stringbed basher device
Impact between ball and stringbed.

Tennis players maintain that changes in either the string tension or material can make large differences in ball speed and its associated "feel." The findings of this experiment show that string tension and material have only a small effect on ball speed, even while having a more significant affect on the feel parameters, at least at lower tensions.

ball basher device
Impact compression with ball basher.

What is the fastest ball? The stiffest? Tennis balls were dynamically tested with a "ball basher" device. Fourteen ball models from 6 companies were tested for energy return, bounce velocity, deformation during impact, stiffness, impact duration, recovery rate, and force load rate at multiple impact energies.

effective mass teaser
Lacrosse ball impact with wood beam.

How does the ball know how fast to bounce from a beam or a racquet? You say that is a silly question — it is determined by the forces of the collision which depend on the properties of the colliding objects. Well, yes, but how do the colliding objects know their own properties? Again, a silly question, you say. They just are. Mass, length, balance, and stiffness of an object just are . . . or so you think.

edge effect teaser
Rubber band stringbed.

The center two mains were tensioned to 60 lb with each adjacent main decreasing in tension by 5 lbs, with the last string at the periphery at 20 lb on each half of the racquet. The cross strings were all tensioned at 50 lb. Did this variable string tension method to produce more spin?

Typical impact
Geometry of impact.

Factors that might affect racquet control were investigated by setting up a simple experiment where a tennis ball was fired from a ball machine to impact a hand-held racquet at rest. Speed, angle and spin were measured for rebounds from the racquet in its natural condition and with mass added.

free standing impact
Free-standing impact.

Experiments analyzing racquet performance during collisions with tennis balls use several methods to secure the racquet — free-standing, hand-held, head-clamped, and handle-clamped. The concern is that the securing method can alter performance. Each method allows or prevents certain degrees of freedom of movement. All have major drawbacks except free-standing and hand-held. The main concern between these two is that although a hand-held racquet allows all degrees of freedom of motion, it may alter the magnitude of these motions and thus the performance and assessment of the racquet. The theoretical consensus is that the ball leaves the strings before it receives any information from the handle on whether it is hand-held or free. We test that here.


Spin Demo

Upward moving racquet.
Video still of upward moving racquet

Just because you swing upward does not mean you will get topspin, swinging down does not mean you will get backspin, and swinging flat does not mean no spin. You can get top, back, and no spin from all swing trajectories. Using 18 videos of simple demonstrations and the technique of drawing "relative path" diagrams, this experiment explains the how and why of rebound spin and direction.


Racquet Recommender Tool

Racquet Recommender Tool.
Racquet Recommender Tool

The Racquet Recommender tool helps players find racquets that have more or less of user designated performance properties: Power, comfort, control, maneuverability, and spin. The player's current racquet is represented by the green line on each slider, and as the sliders are moved, racquets (dots on the graph) disappear or appear. The details and comparisons of these recommended racquets are obtained by clicking the green go button.


Racquet Performance Analysis Tool

Racquet Performance Analysis Tool.
Racquet Performance Analysis Tool

The interactive TWU Racquet Performance Analysis Tool compares the shot performance of every racquet to every other in 9 areas —shot ...

  • Speed
  • Distance
  • Spin
  • Launch angle
  • Flight time
  • Swing speed
  • Power
  • Power zone size
  • Plow through

Racquets can be filtered and compared

  • All at once, or by brand, or choose any 2 or 3 to compare.
  • By headsize, swingweight, weight, balance, twistweight, or vibration frequency.
  • By swinging with the same effort or the same speed for each racquet.
  • By on-center and off-center hits.


  • Similar racquets: Highlight and compare all similar racquets to each other.
  • Customize racquets: Customize racquets and show the result compared to all other racquets.
  • Help buttons, icons and screens: Multiple information sources for explanations, definitions and terminology.
  • Interactive: Simply click on any of the dots or choose from the dropdown menus in the Control Panel.
Tennis all into a headwind.
Tennis ball into head wind

Ever since tennis was first introduced as a serious sport, players have been blaming their racquet or the ball for the mistakes they make. That is why professional players need six identical racquets each match, all freshly strung, and why each ball is closely inspected for fluffiness before it is deemed suitable to be served. Any tiny variation is enough to throw even a professional off balance. How then do players cope with changes in air pressure, temperature, humidity, ball mass and diameter, fuzziness, altitude, wind? All of these factors affect the flight of a ball through the air, so they affect where the ball lands.

Peak deformation of 6 mm foam on top, 15 mm foam in middle, and 26 mm thong foam on bottom.
Impact on layered foams

Cushioning is an important performance characteristic of tennis shoes. It is accomplished by altering the geometry, thickness, location and combination of sole materials. Studying a shoe as an entity allows one to assess the cushioning properties of the shoe as a whole. We, instead, were interested in how singular, uniform foams behave when exposed to dynamic impact loads similar to those experienced during foot plant. Knowing how a foam behaves in the general case allows extrapolations about individual footwear components, shoes, and performance.

Shoe cushioning rig

For tennis players, the primary factor determining purchase of a tennis shoe is comfort. For researchers and manufacturers, injury prevention is a main area of interest, concern and design. For both groups, heel and forefoot cushioning and stability is the common denominator between comfort and injury reduction. The objective of the experiment was to examine cushioning effects at different impact energies for both heel and forefoot, determine how stiffness affects cushioning and the player's response to it, and to compare multiple shoes on all parameters.

ALSO NEW — Two new footwear comparison tools:

Shoe test section

Traction is specific to the particular shoe-surface interface. Different surfaces will create different forces with the same shoe, and different shoes will create different forces with the same surface. These resultant traction forces are required for performance, but the open question is how much traction is required for tennis movements on a particular shoe-surface interface, how much is available, and how much is too much, contributing to many lower extremity injuries? In search of answers, a test rig was developed to measure translational and rotational traction. The initial findings are presented here.

Rod Cross explains experiment setup.
Rod Cross with test apparatus

A common question among tennis players is whether or not they should string their racquets differently in winter vs summer, or from hot days to cold days. It is common knowledge that most strings lose a large percentage of tension with time and play. The amount is highly material specific, with gut, nylon, and polyester losing the least tension in that order. Do variations in temperature exposure affect tension loss in these materials differently? Does it matter when the string is exposed to a temperature extreme — before, during, or after tensioning? How do variances in tension loss due to temperature changes affect the most important property of string — stiffness?

Peak impact of tennis ball on strings viewed through plexiglass window.
Peak Impact

Tennis players have long held that rough-surfaced strings will create more spin. The belief is that the roughened string will more vigorously grab the backside of the ball as the racquet sweeps up and drags the ball around into topspin. Is this true? How much extra spin can a player expect?

It has also been thought that lateral string movement produces extra spin during the "snap-back" phase of the impact. This study shows that "snap-back" is only half the story and is a partial misrepresentation of actual events.

The physics of tennis ball spin is revealed as impacts with a simplified stringbed of four mains (with and without crosses) are viewed through a see-through plexiglass frame.

Impact on a single string as viewed through plexiglass window.
Single String Impact

The interaction between a tennis ball and the strings of a tennis racquet is a relatively complicated process since adjacent strings tend to act independently. All of the strings tend to move in unison in a direction perpendicular to the string plane, but motion in a direction parallel to the string plane can vary from one string to the next. Some strings move tangentially more than others, or in opposite directions, depending on whether the ball lands directly on one or more of the strings or whether it lands between two strings. In order to simplify the physics of the problem, a study was made of the interaction between a tennis ball and a single string.

Digitized swing of light racuet with 200g added at 60 cm.
Experiment setup

What effect does the addition and redistribution of mass have on racquet speed, control, feel and ball speed? Players have many varied and conflicting opinions as to where and why lead tape should be added to the racquet. This in-depth study answers all your questions.


The relationships between racquet weight, balance (center of mass — CM), swingweight (moment of inertia — I), swing speed and shot speed are often topics of much confusion among tennis players. Questions arise such as, "How can a heavier racquet swing easier than a light racquet?"; "How can a head-heavy racquet have a lower swingweight than a headlight racquet?"; "Does making a racquet more headlight increase its racquet tip velocity?"; Is it true that adding weight at the axis of rotation has no affect on the swingweight and thus the racquet speed?"; "Does more mass create more plow-through?"; "Does a heavier racquet create more ball speed?"; "Is there a trade-off between racquet mass, racquet speed, and ball speed?".

String movement
String movement on spaghetti strings

Spin is an essential part of today's game of tennis. Racquet and string advertising proclaim the spin benefits of particular products and players agonize over which racquet-string setup will generate the most spin. But how do you choose? What factors really matter to spinning a tennis ball? Why do they matter? How do you maximize the effect of those causal factors?

Ball impact with an 8 mains 0 crosses pattern on a string pattern rig.
Eight mains, zero crosses

The tennis world has gone topspin crazy and players want equipment that maximizes spin. Over the years there have been many theories as to what equipment configurations will increase spin. The string pattern has often been hailed as the principle spin motor. Is it? What patterns produce the most spin?

Components of spin.

One of the mysteries concerning the kick serve is how the server manages to generate topspin in the first place. If the racquet needs to rise at 30 degrees to hit a good topspin forehand, how can anyone serve a ball with a significant amount of topspin when the racquet head is rising at only a few degrees?

Kettle bell drop on strings to test performance degradation.
Kettle bell drop test

Tennis players often complain that a tennis string, especially polyester string, "goes dead." This phenomenon has various and often contradictory descriptions. Some players experience too much stringbed action and others feel complete loss of stringbed action. And these contrary opinions can be about the very same string. How can two completely opposite descriptions of a string going dead be correct? Well, as this research will show, it seems that they can.

Stretch time and return time in a 34.2 ms impact with a steel hammer.
Deflection time ratio vs tension

Part 1 dealt with several ways that strings "go dead. This experiment addresses another factor not addressed there — loss of elasticity due to an internal fatigue and breakdown of the string. This is often the explanation of players who experience string death as a loss of power. But is it true? Do strings go dead because of loss of elasticity due to an internal fatigue and breakdown of the string? What does science say?

String movement during impact.
String movement during impact

The optimization rules for spin appear to be "Goldilocks Principles": the stringbed must be not too soft and not too stiff, not too closed and not too open, and not too rough and not too smooth. You can have too much or too little of each.


Until recently, the prevailing theory regarding string stiffness and spin has been that the firmness and lateral rigidity of a stiffer string will create more ball-to-string friction due to more squashing and embedding, resulting in more spin. It is true that friction accounts for most of the spin produced by the stringbed, but it does not account for most of the difference in spin between stringbeds. Recent research suggests that lateral main string movement is the most important factor determining this difference. Consequently, the explanation of how string stiffness might affect the spin differential has changed. The question now becomes, "How does string stiffness affect sideways string movement and its associated storage and return of rotational energy to the ball?"

Graph of lubricated vs non-lubricated strings.
Spin with lubed and non-lubed string

Lateral movement and snap-back of the main strings has been shown by TWU to increase spin? Lateral movement is facilitated by using strings with a low string-to-string stickiness. These strings are usually stiff, hard polyester strings with low friction, minimal notching, and no fraying. But what would happen if we added lubrication to any string whether polyester, gut, or nylon, used or new? Would this increase the relative string movement of each string and, with it, the spin of the ball? To find out, we added WD-40 to all strings.

Spinning ball in flight.
Spinning ball in flight

Aerodynamic forces are responsible for both the angular, diving, hopping, topspin shot and the floating, skidding, backspin slice. What makes the ball appear to suddenly change course and dive into the court or to seemingly defy gravity and float through the air?


Tennis is a game of spin, and much attention is devoted to the mechanics and equipment for producing it. Much less attention has been given to what makes spin work for the player — aerodynamics. Other than gravity, it is the flow of air around the ball that determines its trajectory when hit at any given velocity, spin, and angle. Forces created by air flow act to decrease velocity in the direction of motion (primarily the horizontal direction in tennis) and, for spinning balls, act to increase or decrease its vertical or side-to-side speed.

Experiment setup.
Experiment setup

Tennis players have claimed for many years that some tennis strings generate more spin than others. Court-side mythology had it that stiff strings that flattened and embedded the ball as well as grabbed it with high friction surfaces increased spin. This experiment was one of the first to discover that material matters, but not for the reasons most players think.

Spin due to normal force depending on impact location.
Forces by location

Can players maximize spin production by customizing twistweight depending on where on the stringbed they usually make contact? Experiment explores the surprising result that impact location matters and twistweight is the key to how much it matters.

Friction device used in experiment.
Friction sled

Main strings that move sideways and snap-back can create more spin. To achieve maximum spin, you must get the string moving, keep it moving both to and fro, and do so quickly with as little loss of energy as possible. Minimizing friction between strings is essential to this process.

Friction COF of hybrids.
Hybrid friction graph

Strings vary dramatically in string-to-string friction, and hybrid combinations accentuate that difference for better or worse in unexpected and counter-intuitive ways.

String snap-back model of spin production.
Snap-back model

Videos of various stringbed setups show that string movement does indeed occur and that the strings snap back for all setups and they do so, for the most part, in less than 1/1000 second.

Double pendulum in action during a serve
Double pendulum action

The complex interaction between the forearm and the racquet is nicely described in terms of the physics of a double pendulum. Players don't worry about that since they know from experience what happens to the racquet when they swing their arm or use their wrist to position the racquet. However, they may not be aware of some of the fine details.