We all know that as we play, pickleballs start to wear, become soft, lose their ability to bounce, and eventually crack. What causes a worn pickleball to bounce less? How often should the balls be replaced? Let’s look at the science for answers to these questions.
Why Do Worn Balls Bounce Less?
The straight-forward answer to this question might simply be that the balls become softer as they wear, making them bounce less. Is that really true?
Let’s assume that a pickleball can be modeled as a mass on a spring (Figure 1). A force (F) is applied to the pickleball mass (m) causing compression of the spring k1 by a distance x1 below its equilibrium position. When the force (F) is released, the pickleball rebounds to a distance x1 above its equilibrium position (in the absence of damping). The total bounce height is therefore 2x1.
If the spring is replaced with a stiffer spring k2 (Figure 2), the force (F) can compress the spring by a smaller distance x2. On release, the pickleball will rebound to the smaller distance x2 above the equilibrium position, making the total bounce height 2x2. Comparison of the bounce heights in Figure 1 verses Figure 2 clearly indicates that bounce height 2x1 is greater than bounce height 2x2.
This illustration suggests that a softer (used) pickleball will bounce higher than a stiffer (new) pickleball. But wait a second! This goes against our common knowledge that a softer (used) pickleballs will bounce lower than stiffer (new) pickleballs! What gives?
The answer lies in the fact that pickleball stiffness alone may not be the primary factor that governs pickleball bounce height. We need to look at the field of molecular physics to discover why.
Pickleball and Molecular Physics
Pickleballs are plastic, which are man-made materials comprised of long chains of repeating molecules called polymers. The polymer chains contain mostly carbon and hydrogen, with other molecules intermixed within the chains, such as oxygen, nitrogen, and sulfur. The exact chemical make-up of the polymer chains and the arrangement of the molecules depends on the type of plastic. The chains are not independent and are often entangled and interconnected with one another through a process known as “cross-linking”.
The elasticity of the polymer chains and their cross-links define the stiffness of the plastic. Soft plastics, like you would find in plastic wraps or elastomeric seals, have highly elastic chains and cross-links that are capable of a significant amount of strain before breakage. Stiff plastics, like you would find in polyvinyl chloride (PVC) pipes or Plexiglas windows, have very stiff polymer chains and cross-links that can undergo only a small amount of strain (although at a much higher force level) before they fail, usually in a brittle fashion.
As we discussed in a previous article, “Power vs Control Paddles”, when a pickleball is dropped from a height of 78” and allowed to bounce on a smooth hard surface, 57% of its kinetic energy is lost, and the ball will rebound back to a height of only 33”. Where does the lost energy go? Some of the energy goes into deforming the ball, which is partially recovered when the ball springs back to its original shape. Some of the energy is lost in external friction between the ball and the hard surface, but these losses are likely small, since there is very little sliding friction between the ball and the smooth surface. Where does the remaining energy go?
There are two mechanisms by which energy is lost when a pickleball impacts a paddle or the ground. First, the impact force causes elastic deformation of the ball, which causes the polymer chains to elongate and compress. The relative motion among the polymer chains causes internal friction that is lost in the form of heat to the environment. Second, if the deformation is large enough it can cause breakage of the polymer chains and their cross-links, which is an irreversible process – that is, the energy expended to break the chemical bonds cannot be recovered.
Returning to our mass-spring analogy in Figures 1 and 2, the missing element is damping. In an automobile suspension, the vehicle mass would continue to bounce up and down on the springs if it did not have shock absorbers. The shock absorbers add damping, which dissipates energy that would otherwise go into bouncing of the vehicle mass. In the same manner, the internal friction and breakage of the polymer chains in the plastic that makes up the pickleball effectively reduces the bounce height of the pickleball.
As you continue to play with the ball, the damage becomes cumulative. As the polymer chains and the cross-links break, the impact loads become more concentrated in the remaining polymer chains. This causes greater deformation and relative motion among the polymer chains, resulting in more energy loss and greater damping which reduces the bounce height of the ball. The concentration of impact loads on fewer polymer chains also increases the stress in these chains, resulting in accelerated breakage.
This process continues until the ball eventually becomes too soft or cracks. The amount of softening and cumulative damage to a pickleball depends on how hard you play. More casual players would expect their pickleballs to last much longer than intermediate or advanced players. How much does the pickleball stiffness change with usage? To answer this question, we need to run some tests.the
Pickleball Stiffness Reduction through Usage
In South Florida, it is estimated that 75% of the pickleballs used by intermediate to advanced players is the Franklin X-40. Rounding out the remaining 25% are the Onix Dura Fast-40, and the Selkirk SLK Competition Ball, which are all pretty good balls. The popularity of the Franklin X-40 is not entirely obvious because the size, weight, and bounce of pickleballs is regulated by the USAPA. However, it is likely that South Florida pickleball players are used to the feel and sound of the Franklin X-40 when it contacts their paddles. The relatively low price of the ball (about $2.00/ball) may allow them to change it out more frequently than higher priced balls.
A simple test was conducted in which Franklin X-40 pickleballs were used in highly competitive games with 3.5 level players (“bangers”) to determine the stiffness loss after 1, 2, and 3 hours of play. Each hour of play represented about four games. After each hour, the stiffness of the balls was tested using a manual force gauge with a digital readout shown in Figure 3.
In these tests, the balls were compressed up to a force of 30 lbs in increments of 1 lb. An aluminum plate was used on top of the pickleball to spread out the load over a flat surface. Although pickleballs are compressed against only one surface when they are hit with paddles, it was felt that this test would be sufficient to provide a “first-order” estimate of the pickleball stiffness for purposes of comparison.
Figure 4 shows the results of the testing in the form of compliance (in/lb), which is the inverse of stiffness (lb/in). The compliance was used because the displacement readout on the force gauge (Figure 3) was much more stable than the force readout. Each curve was linearized to obtain its slope, and the inverse of the slope was equal to the stiffness (lb/in). As indicated, a new ball provides a stiffness of 93.5 lb/in, and the stiffness is reduced to 84.0 and 75.2 lbs/in, respectively, after 1 and 2 hours of play. This shows that we should expect a reduction of ball stiffness of about 10% for each hour of play!
After three hours (or 12 games), the results were not repeatable because the measured stiffness of the ball changed according to its orientation on the test stand. It was also discovered that under a constant deformation level, the force would relax, meaning that the ball was beginning to take on a “permanent set” and not springing back quickly after deformation. Further examination found that the ball had soft spots where it could be indented by finger pressure (Figure 5).
These results suggest that the usable lifetime of the Franklin X-40 ball would be no more than 4-8 games for “tournament” play and up to 12 games for “competitive” play. After 12 “competitive” games, the Franklin X-40 ball still had a lot of useful life and was subsequently used for at least a dozen more “recreational” or “social” games before it cracked.
These results for the Franklin X-40 ball should not be misconstrued to imply that the ball is “bad”, “defective”, or “inferior”, as all pickleballs will wear out, develop soft spots, and crack after extensive use. Pickleball players should therefore expect changes in ball performance during use and should adapt their games accordingly.
Therefore, if you use Franklin X-40* balls, you should monitor their usage, inspect them for soft spots, and replace them early and often, especially if have played more than 12 games with them. This will help you make more consistent shots with the maximum amount of “pop”.
*Some of the embedded links in our website are affiliate links, meaning that at no cost to you, Pickleball Science will earn an affiliate commission if you click through the link and finalize a purchase. Purchase of merchandise through these affiliate links will help support the website so that we can continue to provide meaningful content to our readers.
What's the Bottom Line?
In general, pickleballs will have certain advantages and disadvantages when they are new and worn:
- One advantage of a new ball is that it will have more “pop” than a worn ball. Players should expect that a new ball will bounce off the paddle faster because it will be stiffer and will spring back to its original shape more quickly than a worn ball.
- One disadvantage of a new ball is that the increased “pop” will also reduce the contact time (see our article, “Paddle Weight and Momentum”). Coupled with the fact that the surface of a new ball will be smooth, the reduced contact time will make it more difficult for players to impart spin to the ball (see our article, “How is Topspin Generated?”).
- A worn ball must be hit harder to achieve the same velocity as a new ball because additional force must be expended to overcome the lower stiffness and higher damping (structural losses) within the plastic. The greater effort can contribute to player fatigue and/or injury.
- A worn ball may have certain advantages because the softer ball will increase the contact time with the paddle. The increased contact time coupled with the rougher (stickier) surface of a worn ball will allow players to put more spin on the ball.
- Surface roughness is a two-edged sword. Although the rougher surface of a worn ball increases the aerodynamic drag and makes the ball travel slower, the increased aerodynamic drag will also make spin more effective.
- One significant disadvantage of a worn ball is that it can cause inconsistent play. That is, if you contact the ball at a “hard spot”, it may make you over-hit the ball out of bounds. Similarly, if you contact the ball at a “soft spot”, it may make you under-hit the ball into the net.
If you are having a “bad day” (if there is such a thing) at the pickleball court where your serves lack “pop”, or if you frequently volley the ball into the net, or if your drop shots float over the net only to be slammed back, you should check your ball to make sure that it isn’t overly-worn or too soft. Speaking from experience, when you get into the habit of replacing worn balls with new balls, your games become more enjoyable and satisfying, and you will have fewer “bad days”.
Future Articles
In future articles, we will look at several different brands of pickleballs to determine their “baseline” stiffness and how the stiffness changes with usage. This will help us to identify which brands of balls have the most consistent bounce properties and to determine the usable lifetimes for each brand of ball. If there are readers (or ball manufacturers) who have a particular ball of interest, please let me know.
