Roof turns Dunedin into kickers' nightmare

MARK GEENTY
Last updated 05:00 18/09/2012
Dunedin Stadium
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Scientist Brian Wilkins says the roof on Dunedin's Forsyth Barr Stadium will continue to make life difficult for goal kickers.

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There could be a scientific explanation for the failure of some international pivots to "get their kicks" in Dunedin's stadium. Mark Geenty reports. 

Dunedin's house of horrors will continue to haunt visiting goalkickers as long as it remains under a roof, a Wellington scientist says.

Brian Wilkins, who has written books on how atmospheric conditions assist a cricket ball to swing, says it's no coincidence that New Zealand's only covered rugby stadium causes nightmares for some of the best kickers.

He says the lack of air turbulence under the Forsyth Barr Stadium roof means any ball hit slightly off centre will deviate further than it would outside.

Springbok Morne Steyn was the latest sharpshooter to discover that in Saturday's test defeat to the All Blacks when he kicked just one from five attempts, some of them skewing wildly.

Wilkins says it relates to the lateral Magnus/Robins force, which makes any spun ball curve when the air flowing around it leaves the surface earlier on one side than on the other.

"In rugby, most kickers don't strike the ball dead centre and their boot doesn't come through straight in line with the target; they come around the ball slightly," Wilkins said.

"They're almost guaranteed to put a slight sideways spin on the ball. It's the same sort of thing as table tennis, when you put side spin on the ball it swerves.

"It's the smooth non-turbulent air [under the roof] which accentuates all these phenomena. It's only got to be a very slight rotation for it to take off and develop into a big curve."

After reading yesterday's story in the Dominion Post on the kicking woes, Wilkins wrote in with his low turbulence theory and offered some free advice for visiting kickers in Dunedin: straighten your approach and if you don't hit the ball low and dead centre, and follow through straight, you could be in trouble.

Springboks captain Jean de Villiers suggested the switch from the Gilbert ball which is used in South Africa, to the adidas ball used in New Zealand, was a factor.

England's Jonny Wilkinson also blamed the ball during his struggles during last year's World Cup, when he kicked seven from 14 in Dunedin.

Wilkins said the atmospheric conditions would play a much bigger part and left far less margin for error, regardless of which ball was used. It was during the World Cup that he took a keen interest in why the kickers struggled in Dunedin and he soon settled on the low turbulence theory.

"Only those who kick straight through to send the ball tumbling straight, under and over, around a horizontal axis, will succeed . . . in low turbulence air."

The statistics back up the theory. At the World Cup, kickers made 32 of 59 attempts (54 per cent) and in this year's Super Rugby it was 64 from 97 (65 per cent).

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Wilkins' previous experiments with cricket balls, using a specially made bowling machine in varying wind conditions, proved they swung much more in still, non-turbulent air.

Another theory offered up at Saturday's test was that gaps at the top and bottom of the stadium, to encourage grass growth, created a tricky breeze. But Forsyth Barr Stadium chief executive David Davies booted any atmospheric theories into touch.

"My view is there was nothing atmospherically [to blame]. The reality is that, at times, people kick badly and are looking for any excuse."

- © Fairfax NZ News

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