It seems that everyone has something to say about the Jabulani. As the official ball for the 2010 World Cup, it has a lot to live up to, but seems to be attracting more criticism than jubilation.
It is now a World Cup tradition that the new ball should be roundly slated before the tournament begins but the bad press for the Jabulani failed to cease once the matches got underway. This was no doubt assisted by a slew of lacklustre performances as players and coaches looked for something to blame. But with the ball being described as ‘horrible‘, ‘supernatural’ and even ‘impossible‘, might there may be a little more at work than mere sour grapes?
What might be to blame for this crisis in confidence? Like the teamgeist before it, much of the behaviour has been ascribed to the ball’s low mass, with journalists continuing to report this even after first hand experience. However, as my colleague Tom Allen points out, the weight of a ball is strictly controlled by FIFA and must be between 420 and 445 grams in order to attain the highest ‘FIFA approved’ standard. Our own tests with a sample of Jabulani balls agree with another report (440 grams) which suggest that the Jabulani is towards the upper bound of this limit, and even if not, it’s perhaps unlikely that a difference of only a few percent would be so noticeable to so many players.
Could the altitude be the culprit? We’ve discussed the effects of altitude before and Prof. Haake predicted it might be an issue as far back as last November. When England goalkeeper Robert Green gifted the USA an equaliser in Rustenburg on the 12th of June they were nearly a mile (1,500 m) above sea level. While the thin air might have been responsible for England’s less than energetic performance (see the Sports Science blog for some excellent articles regarding the effects of altitude on football physiology) the kind of erratic ball movement which players are reporting is less likely to occur as altitude increases. As air thins, its lowering density has less influence on the movement of the ball and if anything, the ball will slow down less and fly straighter the higher you go.
Might we be witnessing an illusion? Reports of the ball ‘bobbling’ or moving strangely in the air are widespread but I’ve yet to witness it convincingly (If anyone has any links to videos or specific examples then I’d be happy to amend this opinion). Quite often the ball is dipping due to top-spin or curving due to side-spin but this is exactly as one might expect. At other times the ball looks to be behaving oddly but this is simply down to significant foreshortening of the trajectory due to the camera angle. For an example of this see the undoubtedly impressive free kick by Roberto Carlos, the effect of the curve is made even more impressive by the camera angle. It is worth noting at the beginning of the video just how far away this free kick was made from the goal. Another potential illusion is that of spin, while recording some high speed video for a lecture a few weeks ago we noticed that when the Jabulani is spinning slowly, it becomes quite difficult to judge how the ball is moving although this illusion disappears when the spin of the ball increases. Tom, Heather and myself tried to recreate this effect in the video below but while we might be able to recreate the trajectory of a ball using differential equations, the ability to actually kick a ball with any skill evades us. It took quite a few tries before we recreated the desired effect but towards the end of the video below the rotation of the ball appears to gyrate slightly.
It’s worth noting that I’m speculating at this point, searching for reasons outside of the effect of a media bandwagon as to why the Jabulani might be getting such bad press and why it might not behave as a player would expect. If the sparse markings on the surface of the Jabulani make it hard for a player to judge the ball’s spin, it might curve through the air or bounce of the ground in a way that the player wasn’t expecting. This is down to a mis-judgement, not due to any problems with the physical design of the ball (although you could argue the markings on the ball are part of this physical design).
There is of course the possibility that the players and coaches are right, that the ball is behaving strangely and making the highly trained international superstars look like a Sports Engineer flailing around on the edge of a car park. If this is the case, what aspect of the ball’s design is responsible?
In 2006, Adidas released the Teamgeist ball to very similar criticisms. More specifically, players reported a strange ‘knuckleball‘ effect in which the ball seemed to move to and fro at low spin (like the baseball pitch of the same name). Sarah Barber (now working at the Swiss Federal Institute of Technology) published a paper looking at the side forces acting on the Teamgeist and other balls during very low levels of spin. She found that small rotations of the ball during flight can create fluctuating side and lift forces which effectively push the ball up and down and side to side. This is an effect that Christiano Ronaldo has since perfected using completely different balls to the Teamgeist, so it’s clearly not an effect restricted to Adidas’ recent contributions. However, the knuckleball effect might have been exaggerated by the particular smoothness of the Teamgeist. The 2006 World Cup ball did without the traditional 32 stitched leather panels and opted for a much more minimalist 14 synthetic thermally bonded panels. This design created large areas of smoothness with relatively sparse, shallow seams covering a much smaller proportion of the surface than usual. The seams of a football, like the dimples of a golf ball perform a very important function, they stir up passing air and drastically reduce drag. The lower the drag acting on a ball the less it slows down as it moves through the air, a higher drag force cause the ball to slow down more quickly.
All ball’s suffer what is termed a ‘drag crisis’, when travelling at relatively low speeds the air passing over a ball is very uniform or ‘laminar’. However, the air separates from the ball’s surface quite close to the front and creates a large low pressure area behind it which creates a large drag force. (For a more detailed explanation on the physics of football see Physics World or this excellent site describing the aerodynamics of a golf ball)
As the ball increases in velocity the airflow begins to become more chaotic and cling to the ball for longer, the size of the low pressure area reduces and the drag force drops. This drop in force tends to happen quite rapidly as the velocity of the ball passes a certain point.
This effect can be observed in wind tunnels and is usually charaterised by measuring the ‘coefficient of drag’ (Cd) of the object (a dimensionless measure of drag) as its velocity increase. In order to be able to compare objects of different sizes, most aerodynamicists use the dimensionless Reynolds number instead of velocity. The diagram below is an example of how the Cd of a football might typically change with an increase in Reynolds number (or velocity).
Generally, the rougher the surface of the ball the sooner the ball passes through its crisis (at a lower velocity). Traditional footballs have a relatively dense and uniform seam pattern so the air tends to becomes chaotic over the entire ball at once. If a ball has large smooth portions (like the teamgeist) then there is the risk that the air will become chaotic in one area of the ball’s surface before another. This discrepancy in flow regimes is thought to be responsible for the uneven forces that cause the ball to dip and swerve in flight, and if the ball changes its orientation slightly then the direction of the force can change rapidly. Before you know it you have an erratic ball flight and a disgruntled world cup squad.
Does the Jabulani behave in the same way? The designers at Adidas and Loughborough University have worked hard to try and create a ball with favourable aerodynamic characteristics although they have continued the trend of working towards a more perfectly round ball. These are two opposing aims with regards to football design. A rounder smoother ball will roll more predictably and feel uniform when kicked, but large areas of smoothness are troublesome when it comes to the ball in flight. The designers of the Jabulani attempted to get the best of both worlds. The design team have reduced the number of panels to only 8, each of which is thermally bonded and moulded in 3D before construction. The result is an incredibly round ball, to try and counteract the negative aerodynamic effects this might introduce, a series of aerogrooves have been added to each panel. Their purpose is to stir the air flowing over the ball and try to ensure that flow is uniform. Have they been successful at creating a super round ball with no nasty aerodynamic behaviour?
In the flesh, the Jabulani does look incredibly marble like compared to a traditional stitched ball, the seams and aerogrooves are distinctly shallower than traditional stitched seams. Although I’ve not performed any direct measurement, work has been done in the past which tries to quantify the ‘roughness’ of a sports ball. Achenbach and Haake both looked at how the size of features on a sphere’s surface (seams in the case of a football) affect how soon the ball enters the chaotic, low drag flow regime. The smoother a ball is the later the transition occurs, a smooth ball experiences higher drag at increased velocities.
Dr Takeshi Asai at the University of Tsukuba has worked extensively in characterising the flow over footballs and has published a number of papers on the subject. He recently tested the Jabulani and Teamgeist balls in a wind tunnel in order to determine how the Jabulani compares. He’s kindly given me permission to put the results on this blog, rather than post raw points, I’ve fitted polynomial ‘best fit’ lines through the data so you can clearly see how the drag acting on the ball changes with Reynolds number.
This data suggests that the Jabulani behaves much more like a smooth ball than even the notoriously smooth Teamgeist. The coefficient of drag for the Jabulani remains higher for much longer than the teamgeist. In order to visualise this, it’s more useful to have a look at how the drag force alters with the ball’s velocity. The drag force acting on the balls can be calculated using the coefficient of drag, the area of the ball, the density of the air and the square of the velocity of air flow according to the equation found here. The velocity can be obtained from Reynolds number according to the size of the football and the viscous properties of the air, according to the equation found here. Transforming the graph seen above gives us the one seen below.
What can we take from this data? (click on the graph for a full-size image) It shows that at low velocities (below 25 mph) the drag force acting on both balls is very similar. As velocity increases, the drag force on the Jabulani is much higher (due to the force being a product of velocity squared). After around 40 mph the force on the Jabulani dips rapidly such that at velocities above around 47 mph the force acting on the Jabulani is lower than that of the teamgeist. If we assume for the sake of argument that the teamgeist behaves similarly to other footballs (and Dr Asai’s previous papers show that it behaves similarly to previous Adidas incarnations) then does this explain the unexpected behaviour of the Jabulani?
At lower speed kicks (for example, passes) the drag force on the Jabulani is high, it will slow down more than expected and dip earlier than the player might anticipate. At higher speed kicks (strikes and long passes) the Jabulani will slow down less, it may travel faster than expected. In the case of longer kicks the ball may slow down considerably during its flight. The sudden increase in drag force as the ball drops below 45 mph might cause the ball to suddenly dip as force rapidly increases. It certainly might not be what a player inexperienced with the Jabulani would expect. The relatively large changes in drag force could go some way to explain why the players are taking time to get used to the ball, sudden dips or decelerations could easily be interpreted as erratic behaviour if you’re not used to it.
Does this data suggest that the Jabulani may be behaving erratically like the teamgeist? Unfortunately there’s not enough data and no tests to show that it does or doesn’t. One might speculate that higher forces at velocities between 25 and 45 mph could cause larger deviations in the flight path should the air flow become unbalanced, however, there’s no evidence to suggest this is the case, the large areas of smoothness present in the Teamgeist have been eliminated with the aerogrooves. So the Jabulani may be smoother than a normal football but it may well be uniformly smooth and it’s hard to know until someone provides scientific evidence for either case.
In addition to Dr Asai, Caltech Scientists have also analysed the aerodynamic characteristics of the Jabulani. Although there is no data to compare, they do say:
As a soccer ball slows down after it’s kicked in the air, the air flow around it changes from turbulent to smooth. It is likely that the details of this transition are different for the traditional soccer ball than for the Jabulani, which from the point of view of a soccer player translates into a ball that’s behaving unpredictably…
…Still, the Jabulani isn’t so unpredictable that players can’t learn to control it. “It seems like anytime the ball is changed, it takes a while for people to adapt,”
So the Jabulani ball isn’t supernatural, and certainly not impossible, it might just take time for players to adapt to its behaviour. Like Ronaldo with the knuckleball, adept players might even learn to exploit its unique characteristics.
I mentioned in the article that there is currently no evidence that the Jabulani is behaving like a ‘knuckleball’ and moving erratically through the air, well NASA have recently joined the debate and have a page describing the strange aerodynamics of smooth balls. I’ve gone through the page and video a couple of times but I can’t make out whether they’re referring to the current Jabulani or older Teamgeist ball a lot of the time. For this reason it’s hard to know whether the movement seen on the video refers to the most current World Cup offering. However, towards the bottom of the page there is a quote directly referring to the Jabulani:
From his research on tennis and cricket balls in wind tunnels, Mehta believes that the Jabulani ball will tend to knuckle at 45 to 50 mph, which coincides with the speed of the ball during free-kick around the goal area.
Heaven forbid I should use someone’s belief as evidence of knuckling behaviour but without knowing what NASA found out when investigating the ball itself, this is the best I’ve got. However it is interesting to note that 45 to 50 mph is in the region of aerodynamic transition and around the speed I proposed that players might consider the ball to be acting strangely. With only a few more days left of the World Cup, the argument is descending in to (further) irrelevance but it will be interesting to see how this controversy affects the nature of the ball’s released for the 2012 European Tournament and the next world Cup.