What Makes a Ski Hum?
- By Joshua E. Brown
Last fall, UVM senior Elli Terwiel was in professor Eric Hernandez’s course, Advanced Structural Analysis, learning about how beams bend under a load—“and I started to think about my skis,” she says. She wasn’t daydreaming.
Instead, she was seeing a connection between her two passions. Terwiel is majoring in civil engineering—and she’s a World Cup skier who raced in the 2014 Olympics for Canada and for UVM’s ski team for several years.
“After class, she started asking me questions about the stiffness, the bending, the damping and the vibration of skis,” says Hernandez. He’s an expert on structures—like buildings and bridges—and how their materials change and fatigue over time. “Everything is a structure,” he says, including skis.
Terwiel had noticed, in her years on the race circuit, that skiers choose which skis to race with—“from a quiver of maybe five or ten pairs,” she says—based on “how they feel, how fast they are in warm-ups, how well they’ve done before,” she says. Some skis, on some days, seemed to hit a “sweet spot,” she says—when others were duds. Terwiel wondered if there was some objective measure of whether a pair of skis was in the sweet spot.
Terwiel also noticed that it wasn’t necessarily her newest skis that worked best. They often seemed to need a break-in period. And after several months of hard skiing—“six or eight hammering runs on ice everyday gives some wear”—the performance of many pairs of skis declined. Two pairs of apparently identical skis—the same manufacturer and model—might give a skier consistently different results, a half second or more. And in alpine ski racing, the blink of an eye can separate the winner of an Olympic gold medal from tenth place.
“I said, ‘we need to look at this,’” Hernandez told her. He’s developing new techniques to measure the structural health of buildings and bridges—probing their inner wear and tear by measuring various types of vibrations, whether from earthquakes or truck traffic. Together, Hernandez and Terwiel wondered if vibration testing of skis—not during manufacturing, but on race day—could help skiers find the right pair.
A few months later, Terwiel and Hernandez clamp a racing ski to a table in a UVM engineering lab. Then she firmly smacks it with a plastic-tipped hammer. Two accelerometers glued to the front of the ski measure the fluctuating motion of the ski as it vibrates from the blow. Terwiel carefully watches the tip bouncing like a just-sprung diving board.
On the other side of the lab, junior Elizabeth Richards, also a civil engineering major, captures the pattern of vibrations on a computer. This is not a class project; it’s a life project. With Hernandez’s help, these two undergraduates are running an independent research effort to see if they can peer into the composite soul of a racing ski—in its pattern of vibrations.
“They’re at the beginning now,” Hernandez says. “We’re not sure what we’re going to find.”
“We tested one ski already,” Terwiel says. “Now we’re about to test all of these pairs,” she says, pointing to five pairs of skis leaning against a table, including the pair she raced in the slalom in the Olympics. So far, the students have discovered that the one ski appears to have four fundamental frequencies—a major one, but then three other lesser ones, like harmonies following the melody.
“Maybe there is a pattern,” Hernandez says. “This kind of frequency, with this kind of damping, on this kind of ski course, will be really fast for that particular athlete. That knowledge could allow a skier or coach to make choices between skis in a predictive way.”
Terwiel straps down another ski.
“BLK15803,” she says to Richards.
“Go ahead,” Richards says.
Terwiel hits the ski—one, two, three, four, five times at different spots—watching each time, for several long seconds, while the vibration fades out. “You can totally see that second mode,” Terwiel says. Each hit shows up on Richards’ computer as a diminishing set of blue squiggles playing across the screen.
“End of test,” Terwiel says.
Olympic medals have been won by one hundredth of a second. “I was second on a World Cup run by four hundredths of a second,” Terwiel says. “The fact that our vibrational frequencies are within hundredths of a second means that they are quite relevant to how you're actually skiing on them,” she says. Because high-level skiers are doing everything in their power to maximize contact between their ski and the snow, tuning into these vibrations—that affect how much of the ski is on the snow—could prove to be an advantage.
If the signals they find here start to show some patterns—some meaningful differences between the skis—the students plan to take this into the field, or, rather, onto the slopes. With wireless sensors, they’d like to run these same kinds of experiments while a racer goes zipping around some slalom gates.
Who knows where this exploration might lead. Accelerometers and other sensors are now tiny enough that they might be embedded into a ski. If a skier or technician could “just plug into a USB port and display something about how that ski is running right now,” Terwiel says, “that actually could be a viable business opportunity.”
Elli Terwiel retired from World Cup skiing last year. “I had four concussions. I had slipped and compressed a disc in my back. I was unable to ski at the level I wanted to, without being in pain everyday,” she says. “But I still love my sport, and I’d like to find some useful information, give something back."