On the ball - the materials science behind ball development

Materials World magazine
,
4 Jun 2011
Computational fluid dynamics output showing velocity vectors

Materials science is making a big contribution to the field of ball development in rugby and other major sports, reports Guy Richards.

When the winners of the 2011 Rugby World Cup (RWC) hold aloft the William Webb Ellis Cup at Eden Park in Auckland, New Zealand, on 23 October, it will mark a culmination of another kind for the firm Gilbert Rugby – its Virtuo ball will be used for the tournament.

Gilbert’s association with the RWC dates back to 1995 and extends, for now at least, to the 2015 tournament. But the company, now based in Robertsbridge, UK, first started making rugby balls in 1842, when founder William Gilbert teamed up with fellow boot-maker Richard Lindon. They both ran shops close to the game’s birthplace at Rugby School in Warwickshire, UK.

At first the balls were made from pig’s bladder inside a leather casing. The pig’s bladder was replaced in 1870 by vulcanised rubber, altering the ball’s size and shape – which were hitherto variable according to the particular pig – to a more uniform egg profile, a change enforced in 1892 by the sport’s governing body, the Rugby Football Union.

That is essentially how things remained until the 1980s, when leather-covered balls were replaced with ones using materials such as synthetic rubber to overcome leather’s susceptibility to waterlogging.

Right: Computational fluid dynamics visuals illustrating how the dimensions of a ball are gained through (from top to bottom) laser scanning, Polygon mesh, nurb patches, and final surface

Getting to grips

These days, rugby ball development is about achieving consistent performance in terms of handling, kicking and grip, says Gilbert Rugby. Its choice of materials are confidential, but the firm does say the Virtuo ball has a rubber exterior (for grip, enhanced by a pimple pattern), a cotton/polyester backing (for a good ratio of strength to energy transfer), and a copolymer bladder to give the resilience of latex but the air retention of butyl.

Gilbert Rugby’s Chief Ball Engineer Ian Savage says, ‘In general, the best grip comes from using a high percentage of natural rubber, but we use a higher proportion of synthetic rubber in some balls to give greater durability. So, for example, the Virtuo ball has a high percentage of natural rubber, while entry-level balls used by schools will have more synthetic rubber.’

The ratio of strength to energy transfer is a comparative one, he says. For an entry-level ball, a three-ply material – two-ply cotton with one-ply polyester – is used. Going up the product range, better-grade materials are required with different weaves that allow more energy in and out of the ball.

In a spin

Development of new prototypes at Gilbert is split into three areas – theoretical, laboratory tests on physical prototypes and player feedback. All physical testing is done in-house but Gilbert takes input on the theoretical side from the Sports Engineering Research Group (SERG) at the University of Sheffield, UK.

Explaining the underlying theory of ball design, SERG Director Dr Matt Carré says, ‘for handling and kicking, the key physics are the tribological mechanisms and the energy return during impact. These affect how the ball can be caught, how much spin can be applied in a pass or kick, and, ultimately, how fast the ball can be launched in a kick. For flight through the air, the key physics at play are lift and drag, which depend very much on how the ball is spinning – in a spiral fashion, say, or end over end – and how fast it is travelling.’

The ball, therefore, has to satisfy a number of requirements, Carré explains. So, if the pimple pattern is altered to improve handling, for example, the aerodynamics might also change. With so many factors at play, it is difficult to model a ball’s aerodynamic performance exactly, although different virtual designs can be compared to predict which will travel further and possibly behave more consistently.

‘However, we do not necessarily want a ball that goes as far, or as high, as possible,’ notes Carré. ‘Players want as much control over the ball as possible, and this is related to the contact time between ball and foot. The longer the contact time, the more the ball surface will deflect, so there will be more surface contact and, therefore, more control.

‘But there is a balance. The effect of control is also related to the ball’s stiffness. If it is too stiff, it can hurt the player’s foot, so there is a comfort issue as well. It is therefore a combination of objective data and subjective feedback.’

Top models

Right, top: Finite Element Analysis
computer model of a rugby ball. Bottom: Finite Element Analysis Output
of a rugby ball

The Sports Engineering Research Group at Sheffield uses finite element analysis to model the ‘solid’ aspects of how the ball deforms and returns to its original shape, and computational fluid dynamics for the aerodynamics.

Over the years it has worked with Gilbert, SERG has built up a series of such models for various designs, which it validates with experimental data. For instance, high-speed video footage of players manipulating a ball is used to compare the predicted flight distances for kicked balls (based on the calculated drag and lift parameters) with actual flight distances measured on the field. This data also provides a basis for analysing the different design parameters – pimple patterns, backing materials and so on – of future balls.

For initial physical testing, Gilbert carries out impact studies by loading the ball statically and dynamically – for example, by letting a weight fall on it under gravity and looking at how the ball deforms. It also conducts a mechanical kick-test using a pneumatic rig, and looks at the effect of changing the pressure inside the ball. Savage will not elaborate on the testing facilities, except to say ‘no one else uses this sort of set up’.

Player feedback comes via Northampton Saints back-line coach and former England fly-half Paul Grayson. As Savage explains, ‘He is impartial, independent and able to kick in various styles – he can also test one prototype versus another, or a current ball versus a prototype. His feedback then goes back into the development process to reach a prototype on which we are happy to get further feedback from other players’.

Feedback, he says, is a subjective process, but making direct comparisons between balls helps to normalise it.

Raising the game

So what impact has ball development had on the game? ‘It is hard to know,’ muses Savage. ‘Goal-kicking averages have increased in this time, but that could be down to many external factors – weather, time of year, players and so on – as well as the ball.’

Gilbert’s development programmes are on a fouryear rolling cycle, to peak a couple of years before each RWC, so it is already well on the way to producing a new ball for 2015. Unsurprisingly, Savage cannot give details of this new ball, except that it is at the fine-tuning stage now, with Gilbert looking at areas such as the backing materials and outer surface compounds.

For the future, he says, we could see some radical new developments but with the drive always towards balancing aerodynamics with handling grip, and producing a ball that represents a constant in the equation of a game. And, as Carré says, ‘there is still a huge amount to explore – an almost limitless range of pimple sizes and shapes, and ball materials, even considering the various limits on these factors.’

Clearly, we have not even reached half-time in the rugby ball’s development. 


Core improvement - golf balls


For quite some time, golf ball development has primarlily been focused on the number of layers with a solid rubber core. However, a four-year collaborative research project between Nike Golf and materials experts at DuPont has resulted in a ball that is said to achieve more distance and accuracy in playing.

The core of the Nike 20XI ball is made from a new resin material, rather than rubber, and is said to be faster and lighter. Tests at Nike Golf have shown high levels of moments of inertia (MOI), which assists in reducing driver spin and maintains spin beyond apex to preserve carry and control. After apex, the higher MOI helps keep the spin at an average of 100-200 rotations per minute, the highest of any ball.

The company claims that combined with the softer cover materials, the steeper spin slope gives more distance from the Tee and greater control around the greens. ‘For many years, golf ball development has primarily been focused on the number of layers with a solid rubber core. We believe that there was not really anywhere else to go as far as technology advancement in these areas, and felt that the next window of opportunity was in the exploration of various materials for the core,’ says Rock Ishii, Product Development Director for Nike Golf.  


On the street - new balls, please


For those tired of the traditional circular shape of recreational balls, a new ball that can be manipulated to bounce unpredictably has been developed by toy manufacturer Waboba, Greenacres, USA (right).

Waboba STREET – has a specially fabricated ‘dub’ or notched surface that allows players to vary the ball’s spin, trajectory and bounce through throwing technique. 

 

Match of the day - the history of balls


Evidence of people playing some form of football stretches back to pre-Christian times, with the ‘ball’ ranging from a pig’s bladder to a human skull. It was not until 1855 that the inventor of vulcanised rubber, Charles Goodyear, produced the first rubber football.

In 1863, the English Football Association was formed, and set the rules governing the ball. Since then, the ball has changed very little. In 1872, its size was fixed – it must be spherical, with a circumference of 27-28in (68.5-69.5cm) – and this rule is still in force with the world governing body, the Fédération Internationale de Football Association (FIFA). Then, in 1937, the weight requirement was changed slightly, from 13-15oz to 14-16oz. Nowadays, FIFA specifies 420-445g (14.5-15.5oz).

What have changed markedly though, especially over the past 30 years, are the materials used in the ball’s construction. Stitched leather balls were eventually replaced in the 1980s with synthetic ones, but the rubber bladder is still used.

Adidas has been the matchball supplier for all official FIFA and Union of European Football Associations (UEFA) games since the early 1970s, and its Jabulani ball was chosen for the 2010 World Cup in South Africa. Adidas declines to give many details about the ball’s material composition but it does say that, internally, it has a latex bladder, an inner carcass for shape stability and a counterweight to the inflating valve for balance. Externally, it is a combination of a polyurethane-based foam, designed to have very little temperature sensitivity, and a one millimetre polyurethane coat. The exterior is thermally bonded to make it virtually waterproof.

Developing a new ball takes about three-and-a-half years. Adidas will not specify which companies it works with but in academia it cites UK universities in Loughborough and Sheffield as collaborators.

All laboratory testing to make sure its balls conform to FIFA rules is done in-house but Adidas also trials new balls with players. Laboratory tests include those for water absorption (pressing and rotating the ball in water 250 times, and measuring its weight before and after), and shape and size retention (firing the ball against a steel plate 2,000 times at 50kph, then checking for air pressure and roundness).

As a result of this development, an Adidas spokesman says, ‘the ball has become more responsive over the years, with the effect that longer passes can be played with more control, and games are less influenced by heavy rain or the cold, as the ball does not take up water any more’. 

Game, set and match - tennis balls


The materials in, and manufacture of, tennis balls have changed little in the 140- year history of lawn tennis. Originally made solely from an unpressurised rubber core, they were subsequently covered with stitched flannel, to improve their wear and playing properties. This has since been replaced by a wool/cotton and synthetic textile blend – either melton, which has a high wool content, or cheaper needle cloth, which has a higher synthetic content.

Pressurised balls were introduced in the 1920s and, in 1972, the now-familiar yellow balls were introduced by the International Tennis Federation (ITF), the sport’s governing body, because they are easier to see on television. Before then, balls were either black or white.

The ITF is responsible for testing and approving all balls for the sport, and there are about 200 brands on its official list, most of which are pressurised and all are given one of three type ratings according to their performance characteristics. Type 2, for example, is the standard ‘medium’ ball intended for play on most acrylic-coated surfaces.

Pressurised balls, whose hardness and resilience result from the modulus of the rubber in the core, as well as the core’s internal pressure, are more elastic than pressureless balls, which derive their hardness and resilience solely from the core. Pressureless balls, with their slightly harder rubber, therefore have more bounce, although the properties of pressurised balls are affected by factors such as ambient temperature and atmospheric pressure. They also have a shorter shelf life, as the internal gas (usually hydrogen) diffuses through the core over time.

So testing of pressurised balls has to be carried out under controlled conditions – at or near sea level ideally, for example, and at 23ºC ± 2º. All approved balls must conform to ranges of mass (56-59.4g), diameter (6.54cm ±0.012cm for Type 2), rebound (134.6-147.3cm for Type 2), and forward and return deformation (0.673-0.736cm).

The rebound test involves dropping a ball vertically from a height of 2.5m and measuring the rebound. The ITF’s equipment for this consists of a vacuum pipe that holds the ball at the correct height before being released, a granite block onto which the ball bounces, and a video camera and light source. Each rebound is recorded and the rebound height measured using real-time bespoke software.

Testing ball deformation was originally carried out on the Stevens Machine, a manual, mechanical device that applies an 18lb (8.16kg) force to a ball placed between two plates, measures the ‘forward’ deformation, continues compression until the ball’s diameter has been reduced by one inch, removes the compression until the load is again 18lb, and then measures the resulting deformation, (the ‘return’). Now computerised, it can test more than 5,000 balls a year.

Ball laboratory with a deformation test device (left), and a spectrophotometer for measuring cloth colour (no longer used) and materials test stand on the right. In the centre is a mass test (an electronic balance) on the left and 'go/no-go' ring gauges in a stand to measure the ball diameter.

Mecmesin image depicts the deformation test carried out by a dedicated materials test stand (right). The carousel on the left holds 12 balls, manipulated by a pneumatic handling system 

Further information

www.gilbertrugby.com
www.serg.group.shef.ac.uk
www.itftennis.com
www.adidas.com