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Squash Player brings you the lowdown on possibly the most
important aspect of our favourite game - the ball !!!
Here are four articles from Squash Player covering everything
you ever needed to know about balls ...

    A short History | How Balls are Made | Balls on Test | Ball Behaviour

A study of ball behaviour by Steve Edgar


New Dunlop range (Nov '99)  |  Pointfore new ball (Apr '02)  Tests

WSF Ball Specification (pdf)  |  String Matters


The game of squash takes its name from the ball with which it is played – or rather from its behaviour when it rebounds off wall and floor. But the fact that they were squashy is about all we know about the balls first used back in the mid-19th century by the chaps at Harrow. Originally it was a case of needs must: a rackets ball was too hard and therefore too fast to be suitable for knocking about in the narrow confines of those early ‘courts’ (nothing more than two or three walls adjacent to the rackets court). Almost anything would do – even a child’s rubber ball.

Trial and error led to certain types of ball being preferred, but when it came to competitive events the choice of ball would depend on the size of the court. As courts became standardised, so did balls. This process was the job of the Tennis and Rackets Association, which took the fledgling game of squash under its protective wing in the 1920s. In particular, one Colonel R.E.Crompton from the Royal Automobile Club in London seems to have been responsible for weighing, measuring and devising means of comparing the bounce of the various balls in circulation. The Avon India Rubber Company, for example, were producing balls of different sizes (from 3.65 cm to 4.3 cm in diameter), some with a matt finish, others dipped in varnish to make them shiny, even one with a hole in it which was known as the Bath Club Holer. The Silvertown Company were also making a glossy ball with a black ‘enamelled’ surface measuring 3.9 cm across. Meanwhile the Gradidge Company (later to be swallowed up by Slazenger) were marketing their so-called nigger ball which, despite its unfortunate name, was available in white or red as well as black.

In 1923, perhaps because of Colonel Crompton’s influence, the Tennis and Rackets Association’s Squash Rackets Representative Committee adopted the RAC standard ball (licensed by them from the Silvertown Company) as the official ball for amateur championships. Nothing is now known of the specification of the so-called Wisden Royal (the company’s records were destroyed during the Second World War) but as early as September 1923 members of the Committee were complaining that it was too fast and suggesting that another company be licensed to produce two new balls, one three per cent and one five per cent slower.

The task fell to the improbable sounding India Rubber and Gutta Percha Company, but it was not until 1926 that the re-named Joint Clubs Squash Rackets Committee, after a further series of tests, finally laid down a specification for squash balls and officially endorsed two balls (manufactured by Silvertown and Gradidge) as worthy to carry the red mark ‘T and RA – Standard’. In fact, even after the formation of the Squash Rackets Association in December 1928, it was the T&RA’s mark which continued for several years to appear on authorised squash balls.

By 1930 the SRA had adopted the Silvertown ball as the single official ball for amateur championships, although when the Women’s Squash Rackets Association was formed in 1934 it opted for the Gradidge. The Second World War dealt several blows to squash. Not only were the London factories of the major ball manufacturers destroyed by bombing, and with them supposedly details of the secret ingredients of the balls, but rubber supplies virtually dried up so that hardly any balls could be produced. The Silvertown ball was never made again and a company called Dunlop took over as the principal supplier.

Further destruction – this time that of the Bath Club by fire – meant that the Amateur Championships had to be moved to the Lansdowne Club, whose courts were significantly warmer and faster. It was for this reason that Dunlop’s post-war balls were slower than the Silvertowns had been, though they also produced medium and fast balls. 1960 saw the introduction by Slazenger of the synthetic ball, made of a substance called butyl whose performance varied less than rubber in hot and cold conditions. There were other problems with the new material, however, and there was a time in the mid-sixties when players were lucky if they finished a match with the ball they had started with, so liable were they to splitting. Early experiments with non-marking balls were similarly unsuccessful and the black ball continued to be preferred for major championships.

Dunlop introduced the familiar coloured dot balls in the early seventies, but they did not always enjoy the manufacturing monopoly they do today. Grays’ Merco ball, for example, was for a time the official SRA ball.

1999 will undoubtedly be seen as a turning point in the history of the squash ball with the introduction of Dunlop’s new range. In a sense though it is a regression to the early days of the sport when balls of different sizes, colours, and characteristics were selected from according to playing conditions and the ability of the players. Perhaps this reversion to first principles will lead to a regeneration of squash in the 21st century. Let’s hope so.

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Above we looked at the development of squash balls over the 150 years or so of their history. During that time the way they are manufactured has also developed into a highly sophisticated process. Here we investigate how Dunlop balls are made.

To begin with, raw rubber from Malaysia is delivered to the Barnsley factory in ‘bales’ of about 25kg – sufficient to make about 1,200 balls. In its natural state rubber is very stiff and difficult to work, so it is first ‘masticated’ to a softer consistency. A variety of natural and synthetic materials and powders are then mixed with the rubber to give it the required combination of strength, resilience, and colour as well as to enable it to cure (or ‘vulcanise’) later in the process. The manufacturer’s ‘recipe’ is, of course, a no less closely guarded secret than that of Coca Cola, and different combinations of ingredients (as many as 15 are used, including polymers, fillers, vulcanising agents, processing aids, and reinforcing materials) produce fast (blue dot), medium (red dot), slow (white dot), and super slow (yellow dot) balls.

The resulting compounds are warmed and loaded into an extruder, which forces them (rather like a mincing machine) through a ‘die’. A rotating knife cuts the extruded compound into pellets, which are then cooled. The pellets, which now have a putty-like consistency, are dropped into a hydraulic press which subjects them to a pressure of 1,100lb per in2 and a temperature of 140–160°C for 12 minutes. The heat causes the material to cure and so retain its shape. Each pellet makes half a ball, known as a ‘half shell’. 50% of these are ‘plains’ and 50% ‘dots’. The mould for the dots has a pin in the bottom to create the tiny dimple which takes the different coloured paints that indicate the balls’ speed. When the half shells are removed from the press, the excess compound (called ‘flash’) must be cut away before the dots can be glued to the plains to make complete balls.

First the edges of the half shells are roughened (‘buffed’) by a grinding wheel to provide a key for the adhesive. The buffed edges are then coated with rubber solution and a measured amount of adhesive is applied in three coats at thirty minute intervals. Both the adhesive and the dot paint are produced in a similar way to the rest of the ball; the adhesive, for example, is also made from raw rubber mixed with various powders before being ground, broken down into a fine web and ‘wet mixed’ for several hours with a solvent. At last the half shells can be stuck together – an operation called ‘flapping’.

The flapped balls are then put through a second moulding, heating and vulcanising process, this time subjecting them to 1000lb per in2 for 15 minutes, to cure the adhesive. Further buffing, this time of the balls’ exterior, smoothes the join and gives the balls their characteristic matt surface. After being washed and dried each ball is inspected. This is one of the few operations which is still carried out by hand, by a team of four ladies, the only other manual operations being the loading and unloading of the presses, the final buffing and washing, and, most importantly, testing.

The balls are tested at every stage in the process and those that are unsatisfactory rejected. Those that pass are stamped with the Dunlop logo, boxed in dozens, and shipped all over the world, but a sample of them is given a final test to ensure that they conform to WSF standards.

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The current WSF Specification for the Standard Yellow Dot Championship Squash Ball as it appears in Appendix 7 of the Rules of Squash dates from October 1990, apart from a minor amendment made in July 1995, and determines the permitted diameter, weight, stiffness, seam strength and rebound resilience of the championship ball. No specifications are set for other types of ball, "which may be used by players of greater of lesser ability or in court conditions which are hotter or colder than those used to determine the yellow dot specification". But how are balls tested to ensure that they meet these specifications?

The testing procedure itself states somewhat confusingly that: "For the purposes of inspection, balls manufactured from the same mix shall be arranged in batches of 3000 numbers or part thereof manufactured in one shift in a day." Fifteen balls are then chosen at random from each batch and divided into three groups of five balls. One group is tested for diameter, weight, and stiffness; another group for seam strength; the third group for rebound resilience.

First the 15 selected balls must be left in the laboratory for 24 hours to ‘condition’ them to a temperature of 23oC. Their diameter, measured perpendicular to the seam, must be between 39.5mm and 40.5mm, and their weight between 23 and 25g. To be measured for stiffness the balls are held between two plates with the seam parallel to the plates and compressed at a rate of 45–55mm per minute. They are compressed by 20mm six times, the test measurement being made on the sixth deformation only. The stiffness of a ball is calculated by measuring the compressive force at the point where it has been deformed by 16mm and dividing that by 16 to give a ‘force per millimetre’. The result must be between 2.8 and 3.6N/mm at 23oC. In other words, the force required to compress the ball by 16mm (i.e. to just over half its original diameter) must be between 44.8 and 57.6 Newtons.

The calculation of seam strength is even more complicated. "The squash ball is first cut into two equal halves perpendicular to the plane of the seam." Then two strips (one from each half of the ball) approximately 15mm wide and 60mm (roughly half the circumference) long are cut, with the seam running across the middle. The average width of each strip is measured before it is pulled apart at a rate of 180–220mm per minute until the seam breaks. The force at the point of breakage is divided by the average width of the strip to give a ‘force per millimetre’, which must be at least 6N/mm. So if the average width of the test strip is exactly 15mm, the minimum force required to break the seam must be 90 Newtons.

Rebound resilience is simply a measurement of the height a ball bounces off a hard surface. The same balls are conditioned first to 23oC and then to 45oC and dropped from a height of 100 inches onto a concrete floor (which in both cases must be at 21–25oC). At 23oC the balls must rebound at least 12 inches; at 45oC between 26 and 33 inches. (The 1995 amendment was to these figures: previously the rebound specification at 23oC was 16–17 inches and at 45oC 26–28 inches.)

Although a compression test is no longer required by the WSF – it was deleted from the ball specification in September 1988 – Dunlop continue to carry out a test in which loads of 0.5kg and 2.4kg are applied to the ball and the resulting deformation measured. The difference in deformation under the two loads used to be specified as between 3 and 7mm, but Dunlop aim at between 4.5 and 5.5mm, just to be on the safe side!

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Why does a squash ball bounce higher when it’s warm?
To understand this we need to do a little physics. Ready? In order for a solid material to be deformed, work has to be done on it. For that work to be done, energy must be expended (in the case of a squash ball, it is hit by a racket). Some of this energy is dissipated (as heat, etc.), but some is stored in the deformed material and is released when the material relaxes. The extent to which a material stores energy under deformation is called ‘resilience’. Some materials (like sprung steel) store a lot of energy and are described as having high resilience; others (like putty) store very little and therefore have low resilience.

Squash balls, being made of a rubber compound, are of fairly low resilience. Unfortunately, as we know, the lower the resilience of an object, the higher the proportion of the energy used in deforming it must be dissipated. When a squash ball hits the racket strings and the wall and floor of the court, some of this energy is transformed into heat in the strings, wall, floor, and surrounding air and some into sound, but most of it becomes heat in the ball itself. This has two effects: the air inside the ball (which was originally at normal atmospheric pressure) effectively becomes ‘pressurised’, and the rubber compound from which the ball is made becomes more resilient. For both these reasons, the ball bounces higher. Obviously, the ball does not continue indefinitely to heat up; eventually equilibrium is reached where heat loss to strings, wall, floor, and air is equal to heat gained from deformation. This point is normally at around 45oC, which is why the WSF’s rebound resilience specification is calculated at that temperature. It also explains why squash balls are designed to have too little resilience at room temperature and therefore why they need warming before play.

But why have balls of different speeds and how are they made?
The actual ball temperature reached in play varies according to two main factors: the temperature of the court and the ability of the players. The point at which the ball temperature reaches equilibrium is in fact an excess over the ambient temperature of the court. So if the court is at only 5oC, the ball may only reach 35oC.

On the other hand, even on a warm court, if the players don’t hit the ball hard or often enough to raise its temperature to that optimum 45oC, the ball won’t perform as it should. To compensate for either factor, players will need to use a ball of higher basic resilience, i.e. a ‘faster’ ball. These are produced simply by making a different mixture of polymers. More elastic polymers create higher resilience; more viscous polymers lower resilience.

So how can you have a ball with ‘instant bounce’?
For a ball not to need warming, it must either lose as much heat as it gains during play and therefore remain at court temperature, or it must be made of a material whose resilience is the same at any temperature. It remains to be seen whether Dunlop’s new Max Progress and Max balls meet either of these criteria. As soon as we have them (they’re now due to be launched in June this year), we’ll be testing them to find out...

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