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This article is from the In-Depth Report Science at the Winter Olympics

U.S. Olympic Skeleton Team Studies Sled Forces in High-Tech Simulator

Team members used test runs in a simulator created at Rensselaer Polytech to determine the best way to beat wind resistance
Olympics, skeleton, Vancouver,RPI



COURTESY OF U.S. AIR FORCE, VIA WIKIMEDIA

In the sport of skeleton, where athletes called "sliders" hurtle face first atop a sled the size of a seat cushion down an iced-over concrete track at speeds upwards of 110 kilometers per hour, the smallest details separate success and failure. Any aspect of a skeleton run that is not completely in sync—including the slider's outfit, helmet, body position or the sled's orientation—can cost hundredths of a second, an eternity in this sport.

The U.S. Olympic skeleton team, which hits the track this week at the Vancouver games, has their approach down to a science, thanks to some high-tech help from researchers at Rensselaer Polytechnic Institute (RPI) in Troy, N.Y. Engineering professor Timothy Wei and his team built a custom-made skeleton simulator to study a slider's greatest opponent—wind resistance.

"As the athlete goes downhill on the track, they push off with their feet, and what propels them from then on is gravity," Wei says. The three factors working against them are wind resistance (which the RPI researchers have measured), the sled's orientation (keeping it straight to avoid friction), and the slider's ability to run the shortest line down the track (a trick mastered by racecar drivers).

"We built a false bobsled track section and used load cells on the track to measure force," Wei says. This simulator mimicked the dimensions of an actual skeleton track, which is about a half of a meter deep and a little over one meter wide. The sled, which can be as wide as 20 centimeters and as long as 120 centimeters, rested on pads equipped with load cells used to measure force exerted downward by the slider, who could also see feeds from two cameras (one looking down on the slider and the other positioned to the side) on a monitor viewable through a hole cut into the bottom of the track (sliders lie facedown on their sleds). "The athlete can see in real time the affect of their movement on airflow and drag," Wei says. "You know the forces, but you'd also like to know what the flow looks like."

The monitor that the sliders could that see through the bottom of the track was crucial to the experiment. "It's a training tool, so wanted the athletes to have the information directly," says Steve Peters, the U.S. skeleton team's technology coordinator. "We were able to identify trends in terms of what positions work better; some of it was even counterintuitive. Some of this we knew already, but we didn't have the data to back it up."

The simulated track sat at the end of a wind tunnel, which blew air at nearly 100 kilometers per hour. To make wind flow more visible, the researchers introduced fog (the kind used in theater productions) lit up by a green laser beam that created a two-dimensional plane directed over the shoulders of the sliders laying in the simulator. The side camera focused on that plane to provide better perspective on the wind flow.

The skeleton team had done some wind tunnel testing in the past, "but wind-tunnel time is very expensive and we are a low-budget sport for sure," Peters says. "Still, we want to be able to test different positions and equipment."

Wei was already known to the Olympic community through his work with the U.S. swim team. For the past several years, he has been using Digital Particle Image Velocimetry (DPIV) and other measurement techniques to better understand how swimmers interact with pool water.

Ten members of the skeleton team tried the RPI simulator, using different helmets, suits and body positions to determine which combination gave them the best body flow and the least amount of drag. The team determined that different helmet styles work better for different sliders, which means it is important that each slider find the helmet best suited to the way they ride the sled. "Skeleton is very unique to the individual so that's why this testing was so important," Peters says.

Skeleton is a spinoff of a form of tobogganing called cresta sledding, which uses a similar sled and face down position. However, cresta riders use skates on their feet to help steer and brake the sled (skeleton sliders use only their bodies to guide the sled). Skeleton is run on the same type of track used by bobsleds and luge, another sport that involves hurtling oneself along an icy track with only a small sled between the rider and the ice. In luge, however, the rider is positioned face up and feet first on the sled, and speeds can reach 160 kilometers per hour. Luge racers were approaching speeds of up to 155 kilometers per hour during training runs last week in Vancouver when Georgian luge racer Nodar Kumaritashvili was killed during an accident.

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