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Dr Ian Plummer

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Technical
Ball Testing

Letter to the President SA Croquet Association - 24th March 1988

Here are the rather interesting results of some tests I carried out on Sunday the 20th, Monday the 21st and Tuesday the 22nd of March 1988. The balls which I tested were 4 of the new Jaques Eclipse Premium (though not Premium selected) and a set of original and only slightly worn Brento Tournament balls, both sets supplied by yourself.

In the bounce test, each ball was bounced on each of its 2 nodes and 4 poles three times, a total of 18 bounces per ball. For measurement, each ball was diametrically across the 2 nodes and 4 poles, a total of 3 measurements.

Mass was taken to the nearest 0.1 gm on a calibrated chemical balance. The results are tabulated on the separate sheet enclosed. Mass is quoted in grams., the specification requiring 454 gm, or a 461 maximum and a 447 minimum. Size is quoted in millimetres, the specification requiring 91.28 minimum and 92.86 maximum, which can be reduced and slightly tightened to the range 91.3 - 92.8 mm.

Bounce rebound height is quoted in inches, and all values were read, from a distance of about 1 foot, to the nearest half inch value rounded off in each case. The bounce test apparatus uses a spring gate, so that on release the ball is immediately in free fall. By this means. it is possible to cause the ball to fall accurately on any particular point. The poles referred to are those points with a milled ring concentric to them. The two nodes are the two points where the milling lines meet at right angles and there is no concentric ring.

Comments

Mass

The mass of the Jaques balls is near perfect and well within specification limits, with a ball average of 454.9 grams. This mass accuracy of the new Jaques balls represents the greatest single improvement on older manufacture. The new masses have advanced from -48 and +39 grams to -0.5 and +2.5 grams, a quite remarkable achievement. The Brento balls obviously are all between 6.5 grams and 1.1 grams Lighter than the specified mass, but still within the allowed limits which allow a lower limit of 457 grams. This is considered a compliment to the precise manufacture of the Brento balls in their own day as the accepted tournament ball.

Size

The Jaques balls show excellent uniformity, and rotundity to within at the worst 0.43mm.

Bounce

The bounce test was most revealing. The style of milling adopted by Jaques is nearly identical to that of the Brento balls, although the slight wear on the Brento balls precludes the statement that the milling is identical. This milling style is, in my opinion, largely responsible for the very uniform bounce result regardless of the milled form of the bounce spot. In general, too heavy or too rounded a milling form gives rise to a lower bounce because of a drop in the coefficient of restitution. This is why, normally, the bounce of an older, "flatter" ball is slightly higher than that of a new ball. In the present case, however, this has been largely overcome by the milling form, using very narrow grooves and leaving flatter and larger surface areas for the impact at any point.

I would say that the marginal increase in bounce of the Brento balls over that of the Jaques, together with the slightly more erratic values of the Brento balls, could be attributed to the slight wear of the Brento balls. This is pure speculation, but if true would indicate that the elasticity of the cores of the two types of ball is virtually identical.

Subsequent to the enclosed bounce tests, I decided to try to "manufacture" a different coefficient of restitution. First, I chiselled the steel plate from the concrete, and was surprised that when the disc finally came free there was a considerable amount of concrete stuck fast to it, which had been ripped out of the bed. Obviously the initial steel to concrete bond had been good.

I then chose another piece of concrete, about .6 inches thick, not reinforced. Instead of embedding, I simply used a very hard setting special cement which I applied approximately half an inch thick on a carefully scrubbed and roughened area of slab. I set the 1 inch thick plate on this cement bed (trade name "Rockset") ensuring that no air was trapped. In my opinion the coefficient of restitution is not affected by "embedding" so much as by close surface to surface contact of steel and concrete, and the inertia and rigidity of the concrete slab itself.

The 18 bounce results taken with the Jaques blue ball gave results about 5% higher than at first. The figures actually achieved were as follows:-

32: 32: 31.5: 31: 31: 31: 31: 30.5: 31.5: 31: 31: 30.5: 31.5: 31: 31.5: 31.5: 31.5: 31.5

The average of these eighteen results is 31.2 inches as compared with the 29.6 of the previous bounce test.

If it is possible to get these balls to bounce substantially higher, say 10% more, or 33.11 instead of 30", then some kind of international standard of concrete base size, rigidity, reinforcement, etc. etc. plus a well defined method of embedding or otherwise laying the steel plate must be formulated. The intention of the original specification, as far as I can judge, is to have a totally rigid bed supporting a mild steel plate of substantial solidity so that the restitution is between an unmoving but elastically (slightly) compressible steel plate and a free falling croquet ball.

The new result also invalidate the previous report on bounce, which results would also have to be increased by the same 5%. I must apologise for any inconvenience or misleading comment in this regard, largely because there is no standard laid down for the concrete block.

Lastly, my comments to you previously must be repeated regarding method of manufacture. Today in the plastics world in which I live, at least two products offer very good substitutes for the present composite balls. These are, amongst others, polyester and polycarbonate. Both suitable for pigment and filler addition to correct bounce and specific gravity to almost any desired value within the croquet ball range, they also offer a homogeneity which virtually cuts out case splitting. Not only this, but the hardness extends to the surface, so that wear is extremely stow. The advantage of this latter property is that old balls can still be used on tournament hoops as if they were new. Not the least advantage is the very great saving in manufacturing cost by whatever means they are made. I am absolutely certain that 5 to 10 years from now, several sources of such balls will exist and eventually not only precisely duplicate but also outclass present balls.

I have seen too much happen in this plastic world to doubt it for a second. Witness I have just got some of our factory "seconds" of a plastic material to make a waterproof set of croquet overclothes which osmotically breathes vapour and gases better than leather, better than my own skin!

Lastly, it is a pity that so many "cobwebs of the mind" exist about croquet balls. "Barlow balls are difficult to get through hoops", "Barlow balls climb all over the hoops", "they have a different friction coefficient" i.e. they are actually milled too deeply and sharply is what is meant. I have heard such comments and mere of a totally unmathematical nature. It is of course true that Tom has not yet got his ball right, but he is about to do so, and I am sure there are others busy along the same lines.

Regarding Tom's [Barlow's] latest balls. I tested them on Saturday 26th March. Again using 18 bounces each, I obtained values of 31.0 for Black, 32.1 for Blue, 32.1 for Red and 31.8 for Yellow. The variation here is very small, from 31 to 32.1. The individual pole variation is greater, due to poor milling. which in any case drops bounce several inches on all balls. Mass was within specification, averaging 458.5 with a variation of only 4 grams while size averaged 92.6 +/- 0.1 mm in the upper half of the specification limits. I. did not measure size or mass to great accuracy because I was at home.

Regarding the bounce tests, I believe that since the latest results are 5% higher than the first table, then that table can be raised by 5% throughout. This would give Jaques balls revised values of Black 31.8; Blue 31.2; Red 29.5 and Yellow 30.9 respectively, very close to Tom's latest.

I would like to suggest that you press for a tighter specification on bounce, as well as a detailed test description. The present 30 - 45 inch limits on bounce would be totally unacceptable in golf, say or tennis. You either drive a golf ball 100 metres with an iron, or 150 metres. In tennis, you would have to show your opponent which balls you were serving with so that he would not be caught unawares by a slow ball or a bullet serve. It seems that the original specification of 27 - 33 inches could be reduced to 28 - 32 quite satisfactorily and that Jaques and Barlow comply already. For Walker balls, it only requires the use of fillers or a possible change to another resin.

Kindest regards,

R. le Maitre  ©1988

Author: R. le Maitre
All rights reserved © 1988


Updated 28.i.16
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