Not quite sure on this, but I think, the deformation of the ball does not allow it to roll on the strings when grabbed, but at departing the back of the sphere is getting pushed faster upwards than the center of mas.
The hit is basically going past the center of mass and creates torque within the radius, how far off the center of mas the force is applied. Spinloft is different from the face angle compared to vertical, cause both the ball trajectory and the face path alters the angle.
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No more on pain meds - all contributed matter and anti-matter are still subject to disclaimer
I've only added a few sentences to this post. Everything in quotes is from Technical Tennis by Rod Cross, assumig you don't have a copy, touching on what you all have been discussing:
"In fact, the racquet slides for an inch or so, and then the strings grip or grab the ball, pulling the back of the ball up with the racquet until the strings shoot the ball out..." One the strips grab, the amount of spin is set. The greater the angle of incidence, the tilt, the approach angle of swing, the farther past 1" the ball can slide before the roll-and-grab, i.e. the more spin that can be generated. Yes, the distortion of the ball is a vital event in the sequence.
"You create more or less spin by altering the speed, angle (vertical velocity), and tilt of the racquet. Chapter 3 discussed how altering these variables changes the angle of incidence. When you change the angle of incidence, you are also affecting the magnitude, duration, and direction in which friction will work on the ball, which determines the spin of the ball."
"In order to change the amount and/or direction of that spin with your racquet, the ball has to be incident at an angle to the strings. That way the ball slides across the strings and then bounces off with spin due to the rotation generated while it was on the strings."
"To prevent skying the return of a high bouncing topspin serve, you can do two things. One is to hit the ball harder, and the other is to hit the ball downward (ed: tilt) attempting to hit it horizontally or slightly up over the net. If you hit the ball harder, it will tend to go where you want it to go—that is, in the direction the racquet head is going. The vertical motion of the ball off the strings due to the incoming ball spin and the kick off the court is unchanged, but it becomes a smaller fraction of the total outgoing ball speed."
"Rolling is just an instantaneous transition between sliding and biting. When the ball bites, not only has the ball come to rest relative to the surface, it also becomes stuck to the surface. In the case of a racquet, that means that the bottom of the ball moves with the racquet as in Figure 4.6c. This happens when the contact part of the ball is moving at the same speed as the strings. At that point the strings “grip” the ball, and the contact point between ball and strings does not change." The ball has already been distorted, assuring spin coming off the strings.
"So spin depends only on the speed of the racquet head in a direction parallel to the string plane (ed: gained by approach angle of the swing, boost in ISR, and tilt). That’s so because it then takes friction longer to bring the ball to bite, during which time more spin is generated."
Cross, Rod. Technical Tennis: Racquets, Strings, Balls, Courts, Spin, and Bounce . Independent Publishers Group. Kindle Edition.
Angle of incidence matters a lot, altered mainly by tilt. But tilt by itself isn't sufficient. Thus:
"Five degrees of tilt by itself is not enough to generate topspin. However, if the racquet head is rising at 30 degrees and is also tilted forward by five degrees (Figure 4.10d), then there is a big increase in the amount of topspin compared with Figure 4.10b. The trajectory is also five degrees lower. Most of the work is done by swinging the racquet upward, which changes the spin from -3,820 rpm to +159 rpm. Tilting the racquet five degrees changes the paltry 159 rpm into a more respectable 628 rpm of topspin."
A sample Fed swing is under discussion next (and the .4 factor below is the balls coeffient of restoration): "It is clear from these measurements that the outgoing ball speed depends mainly on the speed of the racquet head. The bounce off the strings also contributes to the outgoing ball speed. If the ball was approaching a stationary racquet at 29 mph, it would bounce off the strings at about 0.4 x 29 = 12 mph. If the impact point on the racquet head is traveling at 70 mph (the tip travels a bit faster) and strikes a stationary ball, then the outgoing ball speed will be about 1.4 x 70 = 98 mph. However, Roger’s racquet was traveling upward at 31 degrees to the horizontal and so had a forward speed of only about 60 mph. So the expected forward speed of the ball is about 1.4 x 60 = 84 mph if the ball is initially at rest. Adding a rebound speed of 12 mph, since the ball was approaching at 29 mph, gives a predicted outgoing ball speed of 84 + 12 = 96 mph, exactly as measured." And in the ISR at contact the tilt became 8º, the final acceleration took place, and the vertical velocity in the plane of the string bed was substantially increased... (from earlier in the text...)
Cross, Rod. Technical Tennis: Racquets, Strings, Balls, Courts, Spin, and Bounce . Independent Publishers Group. Kindle Edition.
I would guess most players find that gaining tilt via ISR is easier to produce by a repeatable amount than tilt via wrist or pronation. (If you don't normally approach contact with ISR, try it with shadow swings. ESR to give yourself comfortable room for the ISR later. People are often surprised by the instant provision of tilt, RH velocity, and vertical acceleration which is easy to see yourself even in a slow shadow swing. It really is what the pros and well-taught kids do.) The ISR adds tilt, but also upward velocity in the plane of the string bed and velocity in the direction of the swing, which is good, because swinging faster makes the incoming spin less determinative. (That's also true for aim: Taking an incoming cross-court topspin ball and hitting up the line the ball will deviate a further 12º toward the sideline. Hit it harder and the deviation goes down to ca. 5º.
I would have jumped in to the Andy Murray v. Fognini comparison, but I didn't have time. At the moment I'm up till 4 am EST to make a call to Málaga. So...laugh: Andy takes much weaker ESR at the start, and less ISR into contact, hits flatter when compared to Fognini. Of course intense ISR isn't used in every shot...but is essential for high-speed forehands (cross-court, down the middle). Only the best dare go full speed and full ISR on a down the line ball off a cross-court ball, etc. Players eventually learn "less and more ISR" to mix power and control shots.
Well, you needed some reading, no? laugh
, forehand 52rps, backhand 25rps. You’re welcome.
