Analyzing Performance through USSA
and Ski Club vLink Racing Computer Research

Richard Kirby, Inventor of the vLink Racing Computer

This article, the first on our new vLink Sports Science Corner, describes a technique we’ve developed with the U.S. Ski Team to analyze elite skier performance in an entirely new way. While you won’t learn any technique secrets this time, you will learn to understand the helpful vLink racing computer data that will be shared in future articles.

The data collected for these articles comes from a specially-configured vLink racing computer that is currently only available to the U.S. Ski Team. The standard vLink racing computer provides speed and slip data audibly on the hill, in real time. We hope that the additional research provided in this section will help you even better interpret that feedback and apply it to improve your form and speed.

Special thanks go to Wesley Warner and Justin Samuel, two Rowmark skiers that volunteered to be test subjects during the summer of 2007 at Mount Hood, as well as to Rowmark program director Todd Brickson.

Interpreting the Data
When skiing, your skis can move both forward and sideways. When we say “forward,” we mean the component of the speed that is along the axis of the ski. When the ski is flat, this means that the ski is moving along its centerline. When the ski is on edge, this means the ski is carving cleanly. In Figure 1, you can see Justin carving a clean line. Nearly all of his displacement is in the axis of his skis.

In Figure 2, Wes is demonstrating a classic stivot. While her skis are pointed off to her right (pink arrow), her actual direction of motion is almost directly toward us (green arrow).

One way of looking at this is that Wes has a certain amount of forward speed (the pink arrow in Figure 3) and a certain amount of sideways slip (the blue arrow). We can describe Wes’ motion at this point in time with two numbers: 7.4 m/s is the speed she is moving along the axis of her skis and 0.7 m/s is her lateral speed. If at all possible, of course, you want to minimize lateral displacement, as it is equivalent to hitting the brakes…but more about that in future articles.

While this method of describing Wes’ motion makes it easy to understand how much slip she has, it doesn’t reveal her real speed or demonstrate how her slip is related to her speed. A better way of looking at these same numbers is to use something called a vector, which shows speed and direction relative to the axis of her skis. Wes’ vector at this point in time is shown in Figure 4. The vector tells us how fast Wes is travelling (7.4 m/s) and in which direction relative to the axis of her skis (5.2°).

The larger the angle at the same speed, the more slip. Also, the higher the speed for the same angle, the more slip. Since it is normal to slip sideways faster at higher speeds, this method normalizes the slip to the speed ratio and also shows “real” speed (not just the components of speed).

Now, we are able to build photomontages of Wes and Justin, compare them in the same turn, and analyze why one is faster while the other actually looses speed in a stivot.

Although we won’t analyze Wes and Justin in this article, it is easy to see that Justin is arcing this turn very cleanly and, as a result, has a much faster speed.

This is the type of photomontage we will use in future articles to shed new light on the effects of various techniques and tactics.

 

 

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