The Science of Curling (Part 2)

"Delivery is all about balance," explains Dion Warr, Vice President of the Schenectady (N.Y.) Curling Club. After pushing-off from the hack, a thrower "slides out behind the stone for a fair amount of distance". The curler's lead foot, clad in a shoe with a slippery sole, bears most of the weight of the body. The player, with forward knee bent and rear leg trailing behind, may slide as far as 20-feet before releasing the rock with a curl. Leg strength and abdominal strength are also important, Warr adds, and even casual curlers must be "firm and fit" to enjoy success on the ice.

Stones and Hog Lines

Curling stones are made of polished granite and have a circular running surface of just 4-inches in diameter. They also feature a detachable, molded plastic handle that the thrower grips with his or her dominant hand. Typically, a long-handled curling broom is used for counterbalance. After pushing-off from the hack, the player must keep the curling stone in front of the body and in contact with ice. The rules of the game require the curler to release the stone before it reaches the hog line, a boundary that resembles one of the lines in a hockey rink.

Clockwise or Counterclockwise

The curl that the thrower imparts before releasing the rock determines the stone's initial trajectory. Releasing the stone clockwise causes it to curl to the right. Releasing the stone counterclockwise causes it to curl to the left. Regardless of direction, Warr explains, the stone always has a rotation because it travels over a bumpy surface. "When we aim, we never actually aim at where the stone ends up," he adds. Rather, "we aim at a point and let the stone curl there".

Sweeping and Scrubbing

The faster a stone travels, the less its curl. When a stone curls too much, however, players minimize the amount of curl by sweeping the ice with a specialized broom. Traditionally, curling booms were equipped with wooden or fiberglass shafts. Today, Warr explains, "all the top players use carbon fiber shafts" instead. But shaft material isn't the only thing that makes a curling broom different from a commonplace kitchen sweep. In curling, the broom head itself is just a thin pad covered in Cordera, a synthetic material. Most players prefer a broom head that can swivel, Warr adds, "so that they can adjust the angle."

Pebbles, Pads and Particles

During sweeping, the curlers wear down the "pebble", the name given to the bumpy surface that distinguishes the curling sheet from the smoother ice preferred by figure skaters. With curling brooms in hand, sweepers push downward rapidly and repeatedly, scrubbing the ice with their coarse Cordera pads. "Good, powerful sweeping is really a matter of scrubbing the ice as fast as you can," Warr explains. But don't expect the ice to melt much – the players and their brooms don't produce enough heat for that. Curling creates "a microscopic slush of particles", Warr notes, but there's only a "small amount of melting" involved.

Friction and Lubrication

The pebble's phase change from ice to water (or at least from a solid to slurry) provides the moving stone with lubrication. On a ring, the force is the same in every direction. On a heavy curling stone, however, there is a "braking effect" at the front that increases the friction force there. With more of the melted ice moving towards the front, sweeping lets the rock glide on the ice and towards the button of the house - the curler's ultimate goal.

Editor's Note: Click here for Part 1 of this two-part story.