David Ferrer playing tennis

Sports technology: Tennis racquets

With Wimbledon approaching, we show you how to keep up with the professionals in the latest tennis racquet technology.

So, Wimbledon is upon us once again, and like so many others I have been inspired to dig my tennis racquet out of the dark recesses of the garden shed, where it generally stays for 11 months of the year. Its chipped and battered frame and straggly strings are a far cry from the state-of-the-art objects that Federer and co will be wielding on Centre Court.

Tennis racquets were made almost exclusively from wood until the introduction of the steel Wilson T2000 and the aluminium Prince Classic in the 1970s. Then manufacturers experimented with materials as diverse as fibreglass, carbon fibre, titanium, Kevlar, boron and graphene, as used in new professional models from Head.

That said, ProKennex has this season reintroduced wood, in part at least. Its Delta X10 280 CORE model has a wooden spine incorporated into the frame, which is encased by graphite cylinders, creating opposing shockwaves when a player strikes the ball. The wood spine is said to cancel out these shock waves and transmit a clean, responsive feel to the player's hand, which allows for harder shots, but without the vibrations that can lead to fatigue when using a carbon-fibre racquet.

Newer composites

'Traditional' carbon fibre is also seeing regular developments that help to improve racquet performance. For instance, Prince's award-winning TeXtreme Warrior 107T has been re-engineered for 2016.

Prince wanted to find a thinner, lighter and more durable material than the usual woven fibres, and opted for a new generation of carbon fibre called TeXtreme. This features 'spread tow' fabrics which are created with a thinner structure and straighter carbon fibres to make a composite material that is 20 per cent lighter than conventional carbon fibre, but with improved stiffness.

TeXtreme looks set to become the new choice of carbon-fibre composite in sporting goods. It is based on using the above mentioned 'spread tows', which are straighter than the tows used in earlier carbon-fibre composites. This makes it possible to achieve thinner and lighter laminates with less crimp, which optimises the strength of the composite and produces ultra-lightweight, high-'performance sporting equipment.

Tyler Herring, vice president of product and marketing at Prince, says that adding TeXtreme to the frame of the racquet provides superior torsional stability which results in increased power and control.

Prince has also ensured players are aware of the fact that they're using a TeXtreme racquet by making the material visible in the frame. "Prince has always prided itself on having visible technology," says CEO Mike Ballardie. "When we apply TeXtreme to the frames it is clearly visible from the shaft up to the bottom of the hoop. While these new racquets have a bold, fresh look that can't be missed, this is not smoke and mirrors. It's real, it's visible and players will feel the difference when they play."

Nearly all tennis racquets today are made from carbon composites, are 25-40 per cent lighter than their counterparts from the late 20th century, and feature noticeably bigger head sizes. This imparts huge advantages to everyone from recreational player to pro. In particular, the racquet has a bigger 'sweet spot', thus allowing players to hit the ball with more power and accuracy and get away with off-centre hits.

Most players, whether recreational or professional, stick to the tried and tested standard design, with their racquets almost invariably being made in the Far East, where carbon fibre composite mixes are cooked in moulds and then subjected to extensive testing by machines and humans (usually former or current tennis professionals).

Test techniques

The International Tennis Federation (ITF) in London has the world's most advanced tennis-specific research facility. It can test racquets, balls, court surfaces and other equipment; this enables it to determine just how far modern technology can influence the development of racquets and how it might affect the way the game is played.

The ITF has a custom-built racquet performance machine known as 'MYO' (the Greek word for muscle), which is designed to measure the maximum power a racquet can generate.

Each racquet is tested using three different racquet speeds and six different impact locations to identify the optimal power point on the head.

At impact, the ball is fired through a set of light gates, which record its incoming and outgoing velocity to produce a measure of ball speed. The maximum ball speed at all three racquet test speeds will be used to indicate the racquet's power, with the MYO being capable of producing a maximum ball speed of approximately 210km/h.

The aim is to benchmark all performance racquets from major manufacturers to produce an independent database of power values.

Professional players also have a hand in the testing procedures – for example, world number eight David Ferrer has helped play-test racquets for Prince. This involves testing a range of performance racquets varying from stiff to very flexible. These variations would be virtually undetectable by recreational players, but top pros like Ferrer can feel the differences and provide feedback on how they affect performance.

However, frame composition and design is only part of the story. The other essential factor is the way the racquet is strung. Any serious tennis player knows the importance of stringing, and in most countries other than the UK, racquets are sold unstrung to allow players to choose their own set-up.

Early racquets used natural gut, or serosa, obtained from the outer skin of sheep or cow intestines, but synthetic strings have largely replaced serosa as they're cheaper and harder wearing, although they don't have the same 'feel'.

Synthetic strings are made primarily from nylon, polyester or Kevlar (either individually or in combination), using a single solid strand or up to hundreds of small filaments. The fibres are made by an extrusion process – molten polymer is drawn out of a die with small holes, called a spinnerette. The fibre solidifies as it exits the spinnerette and is stretched to align the tangle of molecules in the polymer, greatly improving its strength.

The fibres are then twisted together, with an outer wrap of thinner fibres usually added to protect the core of the string and improve durability, although in some string constructions the core is replaced entirely by smaller filaments twisted together in an attempt to recreate spun natural gut, which is a multi-filament.

Polyester has recently become popular with top level players. Ron Yu, a respected technician with Florida-based Priority One racquet service company, has strung racquets for several of the world's top professionals. He says: "Luxilon Alu Power is a really popular brand. I'd say three out of four top players now use some Luxilon in their string set-up."

Getting the balance right

Some players prefer a mix of synthetic and natural gut strings. Andy Murray, for example, uses Luxilon for the main (vertical) strings and gut for the cross (horizontal) strings, whilst French Open men's doubles champion Edouard Roger-Vasselin uses a mix of Pro Hurricane (a high modulus co-polymer construction string) in the main and gut in the cross.

According to the ITF, "during a typical serve, the strings impact the ball with such force that both deform extensively. Yet within 5 milliseconds they both recover their original shape." However, the contact between ball and strings is not perfectly elastic and some energy is lost in the process as heat and sound, although the most energy loss arises from ball deformation. Looser strings deform the ball less, so less energy is lost and exit ball velocity is increased.

The average tension of the strings in a modern tennis racquet is 25kgf or more, with some players using tensions nearer 35kgf. Old-style wooden racquets often warped under the load of the strings, but the strings themselves are also subject to creep and lose tension over time.

Reduced string tension can have benefits though, if it's achieved in a controlled manner, as lower 'stringbed stiffness' can lead to a slight increase in speed of the ball.

The stiffness of the stringbed is also affected by the arrangement of the strings. If their spacing is increased the face becomes more flexible. Similarly, increasing the length of the strings, by enlarging the head for example, lowers the stiffness of the stringbed.

However, reducing string stiffness has the disadvantage of reducing control of the shot for a number of reasons. First, the hitting surface distorts more during impact, increasing the range of angles at which the ball may fly off. Second, the ball deforms less so the size of the contact area, which helps to direct the ball, is diminished. Finally the 'dwell time' of the ball on the strings is extended, so the racquet can rotate further during impact, which reduces the player's control (although increased dwell time means that the force is spread over a longer period, creating less shock on impact, so less player fatigue).

Top-level players will modify their racquet's string tension slightly according to altitude, heat, court surface and choice of balls, and you may notice that they often change racquets during the course of a match – usually every seven games, when new balls are introduced.

It's not just the strings that can be adjusted, however. Some players will have the racquet's handle stripped down to the graphite and then have a new grip remoulded for their specific hand shape, whilst weights can be added to improve the swing.

This may be approached in a surprisingly utilitarian manner. Ron Yu describes how some players remove the butt cap [at the base of the handle] and place heavy material inside: "I've seen guys use lead fishing weights, silicone, epoxy, even old coins."

Indeed, it's quite heartening to realise that for all the high-tech materials and skilled stringing involved in top flight tennis there are still things that can be improved with an old fishing weight or ten-pence piece.

Further information

Image credits: Corbis, Prince, ITF

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