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SportsTech - tennis racquets

The racquet with which Andy Murray won Wimbledon has come through quite a technological journey.

Despite the authorities of Wimbledon shunning the advances in sport's fashion by insisting that clothing must be predominantly white, there is nothing they can do about the continuing advances in racquet technology.

Head is one of the leading manufacturers of racquets, and it can count six of the current men's top ten players among its patrons – Andy Murray, Novak Djokovic, Tommy Haas, Richard Gasquet, Gilles Simon and Tomas Berdych. The company's head of R&D, Ralf Schwenger, is challenged with keeping the leading players on top.

Back when Rod Laver and Stan Smith strutted their stuff on Centre Court, wood was the material of choice. In fact, wood was the only material available until Wilson launched its metal T2000 racquet in 1967; its long-throated, small-headed frame became the craze.

The head itself remained a constant size until Howard Head, the founder of the Head organisation working for the Prince brand at the time, introduced the first oversized head in 1976. The Prince Classic featured an aluminium frame and oversized head that was 50 per cent larger than the 65in2 of the standard wooden racquet.

Although the racquet was light and benefited from a huge sweet spot and increased power, it caused problems to advanced players as their power would distort the frame causing shots to fly off course. The answer was a new stiffer material – carbon fibre bonded by a plastic resin and dubbed as graphite, although that was technically incorrect. One leading example of this was the Dunlop 200G, used by John McEnroe and Steffi Graf, weighing only 350g.

"Nowadays the weight of a racquet varies a lot," Schwenger says. "Our lightest racquet is around 230g and our heaviest is around 330g, so quite a large difference. I remember my parents playing tennis in the 1980s – my father was a strong man while my mother was a petite woman. However, their coach sold them both the same racquet because there was not the variety. Today we can offer different racquets to different players with different needs."

Power shot

As the power of players increased, the search continued for ever stiffer materials. Wilson got around this problem with its Profile racquet, increasing the thickness of its tapered head to 38mm at its widest.

"With regards materials, the main part is definitely modern carbon fibres from suppliers such as Toyota and Mitsubishi," says Schwenger. "It is not just the carbon fibres, you need the resin and the carbon fibres pre-pregnated. However, you can use the best materials but still not have a very good racquet. Overall, it is the materials but also how you use them."

The so-called pre-preg carbon fibres have been compressed so they are not isotropic, which means they do not behave in the same way in every direction. Instead the properties depend widely on the direction of the force and the angle of the fibres.

"We make tennis racquets by hand in moulds," Schwenger adds. "It is a manual process of doing it layer by layer, and is definitely time-consuming. If I look at a normal racquet I think we have 20 different layers of carbon. First they have to be cut to the right dimensions, then they have to be stacked together and rolled. There is still so much manual work involved.

"The resin gives the carbon fibre some durability," he continues. "Carbon fibre is so interesting because the properties depend so much on the angle of the fibre, and if you are talking about different angles on each of the 20 layers, ranging from 0-45 degree, there is an unbelievable number of combinations to change the playability of the racquet. Simply, this means that if you change the fibre angle of one layer you end up with a different racquet. You will still have the same centre of mass because you don't change the position of the layer but by changing the angle you change the stiffness."

Another material of interest for the R&D boffins is graphene. "This is the mono-layer of the carbon atom, so is really only two-dimensional," Schwenger explains. "For us, graphene has helped us modify the pre-preg. We have a 12-month research project with a Taiwanese research institute and during this time we tried different variations of graphene and variations of modifications."

Maintaining the sport

Racquet technology is controlled by the International Tennis Federation (ITF). "We need the ITF, although sometimes we may not like them," Schwenger says. "Their mission is to protect the nature of the game. Sometimes there might be conflict within the industry but I feel it is important to have the ITF to ensure that tennis as a sport still keeps its character."

The ITF set limitations on what our racquets can achieve. There are limitations in terms of length, the playability – for example it has to have the same properties from left to right. There are also bans on moving parts or electric motors within the racquet.

"The first challenge is to understand the language of the player because they may not use the same terminology that we use," Schwenger says. "Ideally, when talking about what they like or what they want for their game, they will stick to their own vocabulary. We would need to take a phrase such as 'I have trouble with controlling my backhand', and find out how this negative observation could be influenced; what factor of the racquet might be the reason for this, and how can it be fixed?

"Sometimes there is a problem with players, not only the professionals but also recreational players, diagnosing the problem themselves. For example, 'I have too much power in my forehand, I think it is because the racquet is too stiff'. They not only provide you with the observation, but also their interpretation of why. From experience of working with even the very good players, the interpretation is commonly wrong. They may feel the racquet is too stiff, but we can measure it and show that in actual fact it is soft in the hand."

The quest to optimise racquet performance is focused on mass distribution and materials, particularly choices provided by the direction-dependant properties of the pre-pregnated materials. Simulation software is an important tool, allowing visualisation of the fibre's positioning. The aim is to develop racquets that are durable and then targeted towards either power or control.

"We use CAD software for simulation, for example when looking at the geometry of the racquet," Schwenger adds. "When talking about the geometry we mean the formal design, the cross section of the shapes. The cross-section, or the top view, greatly influences the playability of the racquet.

"Cross-sections are interesting in sorting the mechanical properties, but at the same time the designers are interested in the cross section because they want it to look at the dynamics. This can lead to a conflict of interests, but we need to work towards the optimum result together."

Strung up

Strings are an essential part of any racquet. If you have an excellent racquet but string it badly it has bad playability, but on the other hand if you have a bad racquet that is strung well you can improve the playability.

"In the end it is the strings that make contact to the ball," Schwenger says. "The string material is important but the string tension plays a big role. If you are on the court and have a racquet that has lost string tension, then you simply cannot control the ball anymore. Sometimes recreational players can play with a string as long as it doesn't break, however the tension of the string when it strikes the ball has to act in a very elastic way."

He continues: "The string needs to have the resilience to stretch when the balls hits, but to also stretch back once it has gone. Looking at a rubber band, sooner or later it will be fatigued and while it may not break it will no longer be elastic, which is what happens with strings.

"The string pattern is also important. Neither materials, tension nor pattern are more important than the other. It depends on the needs of the player. We currently have Novak Djokovic playing with an 18/20 string pattern, of 18 main strings and 20 cross strings, and he produces massive spin."

Originally strings were gut or nylon, but much like the frames that has moved on considerably. Natural gut string has low tension loss and is very elastic and resilient. However, nowadays more and more players are using polymer strings, such as polyester. "It is a good example of how things have changed in disruptive innovation," Schwenger says.

"Looking back 20 years, all companies tried to copy natural gut strings. Then the blue strings that were un-elastic and much stiffer gained momentum, and now bring other benefits."

As in most engineering design projects it is not just individual elements that ultimately decide a product's suitability. "It is the overall system – the frame, the strings and the player," says Schwenger. "There is the physical properties of strength and height of the player, but then you have the individual technique of fast or slow swings and the strokes.

"Our challenge is to not only engineer the racquet but to look at the system overall. We need to think in terms of it being the best to fit the player in terms physical properties and style."

The future will involve both development of expertise as well as disruptive technologies. "We have to continue to research materials and looking at the whole physics of the racquet, but we also need to continue to deepen our understanding of the bio-mechanics or players and even the psychology of the consumer," Schwenger says.

Head are supposedly working on an electronic racquet. This will convert vibration and motion to electrical energy to dampen the vibration. A circuit board in the handle will amplify the electrical energy and send it back to the piezoelectric ceramic composite material in the frame, causing the material to stiffen. Whether this will pass muster with the authorities remains to be seen but one thing is for sure, the evolution of tennis racquets will continue.

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