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Dr Ian Plummer

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Technical
CA Hoop Tests 2014 Full Report

The following is the full report of the tests undertaken in April 2014, and is aimed at those particularly interested in croquet equipment – shorter versions are being prepared for wider audiences.  The report also draws in places on the inconclusive CA Equipment Committee tests of March 2013, a report of which can be found at http://www.oxfordcroquet.com/tech/hoops.

Table of Contents

Goals of Hoop Tests
Top-level Summary of Findings
An Elite Hoop 'Shopping List'
The Testing
    Inherent Test Problems - Caveat
    Factors to Examine
    Earlier Tests
Designs to Test
The Hoops
    The Comparisons
Test Equipment Previously Used
Test Equipment for 2014
Testing and Test Protocol
Test Results
    A. Critical Angle
        The Effect of Adding Mass to the Crowns
        The Effect of Surface Finish
        The Effect of Upright Rigidity
    B. Running Straight Hoops
Appendix – the Hoops Close Up

Supplement (June 2014)


Goals of Hoop Tests

The search for a more challenging hoop for top-class AC events - especially in typical damp UK soils – has continued for some years.  These tests were undertaken as part of that continuing search.

The primary goal of these tests has been to tease apart the factors that seem to make a difference to the inherent 'runability' of a hoop, leaving aside the external factors of soil type, weather, and setting.

Additionally, the tests looked at some specific hoops of novel (as far as the UK is concerned) design – from Egypt, New Zealand and Canada, also assessing their suitability as an Elite Hoop in UK conditions.

Top-level Summary of Findings

The tests revealed the following conclusions about the factors investigated:

  1. Carrot Shapesquare 'parsnips' are indeed more effective in typical UK soils than round carrots of similar size.  The long cylindrical carrots of the Egyptian hoop also worked well.  Finned hoops can work adequately, but are harder to drive in and pull out of the soil, and in typical UK soils offer no obvious advantage over carrots or parsnips.
  2. Grippiness of uprights – hoops are less forgiving of poor hoop strokes if the uprights' surface finish grips the ball to some extent.  In this regard, 'good' seem to include unfinished 'brushed' steel (which would need to be stainless steel for other practical reasons), rubber-coated, knurled, or square uprights with no chamfers on the edges.  (However, fears over ball damage will probably preclude knurled or square uprights.)  It was shown that adding a paint finish to a stainless hoop made it easier, and removing powder-coating from uprights made the hoop more difficult.  It maybe this factor also accounts for the suspicion in recent years that hoops made with thicker uprights (in the pursuit of greater rigidity and so difficulty) are in fact made easier by the larger radius of the uprights gripping the ball less well than the narrower wires of some older hoop designs.
  3. Mass above ground – it seems to make a hoop more challenging if the mass above ground is significant.  Some of the test hoops (e.g. Egyptian) already had massive crowns or uprights, but for those of a skinny construction, adding lead clamped to the crowns improved the hoops in some – but not all - cases.  Hoop manufacturers might like to investigate e.g. using a double crown bar, or using a heavy square steel section with a normal round section above, so scoring clips can still be used.  Previous experiments with much more mass below ground were confirmed here, with the prototype 'Hopewell on Steroids' performing no better than a typical ordinary hoop, despite weighing 60% extra.

It was also evident that despite the novel and varied design of hoops tested, in fact the differences in 'runability' were surprisingly limited.  All the hoops – except the Trimmer test Hoops #7 and #8 - had a 'critical angle' (maximum angle from which they could be run) of between about 40 and 45 degrees.  This is a disappointingly small range – it would be useful to have an elite hoop which was far less tolerant of poor approaches than a normal hoop, not just a few degrees.

Previous CA tests with hoops with facetted or even square uprights (and other tests of a double-depth hoop, with two sets of uprights welded back to back) confirmed this view that even what appear quite extreme design changes make only modest 'runability' changes in practice.

The Egyptian hoop worked very well.  It doesn't meet the formal hoop specification – being too tall and the crown and uprights over-sized (and you couldn't use a normal scoring clip on it).  It resisted poor hoop shots well, was easy (compared to some of the other novel designs) to set and adjust, and seemed to keep its setting well.  Carrying six around a lawn would be quite a task!

The NZ Atkins Quadway worked very well.  The square parsnips and the unfinished stainless steel uprights contributed to its good results.  Given how much any hoop springs in or out in UK soils, once the hoop clamp is removed (usually due to stones underground), it is unlikely its 'adjustable width' feature is useful in practice, and only adds to the manufacturing costs.

The Canadian Oakley 4-fin hoop performed much like any normal UK hoop, despite its very different design.  It was significantly harder to drive into the ground or to remove from it.

A final conclusion is that we should reconsider the specification for a hoop given in the AC Laws, GC Rules, and WCF Equipment Regulations, as it may be that there are good designs out there (such as the Egyptian hoop) which are technically illegal.  Can the specification be written in such a way as to allow more flexibility and innovation?

An Elite Hoop 'Shopping List'

There are three options it seems for arriving soon at a CA Elite Hoop, for top-class AC events such as the Opens, Mens' & Womens' and President's Cup:

  1. buy Atkins Quadway
  2. see if an Egyptian-style hoop could be manufactured in unfinished stainless steel
  3. commission a CA Elite Hoop prototype with these characteristics:
    • unfinished stainless steel
    • square parsnips
    • uprights of 16mm, not more
    • massive 30mm square crown with a 16mm round crown bar welded on top (to take clips) - the double crown welds should also stiffen the hoop.

It could also be worth widely inviting suggestions for how to make uprights grippier while still being practical, non-ball-damaging and long-lasting, and then having a further round of tests to select the best solution and including this in the Elite Hoop recipe.  This may delay securing an Elite Hoop by a further year, but may yield improved performance.

The Testing

Inherent Test Problems - Caveat

Hoop difficulty is governed by the design of hoop, how narrow and firmly it is set, the type and dampness of the ground, the kind of stroke and the make of ball.  It is very difficult to get meaningful results other than by comparing different designs against each other, when in new holes in the same ground on the same day.  Conclusions drawn here may not be valid to different soil types, or soil that is much drier or wetter, or with different balls, and so on. 

It is also evident that players have all manner of hoop strokes, some imparting top- or back-spin to varying degrees, and at all kinds of pace.  These tests for practical reasons are limited to testing hoops with one standard strength stroke delivered with a similar spin in all circumstances.  So it is possible that in actual play, real players may have quite different findings about the relative merits of these hoops and features.

Factors to Examine

Discounting for now ideas that would require different engineering in the turf, there are 3 factors which seem worthy of systematic investigation:

1). Carrot Shape – square 'parsnips' have already been found to provide some more resistance in damp ground.  Do fins instead of carrots work well in UK soils?  Egyptian GC hoops stand up to much abuse very well.

2). Friction of the Upright – we have already found some powder coating is akin to Teflon™ ('non-stick hoops') and that unfinished steel and stainless steel uprights seem to pose more of a challenge.  Dave Trimmer has now suggested some intriguing alternatives in this direction: not just a move to unfinished stainless steel, but knurled or rubber coated uprights to 'grip' the ball.  The 'grippiness' of the upright might also explain why anecdotally, older hoops with thin uprights often seem harder to run then new hoops with thicker uprights (as the ball may deform around these less?). 

3). Mass Above Ground – previous experiments seemed to show that, perhaps surprisingly, more mass below ground did nothing to make the hoops more difficult (see the results for Hoop #6).  Some crude and quick experiments with adding mass to the top of a hoop seemed to suggest a potential benefit.  The Egyptian hoops also seem to benefit from this effect (30mm square mild steel crown, plus taller and thicker uprights).  Colin Irwin has suggested a refinement - using this extra mass to also increase the resistance to deflection of the uprights – for example by having extended uprights and welding say 2 or 3 cross-bars in the top few inches to stiffen up the hoop.

A reason to defer – at least at this time - other ideas which involve engineering the turf, is that a desirable goal of a new more difficult hoop would be one that could be used 'everyday', or at least was interchangeable with 'everyday' hoops. 

Earlier Tests

Our 2013 tests also investigated square uprights – but the smallest of chamfers on the edges of the uprights (~1mm) was enough to radically alter the playing characteristics.  This might suggest these could be too variable in practice – there was also a concern about potential ball damage longer term.  It may be the un-chamfered edges were in fact just digging into the ball and causing the 'grip' that knurling or rubber might provide instead.  Tests a few years earlier had also shown that uprights with wide facets milled into them at an angle (35 to 55 degrees were tried) did make a discernable difference to difficulty, but not enough to justify the investment in re-equipping.

Designs to Test

In our 2013 tests, we eliminated the US Superhoop as being a usable or effective hoop in UK soils, despite its reported good characteristics when in a stratified sand court.  We also wanted to complete the tests of NZ Atkins Quadway this year, and to undertake testing of the recently-acquired Egyptian hoop.    So the hoops required for the tests are as shown below.

Selection of hoops used in the tests

Individual photos of all the hoops are included at the end. The photo below shows the new "Tonker" used in all these tests to deliver a standard hoop stroke.

Spacer in hoop

All hoops were set to 1/16" in new holes.

The 'Tonker'

The 'Tonker' used to deliver a standard hoop stroke.

The Hoops

Hoop #1 is our 'reference hoop' – a perfectly standard Aldridge hoop, albeit selected at the nominal 1/16" gap rather than 1/8" gap.  Aldridge and Hopewell (cast iron but similar dimensions) are the two top-selling club and tournament hoops in UK at this time.

Ref

#

Hoop & construction

Carrots

Uprights finish

Uprights diameter

Crown

Ht, carrot to crown

Overall length

Mass

Free air gap at half ball

1

Aldridge Championship

welded mild steel

35mm at top, tapering over 215mm

White powder coat

16.3mm

19mm round

285mm

525mm

2605g

94.2mm

2

Egyptian

welded mild steel

28mm cylindrical, tapers only in last 95mm

Turned down from 28mm bar; white paint

20mm

30 x 30 x 156 square bar

330mm

600mm

4355g

95.0mm

3

Aldridge prototype

welded mild steel

square, 34mm at top, tapering over 215mm

White powder coat

16.3mm

19mm round

285mm

520mm

2370g

93.5mm

4

Atkins Quadway

stainless steel, adjustable

square, 30mm at top, tapering over 200mm

Bright stainless steel

16mm

Machined ex. 20mm

285mm

510mm

2510g

93.5mm

5

Oakley

welded stainless steel, 4-fin

none, 4-fins at 90 degrees, spanning 70mm

Brushed stainless steel

19mm

16mm round

288mm

585mm

3430g

93.8mm

6

Hopewell on Steroids

heavy prototype, cast iron

45mm at top, tapering over 260mm

White powder coat

20mm

20mm round

285mm

565mm

4260g

93.0mm

7

Trimmer test hoop A

bolted mild steel

35mm at top, tapering over 165mm

Unfinished steel, thin rubber around half ball ht

16mm

10 x 30 x 130 bar

288mm

475mm

2450g

93.0mm

8

Trimmer test hoop B

bolted mild steel

35mm at top, tapering over 165mm

Unfinished steel, knurled around half ball height

16mm

10 x 30 x 130 bar

288mm

475mm

2450g

93.0mm

3 pieces of lead sheet & clamp

340g each piece of lead + 290g clamp (= 1320g total)


The Comparisons

Comparing Hoops #3 and #4 with #1 will allow us to establish the merits of square 'parsnips' over round carrots.  Hoops #2 and #5 let us consider other below-ground shapes.

Hoop #2 – and the use of a clamp and lead totalling 1320g fixed to other hoops – will allow us to see the effect of more mass at the top of the hoop.

Systematic tests on Hoop #6 will enable us to check whether the anecdotal findings a few years ago that a much more massive hoop overall, with huge carrots, is strangely easy to run.

It is difficult to eliminate other factors when trying to decide whether one hoop with unfinished metal uprights is better or worse (i.e. more difficult or easier to run) than one with powder-coated uprights – but a simple test will be to trial a good stainless steel hoop then immediately spray-paint the uprights and re-test it.  This will show what effect surface finish has, and complements the 2013 test when the powder coating was removed from the uprights on some hoops to see the effect. 

Taking this last point further, increasing the 'grippiness' of the uprights by either knurling them or covering with thin rubber tube will see if this is an area worthy of further exploration.

Unfortunately, that still leaves some factors which are too difficult to tease apart with our level of resources, such as degree of flexion of the uprights – is there a difference between different types of steel?  Is the 'smoother radius' of a 20mm upright ('bad'?) shielding the greater rigidity ('good'?), for example. We did a late supplemental test (see later) which confirmed rigidity was a positive

In a perfect world, a set of test hoops would be constructed which were all exactly the same height, mass and balance (between crown and carrot), in which specific different variables could be examined – but that is beyond our resources.  Instead, we have done our best to interpret the results from test hoops available to determine what factors help, hinder, or make little difference.  The full results are presented in [Result sheet April 2014.xslx] – you might make different interpretations from them.

Test Equipment Previously Used

The Pidcock Peeling Plank.  An MDF plank which clips around the near wire and allows shots to be played at increasing angle to the hoop.  Each stroke is played with a pair of balls initially in contact and resting against the side of the plank.  Shims allow various offsets to be set with respect to the near wire.  The plank has two problems:

  • As the peel has to be struck manually each time, there can be considerable variation in strength of shot.  We found it hard to prevent the tester hitting harder when at more acute angles – as this is what we do instinctively to get balls to go through!
  • It is quite possible to 'pull' the peel some millimetres into or away from the near wire, depending how the peel is struck.  (The presence of a side rail right next to the mallet makes it a challenge to play the stroke completely straight.)

The Ramp.  A 2.4m long ramp made of a pair of timber rails 44mm apart.  The ball is deposited onto the lawn about 350mm in front of the hoop.  This also has two problems:

  • By design, the ramp delivers the ball with top spin – but arguably much too much.  It is possible for balls to run through a straight hoop even when the ramp is set up more than 10mm off-centre – which doesn't match our experience in play!  So we are measuring something that isn't typical of normal play.
  • The ball hits the ground at a 15 degree angle at the foot of the ramp and quickly makes a small dent in a damp lawn.  This, together with a visible track soon worn between the ramp and the hoop, means results change significantly over a set of 10 strokes.  The denting problem has been largely fixed, but the visible track has proved more difficult.

Test Equipment for 2014

A new tester was made which would allow more relevant and objective testing of both angled hoop strokes and of straight on (but off centre) hoop strokes.  The first seeks to find the maximum angle at which it is still possible to run the hoop (a smaller critical angle would be good, given the goals of our tests).  The second seeks to measure how tolerant the hoop is of badly-hit straight hoop strokes (a smaller offset at which it became impossible to run the hoop would be good).

The Tonker (see photo earlier) was designed and built to swing a mallet head as a pendulum, dropped from a controlled height, to strike a single ball aligned to just miss the near wire. This seemed to produce more meaningful results than previous test but it should still be noted:

  • croquet players have a very wide range of hoop strokes, both hard and soft, with top spin or none, on target and not quite on target.  The Tonker instead tries to replicate one specific stroke consistently.  This may not be representative of many typical hoop strokes.
  • there is a fair degree of 'noise' in the results, perhaps due to the imperfections of the lawn, the exact line of the pendulum swing on impact (we noticed the springy shaft on the pendulum allowed some degree of lateral oscillation), imperfections in the ball and so on.  Some of the effects of the various factors being tested may be not much larger than this noise, making it hard to deduce correctly what works and what doesn't.

Testing and Test Protocol

Ipswich Croquet Club lawns are open all year so the grass is kept representatively short.  The testing was undertaken in mid-April, after a couple of weeks of dry weather.  The soil is typical of many English clubs – a clayey loam with only a few large stones but more small stones, and a sandy top few inches.  It drains quite well.  The speed wasn't measured, but was a reasonable, medium pace.  It was found that dropping the mallet head from 300mm produced a stroke where the ball travelled about 750cm across the open lawn.  That was then taken as the standard hoop strokes for all tests.

Each hoop to be tested was driven into new holes, without any cores being taken, using an Oakley hoop clamp and set to a gap of 1/16" using the same Black ball.  All tests were conducted with Dawson balls.

Tests to determine the critical angle relied on a group of at least 4 strokes to ascertain the critical angle beyond which a ball would not run the hoop. The distance through the hoop each ball ran was measured (in cm), and an average for that angle determined.  If the results were particularly noisy, then the group was extended to 8 or even 12 strokes.  The Tonker was hooked around the near wire and set at increasingly acute angles to the hoop until the critical angle was found.  Tent pegs were used to keep the Tonker in position.

Tests for tolerance to off-line straight hoop strokes used a group of at least 4 strokes to measure how far through in centimetres (= how cleanly) they ran a straight hoop at increasing amounts of offset from the centre line.  The Tonker was set up perpendicular to the hoop plane and pegged in position.  A series of shims were added to the sighting-plank on the left side to offset the ball further and further to the right.

Peeling plank and shim

Here, a black 3.4mm shim is held onto the sighting plank using a rubber band; the ball is in contact with the shim and about to be struck.

Test Results

See [Result sheet April 2014.xslx] for the full results – only the averages are discussed further here.

A. Critical Angle

The first test with each hoop was to fire balls at it with the Tonker at increasing angles to the perpendicular, until the angle was reached at which few if any balls ran the hoop (the 'critical angle').  A more difficult hoop would have a smaller critical angle, which would penalise poor hoop approaches.

Critical Angle Results for All Hoops

Critical Angle Results

[In all charts in this section, the x-axis is degrees from perpendicular to the plane of the hoop, and the y-axis is the average distance a group of balls travelled beyond the hoop in centimetres.]

Although hard to read, this chart shows a number of useful findings:

  • The Reference Hoop (#1, a standard Aldridge) was the easiest to run (= 'had the largest critical angle') – which is what would be expected given that all the other hoops are attempts to make things more difficult
  • While some hoops are quite receptive until just before their critical angle, others are 'tricky' for quite some range of angles before you reach 'critical'.  Hoop #7 and #5 (Oakley 4-fin) in particular have a broad range of difficult angles.
  • There is a fair degree of noise in the results, which sometimes can disguise real findings.  The curves for #3 and #8 exhibit this particularly.  Re-testing with larger batch sizes would probably help clarify the true curves.
  • The two hoops with square 'parsnips' (#3 and #4) and the Egyptian hoop with long cylindrical carrots (#2) all have critical angles that seem a bit more difficult than the rest of the hoops (save #7 and #8).  Both shapes seem an improvement on the traditional carrot shape.
  • Hoops #7 and #8 were designed to test very 'grippy' uprights – with either rubber-covered or knurled uprights.  It is significant that they have critical angles some 5-10 degrees smaller than the rest.  Further development of this idea – to find a grippy upright which doesn't either wear out the balls or the covering – could be a route to a really challenging elite hoop.  These prototypes had some issues though:

 

Damage to ball by textured hoop Debris on textured hoop

These photos show the damage to a ball from Hoop 8's knurling, and the fine swarf of coloured plastic that was soon acquired by the knurling!  Clearly, hoop 8's knurling is too aggressive with the balls (but was the gentler of the two forms supplied by Dave Trimmer for testing), while the rubber tube of hoop 7 had worn into a hole by the end of the tests at the primary contact point.

The Effect of Adding Mass to the Crowns

About a kilo of lead was added to the crowns, held in place with a metal clamp (total = 1320g) to see the effect of more massive superstructure, then the critical angle re-tested. 

The effect for many hoops was indistinct – this particularly was the case for hoops which were already massive above ground – such as #2 and #6.

Of those where there was a noticeable effect:

Hoop 1 Hoop 5

 

Hoop #1 (our reference – a standard Aldridge) became more difficult to run sooner – so hoop running was more difficult even at 'easier' angles, and the Critical Angle was reduced.  Likewise the Critical Angle of Hoop #5 (Oakley 4-fin) became about 2 degrees less.

With Hoop #7 (Trimmer test hoop – rubber coated uprights) the effect was significant – probably due to the relatively insubstantial superstructure and carrots compared to the rest of the test hoops (see chart over page).

Hoop 7

There is no obvious explanation for why Hoop #8 didn't also share this characteristic.

The Effect of Surface Finish

It has been suspected for the past few years that modern powder coating is like Teflon™ and helps balls slip through.  Strangely, even quite shiny bare metal (#4 and #5) seem to 'grip' the ball better than powder coated or painted uprights.  This has now been confirmed in two tests:

  1. In the 2013 tests, having found the critical angle and maximum offset at which balls would still run the hoop for the Aldridge prototype (#3) with powder-coated uprights, the coating was stripped off the bottom few inches of the uprights, leaving bare steel.  The hoop immediately became appreciably more difficult to run.
  2. A fascinating result was obtained in the 2014 tests, when the Atkins Quadway (#4 - a good hoop with stainless steel uprights) had the uprights spray painted (car paint) then re-tested while still in the same holes:

Painted versus bare uprights

It had 'magically' become much easier to run!  This was doubly strange as the stainless steel felt almost polished and very smooth, while the spray paint finish wasn't very smooth to the touch – and yet let the balls slip through more easily.

The Effect of Upright Rigidity

Effect of clamping uprightsIdeally, we would like to systematically test the effect of different kinds of cast iron, steel and stainless steel uprights, of different diameters, to properly understand the effect of increasing the rigidity of the uprights. Unfortunately, this is beyond our current resources.

We have already noted that it maybe we see two competing effects with the move towards thicker (19 or 20mm) uprights, instead of the “traditional” 16mm: the greater rigidity (“good”) is offset by the smoother radius (“bad”) of the uprights.

One simple test we added at the end of preparing this report was to fix an Oakley hoop clamp about 15cm above the ground between the uprights. This had the effect of binding the two uprights together and increasing the rigidity of the hoop. (It was checked that the clamp hadn’t altered the width setting of the hoop.)

As can be seen, this method of making the uprights more rigid has reduced how easily balls run as the angle approaches critical – though it doesn’t seem from these few measurements to have greatly altered the critical angle itself.

So a method of making the uprights more rigid, without ideally making them a larger diameter, could well be beneficial.

B. Running Straight Hoops

For these tests, the Tonker was set up perpendicular to the plane of the hoop (= straight in front) but by varying amounts offset to the right.  When the offset was zero or 1.6mm, not surprisingly, the balls would run through by up to 800cm, while with offsets of about 9mm, many balls were sticking in the jaws.  Some balls even got through when 12.4mm off-centre!  The results from these tests are less clear cut than the critical angle tests, but we can still draw some conclusions:

Offset straight hoops

While all seem quite similar, it can be seen that those two with 'grippy uprights' (#7 and #8) allowed the balls to travel through least distance at any given offset, and prevented all shots at 9mm from running, while most hoops allowed some balls to run at 9mm offset and even a few at 12.4mm.

Painted versus bare So grippy uprights seem to resist inaccurate hoop strokes best.

Again, spray-painting the shiny stainless steel uprights of hoop #4 (Atkins Quadway) produced an interesting result:

Despite the apparent slipperiness of the bare stainless steel compared to the not-very-smooth-to-the-touch paint finish, more balls ran through the hoop further after the uprights had been painted.

So bare metal uprights seem to be more 'grippy'.

Adding the lead weight clamped to the crown made little difference to how well balls ran the hoops at various offsets – it's hard to draw any conclusions from the small differences here and there in the raw data.

CA Equipment Committee
April 2014

Appendix – the Hoops Close Up

Hoop 1 on the right, alongside Hoop 6 for comparison.  Hoop 6 weighs two thirds more than our standard reference hoop, #1, and is more massive in every aspect.

Hoop 1

Hoop 2, the Egyptian GC hoop, was taller than normal above ground, had a massive 30mm square crown, and long cylindrical carrots which only taper near the bottom.  Egypt uses an even longer version for their sand courts – this version is used in the silt courts.

Hoop 2

Hoop 3 is a prototype made by Aldridge last year and is effectively a standard hoop's superstructure on square parsnips.  It performed well once the powder coating was stripped from the uprights (here, it has been re-painted for this year's tests).

Hoop 3

Hoop 4 is the Atkins Quadway.  The combination of bare metal uprights and square parsnips works very well to produce a more challenging hoop.  Here it is shown after the uprights had been spray-painted with car paint – re-tests then showed its performance had slipped to be more ordinary.

Hoop 4 - Atkins Quadway

Hoop 5 is the Oakley 4-fin from Canada.  Despite the bare metal uprights and (for the UK) radical design, its performance was not out of the ordinary in typical UK soi l.

Hoop 5 - Oakley 4-fin

Hoops 7 (near/upper) and 8 (far/lower) were produced by Dave Trimmer to test whether we could find ways to make the uprights gripper, and so the hoop more challenging.  Despite the bolt-together construction, these were very effective at reducing the critical angle.  The problem now is to find a method which lasts and doesn't damage the balls.

Hoop 7 Hoop 8

Link: CA Hoop Tests 2014 - Supplement (June 2014)

Author: Martin French
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Updated 16.vii.17
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