Fore/Aft Balance and Physics

[karlo]

Yes Raz, but still shots do not effectively indicate pressure.

The physics of ski technique is too complex to make many or most of the assertions I see made in regards to a still frame

Including, IMO, the assertions made at that seminar in Aspen. It assumes mono-modal bend shape and pressure application.

 

[Mike King]

So you're saying that to keep the inside ski back as much as needed, it needs to be weighted? Probably to flex the boot? Same amount as the outside boot, you're suggesting?

First skier - his snow spray clearly stops before the heel of the boot, so he is clearly pressuring the forebody of the ski, not the middle.

Also, you would largely be correct if the below skier made the technical error that you described, often enough in free ski or courses. He doesn't. It's a common tactical error that got him there - race coaches are probably smiling when seeing that

But to clarify, if there was any doubt at this point about the topic that we're discussing here: he is back, although he is forward in the respective plane...

@Mike King so you are saying that the acceleration of the ski relative to the hips is important. Let's say for the sake of simplifying the argument that I am staying square to the skis. How can the ski accelerate faster than the hips into the fall line if I don't get back in the fore/aft plane?

Watch the skater dropping into the half pipe video posted on the first page.

 

[Mike King]

Can someone tell me what is meant by “accelerating” and “decelerating “? Velocity of what, relative to what?

Yes, I can see that ski will turn “into the fall line” and turn away from it, but accelerate? To me, a turn is an acceleration; their not two different things

Acceleration = change in velocity.

 

[Mike King]

Including, IMO, the assertions made at that seminar in Aspen. It assumes mono-modal bend shape and pressure application.

You know, @karlo, that’s a pretty outrageous assertion from a PSIA level 1 ski instructor about a seminar presented by physicist, level 4 APSI examiner, attended by 4 demo team members. But you’ve never had a problem of being introspective.

 

[James]

Watch the skater dropping into the half pipe video posted on the first page.

Hah, the first time I dropped into a skate half pipe, about 2x that one's height, I went right on my butt. I didn't commit downhill, or pull the feet back enough, so that the skates were under my body as I went down the vert.
It's a simple thing to do it right but a huge leap of faith. Like downhill skiing.

In a snow half pipe, most people when going up the wall don't allow their feet to get ahead or move the body back. That simply to remain more square to the ski fore/aft. What happens is they don't move aft, they end up on the wall with lots of pressure on the tips and have to jump or pole to get the skis around. It seems like pipe athletes use that pressure for tricks, but that's beyond me.
On a banked terrain turn you commit your body inside the coming turn before the skis.

 

[razie]

a seminar presented by physicist, level 4 APSI examiner,

why don't you say so - that explains it. Things spin backwards down there...

 

[Corgski]

In order to keep up with the skis and to keep the pressure point under the center of the skis, the CoM must move forward along the length of the ski. And, similarly, as the skis decelerate as the the skis exit the fall line and traverse across, or even up the hill, the CoM must move aft to keep the pressure distribution centered on the ski.
Mike

Sorry to quote myself quote myself but a year ago I asked this without getting an answer:

My question though is how does this translate to steeper slopes? The physics suggests that if you are accelerating at the maximum possible rate for the slope angle, to keep your weight centered over your foot as described by @LiquidFeet, your body position relative to the skis should be the same as on a flat (meaning that you are leaning forward relative to horizontal). However if the acceleration is less, your body position would need to shift back to keep your weight over the same position over your foot. What feedback does one rely on? I am guessing a more advanced skier can tell from the feedback from their skis where their weight needs to be, is this an effect that the those of us still learning need to think about?

Seems to be the same physics but without the additional turning complexity. My understanding was that when accelerating straight down the fall line (not at all considering acceleration from circular travel), perpendicular to ski would be necessary, which is actually forward due to slope. It is when you are going downhill but no longer accelerating (wind and other resistance) that a centered position will look like being in the back seat. I have no idea how much of this is actually visible during a turn and unfortunately there is not enough acceleration in my skiing to make this much more than a theoretical exercise for me .

 

[razie]

You know, @karlo, that’s a pretty outrageous assertion from a PSIA level 1 ski instructor about a seminar presented by physicist, level 4 APSI examiner, attended by 4 demo team members. But you’ve never had a problem of being introspective.

Hey - I see his lectures have dropped from youtube - last time I watched them, a lot of things didn't make sense - I would be interested to break it all down in a series of articles, maybe here, on pug, if you convince them to put them back up...?

 

[geepers]

In the youtube vids (now removed) John Fahey does an interesting demo. He takes a pole and stands it vertically on an inclined plane and asks the audience: "When I let go the pole, will it:
1. Topple down the slope, top of pole going first?
2. Slide down the slope at the same angle, top and bottom going at the same rate?
3. Or fall backwards, base going first?
The pole of course represents a skier going down a hill.

The audience has supporters for each of those outcomes.

He lets go the pole and it falls backwards, the base shooting out first. He then goes on to demonstrate that the pole must have the right amount of forward lean in order to slide down the incline with same starting angle. And the amount of forward lean depends on the friction - more forward lean for lower friction and less forward lean for higher friction.

Like the difference in friction skiers experience between skiing an icy slope and some fresh pow.

My take is that part of this race coach "get forward" vs ski instructor "in the middle" is also explained by the difference in the snow their students typically ski on. Otherwise I thought OP had a good point.

BTW I thought that John Fahey said he was an engineer rather than a physicist.

Sorry to quote myself quote myself but a year ago I asked this without getting an answer:

Seems to be the same physics but without the additional turning complexity. My understanding was that when accelerating straight down the fall line (not at all considering acceleration from circular travel), perpendicular to ski would be necessary, which is actually forward due to slope. It is when you are going downhill but no longer accelerating (wind and other resistance) that a centered position will look like being in the back seat. I have no idea how much of this is actually visible during a turn and unfortunately there is not enough acceleration in my skiing to make this much more than a theoretical exercise for me .

Well, one hint is don't fall over backwards.

Can do that same experiment as John Fahey with a ski pole sliding down the base of an inclined ski. (Put the strap under the base to change the level of friction.) Vary the steepness of the incline and find out what works.

 

[karlo]

Acceleration = change in velocity.

Exactly. And, acceleration creates g’s, like getting pushed back against the seat when pedal hits the metal. Or, we feel g’s outwards when we turn, like as the ski’s turn. So, I don’t get what the

physicist, level 4 APSI examiner,.

is talking about. Acceleration of what. From what’s been described, it’s not the turn, cause if it were, then there wouldn’t be talk of deceleration at the latter part of a turn, since the latter part of a turn is simply less acceleration with less g’s

So, perhaps you can ask the physicist what acceleration and deceleration is being referred to? There’s got to be something missed in translation.

 

[Mike King]

@karlo, I’m at a loss of how to explain such a simple concept to you. Let’s try it this way

1. Assume you are on a slope with a constant pitch.
2. If the skis are perpendicular to the fall line, edged with a platform angle greater than 90 degrees, and your velocity is zero as an initial state, you won’t move
3. If you angle the skis at 20 degrees to the fall line, you may move if the slope has sufficient gradient to overcome friction. This is because the force of gravity, deflected by the slope angle and modified by the angle of the skis to the fall line, translates the force of gravity into a forward motion.
4. The forward pull of gravity is at its maximum when the skis are parallel to the fall line.
5. So as the skis turn down the hill they accelerate.
6. Conversely as they turn out of the fall line and across the hill, they decelerate (acceleration is negative).

Mike

 

[razie]

@Steve - here are some Canucks doing our fundamental of skiing #5 - carving the outside ski with 100% of weight, just now in Are:

 

[CalG]

Yep. Here's Lindsey, apparently doing everything wrong, on her way to a bronze in the 2018 Olympics.
View attachment 65625
Link: youtu.be/ghXI7t70Bnw 1:51

She did need to play some "catch up" for those depicted moments when her skis were not helping.

I wonder, Is she the kind of person who would blame the ski ? I doubt it.

 

[karlo]

If the skis are perpendicular to the fall line, edged with a platform angle greater than 90 degrees, and your velocity is zero as an initial state, you won’t move

This is just standing side ways to hill?

If you angle the skis at 20 degrees to the fall line, you may move if the slope has sufficient gradient to overcome friction. This is because the force of gravity, deflected by the slope angle and modified by the angle of the skis to the fall line, translates the force of g

This is rotating down the hill 20 degrees?

The forward pull of gravity is at its maximum when the skis are parallel to the fall line.

Skis pointed downhill

So as the skis turn down the hill they accelerate.

I would say that the COM accelerates downhill, as resistance to acceleration is released. The skis are, for now, just part of the total mass

Conversely as they turn out of the fall line and across the hill, they decelerate (acceleration is negative

I a pivot slip, I'd say we apply resistance to the pull by gravity on our COM and we decelerate.

But, I don't think this applies to carving. To me, carving in its purest form results in constant downhill velocity of COM. We most times deviate from that base case and deflect our COM laterally. Even then, I think our downhill vector is constant velocity, and our lateral vectors accelerate and decelerate as we change direction side to side.

So, am I correct that you are referring to the skis themselves, that, in the downhill vector, they accelerate and decelerate during a carve? You are not referring to COM?

 

[Doby Man]

This is why the movement analysis on still frames is a very bad practice to conceive or follow:


1:29 to 1:35 shows a clear example of how the WC SL skier is using, bending and pressuring his inside ski except for a millisecond deflection (micro deflection) where a photo can be taken and technical claims be made that are completely false. At 1:29, immediately before and after said micro-deflection of the inside ski coming off the snow surface we see the inside ski bent even more than the outside ski which is a clear indication of inside pressure. Because the inside ski gets less pressure than the outside, it is the one that gets deflected in a manner that bounces it off the ground.

This is one of many clear examples of why you cannot do a movement analysis of a still frame and why is is so cheesy and irresponsible to do so on here. Still frames are almost meaningless for registering actual ski output which is the goal of all motor patterns. Don’t fall for this common argumentative red herring. It is also just too convenient to use these red herring photos for a lack of the right words.

 

[Mike King]

This is why the movement analysis on still frames is a very bad practice to conceive or follow:


1:29 to 1:35 shows a clear example of how the WC SL skier is using, bending and pressuring his inside ski except for a millisecond deflection (micro deflection) where a photo can be taken and technical claims be made that are completely false. At 1:29, immediately before and after said micro-deflection of the inside ski coming off the snow surface we see the inside ski bent even more than the outside ski which is a clear indication of inside pressure. Because the inside ski gets less pressure than the outside, it is the one that gets deflected in a manner that bounces it off the ground.

This is one of many clear examples of why you cannot do a movement analysis of a still frame and why is is so cheesy and irresponsible to do so on here. Still frames are almost meaningless for registering actual ski output which is the goal of all motor patterns. Don’t fall for this common argumentative red herring. It is also just too convenient to use these red herring photos for a lack of the right words.

@Doby Man, I agree with your post. One thing I do object to is how these threads start to use video of ski racers in gates as demonstrating some point. The fact is that often folk in gates are in recovery and skiing from recovery to recovery. So what you might observe may not be a great example of technique. What's more informative, IMO, is video of these folk free skiing.

Mike

 

[Doby Man]

Well, Mike, I also believe that the same issues occur to making MA's on video where some of what is typically asserted cannot be confirmed without skier intent such as how exactly are movements motored, whether by a muscle contraction that cannot be seen or the invisible external forces at play such as momentum, "centripetal/centrifugal" forces, gravity, etc. There is a youtube video of Marcel hirscher insisting that he is "pushing" his arm forward and "pulling" his inside foot back which, technically, cannot be confirmed by video alone. I would suggest that those "positions" of anatomy were the result of the management of the relationship between the CoM and BoS rather than direct movement inputs of pushing and pulling as suggested. Had I made such assertions, they would have had to accompanied an interview with Marcel. That said, you are right that videos of racers for racers and videos of freeskiers for freeskiers is better. Though, a little cross training where racing and freeskiing are concerned should not be overlooked.

 

[HardDaysNight]

Mike and Doby. I take it you understand that serious coaches review and analyze a great deal of video of their athletes and are unlikely to mistake recoveries and inadvertent screw-ups for actual technique? Also, they not only talk to their athletes about what they might have been trying to do, but generally are instrumental in establishing what they should be trying to do. With this background and experience it is not difficult to review third party footage and not have to guess what’s going on.

 

[Doby Man]

Yes, I agree a coaching relationship with the skier in a video could easily stand in for knowing what the skier intent is going to be based on a specific conversation. I assume, from reading his posts, that Mike does as well.

 

[Mike King]

Well, Mike, I also believe that the same issues occur to making MA's on video where some of what is typically asserted cannot be confirmed without skier intent such as how exactly are movements motored, whether by a muscle contraction that cannot be seen or the invisible external forces at play such as momentum, "centripetal/centrifugal" forces, gravity, etc. There is a youtube video of Marcel hirscher insisting that he is "pushing" his arm forward and "pulling" his inside foot back which, technically, cannot be confirmed by video alone. I would suggest that those "positions" of anatomy were the result of the management of the relationship between the CoM and BoS rather than direct movement inputs of pushing and pulling as suggested. Had I made such assertions, they would have had to accompanied an interview with Marcel. That said, you are right that videos of racers for racers and videos of freeskiers for freeskiers is better. Though, a little cross training where racing and freeskiing are concerned should not be overlooked.

Agreed