Football is the true world sport. It is played in nearly all areas of the world and in all weather conditions. From the hot, dry desert to the rain-soaked fields around the world. From 40°C in World Cup matches, to well below 0°C in Champions League games. Although it is often acknowledged that weather can affect how the sport is played, it is often focused on the athlete. The effect of heat or cold on hydration and muscle fatigue is often a focus, but not how the same conditions affect the main focal point of every athlete, coach, official, and fan – the ball.
The football has gone through a major evolution over a short period of time (you can find a history of the ball on the FIFA website). Initially football was played with a ball constructed of animal skin filled with hair or feathers. Ball construction then transformed into a leather casing with a pig bladder inner layer. In the 1900s, the pigs’ bladders were replaced with a cloth backing to resist the stretching of the leather outer. The next major construction change came in the 1960s when synthetic materials were introduced, adding a resistance to water. The 1986 World Cup in Mexico was the first to be played with a completely synthetic ball, containing no leather.
FIFA is entrusted with ensuring that all footballs used are of a high quality and perform in a consistent way. Each ball faces an extensive series of lab tests to examine dynamic and static properties. The test that examines the balls coefficient of restitution, or COR (coefficient of restitution explained here), is performed at 5°C to 20°C and at a speed of roughly 6 m/s (see the test method explained here). This temperature range is not consistent with the wide range of temperatures that the sport is played, so little data is known on the effect of temperature and velocity on soccer balls, past and present. This quandary led me to question how a ball reacts dynamically to different temperature and speed conditions.
Balls are constructed of viscoelastic materials that are affected by temperature and velocity. It takes less energy to stretch viscoelastic materials at a higher temperature, so less energy is lost at impact. This results in a higher COR, contact time, and more deformation. I performed a test to determine the effect of temperature on the ball being used by the top leagues in world (Wiart et al. 2011). At a higher temperature, there is a higher COR, contact time, and deformation. These higher values cause a penalty kick to be 7% higher and 7% faster to the goal at 40°C compared to 0°C.
How big is 7%? In a game of inches, 7% could make the difference between a goalkeeper’s fingertips and the back of the net, the bottom of the bar and ringing of the center. It could mean the difference between the glory of a game-winning goal or a game-winning save. Is it fair if everyone is playing in the same condition? Is there a positive in practicing in a high temperature or a low temperature? Does it depend on the position?
In the big picture it can seem like a large undertaking to design a ball that performs the same in all temperatures. Even if the ball is designed for temperature ranges, would you then need to design for different altitudes, humidity, and playing surfaces? It is thought by some that adapting to these differences is just part of the game. So I will pose the question to you, is part of the game of soccer to adapt to the conditions, whether it is the team that you are playing or the equipment being used? Cleats have been developed for all playing conditions – why not balls