Pray for headwinds in the discus event

2012-07-21 00:00

FOR an event featured 2 719 years ago in the first pentathlon, the discus throw is long overdue for an analysis more sophisticated than “chuck it as hard as you can”.

Thanks to several new three-dimensional studies in biomechanics labs, the ancient event is finally getting its due. Based on the latest findings, fans hoping to see a new world record should pray for headwinds.

The world’s first Frisbee gets most of its distance from the angle of its rising trajectory and its speed at release.

Rotation also matters: a typical spin of 400 revolutions a minute stabilises the discus in flight, much as a jack or bicycle wheel stays upright as long as it spins.

Release speed is all about power and technique, of course, which come from strength training and practice. But the flight angle makes a less obvious contribution to distance, and so has been the main focus of sports scientists.

For heavy objects like missiles or baseballs, basic equations of motion show that a 45-degree angle produces the greatest distance. But the onrushing air affects the discus much more significantly, said Mont Hubbard, director of the sports biomechanics lab at the University of California, Davis.

One force, drag, slows its motion. But another force, more interestingly, is lift, “exactly the same force that keeps planes in the sky,” Hubbard said. The longer the discus is aloft, the farther it will fly. The key to a winning discus throw is therefore maximizing lift.

Lift is a function of several factors. One is the speed of the object relative to the air: greater speed, greater lift. Another is the shape and size of the flying object: flat discs and airplane wings with lots of surface area generate more lift than a skinny pencil or missile. The third is the object’s angle. A discus flying horizontally with its front edge higher than its back generates more lift than one flying flat.

Athletes can not do anything about the shape of the discus, of course. And while they presumably throw with as much speed as they can, they can increase the discus’s relative wind speed by throwing it into the wind as much as possible.

“With the wind blowing five metres per second in your face, you can throw the discus 10 metres farther for men and 14 metres for women compared to a 10 metres per second tailwind,” said Hubbard.

“Throwing into a headwind generates more lift, which keeps the discus aloft longer.”

The gender difference comes partly from the greater weight of the men’s discus, two kilograms compared to one kilogram for women, and partly from its greater diameter.

The discus’ orientation is also under the thrower’s control. By plugging thousands of combinations of these properties into the equations describing a discus’s flight, Hubbard and Kuangyo Cheng of the National Cheng Kung University on Taiwan calculated the angles producing the greatest distance for a set release speed.

The optimal flight-path angle is 38,4 degrees, flatter than one might expect.

The optimal roll angle — the angle the discus is tilted to the right as seen by the athlete — is 54,4 degrees. That lets the lift force stay nearly vertical the longest, especially near the end of the flight, keeping the discus aloft.

And the optimal “attitude” — the angle of the discus relative to its forward direction, where an angle of zero means flat — is 28 or 29 degrees, said Dapena.

Throwers can gain an edge by playing the wind, Hubbard’s calculations show.

“In a very strong direct headwind, they can get the greatest distance with a slicing strategy: throwing the discus tilted vertically so it slices through the wind like a knife,” he said. “In a strong tail wind, a kiting strategy is better: throwing the discus so the wind pushes it like a kite.”

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