Swedish friction experts on why curling stones curl

Scottish curler Eve Muirhead Eve Muirhead is one of today's top Scottish players

Swedish friction experts claim to have uncovered the secret to what curving path a curling stone will follow.

Researchers from Uppsala University suggest the microscopic roughness of a stone's underside cuts scratches that steer it across the ice.

Scottish team coach and Winter Olympics gold medal winner Rhona Martin said it was well known in the sport that roughness was key to a stone's curl.

She said the condition of the ice was also a crucial factor.

Researchers Harald Nyberg, Sara Alfredsson, Sture Hogmark and Staffan Jacobson said it had long been known among curlers that the roughness of a stone was important to how it slides across ice.

Start Quote

"If the ice is smooth and the stone is smooth it won't curl."”

End Quote Rhona Martin Scottish team coach

But they said the scientific mechanism involved had not previously been explained in published academic literature.

The team sought to explain why a stone spun by a player in a clockwise rotation curls to the right, and if sent in an anti-clockwise motion it will curl to the left.

In a statement, Uppsala University said: "The rotation of the stone will give the scratches a slight deviation from the sliding direction.

"When the rough protrusions on the trailing half shortly pass the same area, they will cross the scratches from the front in a small angle.

"When crossing these scratches they will have a tendency to follow them. It is this scratch-guiding or track steering mechanism that generate the sideway force necessary to cause the curl."

Prof Jacobson said he and the others worked on the research in their free time following an approach from an ice making expert at the Swedish Curling Federation.

He told BBC Scotland that the scratches cut in the ice were only a few thousandths of a millimetre deep and could only be clearly seen through a microscope.

Prof Jacobson said the study was unlikely to be a "game changer".

The scientists' have published their research in Wear, an international journal on the science and technology of friction and lubrication.

Curler checking stone The rough bottoms of the stones help to guide them across the game's ice sheets

Ms Martin, who won gold with the GB team at Salt Lake City in 2002, said how well a stone curled depended on the roughness of a stone's underside and also the condition of "pebbles" on the ice.

The "pebbles" are small frozen droplets of water sprayed on to the surface of the ice.

Players try to curl their stones around those of their opponents to reach the centre of what is called the house - the area within concentric circles at each end of the ice sheet.

Ms Martin told BBC Radio Scotland's John Beattie programme: "The base of the stone is gripping the pebble on the ice and this causes the stone to curl.

"If the ice is smooth and the stone is smooth it won't curl."

The game of curling has a long history in Scotland.

According to the World Curling Federation, the first written record of curling being played was in Paisley in 1540. It involved a contest between a monk and abbot's representative.

The game's first rules were written in Scotland and formally adopted in 1838 by Edinburgh's Grand Caledonian Curling Club.

Top Scottish players include Stirling's Eve Muirhead and Lockerbie's David Murdoch.

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