Seriously about Twist serve

[jwbarrientos]

Originally Posted by toly

To clarify the matter, there is one more example - Ryan Chung hits unreturned twits serve around 8:13
http://www.youtube.com/watch?v=nvGbQraZD98.

I extracted four frames from this video.


Figure 4. Ryan Chung's twist serve

Chung tosses the ball very far to the right. During contact the racquet is slightly closed and he hits the ball above its equator. This is really very complicated serve.

The follow thru at 8:12:56 and contact point at 8:12:66 ... that's why I never could make my twist serve work

[mightyrick]

Originally Posted by SystemicAnomaly

Are you talking about kick serves, in general, or specifically twist kicks? The term, kick serve, is somewhat generic/ambiguous. Some use the terms, kick serve & twist serve, as synonyms. Whereas others classify the twist serve as a specific type of kick serve. In this taxonomy, a kick serve is any serve with heavy topspin that kicks upward on the bounce. If the kick serve curves one way, in pre-bounce flight, and then kicks off in the opposite direction on the bounce then it qualifies as a twist serve. If the kick does not deviate appreciably left or right on the bounce then it would be a topspin kick (or a topspin-slice kick) serve.

Now the ball, on a twist serve, may very well deform a bit differently than it does on other types of serve. However, this is not really the underlying cause of the way bounces. The ball bounces the way it dues because of the way the ball interacts (rolls/skid/etc) with the ground. This interaction is influenced by the speed, the spins and trajectory on the ball prior to the bounce.

Note that (vertical-axis) sidespin does not directly affect the bounce (height or direction) according to physicist, Rod Cross. OTOH, horizontal-axis spins do have an effect on bounce height and direction. The most common of these is topspin and underspin. A ball that bounces off to the left or right, relative to the pre-bounce direction, indicates the presence of spiralspin as toly has indicated. This type of spin accounts for the left/right deviation seen on the bounce of a twist serve.

I've been talking about spiral spin for 4-5 years in these forums -- there are quite a few threads that discuss this type of spin. In his book, Technical Tennis, physicist Rod Cross refers to it as spiralspin, However, in his TWU article on kick serves, Rod refers it as gyrospin. It is sometimes aka Z-axis spin or longitudinal spin. Table tennis players refer to this spin type as corkscrew spin (or corkspin).

http://www.youtube.com/watch?v=Q8egU54BeyU

You make an important point about the need for clarification. I think that most times, when people say "kick serve", they generally just mean any serve that has a significant topspin component which causes the ball to accelerate dow***** with an ever steepening angle of attack -- due to the magnus effect. The downwards acceleration causes a significant bounce. Hence, "kick".

If someone is talking about a kick serve with a significant corkspin component, then this is obviously a twist serve where the resulting ball will also bounce in the direction of the corkspin.

For me, I find that a twist serve feels VERY different than a standard non-twist kick serve. For a twist, I will usually toss at least six inches behind my head. I find it makes it much more difficult to generate pace and get leverage. So these serves are usually much slower than a topspin or top/slice. But the resulting bounce outwards is usually rewarding.

[toly]

Originally Posted by jwbarrientos

The follow thru at 8:12:56 and contact point at 8:12:66 ... that's why I never could make my twist serve work

Thank you so much. I corrected the picture, so you have no excuse and should hit perfect twist serve.

[jmnk]

so essentially you are saying that on twist serve one hits the ball downward? I thought someone was claiming that unless you are like 7' tall you can't really hit the serve downward and expect it to land in the service box?

[SystemicAnomaly]

^ A 6'9" player who jumps should also be able to hit down on the ball.

Not sure that toly is actually saying that most players will be hittng downward on the ball. He does say that the racket face is slightly closed and the contact point is above the equator of the ball. However, if you look closely at his picture from the OP, it appears to indicate that the racket head is still moving upward at/after contact. The tip of the racket is higher in position 3 than it is at position 2 (the instant when the ball is contacted). There is still an upward brush on the ball -- (the contact lasts a mere 4-5 ms).

[jmnk]

Originally Posted by SystemicAnomaly

^ A 6'9" player who jumps should also be able to hit down on the ball.

Not sure that toly is actually saying that most players will be hittng downward on the ball. He does say that the racket face is slightly closed and the contact point is above the equator of the ball. However, if you look closely at his picture from the OP, it appears to indicate that the racket head is still moving upward at/after contact. The tip of the racket is higher in position 3 than it is at position 2 (the instant when the ball is contacted). There is still an upward brush on the ball -- (the contact lasts a mere 4-5 ms).

It's not physically possible to hit a sphere (a ball) with forward tilted plane (a racket) above the equator of the sphere, and --not-- hit down on the sphere.

saying 'the racket face is slightly closed and the contact point is above the equator of the ball.' and saying 'hitting down on the ball' means exactly the same.

[SystemicAnomaly]

Originally Posted by jmnk

It's not physically possible to hit a sphere (a ball) with forward tilted plane (a racket) above the equator of the sphere, and --not-- hit down on the sphere.

saying 'the racket face is slightly closed and the contact point is above the equator of the ball.' and saying 'hitting down on the ball' means exactly the same.

This sounds like a matter of semantics. To my mind, and I suspect to most people, if you say that you are hitting down on the ball, it implies that the racket face is closed and moving in a downward direction at impact. In the case of the serve (and most topspin g'strokes hit with a slightly closed face), the racket is not moving in a downward direction at all but is moving upward during the contact phase.

In fact, in studying slow motion videos of serves/shots hit with a slightly close face in the manner I described, the ball moves upward just after contact, not down. Take a look at the link below about 30 seconds in. This, to me, is a clear indication that we are not hitting down on the ball.

http://www.youtube.com/watch?v=zTwCys9VkLQ&t=30s
.

[toly]

I hope the following pictures can resolve this dispute.


Figure 6. Arm and racquet motions around contact

The fig.6 demonstrates that tip of the racquet moves upward and to the right. The frame #4 is contact point. The arm pronation, wrist flexion, arm and body forward rotation move the racquet to the right and create flat component VF of the racquet velocity VR, see fig.7. The wrist ulnar deviation and elbow flexion move the racquet upward and produce topspin component VTopspin. I ignored horizontal spins, because they are not essential here.


Figure 7. Ball’s trajectory after impact


So, velocity of the racquet VR is facing up and to the right, not downward. The flat component VF is directed downward and to the right.

However, why the direction of the ball VB does not coincide with racquet velocity direction VR? The fact is that some of the energy of VTopspin is spent on topspin production.

About vectors and their components see http://tt.tennis-warehouse.com/showthread.php?t=436086.

[SystemicAnomaly]

^ Not sure that your additional images resolve all the issues. It clearly shows that the racket is travelling upward before and after contact. However, in this case, the ball the ball appears to be falling (at a slower rate than the toss) after contact. Did this serve go over the net? Was the server jumping and how tall is he?

Quite a few slow-motion videos, such as the one I posted above, show that the racket face appears to be slightly closed yet the ball rises for a while after contact before falling. There may very well be some cases where the ball is still falling slightly just after contact (such as a with a very tall server who is jumping). OTOH, many videos show the ball rising. A single case is not necessarily representative of most serves.
.

[toly]

Originally Posted by SystemicAnomaly

^ Not sure that your additional images resolve all the issues. It clearly shows that the racket is travelling upward before and after contact. However, in this case, the ball the ball appears to be falling (at a slower rate than the toss) after contact. Did this serve go over the net? Was the server jumping and how tall is he?

Quite a few slow-motion videos, such as the one I posted above, show that the racket face appears to be slightly closed yet the ball rises for a while after contact before falling. There may very well be some cases where the ball is still falling slightly just after contact (such as a with a very tall server who is jumping). OTOH, many videos show the ball rising. A single case is not necessarily representative of most serves.
.

There are two more examples of the ball direction.


Figure 7. Direction of the balls around contact

Again, the balls move downward.

Videos don’t show server jumps and balls near the net.

IMO, in case of Twist serve ball usually isn’t rising, but I’m not sure.

[SystemicAnomaly]

^ The video I posted and others clearly show the ball rising. When I hit my twist and other kick serves, the ball is also rising just after contact -- definitely more than my flat serve. (BTW, I am 5'10" and probably jump about 10 cm or so on my serves).


.

[jmnk]

Originally Posted by SystemicAnomaly

^ The video I posted and others clearly show the ball rising. When I hit my twist and other kick serves, the ball is also rising just after contact -- definitely more than my flat serve. (BTW, I am 5'10" and probably jump about 10 cm or so on my serves).


.

the only way you can make the stationary ball raise is to hit it below the equator. (I mean that is assuming we disregard the effect of the friction between the strings and the ball. I suppose when friction is considered it could be argued that the ball could be hit above the equator and still pulled up - but I would be surprised if that was in any way significant).
Which is why the tip about hitting from 8 to 2 makes sense (at least to me). But when one says that the contact does not start until around 10, which is above the equator, I just do not see how it would make the ball raise.
Note, that forehand stroke of the incoming ball, the one that is raising and spinning is different than a serve. There's a pretty good article on all of that on TW University (or few papers by Broody).
What you see on the video is someone hitting below the equator. And it is really hard to see where the initial impact happens so sometimes it looks like it is around 10.
But you make a good observation - the ball does raise most of the time. Just look at any of those drills where people serve from their knees, and the ball goes up really high over the net. It is because they hit it below the equator.

[charliefedererer]

Got to agree with SA that the racquet is moving up for twist and kick serves, and that the ball initially rises, even if the racquet face is slightly closed.
(I made a similar argument in posts 67 and 68 in this thread: http://tt.tennis-warehouse.com/showt...=366690&page=4 )


Here is a sequence of Sam Stosur serving that Toly had previously posted:



One could argue exactly which pic ball contact occurs at. My reading is that ball contact occurs at pic 19, but certainly by pic 20 at the latest.

Clearly on pic 25 the ball is off the racquet. But in this pic, notice that the ball is definitely lower than the top of the "25" stamped into the upper corner of this pic.

Now look at pic 26 where the ball is almost even with the top of the "26" stamped into the upper corner.

Then on pics 27 and 28 the ball is higher still!

That ball is rising!

[toly]

Here is a sequence of pro flat serve.


Figure 8. Flat serve

During impact, frame #3 and #4, the racquet moves downward. The racquet doesn’t move up due to the wrist ulnar deviation is locked. So, there is no topspin at all and just a little sidespin. To produce this kind of racquet swing the hand/wrist muscles must be active.

[toly]

Originally Posted by SystemicAnomaly

^ The video I posted and others clearly show the ball rising. When I hit my twist and other kick serves, the ball is also rising just after contact -- definitely more than my flat serve. (BTW, I am 5'10" and probably jump about 10 cm or so on my serves)..

Originally Posted by charliefedererer

That ball is rising!

I checked a lot of videos of different serves and different players. I failed to find any serve with rising ball.

Your examples are optical illusions.

[LeeD]

I am far from pro.
When I hit a twist serve, the ball rises off it's strikepoint, which is around 8'6" or so. For flat serves, I think my ball goes flat out, from a strikepoint around 9'4".

[Netspirit]

In a twist serve, the ball obviously rises. You cannot clear the net with a completely flat or descending trajectory regardless of what serve you hit, if you are not as tall as Karlovic.

You must hit UP. The point of contact is below the balls "equator" and on the left side (for righties). This is why it is thrown behind the head and to the left - to be able to brush up on the left side of the ball.

[toly]

Originally Posted by Netspirit

In a twist serve, the ball obviously rises. You cannot clear the net with a completely flat or descending trajectory regardless of what serve you hit, if you are not as tall as Karlovic.

You must hit UP. The point of contact is below the balls "equator" and on the left side (for righties). This is why it is thrown behind the head and to the left - to be able to brush up on the left side of the ball.

Rod Cross disagrees with you. See please his article – Physics of the Tennis Kick Serve in http://twu.tennis-warehouse.com/lear.../kickserve.php.

I made the picture from his video.


Figure 9. Slice serve - Rod Cross

Try please to find a similar video that could support your statements.

[Netspirit]

They explain it like this: "Even though the ball is struck while the head is rising, the ball travels downward toward the net because of the high ball toss and rotation of the racquet during the impact. The high ball toss means that the ball drops rapidly both before and after the impact."

I am not convinced that a high ball toss and downward ball motion is the only way to produce a twist serve, or even the best way. The ones I have seen (and attempted myself) had quite a lot of kick/topspin on them and curved up-and-down.

[jmnk]

sure, here's a practical video;
http://www.youtube.com/watch?v=j34COa_LFKg&t=4m28s

see at 4.28, but any of those serves are clearly raising.