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March 2005 Article

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New Research on String Tension and Control

Jani Macari Pallis, 
Ph.D. Photo
Jani Macari Pallis, Ph.D.

A very interesting piece of research funded by the International Tennis Federation (ITF) was recently published on the relationship between string tension and control (Goodwill and Haake, 2004a). Clearly, a player's ability to place the ball in a strategic position on the court is critical to their game. String tension has a major effect on the ball's rebound characteristics and contributes to the player's ability to control the ball. The generally accepted rule of thumb is that lower string tension provides greater power (Brody, 1979) and that higher string tension produces greater control.

Goodwill and Haake dissected player perception of control into multiple factors:

  • Higher tension produces a "stiffer" stringbed.
  • The stiffer stringbed has a shorter contact time with the ball.
  • For off-axis impacts, the shorter contact time produces less racquet rotation, which decrease the deviation of the ball from the anticipated path.

The researchers also noted that player belief that a higher tension string "bites" and grips the ball which enables the player to impart more spin on the ball. However, laboratory experiments by Goodwill and Haake (2004b) did not produce these same results. Their experiments showed that about the same amount of spin was produced.

Thus, the purpose of this new study was to further study high string tension and the ability to control and impart spin on the ball and to identify any other factors which affect string tension and control.

An experiment was setup to emulate the creation of topspin. The researchers used an air cannon (ball gun) and fired balls at the geometric string center of the racquet. (Previous research has demonstrated that this is where professional players typically strike ground strokes.) The air cannon was placed at an angle of 36 degrees to the racquet. Two identical racquets with 16 gauge nylon strings were used. One racquet was strung at a tension of 40 lbs. and the other at 70 lbs. The inbound ball velocities were varied from 15 m/s (meters per second) to 40 m/s (about 34 - 90 miles per hour). To reduce the number of variables in the experiment the ball was released from the air cannon without any spin.

Two high-speed cameras were positioned such that one would capture the racquet's impact with the ball and the other would capture the ball-string interaction at impact. Using image analysis software, measurements were made of the ball velocity, angle and spin before and after the impact, the contact time of the ball with the strings, lateral deflection of the strings (string slip) and the lateral distance the ball moved over the stringbed (contact distance). (Lateral deflection of the strings refers to the distance that the string moves when the ball impacts and lateral distance the ball moves over the stringbed refers to the total movement of the ball, not just the vertical distance change.)

The researchers examined the ball rebound velocity and rebound spin as the inbound velocity varied from 15-40 m/s for both the 40 and 70-lbs. string tension racquets. There was basically no difference between the 40 and 70 lbs. strung racquets. The string tension did not affect the rebound velocity or the rebound spin. Yes, as the inbound velocity increased the rebound velocity and rebound spin increased, but it was independent of the string tension.

The researchers did note that the contact time was 20% longer on the racquet strung at 40 lbs. compared to the 70-lbs. strung racquet. Additionally, the ball travel on the stringbed was 30% less on the 70-lbs. strung racquet.

In light of this, the researchers have suggested that while players may have attributed shorter ball travel over the racquet to the strings "biting" the ball, this actually occurs due to the shorter contact time on the racquet with higher tension strings.

To understand this, examine how heavy topspin is generated on a ball: the racquet is moved forward and up over the ball with the racquet face directed towards the ground. The successful execution of heavy topspin is more likely if the ball strikes the center of the stringbed (avoiding the frame). The shorter the distance ball travels on the stringbed, the more likely this will occur.

Thus, the researchers hypothesize that it is not ball "biting" that causes the increase in spin. Rather, since the higher string tension reduces the contact time and hence the distance the ball travels over the stringbed, this provides a better scenario for a properly executed shot.

The reduction in string slip also would explain the professional player's desire to use higher tension, since a more predictable ball rebound is generated.

Until Next Month ... Jani

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This column is copyrighted by Jani Macari Pallis, Ph.D., all rights reserved.

Dr. Jani Macari Pallis is the founder and CEO of Cislunar Aerospace, Inc., an engineering and research firm in San Francisco. In addition to her engineering practice, she has led two collaborations between NASA and Cislunar, creating educational materials on the aerodynamics of sports for pre-college students and educators. As the head of NASA's "Aerodynamics in Sports" project, she has led a team of researchers investigating the aerodynamics, physics and biomechanics of tennis. The group has conducted high speed video data capture at the US Open and research of ball/court interaction, footwork, serve speeds, trajectories and ball aerodynamics. Pallis received a BS and MS from the Georgia Institute of Technology, an MS in mechanical engineering from the University of California, Berkeley and a Ph.D. in mechanical and aeronautical engineering from the University of California, Davis. She is a member of the Executive Committee of The International Sports Engineering Association.

Questions and comments about these columns can be directed to Jani by using this form.


 

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