What is the Effect of Heavier, Larger, or Softer Tennis Balls?

To slow down the men’s game, and thus hopefully to increase its entertainment value for the unsophisticated, the rulers of tennis want to change the balls.

Particularly troubling is the lack of prior consultation with the pros who will be using these balls.  Which of them are in favor?  True, it is rare to find any touring pros who might be called formally educated, so naturally they must expect their opinions to be ignored and their objections overruled.  If they don’t want to play, there are plenty of ambitious youngsters eager to take their place.  However, even though there might be no reason for courtesy or compassion, wouldn’t it be economically prudent to prolong the careers of the marquee players, and not to increase the already alarming rate of injuries among the recreational players?

The ITF has authorized a ball with a 15% greater diameter to be used “on an experimental basis.”  The intention is that the bigger ball will meet more air resistance, therefore play will be slower.  Fluffing the nap (felt covering of the ball) will increase diameter and drag, but apparently the intention of the ITF is to require ball manufacturers to mold a larger rubber core.

The larger diameter of the rubber core, even if the weight of rubber remains the same, will result in a higher rotational inertia for the ball.  That means a “heavy” ball because players will be able to impart a lot of angular momentum (spin).  Angular momentum is the product of the rotational inertia and the rotation speed, and the higher rotational inertia permits a much “heavier” ball at the same spin rate.  High angular momentum of the ball on impact will aggravate Torsion (screwdriver twist on the handle), causing more stress on the arm of the receiver.

Another problem with bigger balls: they will radically change the game in the same way that the “spaghetti string” racquet did, by giving junkballers an edge.  The ITF banned (retroactively) the spaghetti strings which imparted such extreme spin.  The same reasoning should ban these balls.

Yet another problem with bigger balls: if the same ball weight (57 grams) is to be maintained, the rubber of the bigger ball must be made thinner to stretch over the larger surface.  Thinner rubber means that the air will leak out easier, and higher air pressure will be needed to maintain the same ball bounce.   These balls will go flat faster.  They will also be less bouncy in actual pro-level play because of higher hysteresis loss from more air being compressed.   These will be soft balls.

Presently, for professional tournament play, a ball must bounce more than 53 inches and less than 58 inches when released from a height of 100 in.  That means that the coefficient of restitution for the ball itself (apart from the racquet) is between 0.73 and 0.76.  It should be noted that the 100 in. drop height does not approximate the speed of a pro serve, so for testing the hysteresis loss from the bigger ball this test would be inadequate.  Using softer balls, having a bounce at the low end of this range (low c), means higher Shock, Shoulder Pull, Work, Shoulder Crunch, Wrist Crunch, and Elbow Crunch for the players.

As you can see from the formulas, heavier balls (high b) means both higher resultant forces from impact (Torque and Impulse Reaction), and higher Shock, Shoulder Pull, Work, Shoulder Crunch, and Elbow Crunch.  With heavy balls, the game becomes more painful and less accurate.   See the formulas.

Club players can take a lesson here, especially those who play on clay, where the balls get heavier as play goes on. Change balls often to protect your arm.  Tennis balls are a bargain, so leave them on the court.

Leave a comment

Your email address will not be published. Required fields are marked *