Thursday 29 December 2016, 08:36

The physics behind Subri's swerve

When Mohd Faiz Subri’s remarkable free-kick hit the back of the net at the Bandaraya Pulau Pinang Stadium in February, the roof was raised as Penang fans were sent into delirium. But there would likely have been a fair number of supporters inside the ground who bore witness, and then asked themselves and fellow fans: ‘how on earth did he do that?’

The incredible amount of swerve that Subri achieved with his dead ball, struck from the left-hand side of the pitch with his right foot, curving out and back in before dipping past the helpless goalkeeper Mohd Nasril Nourdin in the Pahang goal, seemed to defy physics.

A FIFA-backed study has been investigating the aerodynamics behind the swerve of footballs since October 2015, with the research triggered by at-times anomalous ball movement. Can the experts explain how Subri did it?

“Judging from the curve going to the right, the ball is spinning in a clockwise direction from the player’s point of view, so he probably hits it with the outside of the foot on the left side of the ball,” Kerstin Wieczorek, Project Manager at Germany-based engineering company FluiDyna, who are conducting the study, told “Then, the spin is not only directed sideways, but vertically as well, which could make it dip more as it arrives at the goal.”

Immediately after the goal, a vast number of commentators and fans across the world declared Subri’s remarkable strike a product of the ‘knuckleball’ technique.

“For me, it’s not a knuckleball,”Johsan Billingham, of the FIFA Quality Programme’s Football Technical Development Division, told "A knuckleball occurs as a result of the ball being hit with a very low spin rate. This means that, due to the geometry of the ball and the location of seams, the separation points (where the airflow leaves the ball) change during the flight. This causes the forces acting on the ball to vary in direction which is why you often see a ‘wiggle’ in the trajectory of a knuckleball."

The idea behind the study (titled 'Development of new test methods for the evaluation of aerodynamic performance of footballs') is to create a test that assesses the aerodynamic consistency of footballs. There will always be swerve, but players should be able to practice in set conditions with the same ball, and achieve near-identical results.

One thing that the Quality Programme, and the study, will not be able to control is the temperature and myriad external factors in a game situation. The average temperature in Malaysia is approximately 25-30 degrees Celsius in mid-February, when Subri’s stunner was scored. The end result could have been different, Wieczorek says, if Malaysia’s climate was considerably cooler.

“Temperature and altitude definitely do have an impact on the swerve of a ball,” she explained. “For example, as a rough estimate, if the game was played in the same stadium, remaining at sea level but played at 5 degrees Celsius, the same kick would likely arrive up to 10cm further to the right and not go in the goal.”

Fortunately for Subri, Penang does not get that cold, and his free-kick swerved inside the post, nestling in the back of the net. For Billingham, the goal brought to mind another iconic free-kick, which seemed to swerve inexplicably: Roberto Carlos’s effort for Brazil against France almost 20 years ago.

“It’s reminiscent of the Roberto Carlos free-kick,” Billingham said, smiling as he re-watched the goal on the computer in his office. "If we draw a line between the initial position and the end position, the two goals are actually quite similar. Both are kicked in one direction before the spin causes the ball to go in the opposite direction. What you don’t have in the Roberto Carlos free-kick is the dip. This is because the spin axis in the two kicks are different. While Roberto Carlos has vertical spin axis, Subri’s is tilted forwards which causes the sharp dip at the end."

In fact, a different study published six years ago – which focused solely on Roberto Carlos’s stunning set-piece – gives insight into Subri’s as well.

“If the distance is large – like with Carlos’s kick – you see the curve increase,” Dr Christophe Clanet from the Ecole Polytechnique in Paris described to BBC News in 2010. “So you see the whole of this trajectory.”

In layman’s terms, if you completely removed gravity – and the goal, posts and net - the ball would continue on a ‘snail-shell shaped trajectory’, curving around in a circle. With the power that Roberto Carlos, and also Subri, hit the ball, it reduces the effect of gravity, which lends itself to the wicked swerve on both free-kicks.

There is more to these studies than to encourage ‘what if’ debates among the wider footballing community, fans and players. The FIFA Quality Programme commissions such scientific investigations to make sure all products, including footballs, that are signed off by the programme are of the highest possible standard.

"Essentially, the goal of this particular study is to explore whether it's possible to develop a test which can be used to evaluate the aerodynamic consistency of a ball," Billingham said. "If Cristiano Ronaldo hits the ball with the same ‘launch conditions’, we want it to go in the same place every time, with the aim being to remove inconsistent ball behaviour."

So then, in future, as a result of these studies, the next Subri could potentially practice their remarkable technique and attempt to replicate the wondergoal. For now, though, it is a standalone stunner for the ages, which will surely stand the test of time - particularly if it claims the FIFA Puskás Award on 9 January.