Hard pressure on tips while on steeper terrain (Carved short turns?)

geepers

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Ah right, I meant where on the ski you give the impulse, not how much.

Franko is also skeptical of the idea that skier's get the tips to do more of the turning that the rest of the ski when carving.
He points out that if a force is applied to one end of a rod and not the other end the rod would rotate about the mid point - which we don't see in carving turns. Says there's no technical evidence of it.

Made this comment on FB re the feeling of pressuring the tip.

Jurij Franco On Loading Tip To Turn.JPG
 

Zirbl

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Franko is also skeptical of the idea that skier's get the tips to do more of the turning that the rest of the ski when carving.
He points out that if a force is applied to one end of a rod and not the other end the rod would rotate about the mid point - which we don't see in carving turns. Says there's no technical evidence of it.

Made this comment on FB re the feeling of pressuring the tip.

View attachment 189191
Interesting, thanks. But if stay back, I'm certainly not pressuring the tip, am I?
 

markojp

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Interesting, thanks. But if stay back, I'm certainly not pressuring the tip, am I?

Cuff pressure isn't a zero sum gain through an arc. IMHO of course.
 

Sanity

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Franko is also skeptical of the idea that skier's get the tips to do more of the turning that the rest of the ski when carving.
He points out that if a force is applied to one end of a rod and not the other end the rod would rotate about the mid point - which we don't see in carving turns. Says there's no technical evidence of it.

Made this comment on FB re the feeling of pressuring the tip.

View attachment 189191


This quote by Franko is exactly what I've been talking about in this thread. He's saying, as the edge angle increases, the turning radius decreases which creates a persistent torque on the body to rotate it around. In other words, a torque is applied to create angular momentum around the COM. If the turning radius was constant throughout the turn, then there would be a torque at the beginning of the turn, and then there wouldn't be any torque, because inertia maintains the rotation (another good reason to be on the cuff at the top of the turn). It won't be just the feeling of pressure though. From the skiers frame of reference, it really is pressure pulling up on the front to apply real torque to rotate the body back. Now, what happens at the end of the turn? The exact opposite. The rotation has to stop by the middle of transition. A torque must be applied in the opposite direction to stop the rotation, i.e. take away the angular momentum. Therefore, there will be pressure on the back of the ski as the turning radius gets larger, which is the second half of the turn. A skier will be aft at the end of the turn.
 

AchtungSki

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This quote by Franko is exactly what I've been talking about in this thread. He's saying, as the edge angle increases, the turning radius decreases which creates a persistent torque on the body to rotate it around. In other words, a torque is applied to create angular momentum around the COM. If the turning radius was constant throughout the turn, then there would be a torque at the beginning of the turn, and then there wouldn't be any torque, because inertia maintains the rotation (another good reason to be on the cuff at the top of the turn). It won't be just the feeling of pressure though. From the skiers frame of reference, it really is pressure pulling up on the front to apply real torque to rotate the body back. Now, what happens at the end of the turn? The exact opposite. The rotation has to stop by the middle of transition. A torque must be applied in the opposite direction to stop the rotation, i.e. take away the angular momentum. Therefore, there will be pressure on the back of the ski as the turning radius gets larger, which is the second half of the turn. A skier will be aft at the end of the turn.
I'd be curious to see the context of the quote @geepers posted, but I don't believe Franko is saying that there is an unbalanced moment about the skier's COM when carving and I think that perhaps there's a misunderstanding here. Purely from the frame of reference of the ski, as a shaped ski tips over further and further more space is created between the snow (let's assume super firm icy hard pack for simplicity's sake i.e very flat and little surface deformation) meanwhile the ends of the ski are still in contact and accepting the lateral grip frictional force perpendicular to the edge. An unbalanced moment arm (i.e torque) is therefore created at both the tip and tail of the ski since there is now less lateral grip force acting at the center of the ski from the edge lifting, thus the ski bends and arcs. This is what a skier is perceiving. Once peak edge angle is achieved though the ski does not bend further as the load is now evenly distributed and unbalanced moments resolve. The lateral grip force is the centripetal force that deflects you across the hill and you want to stay aligned with that force in the center of your ski.

Now imagine the opposite, if you had 150cm long 1/4 inch wall square steel tubing attached to your bindings (a "ski" that can't be bent both due to the straight shape and relative stiffness). As you transition your COM across your "skis" into the new turn there would be no unbalanced moment arms as the lateral grip force would remain constant along the length, they would simply continue straight across the hill as you the skier fall downhill of them, ouch. What the position of the skier COM really influences is the profile of the distributed load of that frictional grip force along the length of the ski by increasing it in the place where the skier COM is aligned with. In non-carved turns this causes differential bending along the length depending on where that distributed load resolves to relative to the center of the ski as well as allowing the portion of the edge furthest from where the COM is aligned with to slide. As mentioned previously, for carved turns you want the whole edge working simultaneously bending at the center, thus you need to be centered.

The problem comes given the fact that humans are not a rigid mass on a pole welded to the center of the ski. It requires muscular effort to control your joints to maintain the centered position since your feet/skis are moving faster than your COM is around the arc created by the side cut, the center point of said rotation is not the skier COM though, but the geometric center of the ski's radius. Rotation about the skier COM would necessarily mean tails not following tips as human legs are usually shorter than than 13-22 meter side cut radius of skis ogwink. IMO rotation about the skier COM (looking top down at least) is pretty undesirable, e.g. losing your tails at the end of a carve and flying head first down hill.

My far from expert understanding of what Tom or Paul are talking about with their ever so slight shift from ball to heel through the turn when carving is that it's a real world implementation of this theoretical physics classroom model of carving. A slight bias towards the tips initially gets the tips engaged as you're coming out of transition and gives your brain time to manage the velocity differential of your feet and COM which inspires confidence. The aggressive pull through with the hip flexors at the end is because they're making incredibly athletic turns/transitions where they actually need to accelerate the feet even further to manage the large velocity differential between their COM and the skis' path.
 

Rod9301

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I'd be curious to see the context of the quote @geepers posted, but I don't believe Franko is saying that there is an unbalanced moment about the skier's COM when carving and I think that perhaps there's a misunderstanding here. Purely from the frame of reference of the ski, as a shaped ski tips over further and further more space is created between the snow (let's assume super firm icy hard pack for simplicity's sake i.e very flat and little surface deformation) meanwhile the ends of the ski are still in contact and accepting the lateral grip frictional force perpendicular to the edge. An unbalanced moment arm (i.e torque) is therefore created at both the tip and tail of the ski since there is now less lateral grip force acting at the center of the ski from the edge lifting, thus the ski bends and arcs. This is what a skier is perceiving. Once peak edge angle is achieved though the ski does not bend further as the load is now evenly distributed and unbalanced moments resolve. The lateral grip force is the centripetal force that deflects you across the hill and you want to stay aligned with that force in the center of your ski.

Now imagine the opposite, if you had 150cm long 1/4 inch wall square steel tubing attached to your bindings (a "ski" that can't be bent both due to the straight shape and relative stiffness). As you transition your COM across your "skis" into the new turn there would be no unbalanced moment arms as the lateral grip force would remain constant along the length, they would simply continue straight across the hill as you the skier fall downhill of them, ouch. What the position of the skier COM really influences is the profile of the distributed load of that frictional grip force along the length of the ski by increasing it in the place where the skier COM is aligned with. In non-carved turns this causes differential bending along the length depending on where that distributed load resolves to relative to the center of the ski as well as allowing the portion of the edge furthest from where the COM is aligned with to slide. As mentioned previously, for carved turns you want the whole edge working simultaneously bending at the center, thus you need to be centered.

The problem comes given the fact that humans are not a rigid mass on a pole welded to the center of the ski. It requires muscular effort to control your joints to maintain the centered position since your feet/skis are moving faster than your COM is around the arc created by the side cut, the center point of said rotation is not the skier COM though, but the geometric center of the ski's radius. Rotation about the skier COM would necessarily mean tails not following tips as human legs are usually shorter than than 13-22 meter side cut radius of skis ogwink. IMO rotation about the skier COM (looking top down at least) is pretty undesirable, e.g. losing your tails at the end of a carve and flying head first down hill.

My far from expert understanding of what Tom or Paul are talking about with their ever so slight shift from ball to heel through the turn when carving is that it's a real world implementation of this theoretical physics classroom model of carving. A slight bias towards the tips initially gets the tips engaged as you're coming out of transition and gives your brain time to manage the velocity differential of your feet and COM which inspires confidence. The aggressive pull through with the hip flexors at the end is because they're making incredibly athletic turns/transitions where they actually need to accelerate the feet even further to manage the large velocity differential between their COM and the skis' path.
How do you know they are pulling with the hip flexors?
 

geepers

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Interesting, thanks. But if stay back, I'm certainly not pressuring the tip, am I?

Not sure I get what you are asking. Presumably to engage tip n tail for carving we need to be in roughly the 'middle' of the ski. So why "stay back"?

I'd be curious to see the context

Grabbed that snapshot from a FB post a few years back. Probably hard to find the original thread now.

but I don't believe Franko is saying that there is an unbalanced moment about the skier's COM when carving

He's pointed out elsewhere that if there was the skier would quickly begin to rotate.

Once peak edge angle is achieved though the ski does not bend further as the load is now evenly distributed and unbalanced moments resolve. The lateral grip force is the centripetal force that deflects you across the hill and you want to stay aligned with that force in the center of your ski.

As Franko states there has to be a torque to change the orientation of the skier - from facing one way across the hill to facing the other way. If the skier was in space and used a rocket to simulate the magnitude and direction of the GRF presumably the CoM would move in a curved path but would not change orientation.

How do you know they are pulling with the hip flexors?

Because they say so, demo it, and have drills of it.

It's just the anterior version of pulling the feet back with the ham strings/glutes. Now pull 'em forward with the hip flexors - pull toes to nose (which is another way of saying don't allow the torso to rotate backwards).
 

Sanity

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I'd be curious to see the context of the quote @geepers posted, but I don't believe Franko is saying that there is an unbalanced moment about the skier's COM when carving and I think that perhaps there's a misunderstanding here. Purely from the frame of reference of the ski, as a shaped ski tips over further and further more space is created between the snow (let's assume super firm icy hard pack for simplicity's sake i.e very flat and little surface deformation) meanwhile the ends of the ski are still in contact and accepting the lateral grip frictional force perpendicular to the edge. An unbalanced moment arm (i.e torque) is therefore created at both the tip and tail of the ski since there is now less lateral grip force acting at the center of the ski from the edge lifting, thus the ski bends and arcs. This is what a skier is perceiving. Once peak edge angle is achieved though the ski does not bend further as the load is now evenly distributed and unbalanced moments resolve. The lateral grip force is the centripetal force that deflects you across the hill and you want to stay aligned with that force in the center of your ski.

Now imagine the opposite, if you had 150cm long 1/4 inch wall square steel tubing attached to your bindings (a "ski" that can't be bent both due to the straight shape and relative stiffness). As you transition your COM across your "skis" into the new turn there would be no unbalanced moment arms as the lateral grip force would remain constant along the length, they would simply continue straight across the hill as you the skier fall downhill of them, ouch. What the position of the skier COM really influences is the profile of the distributed load of that frictional grip force along the length of the ski by increasing it in the place where the skier COM is aligned with. In non-carved turns this causes differential bending along the length depending on where that distributed load resolves to relative to the center of the ski as well as allowing the portion of the edge furthest from where the COM is aligned with to slide. As mentioned previously, for carved turns you want the whole edge working simultaneously bending at the center, thus you need to be centered.

The problem comes given the fact that humans are not a rigid mass on a pole welded to the center of the ski. It requires muscular effort to control your joints to maintain the centered position since your feet/skis are moving faster than your COM is around the arc created by the side cut, the center point of said rotation is not the skier COM though, but the geometric center of the ski's radius. Rotation about the skier COM would necessarily mean tails not following tips as human legs are usually shorter than than 13-22 meter side cut radius of skis ogwink. IMO rotation about the skier COM (looking top down at least) is pretty undesirable, e.g. losing your tails at the end of a carve and flying head first down hill.

My far from expert understanding of what Tom or Paul are talking about with their ever so slight shift from ball to heel through the turn when carving is that it's a real world implementation of this theoretical physics classroom model of carving. A slight bias towards the tips initially gets the tips engaged as you're coming out of transition and gives your brain time to manage the velocity differential of your feet and COM which inspires confidence. The aggressive pull through with the hip flexors at the end is because they're making incredibly athletic turns/transitions where they actually need to accelerate the feet even further to manage the large velocity differential between their COM and the skis' path.

From Franko, "But as a skier is facing for example left of fall line at start of a turn and right at the end of a turn, there must be a torque to change his orientation."

When a skier goes around in a circle, the body rotates around the center of the circle. But also, the body rotates around itself as well. Let's say the skier is leaned over to the right. If he keeps traveling around the circle, then he'd be leaning over to the left and backwards after 180 degrees relative to the fall line. You might say, sure that's what it means to go around in a circle. But, you can look at the rotation of that body independently. The body rotated. Body's don't rotate without a torque. Therefore a torque must have been applied. From that quote Franko understands that a body in motion stays in motion, so you really only need that torque just as the rotation gets started, and that's not that interesting. So, Franko goes on to say that as the turn develops, the edge angles continually increase, which gives a steadily increasing rate of rotation. Now that there's a steady increase in rate of rotation compared to a constant rate of rotation, there needs to be a constant torque applied. The body is rotating in free space. The only contact is with skis and snow. Therefore, that constant torque must come from the front of the ski which then levers up through the boot.

To help understand this, imagine if there was no torque on the body, no rotation of the body about itself. Let's say the skier is leaned over to the right with the skis pointing downhill, and he travels around a circle with a 25 meter radius. After 45 degrees of rotation (or any degree) around that 25 m radius, he's still leaned over to the right, and his skis are still pointing down the hill. That position is held through the entire turn. That's what rotation around a circle looks like when there's no rotation around the skier's COM. So, it is indeed desirable to have rotation around the COM, and nearly impossible to turn on skis without it.
 

François Pugh

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Just want to remind everyone not to mix frames of reference. Skier rotates in the frame of reference fixed to the hill, changes in the speed at which he is rotating require torque. Skier does not rotate in frame of reference fixed to ski, no change in speed of rotation (0=0), no torque required. Pick one frame of reference and stick to it, Or start new analysis with new frame - don't carry things over.
 

Rod9301

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Because they say so, demo it, and have drills of it.

It's just the anterior version of pulling the feet back with the ham strings/glutes. Now pull 'em forward with the hip flexors - pull toes to nose (which is another way of saying don't allow the torso to rotate backwards).


The hip flexors are pretty weak compared to the hamstrings, so it would be inefficient to use them.
And it seems to me that using the hip flexors you would move the upper body forward in relation to the feet. Upper body weighs a lot more then the feet, do this move would take longer and require a lot of energy.

This is old school thinking, and you could not see an advanced racer do it.
 

Zirbl

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Not sure I get what you are asking. Presumably to engage tip n tail for carving we need to be in roughly the 'middle' of the ski. So why "stay back"?
Not suggesting you would want to. Suggesting that to pressure the tip as much as the tail, you might need to make movement towards the tip, given how the COM can move back as the turn progresses due to the increased flexion of the inside leg. Whether it has to or not is another question.
 

LiquidFeet

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This is great observation. I'm posting here not to disagree with its substance, but to add some context for recreational skiers on many days in many parts of the world, notably including here in the American NE when there is not much snow or trail acreage to go around, but plenty of people there to use it.

The implied idea that racers are the only ones constrained to a narrow corridor is false. More significantly for this discussion, perhaps, it's very notable how these hot skier demos take place on slopes that tend to be quite wide and are invariably virtually empty. This "arena" disparity presents a very real practical challenge for those of us who like to practice arc to arc turns, but are not yet retired and/or are otherwise constrained as to when we can go skiing. Especially when the slopes we ski on may be narrower to start with. In any case the longer I extend the transition period across the hill the more likely I am to get clocked by an unguided missile coming down from above. This conflict of usage patterns is frequent topic of discussion here on the forum.

That's it. Carry on.
Timely note of reality.
 

LiquidFeet

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+1
And not to mention the surface variations that have to be negotiated. Love skiing those glaciers in Europe with wide open consistent surfaces. But here in the NE we have dumps, bumps, lumps, ice, humps, ruts, nuts - all within the space of a few turns... You have to adjust on the fly and be reactive. Theories and formulas don't work very often.
Even more reality comments are coming in as i make my way through this thread.
I know these Debbie Downers apply to us New Englanders. But how about others?
 

LiquidFeet

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Franko is also skeptical of the idea that skier's get the tips to do more of the turning that the rest of the ski when carving.
He points out that if a force is applied to one end of a rod and not the other end the rod would rotate about the mid point - which we don't see in carving turns. Says there's no technical evidence of it.

Made this comment on FB re the feeling of pressuring the tip.

View attachment 189191
 
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