Technological advancements have played a key role in making power and spin prominent features in the game of tennis. The transition from wood to composite graphite rackets has produced larger sized rackets (in terms of both head and shafts) with frames that are thicker and lighter (Brody 1997), allowing players to hit harder than ever before. Changes in racket construction have altered the tennis serve, to the point where serves can dominate many tennis matches. In order to make match points last longer, scientists and athletes have explored ways to slow the serve and restore balance to the game. One approach has been to engineer new types of tennis balls with properties that can counteract the power and speed of the serve (Haake et al. 2000). Changes in tennis ball construction and interactions between the ball and different types of court surfaces have become primary considerations.
Types of Tennis Court Surfaces
Tennis was first played on natural grass courts. Modern day grass courts consist of a soil foundation with a seeded turf overlay (Miller 2006). While grass is still used at Wimbledon, its use has diminished due to the cost associated with high maintenance.
Clay courts gained favorability in the 1950s and consist of a base layer of crushed stone covered with a layer of rough particle material such as crushed brick (Miller 2006). This produces high amounts of friction between the ball and surface, but low amounts of friction between the player and the surface. On a clay court, the player has a tendency to slide, particularly when slowing down or attempting to change their direction of movement (Miller 2006). Lower injury rates have been associated with players that frequently use clay courts (Dragoo et al. 2010), possibly because of lower impact forces due to the sliding motion. Currently, the French Open is the only major tennis tournament played on clay.
Acrylic hard courts have rapidly gained popularity since their introduction in the 1940s and are used in two major tennis tournaments, the US Open and the Australian Open. These courts utilize either asphalt or concrete as the foundation layer, a rubber mid-layer, and a top coating made of an acrylic paint/sand mixture (Miller 2006). These courts produce the highest amount of friction between the surface and player, and have been associated with the most player injuries when compared to other surfaces (Dragoo et al. 2010).
Tennis Court Surfaces Affect the Speed of the Game
One of the most important considerations in tennis is the influence of the court surface on the ball. Aside from the force of gravity, a bouncing ball additionally experiences normal and frictional forces (Brody 2003). The normal force acts perpendicularly to the surface and the frictional (or sliding) force will act parallel to the surface (horizontally). The combination of these forces impacts the bouncing movement of the ball. The amount of friction generated between the ball and court dictates if the court is considered to be “fast” or “slow.” In particular, the amount of sliding friction that is present, dependent upon the surface type, is of interest.
A “slower” court is one where more friction is generated between the ball and the surface. Clay, with its rough surface composition, has a high coefficient of friction. When more frictional contact is produced, the horizontal speed of the ball is reduced. This reduction in forward motion creates a high vertical bounce. The longer the ball is in the air, the more time a player has to move and react, making clay a “slow” paced court.
A “faster” court produces less friction between the ball and the surface. Grass, with its firm and slippery (even more so when wet) surface composition has a low coefficient of friction making it a “fast” paced surface. With less friction, the ball will slide more easily across the surface, and it will retain more of its horizontal speed. This produces a low vertical bounce. For these reasons, points on “fast” surfaces are often much shorter, as a lower bounce means the player has less time to react and move towards the ball. In an examination of rally lengths in men’s singles tennis, 66% of rallies on clay lasted less than six seconds, but this figure increased to 88% on grass courts (Lees 2003).
Engineering of Tennis Balls
In order to engineer tennis balls with more desirable properties, understanding ball construction is important. The two main components of a tennis ball are the core and covering. The core is typically made of natural rubber that is mixed with powder fillers to produce desirable properties, such as strength and color (Manufacture). The outer surface is made of cloth material; either of a wool-based fabric (Melton) or less expensive cloth (Needle cloth) that contains more synthetic components (Manufacture). In addition, most tennis balls are pressurized, and the amount of internal pressure (ranging from 0-15 psi) will be determined based upon the ball type (Miller 2006).
Tennis balls are manufactured through a series of processes. The first of these processes is an extrusion, where the rubber is forced into a cylindrical shape through an application of pressure (Manufacture). The resultant rubber rod is then sectioned into smaller segments. Subsequent processes include forming the material into a spherical shape (by using a hydraulic press to form two individual hemispheres that are later joined), curing and pressurizing the ball, covering the ball with fabric and finally joining the core and covering together in a molding process that utilizes pressure and heat (Manufacture). The final step is to steam the ball, thus producing a more raised outer covering. Once finished, balls must pass tests related to mass, size, compression and bounce (ITF 2012).
There are three major categories (types 1-3) of tennis balls, each designed for specific use on set court types to speed up or slow down play. The weight and rebound of the tennis balls does not change across type. The standard and most utilized ball type is type 2, as it is suitable for medium paced surfaces. Type 1 balls are the same size as type 2 balls, but are harder, which is reflected by smaller amounts of forward and reverse deformation, when compared to the type 2 balls (Miller 2006). Because type 1 balls are considered to be “fast” balls, they are suggested for use on slower surfaces such as clay.
Type 3 balls differ from type 2 balls only in size. Type 3 balls are typically 6-8 percent larger than type 2 balls (Miller 2006). As demonstrated by Andrew et al. (2003), Type 3 balls are “slow” and travel through the air more slowly than their standard tennis call counterparts. Since type 3 balls are larger in size, they encounter greater drag (resistance) when traveling through the air. Thus, type 3 balls are suggested for use on faster court surfaces, such as grass, to help slow down the pace of play and reduce the dominance of the serve. Furthermore, the additional drag associated with type 3 balls also allows for a larger amount of spin to be generated (Blackwell 2007). Type 3 balls may also be beneficial for new players, as a slower pace allows for more reaction time and increased spin can aid in accuracy.
Although less prevalent, there are also balls designed for use at high altitude. By changing either the internal pressure of the ball or the elasticity of the core material, high altitude balls can be made to bounce lower than type 2 balls (Miller 2006). This is done so that at the lower air density at higher altitude the same bounce height (as that of a type 2 ball at sea level) can be achieved.
Other Tennis Ball Considerations
Despite careful efforts to engineer tennis balls and select a ball that complements court conditions, there are additional factors that impact the speed of the game. One of these is unavoidable ball wear-and-tear, which affects even the most well-engineered tennis balls.
Four distinct phases of tennis ball wear have been identified: new ball, loose fuzz, tufted fuzz, and finally the bald ball (Steele et al. 2006). The ball covering itself is porous, which creates additional instances of drag (Mheta et al. 2001) when compared to a smooth covering. As the surface material becomes worn, the “fuzz” on the ball is depleted, and both lift and drag forces are reduced (Goodwill et al. 2004). The drag coefficient for new tennis balls was found on average to be higher than 0.6. The drag coefficient was reduced to values near 0.5 for worn balls (Mehta et al. 2001). This means that a worn ball will fly faster with less force to push it down when compared to a newer ball, increasing the likelihood that the ball will be hit out of play.
The impacts of a worn ball may be reduced if a player applies spin to the ball. Spin is possible due to the Magnus effect, or a nonsymmetrical distribution of air that flows across the ball surface while it is in flight (Mehta 1985, Miller 2006). When topspin is applied, some of the air (that is flowing in the same direction as the spin of the ball) will interact with the ball surface longer, which has the effect of deflecting the wake of the ball upwards while other forces act in a downwards direction (Mheta et al. 2001, Miller, 2006). By applying topspin to the ball, a player can help a tennis ball fall to the ground and remain in-bounds.
What Players Should Know
Beginning players may be most successful on clay courts, since the speed of play is likely to be slower, and the player is better able to slide
- A type 2 ball is most common, but a beginner may consider using a type 3 because of its slower air speeds
- Old, worn balls do have a noticeable impact on play
By: Lindsay Sanford, University of Utah
Lindsay received her B.S. in Mechanical Engineering from Washington State University and is currently pursuing a PhD degree in Bioengineering. In her spare time, she likes to travel, hike, read, and play with her two year old son. She is also an avid runner and tennis player.
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