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Gear West: Classic Skis and Correlation to Flex Data

Flex data Doug Debold’s 2013 winning Birkie classic ski.

Flex data Doug Debold’s 2013 winning Birkie classic ski.

By: Matt Liebsch at Gear West
At Gear West we have a strong understanding of flex data and how it relates to glide properties of skate skis.  Looking at classic skis has been more involved because there are more variables compared to skating, both kick and glide.  Over the past year we have spent considerable time looking at bad, good and great classic skis.  We look at how the flex pattern changes with load and the static shapes at a given skier body weight.  We take the gathered flex data and draw a parallel to how the ski actually performs on the trails.
New software implements to our flex tester from Josh Dobbert have allowed us to set a pocket length and sum the given load inside the kick zone during kick and glide phases.  This is exciting because we can calculate a numeric “kick force” value for a classic ski.  Ideally, at 50% body weight a ski should not have any load within the kick zone (double poling and tucking) and at full kicking weight the ski needs to have load applied inside the kick zone.
What do we know about “kick force” and classic skis??
In order for a ski to have a chance at kicking the kick force needs to be roughly 33-50% of a skier’s body weight.  Less than 33% and the ski can be very difficult to kick or is a specialty klister ski with a large amount of residual pocket.
Skis that have load inside the kick zone at 50% load are generally too soft and tend to be draggy.  Unless you are specifically looking for a colder powder ski.
Shape matters.  When looking at the dynamic load response of a classic ski the manner in which the pocket closes is very important.  Skis that initiate kick right at the load point/balance point are generally too inelastic within the pocket to kick well no matter what kick force value.  These skis generally “punch” the snow too aggressively and only seem to kick in easy kicking conditions.  Skis with pockets that close from the front of the pocket or both front and back of the pocket simultaneously are very easy to kick due to the manner in which they interact with the snow.  They tend to “paw” the snow.  Pair a strong kick force with the correct pocket action and you will have a brilliant classic ski.
My favorite classic ski has a kick force of roughly 70% yet holds the pocket open at half my body weight.  This is what we at Gear West refer to as “magic skis” I can literally lightly sand my ski in the pocket and the thing kicks.  Yet the ski was picked specifically as a Klister ski.  902/speedmax mold from Fischer.
For another example, let’s look at Doug Debold’s 2013 winning Birkie classic ski that I picked for him out of the Salomon service center in Ogden Utah in summer of 2012.
Things to notice include…
Smooth pressure at 50% load.  This ski is designed for cold snow. But it holds the pocket open = fast.
The kick force for a 180lb Doug Debold is roughly 50%
The pocket closes from the front, makes for easy kicking
One last takeaway is that a highly trained set of eyes and hands can pick very good classic skis.  Our flex tester allows us to take ski fitting one step further and verify our selections.  We can also very easily identify classic skis that just won’t work for anyone… these skis are promptly turned into firewood.

Comments

  1. Matt,
    Thank you for clarifying a somewhat cloudy issue: the “kick force” value. Unfortunately, my own inexperience makes it difficult for me to follow the ‘progression, from 50% of body weight: no load inside kick zone. to “full kicking weight”: full body weight plus ?, to “kick force”.
    The technique and concepts are familiar to me, but I’m struggling with the terminology. What, precisely, does it mean to say that a “kick force” is 50% ?
    Thanks again,
    Mike

  2. gearwestski says:

    1harpoon –
    Reading through the article again, I can see how the terminology we used is a little confusing, so let me try to clarify (note: this is Josh, the flex testing lead at GW).
    When I look at a classic ski, I note 3 things: initial pocket engagement, the final kick force, and the distribution of load at full body weight. I will also watch to see how the ski transitions through the load cycle and get a general feeling of the character of the ski, but the IPE, FKF, and distribution give me specific data points to compare across all other classic skis we’ve looked at.
    Initial pocket engagement is defined as the load at which the any part of the pocket begins to carry any load. Typically, this engagement happens at the front of the pocket due to our loading methodology. We load solely at the ball of the foot, which is admittedly not a realistic model of a gait cycle (when skiing, you roll from your heel or midfoot to your toe and thus engage the back of the pocket first), but it still gives us a pretty clear picture of what will happen on snow. We generally look for this value to be between 50 and 55 percent of body weight, although acceptable values can be higher or lower than this depending on the desired use of the ski (i.e. specific conditions) as well as skier strength and form (weaker skiers may wish to trade glide for easier kick).
    Final kick force is determined under the skier’s full body weight and is the percentage of their body weight that lands within the pocket vs outside. In general, the higher this is, the better, although in very rare cases, the ski can “flip” so hard that it actually becomes difficult to control on snow (there’s not enough stability in the ski).
    I think Matt did a pretty good job of explaining the distribution, so I won’t go over it again aside from saying that a good portion of our current research is going into a better way of quantifying this. Right now, there’s a lot more qualitative judgement calling based on experience than I would like – as an engineer, I prefer the cold comfort of hard numbers :-)
    Hope that helps clarify what Matt was talking about.

    -Josh Doebbert
    Flex Specialist
    Gear West

  3. WI skier says:

    How is the pocket length defined? Before reading this I would have though that the pocket length would be defined by where the ski starts to close with 50% weight (or some other value). If that is the the case IPE would always be between correct by definition. This would lead me to believe the pocket length is defined ahead of time based off of some sort of distance measurement not impacted by weight. Otherwise checking IPE would be kind of meaningless as defined above. Can you clarify?

  4. gearwestski says:

    WI skier -
    Pocket length is measured beforehand by doing a paper test if needed; it isn’t measured or marked on the machine. Importantly, paper tests are measured with the skier rolled back on their heel and with all their weight on the one foot to simulate gliding, but on the flex tester we never look at this type of loading. We only collect data with load applied at the ball of the foot, which would represent the kick phase of a stride. The way that a ski transitions from one type of loading to another can vary wildly from pair to pair and even sometimes within a pair. With that in mind, the IPE is simply a way to measure or quantify how “easily” the ski makes this transition. The FKF, conversely, would be a measurement of how solidly the pocket grips the snow after the transition has occurred.
    Hope that helps.

    -Josh

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