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  1. brokenball is online now
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    #21
    After a week of trying to straighten out what the Magnus coefficient should be for a TT ball it seems that no one really knows exactly. So I picked one that matched the trajectories I see in videos. Especially a Butterfly chopper video and the Sakai fastest serve ever video. I have made a pdf with about 10 different combinations of top and bottom spins with varying initial conditions which include the distance from the net at impact, the height above or below the table top, the initial spin, speed and angle.

    What is clear is that if the ball is hit while close to the table but from below the net, a fast paddle will not be your friend. The initial speeds are in the range of about 15 m/s. This varies depending on the initial angle and spin. 15 m/s not that fast. Some of the simulations use speeds of 20 m/s and 75 rps. This might be on the high end of what is achievable. It isn't hard to flat hit a ball much faster but to brush the ball enough to get 75 rps and also a speed of 20 m/s takes a much faster swing. When back from the table, a faster paddle is better to compensate for the air resistance. Reducing spin keeps the ball from dropping too soon.

    The reason this interests me is that the tables are "too short". Actually I hit the ball just a little too fast with not enough spin to bring the ball down fast enough. I can see the arc of the ball but it often just misses the end of the table. Lowering the initial angle is not good either since the net snags too many of my returns. Another item is keeping the ball low. Keeping the ball low so its peak height is less than 200 mm is the goal 250 mm is OK and probably safer. A ball that has a peak height of 200 mm will bounce up at about 80% of that height. A little more with back spin and less with top spin. Keeping the ball low forces the other guy to loop the ball over the net instead of just drive it from above the net.

    One thing you should notice is that a change of the initial angle of just one degree makes a big difference on how deep the ball lands if it lands at all.

    https://deltamotion.com/peter/TableT...Trajectory.pdf

    BTW, I just saw a Rozena add on Amazon where it claimed to correct for errors in the paddle. How does it do that? It can't.

  2. Zwill is offline
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    #22
    To me the 20m/s sounds about right, but 75rps...? Is that with pips???

    Butterfly just did some videos, 20m/s and 120-150rps doesn't seem like a big deal.

  3. brokenball is online now
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    #23
    [quote="Zwill;369644"]To me the 20m/s sounds about right, but 75rps...? Is that with pips???

    Butterfly just did some videos, 20m/s and 120-150rps doesn't seem like a big deal.

    The max achievable spin is about 150 rps but then the speed is low. 75 rps is easy to achieve with inverted 2*PI*75 is only 9.42 m/s tangential velocity even if the rubber has no spring effect at all.
    The Butterfly video use high speed cameras to measure the spin. In the video the chopper was able to achieve 137 rps.
    https://deltamotion.com/peter/TableT...is%20Video.flv
    It isn't unreasonable for a chopper to return a ball with a high back spin since the chopper is basically just adding a little spin the looper's topspin.

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    #24
    ZERO BRAIN ACTIVITY...
    Today's majority of players seem to me a poor and insipid spectacle. A good many electronic, robotic pupets overflued the game all around.. They seem fearful of playing artistic style, they never make intellectual efforts to play, -- no nifty strokes to attract my eye, no spark of intellect. . None rookei boy, none rookei gal seem to have need for performing the game in brainy rout. So pity.
    Below please find some brilliant examples of the nifty strokes, all gone by now, the game has totally degraded ...
    Last edited by igorponger; 4 Weeks Ago at 06:05 AM.

  5. brokenball is online now
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    #25
    What does that have to do with looping or chopping balls? My document is meant to show how the range of spins, speeds and initial angles for landing a ball is quite small. Also, we figure it out in a fraction of a second through training. The document also shows that maximum speed isn't required or desirable unless playing back from the table or making a line of sight shot. Looping balls from below the top of the net requires just the right speed, spin and initial angle.

    What amazes me is so many on the forum talk about faster rubbers, blades and boosting but I bet most don't play more than meter from the table.






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    #26
    Hi BB,

    Any further progress??

  7. brokenball is online now
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    #27
    Quote Originally Posted by IB66
    Hi BB,

    Any further progress??
    I was expecting people to have questions. I showed what happened when the angle changes by just one degree. No one seem think in terms of how consistent they are. Can you hit the ball consistently at a desired angle? How about speed?

    I showed how small changes in initial speed, spin and angle change the trajectory.

    What do you think the standard deviation of your ball trajectory is?
    If your standard deviation is 1 degree, very optimistic, then 65% of the balls will be within 1% of the desired angle. What about the other 35%?

    I chose to show hitting balls from below net height so a real loop is required since the angle must make the ball go up over the net and hope that the magnus effect makes the ball drop on the table. If the ball travels too fast, it will still fly of the end of the table. Still the young players always want to boost even though there setup is already too fast. Do you ever see videos of young players looping from below net height?

    What is your standard deviation of the speed of your shots? If it is 2% then hitting then trying to hit the ball at 10 m/s will result in 65% of your ball traveling in the range of 9.8 to 10.2 m/s. What difference does that make. Do you think you are that consistent? What about hitting the ball at 20 m/s? Then 65% of the balls will be in the range of 19.6 to 20.4 m/s. What difference does that make to where the ball lands if it does?

    Equipment doesn't have control, people do. However, people are inconsistent. Small differences in consistency make big differences in play. Better players have lower standard deviations.

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    #28
    Equipment can help people to have control... That is the point.
    Steal a little and they throw You in jail, Steal a lot and they make You King... (Dylan)

  9. brokenball is online now
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    #29
    Quote Originally Posted by Lazer
    Equipment can help people to have control... That is the point.
    No! The only difference is that 1% of a low speed isn't as much as 1% of a high speed. How you hit the ball and how consistent you are is up to you.

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    #30
    Quote Originally Posted by brokenball
    No! The only difference is that 1% of a low speed isn't as much as 1% of a high speed. How you hit the ball and how consistent you are is up to you.
    A slow blade is significantly easier to control than a fast blade, everyone who has an ounce of table tennis skill knows this.

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    Steal a little and they throw You in jail, Steal a lot and they make You King... (Dylan)

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    #31
    Quote Originally Posted by Lazer
    A slow blade is significantly easier to control than a fast blade, everyone who has an ounce of table tennis skill knows this.
    However, it needs to be scientifically proven other than the manufactures own lab. 🙈

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    says Equipment matters a lot to scrubs who can't make minor adjustments to their stroke.
     
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    #32
    Quote Originally Posted by brokenball
    Do you ever see videos of young players looping from below net height?
    Does U11 count?
    Does open up count?
    /devnull

  13. brokenball is online now
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    #33
    Quote Originally Posted by Lazer
    A slow blade is significantly easier to control than a fast blade, everyone who has an ounce of table tennis skill knows this.
    Again, a 1% error in speed results in less absolute error at slower speeds.
    The pros have a much smaller standard deviation of error than the rest of us.

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    #34
    Quote Originally Posted by brokenball
    I was expecting people to have questions. I showed what happened when the angle changes by just one degree. No one seem think in terms of how consistent they are. Can you hit the ball consistently at a desired angle? How about speed?

    I showed how small changes in initial speed, spin and angle change the trajectory.

    What do you think the standard deviation of your ball trajectory is?
    If your standard deviation is 1 degree, very optimistic, then 65% of the balls will be within 1% of the desired angle. What about the other 35%?

    I chose to show hitting balls from below net height so a real loop is required since the angle must make the ball go up over the net and hope that the magnus effect makes the ball drop on the table. If the ball travels too fast, it will still fly of the end of the table. Still the young players always want to boost even though there setup is already too fast. Do you ever see videos of young players looping from below net height?

    What is your standard deviation of the speed of your shots? If it is 2% then hitting then trying to hit the ball at 10 m/s will result in 65% of your ball traveling in the range of 9.8 to 10.2 m/s. What difference does that make. Do you think you are that consistent? What about hitting the ball at 20 m/s? Then 65% of the balls will be in the range of 19.6 to 20.4 m/s. What difference does that make to where the ball lands if it does?

    Equipment doesn't have control, people do. However, people are inconsistent. Small differences in consistency make big differences in play. Better players have lower standard deviations.

    Hi BB,
    liked the graphs and info contained, also appreciate the work you have put in to this thread!!

    I’m pretty sure that my consistency leaves a lot to be desired!! It’s great to see how the changes in spin, ball speed and bat angles affect the end results.
    Of course when we play the ball either lands on, flies off or hits the net, we have a choice to change either one part of the stroke, or a few parts to correct an error!!!

    Is there one part of the equation that has a greater influence to effect and reduce errors? Spin for example,

    regarding the Magnus Effect, there was to a certain extent, differing valves and no general consensus for an ‘agreed’ value to use. How much difference would these differing values for the Magnus Effect make towards the outcomes in your scenario’s?

    OSAI system is a great development for collecting stats during a match, evaluate tactics and highlight areas that were more or less successful. What’s needed is a system to sit alongside OSAI (or other similar Apps/systems) that is compact, portable, cost friendly ( LOL !!!) and can record and show spin on the ball, ball speed, bat angle at impact etc, probably a way off yet!!
    However, the VR systems that are now coming through, must have certain abilities to register basic information, which must be similar to these ‘smart’ blades / paddles that are also starting to appear.

    Things are starting to get very interesting as far as the ‘Tech’ side of things are concerned!!


  15. brokenball is online now
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    #35
    [quote="IB66;370901"][p]
    Is there one part of the equation that has a greater influence to effect and reduce errors? Spin for example,
    [quote]
    Good question, I think speed is main factor since it is in almost every term.

    I have since modified the program so there are a slider for speed, spin and angle. This allows me to move the slider and see the trajectory change.

    regarding the Magnus Effect, there was to a certain extent, differing valves and no general consensus for an ‘agreed’ value to use. How much difference would these differing values for the Magnus Effect make towards the outcomes in your scenario’s?
    A lot. If I use the most accepted value for the Magnus constant, the ball does multiple loops when the ball is chopped with a back spin of 137 rps. We know that isn't right. The Magnus force doesn't need to be very strong to accelerate a 2.7 gm TT ball
    I had to reduce the Magnus constant by a significant amount to keep the trajectories looking realistic.

    One thing I could do is make the drag coefficient Cd vary as a function of speed. Most people just use 0.5 as the drag coefficient but it actually changes a lot, it increases, as the ball moves slower. This could explain some of the "drop effect" where the ball just seems to slow down and drop on the table. My modeling technique would allow me to make the drag coefficient variable as a function of speed. Modeling the motion of objects in air is not simple.

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    #36
    Drag through the air -

    A light and smooth TT ball must be very susceptible to air resistance, would a TT ball with dimples, similar to a golf ball fly further? Or is it the light weight that has more importance to drag for a TT ball?

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    #37
    Quote Originally Posted by Tony's Table Tennis
    However, it needs to be scientifically proven other than the manufactures own lab. 🙈
    Why don't you stay on point?!.
    This is thread is about TT ball trajectories.

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    #38
    Quote Originally Posted by brokenball
    Why don't you stay on point?!.
    This is thread is about TT ball trajectories.
    You are slow in replying to me, like a week.....

    Any ways, in terms of ball trajectories, I do it all the times with beginners, I teach them how to generate different kind of ball trajectories and them to play more modern, obviously, with my background, it will be the Chinese style of play.

    Trust me, it is more useful being hands on.
    Even with U15 or U19 players, we still need to make technical corrections. Some things just needs to be practically done and that is what will develop the next generation players and for them to grow further generations onwards (since we in the premier division, a lot of graduates will become coaches themselves)

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