Why does the ball knuckle more this World Cup?

These two new features make the Teamgeist ball smoother than the traditional ball, which in turn affects its aerodynamic properties and hence flight.

A football will tend to knuckle when it is kicked without much spin at a critical speed. In simple terms, the flow will be very close to the ball surface around the apex region and can be affected by the seams between the panels at the critical speed. This can produce a lateral force, which makes the ball knuckle.

This is a similar effect to that obtained on a non-spinning pitched baseball.

The critical speed for a traditional football is about 20 mph. With the smoother surface, the “Teamgeist” ball will have a much higher critical speed, perhaps closer to 40 or 50 mph. In match situations, free kicks are more likely to be hit at around 40 to 50 mph than at 20 mph. Ball speeds of up to 70 mph are encountered in World Cup matches.

So what has inadvertently happened is that the probability of the ball knuckling has increased with this new, smoother ball.

Another point to note is that the magnitude of the lateral force is proportional to the square of the ball speed and so the knuckling effect will be exacerbated at the higher critical speed.

Some of the players have also claimed that the “Teamgeist” ball is lighter. If this were true, then both the knuckling effect and the swerve of a spinning ball would be increased. However, in looking at the ball measurements, it turns out that the weight of the “Teamgeist” ball (441 to 444 grams) is at the upped end of the official FIFA specifications (420 to 445 grams), so the goalkeepers need to cross that off their list of excuses.

In fact, in terms of the ball weight, Adidas claims that the “Teamgeist” ball is virtually waterproof and this will affect the aerodynamics since a ball that absorbs water and becomes heavier, will knuckle and swerve less than a dry ball.

Adidas also claims that the “Teamgeist” ball has a truer round shape and that it will remain so for longer periods. This will tend to help the rolling properties and reduce the chances of the ball knuckling due to an odd shape. So while there is something to all the complaints form the goalkeepers about the weird flight of the “Teamgesit” ball, it seems not all their complaints are justified.

After all, the beauty of football is that both sides play with the very same ball.

The author is a NASA scientist and a sports aerodynamics consultant based in California