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Product Knowledge
March 2014

What’s in a ball outer?

Our cut-out-and-keep series to assist retailers with product knowledge.

With a variety of balls on the market for rugby, soccer and netball, your customers may select one based on what it looks like — the colours used or even the patterns on the ball. You will want to point out that they should also consider the many factors — such as materials, seals, linings, etc. — that affect how a ball performs during play.

In flight, the air close to the surface of the ball is affected by any unevenness — like seams, pimples or panels — that cause an asymmetric flow of air around the ball, dr Rabi Mehta, an aerospace engineer at NASA Ames, demonstrated by testing balls in wind tunnels. The air around the ball is ‘broken’ by the seams of the ball, which causes the ball to swerve (or ‘knuckle’) and move unpredictably.

• When a ball is kicked, the air around it forms a laminar (smooth) or turbulent (disrupted) boundary layer. During flight the pressure in the front half of the ball decreases, while the pressure in the back half will separate from the surface and equalise, resulting in a drag force that reduces the ball’s speed.

• When a ball is kicked with little force (at low speed) the boundary layer will be laminar and air layers flowing around the ball will be smooth and parallel to one another. When a ball is kicked with great force (at high speed) the boundary layer will be turbulent creating chaotic air flow that withstands the adverse pressure for longer and causes the turbulent boundary layer to separate later than with the laminar layer. This later separation creates less drag on the ball and allows it to stay in flight longer.

• Air flowing smoothly around a ball and sepa- rating early, leaves a large drag in its wake. When the air around the ball is turbulent (e.g. as a result of panels and stitching), air clings to the ball for longer, separating later and leaves less drag in its wake. ‘Tripping’ the laminar boundary layer into action (using panels) makes ball flight more predictable and longer.

• In soccer, when a ball (spherical in shape) is kicked with great force, the air around the ball is turbulent, causing little drag. o The ball spins toward the side that has the least opposing force (following its nose) — this is known as the Magnus effect — and as it continues on its flight path, its travelling speed will drop - causing the air around the ball to become laminar and its drag to increase.

o As the ball slows down, it will move in the direction with the least opposing force — or look like it is curving.

o Panels or seams on the ball’s side, may cause a curve to become even more pronounced. This is how many soccer players are able to curve a ball toward the end of its flight and ‘change’ its trajectory.

The role of panels

Panels make up the outer covering of the ball and the number of panels (sections) can have an impact on the flight of the ball, as the seams “disturb” air during flight. The design of the panels can also affect the predictability of the flight.

• If a ball has fewer panels, which results in large smooth surfaces, it will be more unpredictable in flight and will not travel as far as a ball with many seams.

o When a smooth ball is kicked, the air around the ball is ‘unbroken’ by the ball’s surface and therefore the ball will slow down quicker as a result of the pressure of the air around it.

o When a rough surfaced ball (with many panels and seams) that is able to ‘break’ the air around the ball is kicked, the ball will travel further because the pressure of the air around the ball has been ‘broken’ and allows it to travel further.

• A rougher surfaced ball becomes turbulent much faster than a smoother one and allows the ball to curve more.

o Typical soccer balls have 32 panels that each have seams, ensuring a rougher surface than a smoother ball without seams.

o The hexagon shape of panels on a soccer ball also causes turbulence to set in much faster, enabling the ball to travel further and maintain predictable flight.

The number of panels

• There are various panel designs used in soc- cer balls — 32, 18, 16 — panel constructions, etc. — but the most common design is 32-panels. This design, known as the Buckminster, features twenty hexagon (six-sided) and twelve pentagon (five-sided) pieces that cover the ball and give it a sphere shape once it is inflated. The shape of the ball allows it to roll and spin evenly and smoothly. This is also the most popular panel design for professional match balls.

• Rugby balls are constructed with four-panel designs that give them an oval shape. They are oval shaped (rather than round like soccer balls), because it is easier to catch, hold and run with the ball than it would be to do with a rounder ball. The shape of the ball also does not allow it to roll as far as a soccer ball, which is ideal for throws and passes that occur in the game.

• Netball balls, much like soccer balls, can have various panel constructions: 32, 18, etc.

Pimples and performance

Pimples (also known as dimples) can be described as little bumps or protrusions on the surface of a ball.

• These add roughness to the surface and, like panels, they can affect the flight as well as the distance the ball may travel.

• The pimples create grooves that ‘break’ the air around the ball that allows it to remain buoyant for longer and therefore travel further. A smooth surfaced ball will drop faster, because the ball will once again have ‘unbroken’ air pressure forcing it to fall quicker than a ball that has a rough surface (see above).

• Smaller pimples improve the distance a ball travels, retain less water in wet conditions, but offer less grip for the player. In dry conditions, however, small pimples will offer the grip needed by players.

• In wet conditions larger pimples offer better grip as they would protrude above any film of water that develops and still offer the necessary grip.

• Pimples ensure better grip of a ball, but may increase drag on the ball — the larger the size of pimples (in diameter), the more drag it will cause. Many rugby ball manufacturers place pimples of different sizes (sometimes grouped in various shapes) in strategic places so they do not disrupt the flight of the ball and still offer the grip needed by players.

Materials

There are a number of materials that can be used to make ball outers, but those most commonly used are synthetic leather, polyurethane (PU) and Poly Vinyl Carbonate (PVC).

• PU offers a harder feel than synthetic leath- er, but is softer and more responsive than PVC. It usually requires a Thermoplastic Polyurethane (TPU) water resistant coating that prevents wear on the material. This material also scuffs and wears down faster than PVC.

• PVC can be considered the most durable, because it does not scuff or wear as easily as the others. It is however harder and offers less control than the other materials. These material covers are also usually coated in TPU to improve water resistant capabilities and prevent wear on the ball.

• Rubber balls do not absorb water, offer good grip and enable balls to bounce on hard surfaces.

• Synthetic leather covers offer a soft feel, is responsive and give players increased control over the ball. This material is usually more expensive than the others, but does not offer the same strength or durability and may absorb water as time passes.

Stitching and construction

Balls can be constructed by gluing, hand-stitching, machine stitching or thermally moulded. The tighter and more securely a ball is stitched, the longer it will last.

• Glue is usually used to bond lower-end or practice balls. The panels are glued to the ball’s lining and creates a harder feel than that caused by other methods of bonding.

• Balls are usually stitched with difference strengths of polyester thread, which is durable and does not absorb water. 5-Ply polyester is the strongest and offers the best quality. Stitching can be reinforced with Kevlar to improve durability.

• Hand sewn balls offer stronger and tighter seams that improve the durability of the ball, but cost more than machine stitched balls. Machine stitching is often used for lower end balls that are less durable.

• Thermally bonded balls’ panels are fused to- gether to create tight seams that offer higher resistance to water absorption and greater durability. These balls are more expensive than other types of balls and are normally used in professional matches.

Valve placement

Where the valve is placed in a ball can affect its balance and therefore its flight.

• Valve placement in rugby balls has a direct effect on the balance, because the area where the valve is placed is heavier. If a ball is unbalanced, it can affect the predictability of the flight of the ball. Many manufacturers place valves in the seam of the ball so that it does not affect its balance.

• The placement of the valve allows the ball to self-correct by 10-15% if it is off target, thus improving accuracy when scoring goals. This placement of the valve also allows the player to align the valve to the target and use the weight of it to keep the ball in line.

• It is vital to use the lightest (possible) valve in a rugby ball bladder, as it will help maintain the balance of the ball. To ensure the most accurate balance is achieved, manufacturers should put a counter weight (the exact same weight as the valve) on the opposite side of the valve.

Linings

Linings are the layers placed between the bladder (that holds that air) and outer cover of the ball.

• These are usually made of cotton or poly- ester materials that assist in maintaining the shape, bounce and strength of the ball. Cotton linings offer softness while polyester makes balls more durable and responsive.

• Balls can have different numbers of lay- ers, depending on the type of ball or purpose. Lower end or practice balls are usually constructed with two or more layers of polyester. These linings are durable, but harder and less responsive than cotton.

• Higher quality, professional soccer balls are usually lined with four layers of lining, made of cotton and polyester, which helps the ball retain its shape and bounce throughout its lifespan.

• Some balls may even have an added foam layer that offers more cushioning and control.


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