Kick Serve - Ball contact position

Chas Tennis · Jan 8, 2017

These posts relate to where the ball is first contacted by the racket and how closed the racket is. How the racket first contacts the ball is of interest for the kick serve - the subject of this thread. Posted for reference.

Earlier post.
https://tt.tennis-warehouse.com/ind...release-on-the-fh.568436/page-3#post-10754089

Where on the ball is first contact for a closed racket with given angle?

Working on this and just found the interactive circle.

When considering the racket impacting the ball, first contact is like a plane touching a sphere. When a plane touches a sphere the plane is always perpendicular to the radius of the sphere at the contact point. This fact allows us to know where on the ball the contact point is located.

For example, if a racket face is closed by 10 d. how far into the top half of the ball is the first contact point located? ("Closed 10 d" means that the top edge of the racket is forward of vertical so that the racket has an angle of 10 d to the vertical.)

Here is an example of calculating the height Y.

Below is an "interactive circle" to give you the result instantly.

1) The circle represents the ball,
2) the green line represents the racket angle,
3) the red line is the height into the top half of the ball for first contact.
4) the angle indicated is how closed the racket is. Work only in the 0 to 90 d angle range.

Place and move the arrow so that 10 d. appears in sin (10) and the red line gives you the height for a circle of radius 1. (the 'unit circle') For a tennis ball, multiply by the radius of the tennis ball. The interactive circle, move the angle around -
https://www.mathsisfun.com/algebra/trig-interactive-unit-circle.html

When the racket angle is closed by 10 d the ball is impacted just above, Y, the horizontal line around the ball -

Y= ball radius X sin 10 d
Y= 1.3" X 0.174 = 0.22" or 0.6 cm (it's going to look neither high or low for 10 d.)

Fortunately, the racket angle itself indicates the first contact point so that we don't have to video this small spot itself to know where it was.

The racket angle should be closed for a kick serve at first contact - but we don't know by how much.

This works as well for the inside out forehand, where the racket angle is as seen from above. You have to define where 0 d is. Based on ball trajectory or court lines............?

----------------------------------------------------------------------------------------

Earlier Post.
https://tt.tennis-warehouse.com/ind...release-on-the-fh.568436/page-3#post-10754837

Draw a horizontal ring around the tennis ball, a diameter.

A racket that is closed 10 d should contact the ball at a point 0.22" above that horizontal ring. You could draw a line around the ball on a diameter. Touch the Sharpie to the ball at 0.22" above the line.

The racket could approach the ball from various side angles (azimuthal angles). If closed 10 d the contact points would all fall 0.22" above the horizontal diameter.

Someone could plot the distance vs closed angle for various angles of a closed racket.

It is just like the lines of latitude on the earth.

See how close 10 d is to the Equator.

But we still don't know high into the top half of the ball the typical kick serve is first contacted.

I think this might imply that string type and tension will have a larger effect than the first contact angle. ?

mntlblok · Jan 8, 2017

Chas Tennis said:
A point for the discussion.

In physics and other technical subjects, often the orientation of a plane surface, such as the racket strings, has to be described in a clear way. The usual way is to consider the normal to the surface and give the direction of that normal as you would a vector direction, with components. In other words, you will have trouble describing how a plane is oriented unless you first picture an arrow normal to the plane and then describe the arrow's direction.

Ain't "normal" science-speak for perpendicular? Not sure I woulda gotten that from the explanation, and suspect it might not be a universally understood use of that term. . .

Chas Tennis · Jan 8, 2017

mntlblok said:
Ain't "normal" science-speak for perpendicular? Not sure I woulda gotten that from the explanation, and suspect it might not be a universally understood use of that term. . .

https://www.google.com/search?q=plane+normal&ie=utf-8&oe=utf-8

If I write something that is not clear please Google a few of the words and you should find a much better description.

The normal to a surface is a clear and useful general way to describe the direction of a surface at a point, even a curved surface. The normal to a ball at a point on it's surface is understood and usually 'normal' is used in technical discussions. The 'perpendicular to a ball's surface' is not used so much and maybe it breaks down for curved surfaces. ?

2D
When a line touches a circle the line is perpendicular to the radius of the circle where they touch.

3D
When a plane touches a sphere the normal of the plane where they touch goes through the center of the sphere.

mntlblok · Jan 8, 2017

Chas Tennis said:
I have never seen a 6 to 12 path high speed video of a top spin serve. I also am never sure of the clock face orientation when looking up sharply at a sphere....... Is the clock face always vertical or does it face the server?

It could be/do both.

3fees · Jan 8, 2017

Top spin, Kick Serve, Twist, Top Spin Slice are all different serves with different actions resulting when the tennis ball hits the Ground.
Right hander-Left hander reverse it.
top spin- will hit the ground and kick fast forward and stay fairly low to the ground--this unconfuses why its called kick.

,,me.me.me.
Kick Serve- It comes into the court slower, hits the ground and kicks slightly right and the Bounce is generally much higher than top spin.
Top Spin Slice- it comes into the court with top spin and slight slice and when it hits the ground kicks more than slightly to the left of what its path was coming into the court and has a bounce similar to topspin.

Twist Serve- This is the back breaker in Tennis, its the most wicked serve ever done and few have ever been able to hit. When the Ball hits the ground it will hover over the ground by inches and do an almost 90 degree movement from point of impact---this is the real twist serve, not this watered down junk peddled as twist serve by those wanting to play follow the leader stuff that is wrong to begin with.

I have seen one pro and one teacher do this, the rest,,better luck next time, if there is one in this serve,,lol.

Cheers
3Fees

Chas Tennis · Jan 8, 2017

3fees said:
Top spin, Kick Serve, Twist, Top Spin Slice are all different serves with different actions resulting when the tennis ball hits the Ground.
Right hander-Left hander reverse it.
top spin- will hit the ground and kick fast forward and stay fairly low to the ground--this unconfuses why its called kick. ,,me.me.me.
Kick Serve- It comes into the court slower, hits the ground and kicks slightly right and the Bounce is generally much higher than top spin.
Top Spin Slice- it comes into the court with top spin and slight slice and when it hits the ground kicks more than slightly to the left of what its path was coming into the court and has a bounce similar to topspin.

Twist Serve- This is the back breaker in Tennis, its the most wicked serve ever done and few have ever been able to hit. When the Ball hits the ground it will hover over the ground by inches and do an almost 90 degree movement from point of impact---this is the real twist serve, not this watered down junk peddled as twist serve by those wanting to play follow the leader stuff that is wrong to begin with.

I have seen one pro and one teacher do this, the rest,,better luck next time, if there is one in this serve,,lol.
.......

I have seen the flat, slice and kick serves in high speed videos and can often identify many when conditions are right. The Top Spin serve is hard to distinguish from a Kick Serve, if I see the side bounce I call that a kick. Upward racket at impact and ball bounce high to the right is a Kick Serve; upward racket motion and high bounce neither right or left is a Top Spin Serve. I have never seen a bounce as you describe for the Twist Serve, looking "almost 90 degrees" to the side and hovering over the court.

The best research that I have found shows measurements of the ball spin and axes for flat, slice and kick serves. See post #41. I have not found equivalent videos or data for the other serves that you list. When I see serves that pop up high but do not bounce to the side, I have assumed that they are Top Spin Serves - different than your "low" bounce.?

Clear Theory. The reference book Technical Tennis, Cross & Lindsey, describes where the racket is expected to contact & move across the ball and the description seems completely reasonable. Technical Tennis has a very creditable general explanation of the Kick and Top Spin Serves with racket-ball paths and bounces and also other serve types. But the ball and strings distort so much that the complete impact is probably much more complicated than the simple arrows in Technical Tennis as shown in earlier posts.

A simpler question than the full impact, that can be answered is - How does the racket first touch the ball? This can be seen in high speed videos. Knowing what the video should look like might be very useful feedback for developing the kick serve technique.

Looking for data and high speed videos that provide some firm information.

Would you have any high speed videos that show the Twist Serve, Top Spin, Top Spin Slice trajectories and bounce or racket first touch? Knowing the serve types in a video is difficult.

mntlblok · Jan 9, 2017

The serves that I've faced that did the major league "kicking" had in common a ball coming at me that appeared oblong in shape. Disconcerting.

SinjinCooper · Jan 9, 2017

3fees said:
Twist Serve- This is the back breaker in Tennis, its the most wicked serve ever done and few have ever been able to hit. When the Ball hits the ground it will hover over the ground by inches and do an almost 90 degree movement from point of impact---this is the real twist serve, not this watered down junk peddled as twist serve by those wanting to play follow the leader stuff that is wrong to begin with.

Please stop playing video game tennis and come back to the real courts, where the immutable laws of physics still hold sway.

Chas Tennis · Jan 10, 2017

The angle of closure for the racket at first touch for a Kick serve would be very useful to observe as feedback for developing a kick serve.

Here is what appears to be a Kick or Top Spin serve based on the rising racket at impact. Unfortunately, whether a fault or good is not known and the bounce can't be seen. The time is in milliseconds and counts down to "0" milliseconds at impact. The frame before impact is - 4 ms.

To do stop action, single frame on Vimeo, click Vimeo & go to full screen, hold down the SHIFT KEY and use the ARROW KEYS. (Sometimes the Vimeo processing may skip frames.)

To analyze with Kinovea, a blue grid was aligned on the side line to get a vertical reference near impact. The forward tilt of the racket was measured by aligning one line of Kinovea's angle tool with the vertical and the other line with a line from the racket's butt cap to the black ring around the handle. The frame before impact showed 13 d and the frame during impact showed the racket had closed farther. I have not evaluated the accuracy of the measurement.

My interpretation of the drawings of Technical Tennis, Cross and Lindsay, is that this would be a Kick serve. My interpretation also is that a Top Spin serve would have the racket face less closed, about vertical, but I don't know. (The racket closes a few degree during impact in both cases.)

The height on the ball that is first contacted, for a serve with a 13 d racket forward tilt, would be about 25% of the distance from the ball center to its top. Compare to the Technical Tennis ball drawings in post #41.

mntlblok · Jan 10, 2017

Chas Tennis said:
The angle of closure for the racket at first touch for a Kick serve would be very useful to observe as feedback for developing a kick serve.

Here is what appears to be a Kick or Top Spin serve based on the rising racket at impact. Unfortunately, whether a fault or good is not known and the bounce can't be seen. The time is in milliseconds and counts down to "0" milliseconds at impact. The frame before impact is - 4 ms.

To do stop action, single frame on Vimeo, click Vimeo & go to full screen, hold down the SHIFT KEY and use the ARROW KEYS. (Sometimes the Vimeo processing may skip frames.)

To analyze with Kinovea, a blue grid was aligned on the side line to get a vertical reference near impact. The forward tilt of the racket was measured by aligning one line of Kinovea's angle tool with the vertical and the other line with a line from the racket's butt cap to the black ring around the handle. The frame before impact showed 13 d and the frame during impact showed the racket had closed farther. I have not evaluated the accuracy of the measurement.

My interpretation of the drawings of Technical Tennis, Cross and Lindsay, is that this would be a Kick serve. My interpretation also is that a Top Spin serve would have the racket face less closed, about vertical, but I don't know. (The racket closes a few degree during impact in both cases.)

The height on the ball that is first contacted, for a serve with a 13 d racket forward tilt, would be about 25% of the distance from the ball center to its top. Compare to the Technical Tennis ball drawings.

Another clue that it is a "topspin-type" serve is that her arm goes mostly to the right after contact rather than mostly towards the target. I suspect that a much faster camera would be needed for determining much about what's going on with the string bed/ball interaction, though.

atp2015 · Jan 10, 2017

nothinButTennis said:
i experimented on this yday, here are my initial thoughts

1. Bottom of the ball - Higher arc over the net and good kick but misses service line marginally and hit frame few times...my racquet speed is NOT good on kick serve so this could be a factor.

2. Back of the ball - Seem to going in most time and kick is okay not as good as option 1 ("bottom")

will experiment more in coming days

Try top of the ball with low to high swing path.

mntlblok · Jan 10, 2017

My understanding of the various topspin serves can be explained by the diagram in the upper right of the above array. The strings passing along the black arrow will yield topspin and a kick to the right. Passing along the red arrow would yield topspin and a kick to the left. The black arrow strike would cause clockwise spiral, the red arrow counterclockwise spiral. A strike straight up the middle would yield a straight bounce and no spiral spin.

The diagram in the *lower* right yields a kick (American twist) serve that bounces to the right. A similar, parallel red arrow on that one (to the other side of the center) would yield a "topspin slice" serve - one that bounces to the left. Both would curve to the left during flight.

I've been wrong before, but that's what makes sense to me.

kb

Chas Tennis · Jan 10, 2017

atp2015 said:
Try top of the ball with low to high swing path.

The racket indicates how closed the racket is. High speed video can show you the racket angle. Using just your eye or feeling might be tricky and depend on how far into the court impact is.

mntlblok · Jan 10, 2017

atp2015 said:
Try top of the ball with low to high swing path.

That reminds me. There've been a couple of situations where I couldn't get a serve to act the way I wanted, and both involved balls other than a standard tennis ball.

When I tried platform tennis (paddle to many), I couldn't make the ball curve down into the service box the way that I can with a tennis ball. Experimentation led me to try your "top of the ball" pass of the paddle to get it to act right. That seemed to work. I do more of the "straight up the back" of the ball thought for tennis topspin serves.

The other situation that had the identical problem and solution was an outing where we played with one of the softer, kiddie-type Under 10/Quick Start balls. It just wouldn't take the spin. . .

I have, on occasion, fiddled with that mental picture for topspin serves. It yields less margin for error and greater pace, but less kick, if I recall correctly. (IIRC)

kb

mntlblok · Jan 10, 2017

Chas Tennis said:
The racket indicates how closed the racket is. High speed video can show you the racket angle. Using just your eye or feeling might be tricky and depend on how far into the court impact is.

Seems a potential problem with the "angle" comes from _Technical Tennis_. They say the racket face goes through something like 10° whilst the ball is on the strings for those 4 milliseconds. Tricky stuff.

Chas Tennis · Jan 10, 2017

mntlblok said:
My understanding of the various topspin serves can be explained by the diagram in the upper right of the above array. The strings passing along the black arrow will yield topspin and a kick to the right. Passing along the red arrow would yield topspin and a kick to the left. The black arrow strike would cause clockwise spiral, the red arrow counterclockwise spiral. A strike straight up the middle would yield a straight bounce and no spiral spin.

The diagram in the *lower* right yields a kick (American twist) serve that bounces to the right. A similar, parallel red arrow on that one (to the other side of the center) would yield a "topspin slice" serve - one that bounces to the left. Both would curve to the left during flight.

I've been wrong before, but that's what makes sense to me.

kb

I don't think that the diagrams are for serves only. I think all those contacts, like your red line, are probably possible for ground strokes. For the serve, I believe that the strings must travel up and to the right for a kick serve because of the way the racket head moves up with wrist motions. I have videos showing fame before, during and after impact and have posted many times. I have never seen a video of a high level serve where the racket strings went straight up on the right or left of the ball.

Find a video of the strings going straight up.

mntlblok · Jan 10, 2017

Chas Tennis said:
I don't think that the diagrams are for serves only. I think all those contacts, like your red line, are probably possible for ground strokes. For the serve, I believe that the strings must travel up and to the right for a kick serve because of the way the racket head moves up with wrist motions. I have videos showing fame before, during and after impact and have posted many times. I have never seen a video of a high level serve where the racket strings went straight up on the right or left of the ball.

Find a video of the strings going straight up.

*Precisely* - thus my description of the diagram on the lower right.

Just thought the explanation might be simpler with the one on the top right.

Dragy · Jan 11, 2017

mntlblok said:
My understanding of the various topspin serves can be explained by the diagram in the upper right of the above array. The strings passing along the black arrow will yield topspin and a kick to the right. Passing along the red arrow would yield topspin and a kick to the left.

Offcenter hits to produce black or red arrow spin direction will require different racquet face (plane) orientation resulting in different ball flight direction. We discussed it, the ball departs close to square against the stringbed with lean towards the swingpath direction. I believe the only decent orientation to apply when discussing the offcenter hits is the departing ball direction. To hit a flat serve from the mid hash down the T one needs racquet face to be paralel to the baseline and slightly closed at impact. To hit a slice serve leaving the stringbed towards the T (it will then curve leftward) one needs racquet face to be barely turned leftward at impact and left-to-right swingpass. To hit kick serve in a similar manner (one also curves leftwars before bounce; I don't consider "pure topspin" as nonexistent), one will have upward and left-to right swingpath around contact, racquet face plane barely turned leftward and tilted forward (to compensate for "launch angle"). The more tilted the racquet face and more accross the swingpass, the more pronounced the spiral spin will be.

Once again, all orientations described related to the departing ball path, not the body, or head orientation or hand swingpath.

Chas Tennis · Jan 11, 2017

Dragy said:
Offcenter hits to produce black or red arrow spin direction will require different racquet face (plane) orientation resulting in different ball flight direction. We discussed it, the ball departs close to square against the stringbed with lean towards the swingpath direction. I believe the only decent orientation to apply when discussing the offcenter hits is the departing ball direction. To hit a flat serve from the mid hash down the T one needs racquet face to be paralel to the baseline and slightly closed at impact. To hit a slice serve leaving the stringbed towards the T (it will then curve leftward) one needs racquet face to be barely turned leftward at impact and left-to-right swingpass. To hit kick serve in a similar manner (one also curves leftwars before bounce; I don't consider "pure topspin" as nonexistent), one will have upward and left-to right swingpath around contact, racquet face plane barely turned leftward and tilted forward (to compensate for "launch angle"). The more tilted the racquet face and more accross the swingpass, the more pronounced the spiral spin will be.

Once again, all orientations described related to the departing ball path, not the body, or head orientation or hand swingpath.

Unfortunately, I have misplaced my copy of Technical Tennis so I'm speaking from memory. I believe that the text says the arrows refer to the path of the strings and where on the ball the path is located. The spin axis direction is different but closely related. Probably the racket path or the last part of the path is roughly perpendicular to the resulting spin axis shown below.

I consider pure Top Spin ground strokes possible but not pure TS serves for a high level serving technique. In a high level serve technique, including the Top Spin serve, the racket path is going forward and also up and to the side at a considerable angle during impact. The up and to the side will not produce pure top spin.

According to the publication in post #41, these are representative measured rotation rates, spin axes and components. AVz is 'sidespin' component, AVy is top spin component and AVx is the gyrospin component. I believe that the directions are based on the court and the serves were directed down the T (top spin and gyrospin are roughly aligned with the court lines). See report.

If you look at the components of the spin axis for the flat, slice and kick serve you find:
1) The largest component of the spin axis for all serves - including the kick serve - is in the direction of side spin.
2) That means the kick serve has a larger sidespin component than its topspin component.
3) All three type serves have similar magnitude components in the forward direction of gyrospin (or spiral spin). (The kick serve's bounce to the right comes from the angle of incidence onto the court along with the gyrospin.)
4) The kick serve has a greater sidespin component than the slice serve.

After seeing this publication, it took me a year or two to fully realize these things. I noticed them because Cross in his TW article pointed out that the kick serve had a larger sidespin component than its top spin component. That seemed very strange. The common names for the different type serves are part of the Tennis Serve Nuthouse and are very misleading when taken literally.

I have assumed the Top Spin serve has less gyrospin than the kick serve and so that it bounces less to the right. I'd like to verify the characteristics of the kick and top spin serves with high speed video. (Anybody have a high level TS or kick serve?)

You have some interesting comments. Do they agree with the video of the racket and ball impact for what I believe is a kick serve attempt in post #59? Particularly "....racquet face plane barely turned leftward and tilted forward (to compensate for "launch angle").." See the magnified box.

The racket rotates so rapidly during impact that I don't see how we can describe its orientation with one orientation or accurately estimate it in degrees or measure it easily. When would the 'orientation' be given? At last string contact when there are hardly any forces? Impact is complicated and involves the racket's initial orientation, path, speed, rotations, string, impact location on the racket face and other factors. Should this be described in discussions in a few words or by narrating high speed videos?

The initial orientation at first touch can be measured. The video above is an example for racket closure angle just before impact. I don't know that the initial orientation of the racket has been measured yet.

Dragy · Jan 11, 2017

Chas Tennis said:
The racket rotates so rapidly during impact that I don't see how we can describe its orientation with one orientation or accurately estimate it in degrees or measure it easily. When is the orientation given? At last string contact when there are hardly any forces. Impact is complicated and involves the racket's initial orientation, path, speed, string, impact location on the racket face and other factors.

I love to dive deep into researching impacts and interactions when looking onto sports mechanics, but also trust knowledge is important to the extent it can be utilized. So try to stay on surface... well, sometimes slightly deeper)
I believe producing various spin serves, directing them left/right, getting them over the net and landing them inside the service box - all shall be covered and guidelines shall be built up from simpler models. We shall agree stringbed/ball iteraction also being complicated and including lots of deformation, shall be modeled as plane vs sphere interaction with some friction component. We shall agree that angle change during impact is uncontrollable, opposed to swingpath and through/accross ratio. With these agreements we shall agree that the ball departs with some lean from square to stringbed plane, lean towards the swingpath direction. Spin shall be towards the swingpath direction. It's also rather straight forward that spin effect on the trajectory (including sideways bounce) is higher for higher spin/speed ratio, which practically is achieved by trading pace for spin through applying more accross swingpath. All provided we aim for more or less same direction, not hitting flat down the T to the deuce court or slice out wide...

Chas Tennis · Jan 11, 2017

Over the years there have been many word descriptions on how the racket contacts the ball in these forums. But evidence is scarce, just about non-existent.

I'd like to get just one creditable brick in place.

What angle of closure is the racket face at first contact with the ball for a kick serve?

Dragy · Jan 11, 2017

Chas Tennis said:
Over the years there have been many word descriptions on how the racket contacts the ball in these forums. But evidence is scarce, just about non-existent.

I'd like to get just one creditable brick in place.

What angle of closure is the racket face at first contact with the ball for a kick serve?

You have a still from henin video. There's a good video of fed showing similar angle. Shall you measure some exact value, or say 10-20 deg range would suit you?

Chas Tennis · Jan 11, 2017

I don't know if the Stosur serve was a kick serve, a fault or good. I remember a recent Federer kick serve with a similar angle, identified as a kick serve, but I don't know about it either.

If we had a confirmed kick serve and a confirmed top spin serve and the right camera angles, a high speed video with small motion blur can measure racket angles before contact.

There will be a range of angles and maybe even different ways to hit the kick serve. ? The good news is that because of the plane touching the sphere information, the racket shaft angle indicates how high up on the ball the first contact point is.

How far around the side of the ball is harder to observe. Ideally, the camera would be above impact. Observing the side contact point may not be as practical.

The first goal is video feedback when learning the kick serve, optimizing it and checking it when you lose it. For example, if you don't get the kick to the side, video your serve and see what the racket closure angle is just before impact. I'd like a practical way to optimize and check a kick serve using high speed video. Needs small data base and simple video technique. The video technique may be similar to the kick serve video shown.

It just occurred to me that the 3D motion capture systems measure racket angles and could make more complete racket angle measurements. Publications with data?

DavidGibson · Jan 11, 2017

Tight Lines said:
It's not as difficult as you make it out to be. It's all in the toss IMO. Approximately 11 o'clock for kick, 12 o'clock for topspin, 1 o'clock for top/slice or slice. If you need more power, then toss the ball more into the court. But, nobody practices tossing.

I don't think you understood Donny's post. He was essentially saying that no one, including the pros think about hitting the ball in 6 different ways and he is correct. There are only three serves and three racket paths, all the rest is just different degrees of those three serves.

Flat serve, Swing into the court. Top spin, swing up and out to the side. Slice, instead of brushing up, brush out and swing to the the corner of the court. Sounds easy enough!

Chas Tennis · Mar 23, 2017

Chas Tennis said:
Unfortunately, I have misplaced my copy of Technical Tennis so I'm speaking from memory. I believe that the text says the arrows refer to the path of the strings and where on the ball the path is located. The spin axis direction is different but closely related. Probably the racket path or the last part of the path is roughly perpendicular to the resulting spin axis shown below.

I consider pure Top Spin ground strokes possible but not pure TS serves for a high level serving technique. In a high level serve technique, including the Top Spin serve, the racket path is going forward and also up and to the side at a considerable angle during impact. The up and to the side will not produce pure top spin.

According to the publication in post #41, these are representative measured rotation rates, spin axes and components. AVz is 'sidespin' component, AVy is top spin component and AVx is the gyrospin component. I believe that the directions are based on the court and the serves were directed down the T (top spin and gyrospin are roughly aligned with the court lines). See report.

If you look at the components of the spin axis for the flat, slice and kick serve you find:
1) The largest component of the spin axis for all serves - including the kick serve - is in the direction of side spin.
2) That means the kick serve has a larger sidespin component than its topspin component.
3) All three type serves have similar magnitude components in the forward direction of gyrospin (or spiral spin). (The kick serve's bounce to the right comes from the angle of incidence onto the court along with the gyrospin.)
4) The kick serve has a greater sidespin component than the slice serve.

After seeing this publication, it took me a year or two to fully realize these things. I noticed them because Cross in his TW article pointed out that the kick serve had a larger sidespin component than its top spin component. That seemed very strange. The common names for the different type serves are part of the Tennis Serve Nuthouse and are very misleading when taken literally.

I have assumed the Top Spin serve has less gyrospin than the kick serve and so that it bounces less to the right. I'd like to verify the characteristics of the kick and top spin serves with high speed video. (Anybody have a high level TS or kick serve?)

You have some interesting comments. Do they agree with the video of the racket and ball impact for what I believe is a kick serve attempt in post #59? Particularly "....racquet face plane barely turned leftward and tilted forward (to compensate for "launch angle").." See the magnified box.

The racket rotates so rapidly during impact that I don't see how we can describe its orientation with one orientation or accurately estimate it in degrees or measure it easily. When would the 'orientation' be given? At last string contact when there are hardly any forces? Impact is complicated and involves the racket's initial orientation, path, speed, rotations, string, impact location on the racket face and other factors. Should this be described in discussions in a few words or by narrating high speed videos?

The initial orientation at first touch can be measured. The video above is an example for racket closure angle just before impact. I don't know that the initial orientation of the racket has been measured yet.

Toly had a detailed post in 2012 especially related to the first contact point on the ball and the forward racket tilt. He also has some of the pictures that I was looking for in the OP showing the racket tilt angle at impact. (This forward racket tilt was also discussed and illustrated in the book of R. Cross & Lindsey, Technical Tennis.)

toly said:
Anatoly Antipin

Twist Serve

To create twist serve we have to brush a ball approximately from 8:00 to 2:00 o’clock, but above the ball’s equator. This kind of brushing motion creates topspin, sidespin, and spiralspin (gyrospin).

The spiral/gyro spin would be in clockwise direction causing the ball bounce deviation toward the right. The gyrospin motion rotates the ball about an axis pointing towards the target. That sort of spin is used to throw a gridiron football, but it has no effect on curvature of the ball through the air.

We can hit the ball above its equator if and only if the racquet face is tilted forward - slightly closed.

There is example of the racket motion around impact in case of twist serve.

Figure 1. The racquet’s movement around contact

The frame #2 is point of contact. The arm itself moves relatively slowly forward, to the right, and downward and practically doesn’t affect racquet brushing motions.

On the other hand, the wrist ulnar deviation rotates, very fast, the racquet upward and to the right almost perpendicular to the ball outgoing direction.

It seems, the wrist ulnar deviation produces the main contribution to the topspin, sidespin, and gyrospin. All others motions of the body (arm pronation, wrist flexion, and so on) create mostly translational motion of the ball.

Figure 2. The tilted forward racquet

The theta (ϴ) angle defines magnitude of the gyro and side spins.

Figure 3. Vector of the spin component along with its components

About fig.3 see also http://tt.tennis-warehouse.com/showthread.php?t=436086.

Assume that the racquet face tilted forward with theta angle (ϴ). Then gyrospin, sidespin, and topspin racquet's velocity components would be:

VGyrospin = VSpinHor*sin(ϴ) (1)

VSidespin = VSpinHor*cos(ϴ) (2)

VTopspin = VSpinVer (3)

The more we tilt the racquet face forward (increase ϴ), the more efficient would be gyrospin and less effective sidespin.

If ϴ =45° then VGyrospin = VSidespin , but I doubt that one can hit successful twist serve with so large theta angle.

Btw, topspin component isn’t affected by ϴ.

The theta angle also determines coordinates of the point of contact, see picture below.

Figure 4. The tilted forward racquet and ball’s equator

IMO ϴ should be less than 30° and thus point of contact would be next to ball’s equator, otherwise the ball goes into net.

Chas Tennis · Mar 23, 2017

Why is this very useful? Feedback.

If you take a high speed video of your kick serve you should see this racket tilt angle (~15d ?) - if the ball bounces to the right after the bounce.

High speed video with small motion blur taken from the right camera angle is required.

If the ball bounces high but not to the right, I'd describe that as a top spin serve.

Raul_SJ · Mar 23, 2017

Chas Tennis said:
Why is this very useful? Feedback.

If you take a high speed video of your kick serve you should see this racket tilt angle (~15d ?) - if the ball bounces to the right after impact.

High speed video with small motion blur taken from the right camera angle is required.

If the ball bounces high but not to the right, I'd describe that as a top spin serve.

What camera frame rate is desirable to see the precise moment of impact (Toly's 15 degree tilt angle and hitting above the equator) ? I believe the dwell time can be up to 4 or 5 milliseconds, so anything less than 5000fps would be inadequate, and that looks to be beyond the scope of consumer cameras. Consumer cameras like the latest Galaxy 8 only go up to 1000fps.

Does anyone know which camera Toly is using?

Chas Tennis · Mar 23, 2017

During impact the racket tilt angle changes over the 4 milliseconds of ball-string impact and also the racket strings and ball distort. Those are difficult to deal with.

I only consider the simple first touch or close to first touch, for racket tilt angle. The Stosur picture in post #69 is just before impact. Before impact, the closing angle as the racket moves forward over a few centimeters or milliseconds is predictable. We can see from the video how many degrees the racket closes as it moves 1 millisecond or a given distance, and use that information to figure how closed it would be at first touch.

At 240 fps a video camera captures one frame every 4.2 milliseconds. Half the serve videos will capture the ball within 2 milliseconds of first touch. The closer to first touch the better but I think that all 240 fps videos would be usable for estimating the angle of closure for the racket at impact. The shutter speed has to be fast to get an image with small motion blur.

I use fast shutter speeds of 1/10000 sec or faster when light allows. During the 4 milliseconds of ball string impact, 0.01 milliseconds is the capture time. (there is some Jello Effect distortion with most cameras to be tested)

mntlblok · Mar 24, 2017

Chas Tennis said:
During impact the racket tilt angle changes over the 4 milliseconds of ball-string impact and also the racket strings and ball distort. Those are difficult to deal with.

I only consider the simple first touch or close to first touch, for racket tilt angle. The Stosur picture in post #69 is just before impact. Before impact, the closing angle as the racket moves forward over a few centimeters or milliseconds is predictable. We can see from the video how many degrees the racket closes as it moves 1 millisecond or a given distance, and use that information to figure how closed it would be at first touch.

At 240 fps a video camera captures one frame every 4.2 milliseconds. Half the serve videos will capture the ball within 2 milliseconds of first touch. The closer to first touch the better but I think that all 240 fps videos would be usable for estimating the angle of closure for the racket at impact. The shutter speed has to be fast to get an image with small motion blur.

Don't know how valuable it is to know this stuff (I get a kick out of it just cause I like to know stuff), but I *think* I recall that my smart phone recording at 240 fps must vary the shutter speed on its own based on how much light is available. I never notice any motion blur during daylight recording, but I *think* I recall seeing some from some of the "under the lights" video I've done. Another possibility is that it automatically changes the ISO, still avoids motion blur, but yields a "grainier" video. Pretty close to the edge of what I know anything about.

Anyway, I'm glad someone did the division and informed me that one frame of a smart phone video set on slow motion is about the same as the amount of time that the ball stays on the strings.

Thanks!

It's also nice to know about how "closed" the racket face is at serve contact. If 15° is for a topspin serve, then I'd guess that it would be greater than that for a "flat" serve, what with the toss being further out into the court, no?

I also recall from _Technical Tennis_ that the racket face angle has a significant change during the time that the ball is in contact with the strings. Too lazy to look it up right now.

mntlblok · Mar 24, 2017

Chas Tennis said:
Why is this very useful? Feedback.

If you take a high speed video of your kick serve you should see this racket tilt angle (~15d ?)

alt+0176 = °

Chas Tennis · Mar 24, 2017

mntlblok said:
.....................my smart phone recording at 240 fps must vary the shutter speed on its own based on how much light is available. I never notice any motion blur during daylight recording, but I *think* I recall seeing some from some of the "under the lights" video I've done. Another possibility is that it automatically changes the ISO, still avoids motion blur, but yields a "grainier" video. ......................................

Almost all high speed videos cameras have automatic exposure control with a computer program to control the camera settings. This leaves the shutter speed a mystery that you can't determmine but you can test after you have bought the camera. Most smartphones have very fast shutters in bright direct sunlight (with sharp shadows). Some high speed video cameras may not have very fast shutters in bright sunlight. ? You cannot tell from the camera specs the shutter speed for certain. Here is a smartphone issue- in very low light some smartphones will even slow down the frame rate - from 30 fps to below 30 fps, etc. I would not expect that at 240 fps but it should be checked for critical timing.

mntlblok said:
............... one frame of a smart phone video set on slow motion is about the same as the amount of time that the ball stays on the strings. ...........

That is very useful for 240 fps because it means that just about always your video will catch the ball on the strings. That is what I have seen. But you never know at what time during string-ball contact you have caught the ball. You don't know if early or late during the 4 milliseconds of string-ball contact. ....but you probably could estimate it by using the racket position on the previous frame...?

mntlblok said:
It's also nice to know about how "closed" the racket face is at serve contact. If 15° is for a topspin serve, then I'd guess that it would be greater than that for a "flat" serve, what with the toss being further out into the court, no?

No. Kick serve 15°closed (first rough estimate). Topspin serve around vertical but data is non-existent for serve as far as I have found. Since the racket is around vertical for TS and closing as it moves forward, you would catch the racket head around vertical, +/-, requires more accurate measurement that just seeing a kick serve tilt of 15°. The string face is what's important but the handle is easier to see. This stuff has not been measured or presented in any serious way as far as I know.

mntlblok said:
..... from _Technical Tennis_ that the racket face angle has a significant change during the time that the ball is in contact with the strings. .....................

I somehow misplaced my copy and have been looking for it for months!

I believe the bottom line is that your kick serve will never bounce to the right if you hit it near the rear most spot of the ball. Also, there are very few high speed videos to show the tilt of the racket strings when the ball is struck for a confirmed kick serve.

Pete Player · Jan 8, 2018

Did not quite figure, under which of the serve topics my question would land, but chose this.

Yet read some of the studies, I haven’t come accross the comparision between horizontal velocity and the amount of spin needed for the gyro effect to happen and neither have I been able to find the treshold rpm for visible gyro effect on a tennis ball.

However have come to a conclusion, that it is the curvature between impact and bounce, that makes the ”spiral spin”, what ever that actually is. In flight the hig revolving ball is trying to maintain the spin axis it got at impact travelling thru the air, while gravity and the lift towards the spin direction affect the ”gyroscopic ball” and make the spin axis to twist just like a spinning wheel would do, had a force effected its axis. However both those too are quite small forces compared to the counter force at the bounce, which will make the difference between a kick to a side and forward towards the initial direction of the trajectory.

I’ve been able to hit kicks myself. So this is not only a hypothetical question.

——————————
On pain meds - all contributed matter and anti-matter subject to disclaimer

Raul_SJ · Jan 8, 2018

nothinButTennis said:
i experimented on this yday, here are my initial thoughts

1. Bottom of the ball - Higher arc over the net and good kick but misses service line marginally and hit frame few times...my racquet speed is NOT good on kick serve so this could be a factor.

2. Back of the ball - Seem to going in most time and kick is okay not as good as option 1 ("bottom")

will experiment more in coming days

Chas Tennis said:
I remember in a TW post a comment by Rafter saying that for a kick serve the racket path on the ball was from '11 to 5'. I still have no idea what was meant. ?? (Once I tried it in a state of confusion and it seemed to work...... ??)

.

I also find the clock-face cues very confusing, particularly as we are using 2D clock-face cues for a 3D sphere.

I am leaning towards the following approach:

Try out the Rafter "11 to 5" and all the the other clock cues out there ("bottom of ball", back of ball" etc. ) If you get results with that image, stay with it, keeping in mind what you feel does not necessarily relate to what is physically happening.

The first step is to get the kick spin. Afterwards, one can figure out what is actually happening physically, if one is interested in knowing that.

Gazelle · Jan 8, 2018

Raul_SJ said:
I had the same confusion. Many of the coaching instructional cues, "Get under the ball..." might relate to sensation rather than what is physically happening .

The Dave Smith "Kick Serve" video says "We are not coming up on the back of the ball. We are coming up on the inside left part of the ball. The part of the ball that is facing us as we face the right fence."

You can see from the pic that he is not brushing up the back of the ball facing the back fence.

And that is the way I think of it visually. Come up on the inside left part of the ball (the portion facing the left fence). Although I am not sure what is actually happening physically.

Interesting. I'll try this next time, see what happens.

mntlblok · Jan 8, 2018

Gazelle said:
Interesting. I'll try this next time, see what happens.

As you compare results, try to note the ball toss position. It will have a great bearing on the result. Also, the usual stuff about tossing a bit to the left, grip over towards the backhand a bit from Continental, and accelerate massively up and to the right, making sure that the elbow completely straightens out from that acceleration.

Chas Tennis · Jan 8, 2018

Pete Player said:
Did not quite figure, under which of the serve topics my question would land, but chose this.

Yet read some of the studies, I haven’t come accross the comparision between horizontal velocity and the amount of spin needed for the gyro effect to happen and neither have I been able to find the treshold rpm for visible gyro effect on a tennis ball.

However have come to a conclusion, that it is the curvature between impact and bounce, that makes the ”spiral spin”, what ever that actually is. In flight the hig revolving ball is trying to maintain the spin axis it got at impact travelling thru the air, while gravity and the lift towards the spin direction affect the ”gyroscopic ball” and make the spin axis to twist just like a spinning wheel would do, had a force effected its axis. However both those too are quite small forces compared to the counter force at the bounce, which will make the difference between a kick to a side and forward towards the initial direction of the trajectory.

I’ve been able to hit kicks myself. So this is not only a hypothetical question.

——————————
On pain meds - all contributed matter and anti-matter subject to disclaimer

The Physics of the Kick Serve, Rod Cross
http://twu.tennis-warehouse.com/learning_center/kickserve.php
because the impact angle determines which part of the ball's spinning surface contacts the court.
This article is full of important and insightful information but it is scattered here and there. I have trouble relocating things. Suggest that you make a paper copy and highlight the interesting information. In this article, Cross emphasizes gyrospin as the reason the ball bounces to the right for the kick serve (right handed server.) But also the steeper impact angle of the high spin rate kick serve is equally important to the bounce To clarify, the slice serve also has considerable gyrospin as shown in the ball/spin vector diagram. But the slice impacts the court near where the spin axis is located on the ball. The velocity of the ball's felt so close to the spin axis does not give very much impetus to the side bounce. PROBABLY WILL CORRECT THE PART IN RED LATER- PROBABLY WRONG.

Gyrospin component is shown on the X axis (forward). Slice serve and kick serves have similar AVx values, similar components of gyrospin. The spin axes are tilted in the forward direction.

It is probably helpful to hold a tennis ball and rotate it to visualize how the spinning ball contacts the court for various impact angles. Maybe a pencil could be stuck through holes drilled in the ball to show the spin axis.

I believe that if the ball bounces to the right for a kick serve, then the racket was closed about 15 d at impact. To see this racket tilt at impact the camera angle must have the right viewing angle, the camera must be high speed video and the motion blur must be small.

Pete Player · Jan 8, 2018

Had a quick glimbse on the paper before rushing to my practice.

Unfortunately found already something, that is slightly controvercy to the laws of aerodynamics. May be it is just written poorly. Some other pragraphs may confirm that.

Shall return and read it thoroughly later.

——————————
On pain meds - all contributed matter and anti-matter subject to disclaimer

mntlblok · Jan 8, 2018

Chas Tennis said:
The Physics of the Kick Serve, Rod Cross
http://twu.tennis-warehouse.com/learning_center/kickserve.php

This article is full of important and insightful information but it is scattered here and there. I have trouble relocating things. Suggest that you make a paper copy and highlight the interesting information. In this article, Cross emphasizes gyrospin as the reason the ball bounces to the right for the kick serve (right handed server.) But also the steeper impact angle of the high spin rate kick serve is equally important to the bounce because the impact angle determines which part of the ball's spinning surface contacts the court. To clarify, the slice serve also has considerable gyrospin as shown in the ball/spin vector diagram. But the slice impacts the court near where the spin axis is located on the ball. The velocity of the ball's felt so close to the spin axis does not give very much impetus to the side bounce.

If the spin axis is near where the ball contacts the court, then there can't be much top nor gyro spin, right? I can definitely hit a version of a "slice" serve (a goofy version) that will kick hard, dead left. It's really more like an overhead or sidearm drop shot, though.

kb

Chas Tennis · Jan 8, 2018

For the kick serve, the trajectory is strongly changed by aerodynamics as the ball travels. This changes the angle of impact for the bounce.

The original spin direction is maintained as the serve travels according to Cross.

I'm considering now if the changing trajectory direction of the kick serve has anything to do with how far from the spin axis the court is first contacted? I said that in an earlier thread.

If the spin axis is fixed as the ball flies but the ball's trajectory is being changed by aerodynamics, does the location on the ball that is first contacted by the court change relative to the spin axis location on the ball? I now don't think that it does.

Preliminary, this is likely to be corrected -

Look at the ball/ spin vector diagram from post #86.

Velocity - Felt Meets Court from Ball Spin

1) For illustration the above shows the spin vectors placed at the center of the ball.
2) The real spin axis would go through the ball's surface at two locations and the center. For the kick serve spin vector (dashed line), draw a line along the dashed line but going also through the bottom half of the ball. That dashed line now represents the spin axis where it intersects the of the ball in 3 points (shown sort of 2D but would really be 3D) .
3) The court will always contact the lowest point of the ball regardless of the trajectory's impact angle. That will be some distance from the spin axis that we drew.
4) A line from the first court contact point perpendicular to the spin axis is the radius for the ball's felt contacted. That is, the distance of the ball's felt from the spin axis.
5) That radius multiplied by the spin rate gives the velocity of the ball's felt just before impact with the court.

Do the same for the slice and flat serves and you can see that the kick serve has a higher velocity of the felt on the court at first touch.

-------------------------------------------------------------------------------------------------------------

After-impact effects are not as simple to consider as estimating the velocity of the ball felt that first impacts the court as above.

1) What effect does the trajectory impact angle have on the bounce direction?
2) What effect do the distortions of the ball have on bounce direction?

Impact with ball distortions is too complicated.

mntlblok · Jan 8, 2018

Chas Tennis said:
If the spin axis is fixed as the ball flies but the ball's trajectory is being changed by aerodynamics, does the location on the ball that is first contacted by the court change relative to the spin axis location on the ball? I now don't think that it does.

I wouldn't think that it would, either.

kb

IowaGuy · Jan 8, 2018

Chas Tennis said:
1) What effect does the trajectory impact angle have on the bounce direction?

Perhaps this could be studied by comparing the kick serve of a 6'8" player to that of a 5'6" player? i.e 5'6" player would in theory have a more horizontal-type kick serve, whereas the 6'8" player would have more of a vertical-type kick serve (and therefore probably higher bounce upon impact).

Pete Player · Jan 8, 2018

Chas Tennis said:
For the kick serve, the trajectory is strongly changed by aerodynamics as the ball travels. This changes the angle of impact for the bounce.

The original spin direction is maintained as the serve travels according to Cross.

I'm considering now if the changing trajectory direction of the kick serve has anything to do with how far from the spin axis the court is first contacted? I said that in an earlier thread.

If the spin axis is fixed as the ball flies but the ball's trajectory is being changed by aerodynamics, does the location on the ball that is first contacted by the court change relative to the spin axis location on the ball? I now don't think that it does.

Preliminary, this is likely to be corrected -

I think you got it right. And the spin axis will change relative to the trajectory, because the combined lift (pulling the ball down at the start) and gravity will make the gyro effect mid-air, making the spin axis rolling slowly like a propeller.

I think, basically know by instinct - living with aviation instruments over two decades, that the spin axis may have changed so much at first contact with the court, that on the best kick serves it may have turned the ball close to ”upside-down over the top” relative to the orientation it left off the racket.

Since the best kicks pops so much high and to the right (righthanded), the ball reminds really much a cornering roadracing bike first losing grip in the front and then, when the grip suddenly returns throwing the bike sideways up onto the high side and driver off to the ouside. That effect would require the spin direction be sideways and spin axis tilted towards the service box and ball spinning towards the sideline.

——————————
On pain meds - all contributed matter and anti-matter subject to disclaimer

mntlblok · Jan 8, 2018

IowaGuy said:
1) What effect does the trajectory impact angle have on the bounce direction?

In my experience, the direction (left/right - relative to the ball's flight path) of the bounce is strictly a function gyrospin - no gyrospin, no change in direction. Steeper downward trajectory, higher bounce. What's happening with the friction of the nap and the distortion of the ball shape against the court surface is far out of my league.

kb

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