Transitions

[markojp]

Guided discovery.... I'm not going to tell you directly what I'm trying to get you to do, but I know exactly what it is, and you have to figure it out. Once you snatch the day's pebble from my all too gracious hand, you can credit yourself for having pleased me.... a veritable Stockholm syndrome of fun and excitement on the slopes.

 

[mister moose]

dominant edge (due to weight and angle) changes.

Yes! Best description so far, IMHO. Notice I deleted the wedge qualifier, it's universal. The edge can change from left to right, or the edge can change from equal to dominant. No mention of COM inside the turn, whatever that means, as the COM in a wedge stays between the feet, for mere mortals anyway.

Some folk may want to divide it into two separate parts, left accelerating force stops, right accelerating force begins. Typically the old turning force stops first, then the new turning force begins, but it doesn't absolutely have to be done in that order; we can all do some tricks.

And then you lost me. Sure you can talk about micro movements, but the sum of the parts has to be either moving in a leftward rotational motion, or straight, or a rightward rotational motion. There is no both.

 

[François Pugh]

And then you lost me. Sure you can talk about micro movements, but the sum of the parts has to be either moving in a leftward rotational motion, or straight, or a rightward rotational motion. There is no both.

Don't worry about it; it's just a trick. Technically you CAN start the right turning force before killing off the left turning force. The net force will switch from left to right, but if you break it down into separate forces, the timing of the forces is up to you. I wouldn't recommend it (having forces in two directions at the same time with force varying with time such that net force changes direction), but it is possible. Gotta cover all the bases.

 

[Mike King]

... No mention of COM inside the turn, whatever that means, as the COM in a wedge stays between the feet, for mere mortals anyway.

In a wedge turn, the CoM is inside the turn since a wedge is formed by laterally moving the feet from the centerline of the body.

If the CoM is not inside the turn, centripetal force cannot act upon it to alter its path. And that's the definition of a turn.

Mike

 

[mister moose]

In a wedge turn, the CoM is inside the turn since a wedge is formed by laterally moving the feet from the centerline of the body.

If the CoM is not inside the turn, centripetal force cannot act upon it to alter its path. And that's the definition of a turn.

Mike

Part of what I'm after here is "CoM is inside the turn" is tough for me to grasp what you're saying. It's a very squishy term that means something to you, but I don't know what it refers to, and I think by now you know I have a good background in rotational dynamics. "Inside the turn" is a place I'm not sure what you mean, "the turn" is not a point. I'm belaboring this to try to get my point across, and also to say if I'm having trouble visualizing your description, your students might be too. Another part I'm after is I think it's just one side of the equation. To me it says, "If you have rotational forces (ie centripetal force occurs) , then you're in a turn. That's just restating the premise a different way. Rotational forces means a turn is occurring. One is the other. Put another way, the question as I see it is what has to happen to change from a left to right turn, and I don't think saying a turn must exist in order to declare that a new turn has occurred gets us very far. Thirdly, I think you're wrong on COM moving. In a wedge turn, not a dynamic wedge turn, I believe all day long you can turn WITHOUT moving the COM. In a wedge turn you can hold your body rigid like a car, and a car's COM does not move in a turn. (Let's not get into sprung and unsprung masses) Which is why I said tricycle earlier. Fourth, I just used you as an example of added complexity over FP's nicely condensed and simplified description, it had to be somebody.

 

[geepers]

I'm paid pretty darn well to help people with their skiing. I'm happy to do this on the hill. It matters to them . It matters to me as well. We have fun and enjoy working together. In these instruction threads, words twisted around words spun into paragraphs into pages of endless bickering, chest beating, and verbose molehills posing as mountains. Watching a couple of videos of great skiing and doing some basic MA would put the confusion and BS out to pasture.... and it would be fun. Yes, there are some good nuggets to be found, but we need to be careful not to suffocate the pony under the proverbial pile of dung.

Oh, golly gosh. Would it be over the top to point out that "I'm paid pretty darn well to help people with their skiing" is a prime example of chest beating?

People learn different ways. Some have to spend time thinking/reflecting/absorbing before the gritty bits of on-snow wisdom carefully doled out by the ski gawds become pearls of skiing motion.

Having an instructor/trainer who makes it fun is kind of nice. But it's not essential (for me). About the best single session on snow I can recall was with the gruffest instructor I ever had. (I was to learn later that inside that gruff exterior was, apparently, an equally gruff interior....) The dude knew his stuff, it helped the skiing and despite the relatively low fun level - relatively low as we're talking about time spent skiing and how bad can that be - I would be damned pleased to pay for more instruction from him any time. OTOH I've had a couple of lessons where the instructors were barrels of laughs and thoroughly entertaining. Didn't happen to get any skiing insights (and I'm quite easy to please). Nice folk to drink coffee/beer with but not going to actually pay for lessons with them.

Posting in a ski blog is (mostly) harmless. It's not like folk are going at pace down the hill with no control taking out innocent bystanders. However weird opinions may be.

Be useful if you could hang around and offer the occasional pointer. It's unlikely many of us will end up with snow time in your class so occasional written nudges back in the right direction is about the best we're going to do.

 

[François Pugh]

Part of what I'm after here is "CoM is inside the turn" is tough for me to grasp what you're saying. It's a very squishy term that means something to you, but I don't know what it refers to, and I think by now you know I have a good background in rotational dynamics. "Inside the turn" is a place I'm not sure what you mean, "the turn" is not a point. I'm belaboring this to try to get my point across, and also to say if I'm having trouble visualizing your description, your students might be too. Another part I'm after is I think it's just one side of the equation. To me it says, "If you have rotational forces (ie centripetal force occurs) , then you're in a turn. That's just restating the premise a different way. Rotational forces means a turn is occurring. One is the other. Put another way, the question as I see it is what has to happen to change from a left to right turn, and I don't think saying a turn must exist in order to declare that a new turn has occurred gets us very far. Thirdly, I think you're wrong on COM moving. In a wedge turn, not a dynamic wedge turn, I believe all day long you can turn WITHOUT moving the COM. In a wedge turn you can hold your body rigid like a car, and a car's COM does not move in a turn. (Let's not get into sprung and unsprung masses) Which is why I said tricycle earlier. Fourth, I just used you as an example of added complexity over FP's nicely condensed and simplified description, it had to be somebody.

We all have our own obvious meanings, and sometimes it's not obvious to other people. For example "rotational forces" does not mean the same to me as what I think it means to you. You can think of "inside the turn" as to the left in a left turn and to the right in a right turn. Also it's hard for a ski to push something left unless that something is to the left of it. Skis, most often when used correctly are pushing us, not pulling us.
Hence, I agree with @Mike King the CofM must be on the proper side of the ski to be pushed. That's what @Mike King 's post means to me. In a wedge turn the CofM is inside the turn relative to the edge that's doing the pushing (the dominant edge).

 

[markojp]

Oh, golly gosh. Would it be over the top to point out that "I'm paid pretty darn well to help people with their skiing" is a prime example of chest beating?

.
I hear what you’re saying. But it doesn’t mean we shouldn’t care. People spend alot of money to ski with some of us.

Geepers, my 'chest beating' was in response to Loki's post . ^^^^

To shamelessly steal a marketing line from a major ski manufacturer, I like to think of solid, useful, informative, memorable coaching as 'serious fun'. I suppose if I don't add at least three paragraphs explaining 'fun's vs. 'serious fun', there'll be ample enough room for confusion to drive another 4-5 pages of discussion.

Skiing is simple. Skis on edge change the direction and/or speed down the hill. Flattening one's skis makes them point down the hill. There are several ways to flatten a ski. Some are effective/efficient, many are not. Skiing 'well' , that's the rub.

Anyhow, apologies for my crankiness and distracting from the primary conversation. I'll check out.

 

[Mike King]

We all have our own obvious meanings, and sometimes it's not obvious to other people. For example "rotational forces" does not mean the same to me as what I think it means to you. You can think of "inside the turn" as to the left in a left turn and to the right in a right turn. Also it's hard for a ski to push something left unless that something is to the left of it. Skis, most often when used correctly are pushing us, not pulling us.
Hence, I agree with @Mike King the CofM must be on the proper side of the ski to be pushed. That's what @Mike King 's post means to me. In a wedge turn the CofM is inside the turn relative to the edge that's doing the pushing (the dominant edge).

Thank you.

Part of what I'm after here is "CoM is inside the turn" is tough for me to grasp what you're saying. It's a very squishy term that means something to you, but I don't know what it refers to, and I think by now you know I have a good background in rotational dynamics. "Inside the turn" is a place I'm not sure what you mean, "the turn" is not a point. I'm belaboring this to try to get my point across, and also to say if I'm having trouble visualizing your description, your students might be too.

There's no doubt that there isn't an obvious and easily calculable position of either the CoM or the BoS. For convenience, use the path of the skis as describing the position of the BoS and the path of the body as describing the path of the CoM. This diagram below, stolen from somewhere on the internet (http://actionskiing.blogspot.com/2017/06/the-anatomy-of-turn.html) shows that the CoM (body) is inside the BoS (skis) when centripetal force is changing the path of the skier (CoM).

Another part I'm after is I think it's just one side of the equation. To me it says, "If you have rotational forces (ie centripetal force occurs) , then you're in a turn. That's just restating the premise a different way. Rotational forces means a turn is occurring. One is the other.

Not necessarily. You can rotate the body, or a portion of the body, without changing the path of the CoM. That's rotation, but it Isn't a turn. A turn describes force operating on the CoM to change it's direction.

Put another way, the question as I see it is what has to happen to change from a left to right turn, and I don't think saying a turn must exist in order to declare that a new turn has occurred gets us very far.

I'd agree with that. But the question presumes you are in one turn (lateral force is being exerted on one side of the turn) and you are moving into another turn (the lateral force will switch). The CoM has to be in a position to accept OR create the new force. In a wedge turn transition, the CoM is already there because the feet are displaced laterally from the normal reference. In parallel turn transitions, the CoM has to cross the feet to accept/create the lateral force of the new edged ski.

Thirdly, I think you're wrong on COM moving. In a wedge turn, not a dynamic wedge turn, I believe all day long you can turn WITHOUT moving the COM. In a wedge turn you can hold your body rigid like a car, and a car's COM does not move in a turn. (Let's not get into sprung and unsprung masses) Which is why I said tricycle earlier. Fourth, I just used you as an example of added complexity over FP's nicely condensed and simplified description, it had to be somebody.

A tricycle is just like a wedge -- the back wheels are laterally displaced, so the CoM is already interior to the turn. But just like a wedge turn, if too much lateral force is generated, the CoM crosses and topples out of the turn.

The CoM has to be inside the turn. In most skiing that we care about, it will have to move to get there. I don't think we really spend much time talking about the transition for wedge turns, although those trying to pass their wedge Christies may need some attention there. But probably the issue that will most affect the performance of your turns is what happens in the transition. And a huge part of that is the dirt of what you do to affect the CoM in the transition.

 

[pchewn]

Thank you.


There's no doubt that there isn't an obvious and easily calculable position of either the CoM or the BoS. For convenience, use the path of the skis as describing the position of the BoS and the path of the body as describing the path of the CoM. This diagram below, stolen from somewhere on the internet (http://actionskiing.blogspot.com/2017/06/the-anatomy-of-turn.html) shows that the CoM (body) is inside the BoS (skis) when centripetal force is changing the path of the skier (CoM).

View attachment 110024

Very nice diagram. In my mind, the single point defining the "transition" is the point on the ski curve where the curvature changes from concave-right to concave-left. (i.e. The tangent point of the two turns.) That correlates to the green dotted "TRANSITION" line above.

 

[Disinterested]

Just a quick point on that diagram: the re-alignment of skier with skis rotationally is more situational than that. That diagram describes a very short radius turn. You tend to see in a turn of greater radius that the skier will be closest to square with the skis high in the arc of the turn, for the simple reason that the hip is turning more in such a turn.

It's an interesting question to ask what are the bare physical necessities to create a turning effect, but I think part of what's happening here is that there's potentially a difference between what is practically necessary for human beings and what might be theoretically possible for a ski manipulated by some other means to do. All skiers in reality do move inside their feet and change their edges to accomplish turning. I cannot think of practical human examples where this is not happening.

In a wedge of any description it is a matter of a person manipulating two different turning platforms to create a net effect, both of which are set up to behave like an outside ski conventionally would, while being inside of both. It's certainly an option.

As it happens, you can create any of these behaviors by just supplying a very strong active turning effort to the skis, and, in most situations, the rest of these things will happen, albeit not necessarily very efficiently.

 

[Steve]

With the discussion on the graphic design thread it got me thinking. I wonder what people think are the absolutes in transition. What has to happen and what are things that do happen but aren’t required. So I guess we should start with a “definition” of a ski turn. We may agree or disagree but for this question, a ski turn consists of crossing the fallline twice on the same edge. Doesn’t matter which edge but is has to cross the falling twice on the same edge. Is that fair? Hopefully. So with that in mind what are the absolutes and what are the desirables in this case?

The change in balance from one leg to the other.

 

[Mike King]

The change in balance from one leg to the other.

@Steve I think that's a good element for desirable skiing, but I don't think it is necessary to turn. After all, you can make turns and transition between them on one leg or execute a White Pass turn. But maybe you are really meaning to change the balance from one side to another?

Mike

 

[Steve]

@Mike King yes I agree with that. In most cases however changing from one leg to the other is changing from one side to the other, so exceptions aside, it's not a bad "one thing that has to happen at the transition."

 

[Loki1]

I think this thread is starting to fizzle out. With that in mind I will tell you my absolutes. You have to be somewhat upright, you have to moving in a direction supported by the hill, You have to change edges. Other than that everything else is arguable. Those are 3 things the absolutely need to happen to transition. Nothing else matters if that is your only goal.

 

[François Pugh]

...... you have to moving in a direction supported by the hill, ....

Can you explain what that means?

 

[mister moose]

A tricycle is just like a wedge -- the back wheels are laterally displaced, so the CoM is already interior to the turn. But just like a wedge turn, if too much lateral force is generated, the CoM crosses and topples out of the turn.

Well, what if "too much force" isn't generated. You are setting conditions when the question was general in scope. Yes, "the COM is already interior to the turn", so that is something that doesn't have to change. Again, the question was what is the minimum that has to happen in transition.

The CoM has to be inside the turn. In most skiing that we care about, it will have to move to get there. I don't think we really spend much time talking about the transition for wedge turns, although those trying to pass their wedge Christies may need some attention there. But probably the issue that will most affect the performance of your turns is what happens in the transition. And a huge part of that is the dirt of what you do to affect the CoM in the transition.

You are setting further conditions to bolster your answer. With your added conditions, I agree with you. But I could add more conditions and generate additional requirements too. I don't think that's the direction to go when the question is what is the minimum.

The single ski question drills down on this, and the wedge turn question drills down on this. What changes is the dominant edge. By using the word dominant, François Pugh nailed it; that description covers 2 skis or one, wedge or parallel. COM moving is optional or secondary.

 

[Mike King]

@mister moose, it is you who is splitting hairs, not me. That being said, this thread, and I, am done.

 

[markojp]

I've yet to ride a tricycle down a ski slope, but it sounds oddly exciting from a spectator's point of view.

 

[razie]

The femur doesn't go stright into the hip socket. The femur has an angle to it at the hip joint. So when we internally rotate the femur there is and added adduction movement. As well as when we externally rotate the femur there is and added abduction movement. When you see the knees adducting or abducting that is the femur rotating in the hip socket.

Hey - you're making stuff up now... the fact that the femur is attached to the hip via a ball and socket joint, it doesn't mean that it's "femur rotation" no matter how you move! If you attach a stick to the ceiling in whatever way you want, it doesn't mean the "stick is rotating" when you push it from side to side...

Here's femur rotation vs adduction/abduction:

Note how femur rotation moves the feet side to side with a flexed knee, while the knee stays in one place!

Again Razie you are wrong. The movement you are describing, while it includes adduction, comes from femur rotation. The femur doesn't go stright into the hip socket. The femur has an angle to it at the hip joint. So when we internally rotate the femur there is and added adduction movement. As well as when we externally rotate the femur there is and added abduction movement. When you see the knees adducting or abducting that is the femur rotating in the hip socket.

Actually no, I'm not wrong - the example I gave was specifically femur adduction/abduction, as in:

The rotation is minimal, because we keep the feet in one place, as opposed to a simple long legged adduction/abduction:

Without adduction/abduction, you can't keep the feet in one place... evident when compared to a simple femur rotation, which would be:

Please reference only normal human biomechanics. If you want to define your own alternative biomechanics where everything we do is a result of a single movement aptly named "femur rotation", just please label it as such!



The exercise I gave demonstrates that the main movement is adduction/abduction and that the rotation is minimal and passive, induced be the knee joint from keeping the feet in one place. The only active movement one makes is adduction/abduction, in that exercise, i.e. moving the knee side to side while sitting on a chair. It doesn't get simpler than that...