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Explaining serve underspin variation by contact

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Josepher3 View Drop Down
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    Posted: 09/09/2022 at 12:08am
It is well known that leading edge contact generates stronger underspin than trailing edge contact. Experts do not always offer a reason for this. When offered, it takes this form:

Contact beginning on the bottom edge has more room to go under the ball, but the trailing edge offers a much reduced space. The premise would be that the ball slides up somewhat on the rubber, so the more room the better.

However, the rubber is made to avoid a slide. Contact occurs on a single point on the rubber with no spreading out. Besides, the more slide, the less friction, exactly the opposite of what it takes to produce spin.

An alternate analysis: This is referred to as "gear effect", and golf club manufacturers design woods (incl metal woods) to compensate for the gear effect created by toe and heel hits. If the driver face was perfectly straight, a ball hit on the toe of the club will deflect the toe backwards (around the center of gravity), but now here's the thing. Ball and clubhead are in contact for some time, and during this time, while the toe of the club is deflecting backward, the ball feels it and the two act as gears, both rotating in opposite directions to each other. If the face was straight, the ball would start straight and the hook spin would take it left. For this reason, woods are designed with radiused faces so that a toe hit would start out to the right, compensating for the resulting hook spin. The case for a heel hit is understood in the same way, only opposite.

This may be an explanation for the difference in spin on leading and trailing edge contact in table tennis. In serving underspin, the bottom hit gear effect separation of ball from rubber increases underspin, and the top hit gear effect separation of ball from rubber decreases underspin. The effect will be smaller than with a driver hit in golf, but it is not negligible. We can feel it.

Picture the racket being held over the table with the face perfectly flat. Now imagine it actually suspended over the table that way, with no forward movement. Send a ball to the top edge and it will rebound with topspin. Send a ball to the bottom edge and it will rebound with underspin.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote anubhav1984 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 1:01am
I always imagined that if one would keep their racket perfectly horizontal and just used ONLY front to back motion of your hand to generate the backspin, then the amount of backspin you generate would always be identical. 
So perhaps, this difference is explained by the angle at which you are able to contact the ball with the lower part of the face vs the upper. Inevitably, the lower part of your racket would be more tilted than the upper part and thus the contact perhaps exaggerates the amount of backspin generated.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 2:11am
You said, 


So perhaps, this difference is explained by the angle at which you are able to contact the ball with the lower part of the face vs the upper. Inevitably, the lower part of your racket would be more tilted than the upper part and thus the contact perhaps exaggerates the amount of backspin generated.

This doesn't seem to apply to the case discussed, which must assume that the face angle is constant. When it is said that more underspin is generated from the bottom hit (and it is always said), it would not make sense to assume other than the similar face angle for the top hit.

Since it is commonly accepted that the bottom hit makes more underspin, we ought to have a convincing explanation for it if it is really true. The ball having room to slide on the rubber can't be it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote balldance Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 3:14am
I agree that ball sliding on the rubber is bullshit. I swear I’ve seen so many people/coaches used that explanation but when they demonstrated the serve, they just tilted their racket more when serving no spin. 

There’s no magic thing such as you just hit with the upper part of racket and produce no spin. I can produce good under spin brushing the ball with the upper part of racket, so I don’t believe it.

I think it’s about the acceleration and angle of the racket. 
Assuming when the lower (leading) part of the racket touches the ball, the acceleration is highest and with the same motion, acceleration is lower when the upper part of the racket touches the ball. This would create the effect of high/low under spin. 

Angle: the angle of the racket can change during serve motion and I assume when the upper part of the racket touches the ball, the racket angle is more closed. Or people just tilt their racket more when they serve no spin, sometimes subconsciously.

In my observation, most people use either or both of the above methods.



Edited by balldance - 09/09/2022 at 3:18am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 11:12am
Any ball hit with a forward motion and an open face will produce backspin. What we are talking about is the varying rates of spin. Even a no spin ball will start out with underspin. With bottom and top hits we are talking about subtle differences in the rate of spin, which, if misjudged, will directly affect the receive.

I cannot agree that, 

Assuming when the lower (leading) part of the racket touches the ball, the acceleration is highest and with the same motion, acceleration is lower when the upper part of the racket touches the ball,

because the trailing part of a train moves at the same speed as the leading part.

The question remains, will a trailing edge hit produce less underspin (assuming everything else is constant) than a leading edge hit. (The difference doesn't have to be great...just different) Very few people will disagree that it doesn't. There is a way to investigate this which I might try. 

Hold the racket in a fixed position (bottom edge down, top edge up) over the table with a perfectly flat face. Would be best to have racket secured somehow, maybe a vise grip. Then carefully, by hand, send a lined ball with no spin at all toward the upper and lower portions of the racket. This should be captured with a slo-motion setting on the video. This would check for unintentional spinning of the ball by the hand throw. If done correctly, observing the ball as it separates itself from the rubber may (or may not) prove the gear effect that is proposed.





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Post Options Post Options   Thanks (0) Thanks(0)   Quote balldance Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 11:47am
Originally posted by Josepher3 Josepher3 wrote:


I cannot agree that, 

Assuming when the lower (leading) part of the racket touches the ball, the acceleration is highest and with the same motion, acceleration is lower when the upper part of the racket touches the ball,

because the trailing part of a train moves at the same speed as the leading part.

the difference is the position, yes, the trailing part moves at the same speed as the leading part, but it's trailing behind. Imagine if you have the exact same motion, same acceleration, same hitting position, if you hit the ball with the trailing part, the timing (to touch the ball) must be a fraction later (in the whole motion) when the speed can be already reduced. If you say the trailing part hit the ball with the exact same speed, then you must change the motion/acceleration to do that.
The same concept for changing the angle of the racket, same serve motion, but when the trailing part touch the ball, the angle has already changed. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 12:05pm
If the trailing part moves at the same speed as the leading part, it does not matter if it is trailing. Besides, that fraction of a second you speak of "when the speed can be already reduced" is likely to be insignificant.

What I'm asking you, and others, is to investigate solely the difference in contact point interaction and control for outside influence by assuming everything else is identical. The "gear effect" interaction is a real thing. The question is, is it applicable for the situation under discussion?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote anubhav1984 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 12:12pm
Originally posted by Josepher3 Josepher3 wrote:

If the trailing part moves at the same speed as the leading part, it does not matter if it is trailing. Besides, that fraction of a second you speak of "when the speed can be already reduced" is likely to be insignificant.

What I'm asking you, and others, is to investigate solely the difference in contact point interaction and control for outside influence by assuming everything else is identical. The "gear effect" interaction is a real thing. The question is, is it applicable for the situation under discussion?
Can't think of any scientific reason or explanation why there would be any difference if all the other variables are same. My understanding of this was always rooted in the racket angle but once you take that out of the equation, there isn't anything that changes, apart from the distance the ball travels after hitting the leading vs trailing edge which would ultimately result in slowing of the ball but it is so insignificant that i dont think we want to consider it. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 3:20pm
"Can't think of any scientific reason or explanation why there would be any difference if all the other variables are same."

i've just offered one, due to hits distant from the center of gravity, a gear effect occurs. In the interaction between ball and rubber during the time when they are together, each rotate in opposite directions (gearlike, hence the name) so upon separation the effect can be seen. Maybe.

Equipment manufacturers would have the equipment to check if the effect is present. Their R&D departments are monitoring trajectory and spin for new product development. It would be easy for a company to design an experiment to check for the effect. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote anubhav1984 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 3:56pm
Originally posted by Josepher3 Josepher3 wrote:

"Can't think of any scientific reason or explanation why there would be any difference if all the other variables are same."

i've just offered one, due to hits distant from the center of gravity, a gear effect occurs. In the interaction between ball and rubber during the time when they are together, each rotate in opposite directions (gearlike, hence the name) so upon separation the effect can be seen. Maybe.

Equipment manufacturers would have the equipment to check if the effect is present. Their R&D departments are monitoring trajectory and spin for new product development. It would be easy for a company to design an experiment to check for the effect. 
If we are talking about the front most and the back most part of the racket then distance from the center of gravity will be same as well, no? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 4:41pm
Yes, and may produce similar, although opposite, rotations due to gear effect. I say "may" because it has to be tested.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lightspin Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 5:22pm
There was a video of someone with a super high speed camera and he tried to test this theory.  If the racket is going at the same speed at contact, he concluded that it was irrelevant where you contact the ball on the racket for spin rate.  As long as the racket was traveling at the same speed, hit it in the front or back you will get the same spin. 

So why for some people is there a variation? My guess is that when you contact the ball at the front of the racket, you are worried about getting under the ball so the racket travels faster.  When you contact the ball at the back of the racket, you are worried about missing the ball so you are traveling slower. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 5:44pm
I won't assume that the video you cite is wrong in its particular conclusion, only that it may not have been designed to test for possible gear effect.

What makes this discussion so interesting is the lack of commentary on a prevailing wisdom (bottom more, top less), cited by so many top level people in the sport, that they do not offer an explanation for.

Yet, when they claim toe hits (away from handle) generate more spin than heel hits (close to handle), they eagerly, and always, provide the proof. Why then, for that simple case, but not now? They feel the difference in the spin, but attribute it solely to contact location when they describe it. If the location matters, why does it matter?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote blahness Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 8:12pm
I hit my heaviest underspin serves in the middle of the racket, same as my no spin serves - it's too easy for the opponent to read if you serve near the bottom edge for underspin and top edge for no spin. Middle of the racket is also best for control and consistency.

I think the main difference in bottom edge vs top edge is the location relative to the fingers on the grip. But I haven't really went into detail on this tbh
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Josepher3 Quote  Post ReplyReply Direct Link To This Post Posted: 09/09/2022 at 10:26pm
High level people in the sport are of the opinion that more underspin can be produced with the leading edge than with the trailing edge. They don't seem to hold the opposite opinion, that the trailing edge will produce more. 

When they voice this opinion, it does not include reference to speed or acceleration. It is usually just flatly stated as a matter of fact. We know that many attribute it to the ball having more room to slide.  


They may be wrong about the reason, but it's doubtful they are wrong in their opinion. It would be satisfying if there was an explanation. The gear effect, if it applies, would be a nice one.

The radiused faces in woods (or metal woods) in golf compensate for the well known gear effect producing hook spin (toe hits) and slice spin (heel hits). To go slightly further into this, irons are not manufactured with radiused faces, but not because there is no gear effect. The reason, according to the leading researcher on the subject, Ralph Maltby, is that the twisting is not as dramatic on irons because the center of gravity is closer to the face than it is on a wood.

It's close to the face on a table tennis racket also, but we can be certain that an off-center hit will deflect the racket, however slightly. Once we agree on this (I just said it's a certainty!), we should agree that ball and rubber are partners in travel for a time (another certainty), and that during that time the ball feels the effect of the blade's deflection and peels off in an opposite direction, gear like. 

We may not agree on this last. Still under consideration. But it would be nice in that it would provide an explanation for what high level people in the sport agree on.
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