When Pete Sampras and Lindsay Davenport step up to serve, they, and all the other professionals, will want the confidence that the rackets and strings they use have been designed and tested to sufficiently withstand the rigours of modern tennis.
The Life of a Tennis Racket
From rest to brutal 140 mph impacts, a racket is subjected to a torrent of punishment throughout a game. Millions of viewers will soon witness once more the cyclic testing during rallies, the critical impact tests from those killer returns and even the occasional drop test following an angry outburst.
Over the last 20 years, racket heads have grown larger, on frames that have become lighter. With an ever increasing drive to offer the latest advances in strong super-lightweight materials there are currently more than two thousand different models on the market. Materials testing solutions offer racket manufacturers the ability to assess quality during production, and the consumer the possibility of buying a racket with that elusive ‘playability’, by quantifying the characteristics that affect how it feels in play.
Mechanical Testing for Rackets and Strings
Measurements such as impact strength, durability, fatigue, torsion strength, flex and standard bench testing of weight and balance are recognised by leading manufacturers of tennis, badminton and squash rackets as important tests for quality assurance. Strings, whether natural gut or synthetic, monofilament or multi-filament, can be easily tested in tensile or relaxation modes, often using purpose built devices that adapt to versatile test apparatus.
Dynamic Stresses and Mechanical Testing
But despite the available testing technology, setting up a materials testing regime to faithfully replicate the dynamic stresses and distortions experienced by racket frames during a game is a real challenge. The compression of the frame, the flexibility of the head, the torsional twist of the racket in the hand upon impact - are all dynamic conditions that affect performance. It seems that players have many factors to consider when choosing a racket. The power to weight ratio, the ability to manoeuvre the racket with ease, the control of the shot, precision, power and the size of the ‘sweet spot’ on the string face all affect performance.
Materials Evolution in Racket Sports
Up until the 1970’s, virtually everyone engaged in racket sports was using wooden rackets with leather gripped handles and natural gut strings. The introduction of aluminium and steel frames paved the way for increasingly lightweight and highly durable materials. Today most racket frames are made from light-weight graphite or graphite composites that incorporate materials such as titanium, kevlar or fibreglass, giving added levels of frame flexibility, while remaining cost effective.
The Importance of Racket Weight
The manufacturers’ ability to produce rackets with a high strength-to-weight ratio means that weight is not really the issue for materials testing purposes. Of greater concern is how a racket’s weight and its distribution affect balance and control. Lighter rackets are more manoeuvrable and permit a much faster swing, but require a more violent action to generate pace. Heavier rackets, on the other hand, provide more power for less effort, but with reduced control. Frame flexibility is also an important consideration. Flexible frames have greater shock absorption properties to improve shooting precision, stiffer frames provide more power by keeping the head rigid and truer to the incoming ball but the vibrations must be absorbed elsewhere, such as in the arm.
A number of specialist test instruments are currently used to measure flexibility in terms of a ‘stiffness index’ of a racket frame. Essentially this involves applying a load to the frame and assigning a flex value following its deflection. The higher the flex number, the stiffer the frame. Such a static measurement may also be replicated in tensile or compression modes on instruments such as Lloyd’s LRX tester. With the LRX’s Nexygen software running there is the added flexibility to carry out detailed materials testing analysis on racket behaviour and performance.
Modern Racket Sizes and Shapes
Modern rackets are made in a variety of shapes and lengths, and testing grips can be adapted to accommodate this. In January 1997, the International Tennis Federation introduced manufacturing guidelines covering design dimensions. While a modern 135 square inch (340cm2) head on a 29 inch (73.5cm) long racket remains legal, it is still twice the head size of the older wooden rackets. This has allowed manufacturers to open up the world of tennis to a wider market. ‘Widebody’ type rackets provide greater power and possess a larger sweet spot than conventional rackets, making them more popular with recreational players - despite being more difficult to control because of the more pronounced ‘trampoline’ effect of the ball on the strings.
Evolution of Racket Grips
The quality control department needs to be aware that even the smaller details on a racket have a noticeable affect on performance. In the case of handle grips, leather is more or less obsolete now because of its varying qualities in relation to friction resistance. Today, synthetic fabrics are more widely used because they can be textured or patterned on the surface to further improve friction. Texture analysis of various fabrics is one test that could be used to assess how different patterns of handle grip affect performance.
The shaft, connecting the racket head to the handle, is another important feature. Flex ratings for the shaft can be determined similarly to the head. A flexible shaft will absorb shock vibrations better, a firmer shaft will deliver greater power by holding the head and hence strings correctly. Whatever their choice, players are affected by the flex of the shaft as well as that of the head, and the characteristics of that flex vary according to the materials used.
Testing of String Dampers and other Racket Components
Even low force compression testing on the rubber string dampers, or testing the flexibility of the small plastic grommets - used to absorb subtle string vibrations in the racket head - could all help manufacturers understand the mechanics of each facet of the racket and help with future design criteria. Some manufacturers have developed a range of test equipment, grip attachments and materials testing software to enable manufacturers to perform these and other tests.
Of course no amount of racket technology would be much use without proper control, technique, poise, balance and footwork on the court. Only with the right level of coaching can a player understand the subtleties of racket behaviour and maintain a consistent technique. The innovation in racket technology that continually hit the market mean that materials testing solutions need to respond accordingly. By applying numbers to players’ feelings about the ‘playability’ of a racket, materials testing will surely be able to serve up solutions to the demands of both consumers and manufacturers.