The Importance of Femur Rotation in Carving

Discussion in 'Ski School' started by Mike King, Aug 19, 2019.

  1. Mike King

    Mike King AKA Habacomike Instructor

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    An amazing video from Checkpoint. I love the visual example of the guy on the bungee and two slippery surfaces. What I take away from this is the importance of creating the lateral force toward the inside of the turn, which as I see it comes from femur rotation.



    Feel free to disabuse me of my interpretation. It is summer after all.

    Mike
     
  2. LiquidFeet

    LiquidFeet Making fresh tracks Instructor

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    I've always loved this sequence. Femur rotation shows up well in these turns,
    but it doesn't cause carving with tails following tips. It redirects the skis while they are light/airborne.

     
  3. LiquidFeet

    LiquidFeet Making fresh tracks Instructor

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    Agree, to a point. Femur rotation allows this skier to manipulate platform angle to be at or less than 90º since it moves the heavy torso to the outside of the turn, over the outside ski. That causes the skis to grip not slip.
     
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    Mike King

    Mike King AKA Habacomike Instructor

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    I don't think it is just moving the body to the outside. It is also creating a lateral force to the inside. Watch the video at 3:30.

    Mike
     
  5. LiquidFeet

    LiquidFeet Making fresh tracks Instructor

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    Yeah, it's like his lower legs could push the slider away, but his upper body is counteracting that move, effectively making his body go in a sideways V. The two movements sum to zero and the feet stay where they are.
    Clumsy wording on my part, but I think this is what the video is getting at.
    Not sure platform angle is not an integral part of that, though.

    Where are our engineers who can make the mechanics of this easy to understand without equations or unfamiliar physics-style technical terms?
     


  6. Erik Timmerman

    Erik Timmerman Making fresh tracks Instructor

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    Cool video. I'm going to digest it a bit more before commenting.
     
  7. karlo

    karlo Out on the slopes Skier

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    When inclining, place both feet on the upper plate, the one sliding on the lower plate. When angulating, place one foot a bit off the edge of the top plate, so when one angulates, the edge of that foot is on the non-sliding plate. Oh, and when doing that, make sure to pressure the inside foot. The video demonstration is phony.

    Edges of skis have started to cut in early in a turn. Once on edge and COM properly balanced on the edges, they will cut in and hold. The skis are not on a sliding plate. Nevertheless, I think adductors and abductors are needed to achieve proper angulation, but I'm no physiatrist, nor a biomechanical engineer.
     
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    Mike King

    Mike King AKA Habacomike Instructor

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    @LiquidFeet, I think you are describing angulation, but I think this is not just angulation, but it is the specific moves that are necessary to "pull" the feet back under the body. That is, by activating the abductors on the outside ledge and the adductors on the inside leg, you create a pull of the feet and legs to the inside of the turn. It is related to angulation, as the result will assist in creating angulation, but its also a way of bringing several other pieces together. Such as the "strong inside half:" the adduction of the inside leg pulls the inside foot into the turn. This results in JF's inside ski is the driver, outside is the rider.

    Anyway, perhaps I'm splitting hairs, but I do think this is a bit more than angulation.

    Mike
     
  9. James

    James Skiing the powder Instructor

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    Witherell talked a little about this in his 1992 book. Using the rotation to drive the tip in.

    I don’t think I buy the “crystal clear” demo. Looks clear to me when he doesn’t slide he angulates first, when he slides he just banks in. Has little to do with the rotation. Unless the claim was rotation as relates to angulation, which it wasn’t. Also don’t think I buy the demos using plastic ski models.

    It’s also not necessary to carve as he implies. Whether it’s desireable and how much is a point of debate.

    Btw, if the text doesn’t show, watch it on youtube and adjust the playback settings.
     
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    Mike King

    Mike King AKA Habacomike Instructor

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    @James, I don't think he's trying to engage the tips -- look at where he is on the ski, particularly as he is obtaining the rebound and it looks to me like he is very much in the center, not forward on the tip. I think it is about trying to engage the entire edge of the ski so that it bites, not that you are engaging the tip and having it drag the ski into the turn.

    Mike
     
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    Mike King

    Mike King AKA Habacomike Instructor

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    Sorry, around 4:25.
     
  12. François Pugh

    François Pugh Making fresh tracks Skier

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    It is angulation with details. With angulation, the bodies weight is transmitted to the ski in the proper direction through the muscles of the legs pushing towards the snow, as detailed in the video.
     
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  13. Thread Starter
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    Mike King

    Mike King AKA Habacomike Instructor

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    That seems reasonable to me.
     
  14. LiquidFeet

    LiquidFeet Making fresh tracks Instructor

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    Yes, agree, and....

    Angulation can be accomplished several ways. I like pulling the inside foot back up under me to increase edges, shorten radius, and maintain hold by assuring that platform angle is sufficient for grip. This sounds like what you are describing.

    I/you/others can also knee-wag for knee angulation. Aka femur rotation. Skis will carve.
    Or one can move the hip in, then shorten the inside leg for additional radius-control.

    Others lean outward with the shoulders, or lift the inside hip/shoulder/half. It works, but leaves out the important role of the inside leg.

    The video also points out that ankle-tipping is part of this whole thing. Agree, totally.
     
  15. James

    James Skiing the powder Instructor

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    I’m going from the description and demo off snow. I didn’t watch the skiing. I wasn’t talking fore/aft. Look at the image below.
    You’re driving the front of the ski more into the snow, no? How else does rotation affect a well edged ski?
    Are we saying that there’s no effect on the ski and it only increases angulation?
    06E5F3BF-0BEE-4130-BBAF-AB384BEE3DC1.jpeg
     
  16. Scruffy

    Scruffy Out on the slopes Skier

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    Here ya go: read at your own risk. ( note: this is the middle of a looong blog discussion. The author is very knowledgeable and knows his shit, but doles it out in very small doses. You need to read his whole web site to get anything out of it. Also at risk is: you may alter you boots beyond normal convention. )

    https://skimoves.me/2019/02/03/the-mechanics-biomechanics-of-platform-angle-part-6/
     
  17. LiquidFeet

    LiquidFeet Making fresh tracks Instructor

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    No no no please not that guy.
     
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  18. mdf

    mdf entering the Big Couloir Skier

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    "part 6"?
     
  19. James

    James Skiing the powder Instructor

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    There's like 12 if you can find them.
    Reading is another thing.
    IMG_6643.JPG
     
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  20. razie

    razie Sir Shiftsalot Skier

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    They produce a lot of good videos that I enjoy, but I have to disagree with this one. The force in that "opposing" direction comes from decomposing the "diagonal" force from the skier into normal (vertical) and outwards. The platform angle got enough coverage and the ski is designed to engage and grab and the component that's normal to the surface makes it grab (G in the photo below) :

    [​IMG]

    So, simply by increasing the edge angles sufficiently for the ski to engage, before increasing the lateral force (I think WW coined this in much better words) we get the ski engaged and sliding, i.e. carving - and I think that's pretty much also what they're demonstrations in the gym show.

    Femur rotation is, of course, important, but it's not the cause that keeps the ski slicing - that's a common mis-conception in my mind. It's worth looking into the difference between "active" and "passive" femur rotation - there's a detailed discussion here. In fact, in the video, seems to me he's mentioning the adductors and abductors sort of leveraging the skis into the snow and keeping them "in", rather than rotations. Of course we'll keep the feet where we need them and abductors/adductors have important roles to play in this, but not to create relevant "shear" forces.

    cheers
     
    Last edited: Aug 19, 2019
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