than typical on court impacts of 4-6 ms. That's because the object striking the strings in the tests for stiffness, tension, deflection, etc. is a test hammer, not a ball. This is takes the ball out of the equation, and gets to the heart of what the string is doing. Balls lose pressure every second, change texture with repeated impacts etc... not good for getting clean data. One might choose to de - ligitimize the measured dwell times for this reason, but you'd have to throw out just about everything else we've measured too (stiffness, tension loss, deflection, energy return) in exactly the same way. Also worth a mention, notice the "actual pre - impact tension". This column demonstrates the tension after the string had a good number of whacks (I forget how many) with the test hammer, at levels simulating a 120 mph serve. Poly loses a bunch of tension in those pre-test whacks, while gut retains much more, yet still manages a deeper pocket and incrementally longer dwell times. Gut, thou art amazing.

1c. Pocketing and dwell times are often inversely related. You'd think that when the ball sinks very low into the strings, then it's going produce a longer dwell time. That's not always the case. QUOTE: "It's counter - intuitive but the harder you hit the ball, the shorter the dwell time. You somehow have a picture in your mind of a harder hit ball sinking deeper into the string bed, and therefore taking a longer time to come out. What actually happens is that the harder you hit the ball, the stiffer the strings get, as does the string plane. Even though the ball sinks in more, it also snaps back sooner due to the increase in string plane stiffness"-- Howard Brody, The Physics And Technology Of Tennis, Chapter 26.






[..]

Issue 2 : Dwell Time and Spin, IE.. "wrapping around the ball like a belt on a pulley"

2a. I follow your line of reasoning, and its a common thought process. You are thinking that deeper pocketing provides better grip on the ball, and better grip provides for more spin. It's a common conception but you have got some slightly faulty mental imagery. In the sport of table tennis, in which the ball does not squash, and the incidence angles are very steep, ball grip is a very big factor. In tennis however, not so much. Consider that the ball squashes flat like a bug on a windshield to roughly 1/2 its original size on impact. When you are able to see what actually happens with ultra high speed cameras, (See ITF Technical Video Provided) the tennis world becomes a strange and beautiful place indeed. The ball is a whole lot of shapes when it smashes into the racquet, but one shape it isn't, is round!. If the forces are great enough the ball can squash so firmly into the string bed that it's nearly flat, with the back of the ball bouncing off the front of the ball. The ball jiggles, the ball shakes and wobbles like a bowl of jello as it exits the string bed. Within this 4-6 ms time frame, all manner of strange things are occurring, and we... are utterly oblivious.

2b. We've learned within the last 5 years that spin enhancement is a product of 2 factors. ball-string friction (more is better), and string-string friction (less is better) Turns out, low inter string friction, is a much more important factor than ball "bite"

QUOTE 1: "Until recently, the prevailing theory regarding string stiffness and spin has been that the firmness and lateral rigidity of a stiffer string will create more ball-to-string friction due to more squashing and embedding, resulting in more spin. It is true that friction accounts for most of the spin produced by the stringbed, but it does not account for most of the difference in spin between stringbeds. Recent research suggests that lateral main string movement is the most important factor determining this difference." -- Crawford Lindsey, March 14, 2011

QUOTE 2 : "Recent experiments (Spin and Material, Spin and String Movement, Spin and String Pattern and String Snap-Back and Spin) have also demonstrated that sideways motion of the main strings during contact with the ball actually contributes to increasing spin. This sideways movement exerts a torque on the ball when it snaps back into position, thus causing topspin. Polyesters have been shown to add about 20% more spin than nylons. Polyesters can differ up to about 15% from each other in spin production while nylons might vary by 20% from each other. And the difference between the spiniest polyester and stickiest nylon is almost 50%. Virtually all this difference is attributable to the amount of torque supplied by the sideways movement and snap back of the main strings.The role of ball-string friction in this process is that it influences both the amount of lateral string movement and the torque the snap-back exerts on the ball (though the magnitude of its contribution is yet to be ascertained by experiment). It is here that ball-string friction contributes to the difference in spin performance between string models, but only if the strings move. Otherwise, ball-string friction produces about the same spin for every string. And whether the strings move or not depends on the static and sliding friction between strings. " -- Crawford Lindsey, August, 2011, Static Friction and Spin

QUOTE 3 : Summary Observations:
• All the polyesters had more spin than any of the nylons (a gut snuck in above some polys however).
• The average polyester spin compared to the average nylon is +20.2%.
• The difference in spin between the highest and lowest poly is +15.9%.
• The difference in spin between the highest and lowest nylon is +26.8%.
• The difference in spin between the highest string and the lowest string is +49.3%.
-- Crawford Lindsey, String movement and Spin


Links: Static Friction And Spin , How String-to-Ball Friction Affects Spin , String Snap Back and Spin , Which Strings Generate The Most Spin?

2c. Getting the string to grip the ball is not a problem!. (See Video Below) The lab testing was performed by the International Tennis Federation. I've seen many slow motion impact vids. This one is the most dramatic. The others show the ball smashing to about to about 1/2 original size.



Video Link Here:
http://www.youtube.com/watch?v=pufedQgGYzg&feature=plcp

[..]

Issue 3: Dwell Time And Control:

3A. Some players opine that longer dwell times increase control. Not only is there no experimental evidence for this, it runs counter to the age old truism accepted by both academics and players ... "string tight for control, loose for power".

- Jack
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Last edited by ChicagoJack : 11-05-2012 at 08:32 PM.
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