Good vibrations

OLYMPIAN TERWIEL TAKES HER SKIS INTO THE LAB

Give it a smack: Elli Terwiel hits her ski with a hammer to test the vibrational frequencies. - Photo by Joshua Brown
Give it a smack: Elli Terwiel hits her ski with a hammer to test the vibrational frequencies. – Photo by Joshua Brown

A local Olympic skier has taken her passion for skiing into the lab to find out if there is a pattern that could help racers choose their fastest skis during competition.

Elli Terwiel, who grew up racing with the Sun Peaks Alpine Club, is testing the vibrations of skis as she works to complete a civil engineering degree at the University of Vermont. The research idea came to mind during Terwiel’s classes on how structures vibrate during earthquakes or extreme events.

“When we’re doing structural analysis of buildings, a lot of time you’re looking at a beam and it’s supported on the ends by two columns and it’s got a load on the top of it that makes it bend, and I started thinking about how my skis bend and it kind of went from there,” Terwiel said.

I would like to definitely test some more, get some different brands in there as well to really see if we can put together a real sequence of ‘this is what a ski’s vibration looks like.’

She teamed up with her professor Eric Hernandez and fellow civil engineering student Elizabeth Richards to start testing skis’ vibrations.

“My expertise is obviously in skiing, (Hernandez’s) expertise is in research and vibrations so without his knowledge of how to look at vibrations and really understand what they mean, this research would not be nearly as effective,” Terwiel said.

During testing, the ski is strapped to a solid surface at the toe of the binding with the front of the ski overhanging a ledge. Then Terwiel hits it with an impact hammer. Two accelerometers attached to the ski measure movement resulting from the different hits.

(Left to right) Elizabeth Richards, Professor Eric Hernanzez and Elli Terwiel test the ski's vibration. - Photo by Joshua Brown
(Left to right) Elizabeth Richards, Professor Eric Hernanzez and Elli Terwiel test the ski’s vibration. – Photo by Joshua Brown

So far they have tested four pairs of skis, which Terwiel raced on for various lengths of time, ranging from a brand new set to ones with an entire season’s use. Testing is still in preliminary stages, but they have noticed four fundamental frequency modes.

“There’s a really strong spike that represents the first mode that looks like doesn’t get dampened quite well and then there’s three modes after that that seems to get dampened much more quickly by the ski,” Terwiel said. “That seems quite reasonable. The first mode would be related to the actual smack of the hammer. When you’re skiing it’s not like someone is smacking a hammer on your skis, it’s much smaller vibrations, so the way the ski is dampening seems to line up with how it would react on the hill.”

Terwiel said she wants to test more skis to compare the difference between the left and right ski, different brands of skis and skis with different amounts of use.

“I would like to definitely test some more, get some different brands in there as well to really see if we can put together a real sequence of ‘this is what a ski’s vibration looks like’. Not just this one particular ski, but a general idea and being able to decipher from there which ones are more fatigued skis from their frequencies,” she said.

…the way the ski is dampening seems to line up with how it would react on the hill.

The next step would be to attach accelerometers to skis on the mountain and test the differences between what was seen in the lab and what is noticed while skiing. One day, Terwiel said she hopes it can become a tool for ski racers to help them choose which ski to use for a particular race.

“If you could attach a sensor to your ski on the training day before you race and you are able to see that this ski is actually in the mode or it’s exhibiting the characteristics of the frequency you know you are fast on, you’d be able to choose that ski for your race day,” Terwiel said. “I can see where it would go. We’re in such a preliminary stage of research that’s definitely a long ways away, but it’s fun to play with.”

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