Is tennis spinning out of control?

Yesterday, the Sunday Times reported that some professional tennis players (allegedly) spray their strings with silicon to help them generate spin (Link to report in The Australian due to Times’ paywall). Spin plays an important role in Tennis, top-spin generates a vertical force which cause the ball to dip, this allows a player to keep hard, fast shots within the bounds of the court. The larger the amount of spin the greater this force and the more severe the ‘dipping’ effect. Some of the game’s biggest personalities are speaking out against an increase in ball spin due to the way it changes the nature of the game.

Advances in racket technology and increases in ball speed don’t favour the traditional (and arguably more exciting) player who attacks the net and plays off the volley. Large amounts of spin put this type of player at an ever greater disadvantage. This controversy has arisen before, the spaghetti strung racket used a mechanical system of strings and rollers to greatly lower the effective friction between interlocking strings (as we have written about previously).  The reduction in friction allows the strings to move past each other much more easily during a shot. On a high friction stringbed (with traditional strings and stringing methods) an impact with a ball causes the strings to slide over each but they remain in their distorted position (you could often see players putting their strings back into position in between points), this effectively absorbs some of the energy from the impact. When this friction is lowered sufficiently, the stringbed can slip back into its original position. Crucially, if the stringbed can return to its original position while the ball is still in contact, there is a catapult effect which accelerates the side of the ball in contact with the stringbed, generating spin. This mechanism effectively transfers some of the energy of impact into ball spin, the lower the friction the more energy is absorbed and the more spin is generated. (Our own Prof. Haake has published on this very subject)

The videos below illustrate the differences in stringbed movement between a normal and spaghetti type stringbed. As friction is lowered through the use of new string materials or lubrication such as silicon, more stringbed movement can be expected.

High speed video of an impact on a normal stringbed
High speed video of an impact on a spaghetti stringbed, clear movement of the stringbed is visible.

The spaghetti strung racket was outlawed after protests from players, if the latest allegations develop further the ITF may have to act to ensure the game of tennis isn’t changed for the worse. Recently polyester strings have been favoured by players because of the lower friction and higher spin. If players are using silicon to further lower friction there is a chance that very large amounts of spin are being generated, research by Professor Kawazoe showed that lubricated strings could create very large amounts of spin.

The difficulty comes in answering the question, how much spin is too much? My colleague Dr John Kelley has worked with the ITF in monitoring ball spin and reported that the maximum spin (off a forehand) he recorded at the 2007 – 2009 Wimbledon qualifying tournaments (using high speed video) was 3,772 revolutions per minute (rpm) which according to the Sunday Times article is roughly equivalent to that produced by Rafa Nadal and Roger Federer.

  Median Maximum
First Serves 2177 4345
Second Serves 3762 5080
Forehand topspin 1910 3772
Backhand topspin 1633 3867
Slice shots 2760 3896
Return shots 1423 3858

A results summary from Dr John Kelley’s work at the 2007-2009 Wimbledon Qualifying tournaments

The article in the Sunday Times goes on to claim that this has risen considerably since even the late 1990’s (Thomas Muster – 2,400 rpm). However, research can be found online which has recorded spin at the 1997 US open and gives spin values in excess of 3,000 rpm in the groundstroke and 5,000 rpm from Sampras in the second serve! These values were obtained using early high speed video cameras. If we move back to the 1970’s and earlier, all reports of ball spin are purely anecdotal and it becomes very difficult to judge how spin values have changed over time. This isn’t to say that silicon spray doesn’t have an effect and isn’t detrimental to the game. The ITF has some great technology at its disposal to monitor spin levels during play and also perform controlled experiments with different string types to gauge effects. I’m very interested to see if this develops any further and whether any actions will be necessary. What kind of actions could be possible? The difficulty comes in detecting foul play. With other rules in tennis it is quite easy to detect whether a non-regulation racket or stringing method has been used. However a coating of silicon lubricant could be difficult to detect. They could always observe spin levels during a game using technology and then launch an enquiry if levels of spin seem suspicious but this requires significant amounts of investment at every level of the game (high speed videos to observe ball trajectories during play).

To throw in a wildcard, this article has so far assumed that the current pundit’s word is gospel and that tennis could be damaged by excessive spin. Warning sirens have been sounded before with regards to serve speed and over time players adapted and the game evolved. It’s still very early to say whether the use of silicon sprays will be a problem at all.

Simon Choppin

About wiredchop

Simon Choppin Simon’s sports engineering career began at the age of six when he loosened the wheels of his skateboard in order to make it go faster. While the experiment was chalked up as his first failure, his resulting dimpled skull has provided an aerodynamic advantage in more recent sporting pursuits. Academically, Simon completed a degree in Mechanical Engineering with Mathematics at Nottingham University before joining the Sports Engineering Research Group at Sheffield to start his PhD. His main interests include work with high speed video, mathematical modelling of various sorts and experimental work involving machines with big buttons. As a sportsman, Simon has an unfortunate lack of talent for anything requiring skill, tactical awareness or the ability to learn from mistakes. He does however seem to posess the ability to move his legs around for a long time until other people get tired, for this reason you’re most likely to see him on a bike of some sort or running up a hill in offensively small shorts. Simon was fortunate enough to have a stint at the Guardian newspaper as part of the BSA’s media fellowship, which gave him the idea for this blog. Other than this, his writing experience includes his PhD thesis and various postcards to his Mum.

3 Responses

  1. alin

    Great articles, this one and all others on tennis issues, so refreshing to see someone doing some real scientific analysis and hard facts, there’s enough fluff about wonder technologies, amplified feel etc etc. Along the lines of these findings, would it make sense to assume the perceived effect of increased spin of more ‘open’ string patterns could also be a result of increased string movement (and not necessarily the number/ area of string in contact with the ball as some are suggesting). What about string gauge (thinner string – less friction?)

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