Undergraduate engineering students at the University of Vermont have teamed up with a professor to find a technique that could help performance skiers select the ultimate pair of skis.
It all started last fall, when 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 said.
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 starting asking me questions about the stiffness, the bending, the damping and the vibration of skis,” said Hernandez, an assistant professor in the College of Engineering and Mathematical Sciences. He’s an expert on structures—like buildings and bridges—and how their materials change and fatigue over time. “Everything is a structure,” he said, 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,” based on “how they feel, how fast they are in warm-ups, how well they’ve done before,” she said. Some skis, on some days, seemed to hit a “sweet spot,” she said—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,” she said—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.
First tests
A few months later, Terwiel and Hernandez clamped a racing ski to a table in a UVM engineering lab. Then she firmly smacked it with a plastic-tipped hammer. Two accelerometers glued to the front of the ski measured the fluctuating motion of the ski as it vibrated from the blow. Terwiel carefully watched the tip, which was bouncing like a just-sprung diving board. On the other side of the lab, junior Elizabeth Richards, also a civil engineering major, captured the pattern of vibrations on a computer.
With Hernandez’s help, these two undergrads were running an independent research effort to see if they can peer into the composite soul of a racing ski—through its pattern of vibrations. “We tested one ski already,” Terwiel said. “Now we’re about to test all of these pairs,” she said, pointing to five pairs of skis leaning against a table.
The students have discovered that the one ski appears to have four fundamental frequencies—a major one and three lesser ones, like harmonies following the melody.
“Maybe there is a pattern,” Hernandez said. “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.”
Plug-in skis?
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 said. “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 said. Because high-level skiers are doing everything in their power to maximize contact between their ski and the snow, tuning in to these vibrations, which 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 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 in 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 said, “that actually could be a viable business opportunity.”
