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

LiquidFeet

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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.
 pull feet back .gif
 

AchtungSki

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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.

This is only true if the frame of reference is a fixed rectilinear orientation with respect to the hill e.g. the x-axis of the coordinate system is the fall line. Choosing this frame of reference though makes analyzing the system complicated since now your force vectors are constantly changing throughout the circle. In this frame of reference all that is required for a change of direction is an unbalanced force, which is the centripetal lateral grip force pushing against the curved length of the ski, thus forcing an arc. Importantly, the ski is also rotating around itself to the same degree the skier's body is, both remain pointed perpendicular to the centripetal force. The ski is a tool that constantly redirects centripetal force towards the center of an arc similar to a banked track or a bowl with a marble rolling around it.

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.

Sticking with the aforementioned reference frame then yes this is true, the skier would basically just be translating around the arc which obviously does not make much sense. An object following a curved path where its "nose" stays oriented tangent to the path necessitates both translation as well as orientation rotation. I don't think this frame is useful for us as skiers though because we don't ski via third person skycam. A ski or skier COM centric coordinate system makes it easier to analyze the relevant forces under angular motion. Using this frame, assuming good technique and staying centered over the ski, there is no rotation around the COM because the coordinate system itself is moving with the skier around the arc.

So, Franko goes on to say that as the turn develops, the edge angles continually increase, which gives a steadily increasing rate of rotation.

He never says the part in italics though and explicitly says COM position is irrelevant, which is where I think you're going wrong. As edge angles increase, radius decreases, centripetal force increases as a consequence of F = (mv^2)/r thereby generating larger torques at the tip and tail until the ski fully bends at peak edge angle at which point momentum has been successfully redirected. That's why I said it would be nice to read the context of the conversation because it's a rather condensed comment and "torque" relative to his frame of reference isn't specified, therefore we can only speculate. IMO, given the rest of the comment and his podcast with TG, he's talking about the torque in the ski frame of reference bending the ski which we perceive and generates the arc that changes skier orientation from left to right from a static observer's frame of reference.
 

geepers

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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.

Whatever it is you imagine they are doing.... that's not it.
 

geepers

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I was responding to a comment that implied that you can get forward using the hip flexors

Well, we sure were not on the same ski run. We weren't even at the same resort.

The involvement of hip flexors is to (in sequence):
1. Keep the outside foot between the CoM and the GFR from the snow
2. Load the aft part of the ski
3. Continue into a low, flexed transition.

It is not to get forward.
 

Rod9301

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Well, we sure were not on the same ski run. We weren't even at the same resort.

The involvement of hip flexors is to (in sequence):
1. Keep the outside foot between the CoM and the GFR from the snow
2. Load the aft part of the ski
3. Continue into a low, flexed transition.

It is not to get forward

Sounds to me that 1 means get forward, but whatever.

I don't think hip flexors play much of a role when why'skiing, they are pretty weak in most people
 

Sanity

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In this frame of reference all that is required for a change of direction is an unbalanced force, which is the centripetal lateral grip force pushing against the curved length of the ski, thus forcing an arc. Importantly, the ski is also rotating around itself to the same degree the skier's body is, both remain pointed perpendicular to the centripetal force. The ski is a tool that constantly redirects centripetal force towards the center of an arc similar to a banked track or a bowl with a marble rolling around it.

This is true in a constant state of rotation. What we're talking about are changing rates of rotation.

He never says the part in italics though and explicitly says COM position is irrelevant, which is where I think you're going wrong. As edge angles increase, radius decreases, centripetal force increases as a consequence of F = (mv^2)/r thereby generating larger torques at the tip and tail until the ski fully bends at peak edge angle at which point momentum has been successfully redirected. That's why I said it would be nice to read the context of the conversation because it's a rather condensed comment and "torque" relative to his frame of reference isn't specified, therefore we can only speculate. IMO, given the rest of the comment and his podcast with TG, he's talking about the torque in the ski frame of reference bending the ski which we perceive and generates the arc that changes skier orientation from left to right from a static observer's frame of reference.

As the edge angles increase, radius decreases we get a change in the rate of rotation which requires a torque for the rotation of the mass around the COM to match the rotation of the mass around the center of the turning circle.

Let's say you have a roller skate on it's side on the floor of your car such that the wheels are horizontal. The ball bearings are amazing with zero friction. Now, drive your car around a turn. What happens to the pair of wheels touching the floor? What happens to the pair of wheels hanging in the air only by frictionless ball bearings? From your frame of reference the wheels in the air will spin. The wheels will spin relative to the skate. The wheels on the floor will not be spinning. Now what happens if you reach your finger down while you're turning and stop the front wheel from spinning? You apply a torque to the wheel, and it will stop spinning. That's the only way you can stop that wheel from spinning is if you apply a torque to it. So, now you have two wheels on the floor not spinning. The front wheel is not spinning, but the back wheel is spinning and you stop turning with your car and drive straight. What happens to the wheels? The back wheel that was spinning stops spinning all on it's own. The front wheel that wasn't spinning is now spinning, and it will spin forever until you reach your finger down and apply a torque to stop it from spinning.

Or another experiment, if you have a stick on the end of a string and you're swinging it around in a circle and then let it go, the stick will spin. The only way for the stick to stop spinning after you let it go is if a torque is applied to it.

A mass going around in a circle will only rotate around it's COM if a torque is applied to it. Once that torque is applied it doesn't need to be applied anymore to keep the mass rotating around the COM (that's the part you're talking about). Anytime there's a change in the rate of rotation, a new torque will need to be applied otherwise the rate of rotation around the COM won't match the rate of rotation around the center of the turning circle. This is relevant to skiing, because the skier will feel that pressure on the front of the ski at the beginning of the turn, and the pressure on the tails of the ski at the end of the turn. Though, you'll probably only notice it for rapidly changing turns with high edge angles.
 

geepers

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That's why I said it would be nice to read the context of the conversation because it's a rather condensed comment and "torque" relative to his frame of reference isn't specified

Feel free to search FB for "Jurij Franko Newton not me". Hours of happy fun reading ancient entrails of ppl insulting each other online. May even find the comment from the image.

This "pressuring the tips doesn't help the tip bite to tighten the turn radius" has been a constant discussion point by Franko. There's a couple of Franko living room lectures where he gets more in depth about it. Vimeo. Not going to post here as they are extremely dull.

The ski/skier changes orientation because the groove the ski makes in the snow forces it to. It's the same as a vehicle going around a banked track - the difference is the ski constructs the bank as it goes.


This is relevant to skiing, because the skier will feel that pressure on the front of the ski at the beginning of the turn, and the pressure on the tails of the ski at the end of the turn. Though, you'll probably only notice it for rapidly changing turns with high edge angles.

Don't believe the skier will feel these pressure at start/end from something automatically happening.

When we turn into the fall line the the angle of the pitch over which the ski is travelling gets a bit steeper. Have you seen the John Fahey (from Aspen) vid of sliding a ski pole down an inclined plane? Unfortunately it's no longer available but it is instructive in this case. Fahey puts the pole on the plane in a vertical position (hand grip down) and asks ppl to predict what will happen when he lets go. No-one gets it right. What happens is the the grip end slides down the plane but the top of the pole lags behind and the pole falls backwards. Fahey then starts the pole with pole with progressively more forward tilt (top in front of base) until finally it slides down without toppling backwards.

(BTW can do this on snow to amuse buddies using an upside down ski.)

That falling backwards is exactly what will happen to a skier if they launch off a drop off or start turning into the fall line without taking anticipating action of getting leaning in to the incline. But it requires a conscious action (or to be an outcome of the skier's technique).

It's the opposite in the last half of the turn. As we turn out of the fall line the pitch gets less steep. It's like setting the pole up with enough forward lean to travel down an inclined ski but then reducing the incline - the pole will topple forward. So the skier needs to make the compensating action. Again nothing automatic about it.
 

AchtungSki

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Ok I feel like we are talking about two distinct things that are being interchanged freely when they cannot be. There are two types of motion relevant here but first the assumptions; our model skier is making a single edge-locked, park and ride carve from the top of the turn until he's facing uphill and gravity stops him. His technique and fitness is perfect such that he is always able to keep his balance at precisely the center of the ski, thus load is evenly distributed along the length of the ski. The surface is also perfectly firm and even.

Circular motion: rotation of a body around a fixed axis outside of the body's COM. This is the arc of the ski with some radius X, the axis of the turn being located at the center of that radius. This motion is what gets our model skier from facing left to facing right relative to the fall line. All that is required for this motion to occur is centripetal force generated by the ski on edge, assuming the skier has some speed already. However, we still do have torque (force times distance) in this system from the force of gravity acting through the skier's COM at rN distance from the rotational axis as poorly illustrated by myself in the image below. Hence why we accelerate both in linear velocity as well as angular velocity ω as we pass through the fall line but decelerate in both as we continue a turn past 90deg to the fall line and uphill.

skiertorque.png


Now, a ski being tipped further on edge results in more centripetal force at a smaller radius. This increase in force explains the linear velocity change we observe with the ski in circular motion undergoing a radius tightening, as you note though there is also a corresponding increase in angular velocity ω (rate of rotation about the axis of the turn). A change in ω requires torque this is true. As mentioned previously, we already have this torque as a consequence of the constant force of gravity on the skier COM at varying radii from the axis of rotation while in circular motion. Side note, from this you can also sort of intuit that there's a maximum rate of increase for ω and therefore centripetal force, meaning that if edge angles are built too quickly the amount of centripetal force able to be created to hold the skier up in the turn can be exceeded and you fall inside, something most of us have experienced at some point.

Rotational motion: rotation of a body about an axis passing through its COM. This is almost assuredly not happening to our skier. Remember load is evenly distributed along the length of a ski in a carve and this will resolve directly under the skier's COM. Gravity is acting directly through the skier's COM. There is a frictional force along the base of the ski that theoretically represents a moment about the COM but this is easily overcome by gravity in a turn and our bodies can generate sufficient internal forces so as to prevent our legs from moving separately from our upperbody/COM. To be clear this, force balance absolutely goes out the window when skidding is introduced and the ski is asymmetrically loaded.

This difference is why I think the torque Franko was talking about it is in reference to the ski itself bending, nothing to do with changes in angular velocity, ASSUMING he's talking about a carving ski not parallel or short turns. The ski itself is not generating any asymmetric torque on the skier to influence angular motion in either regime of motion.

Anytime there's a change in the rate of rotation, a new torque will need to be applied otherwise the rate of rotation around the COM won't match the rate of rotation around the center of the turning circle. This is relevant to skiing, because the skier will feel that pressure on the front of the ski at the beginning of the turn, and the pressure on the tails of the ski at the end of the turn. Though, you'll probably only notice it for rapidly changing turns with high edge angles.

Given all the above, I'm confused by what you're saying here. What mass is rotating about the COM? Are you talking about the skis/feet moving separately from the COM/upper body? For short/parallel turns I would agree with you certainly then that there is rotational motion occurring from the tips being loaded more initially which rotates the feet/skis about the COM causing the turn to occur much faster/sharper than is possible via centripetal force and gravity. The skier then naturally uses the tails a bit to arrest the rotation enough to begin the next turn.

This all is of course very grey in the real world where skidding/edging is blended, imperfect technique, the skier manipulating the motion of their COM through muscular effort instead of passively accepting forces, snow conditions etc.
 

Sanity

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Ok I feel like we are talking about two distinct things that are being interchanged freely when they cannot be. There are two types of motion relevant here but first the assumptions; our model skier is making a single edge-locked, park and ride carve from the top of the turn until he's facing uphill and gravity stops him. His technique and fitness is perfect such that he is always able to keep his balance at precisely the center of the ski, thus load is evenly distributed along the length of the ski. The surface is also perfectly firm and even.

Circular motion: rotation of a body around a fixed axis outside of the body's COM. This is the arc of the ski with some radius X, the axis of the turn being located at the center of that radius. This motion is what gets our model skier from facing left to facing right relative to the fall line. All that is required for this motion to occur is centripetal force generated by the ski on edge, assuming the skier has some speed already. However, we still do have torque (force times distance) in this system from the force of gravity acting through the skier's COM at rN distance from the rotational axis as poorly illustrated by myself in the image below. Hence why we accelerate both in linear velocity as well as angular velocity ω as we pass through the fall line but decelerate in both as we continue a turn past 90deg to the fall line and uphill.

View attachment 190147

Now, a ski being tipped further on edge results in more centripetal force at a smaller radius. This increase in force explains the linear velocity change we observe with the ski in circular motion undergoing a radius tightening, as you note though there is also a corresponding increase in angular velocity ω (rate of rotation about the axis of the turn). A change in ω requires torque this is true. As mentioned previously, we already have this torque as a consequence of the constant force of gravity on the skier COM at varying radii from the axis of rotation while in circular motion. Side note, from this you can also sort of intuit that there's a maximum rate of increase for ω and therefore centripetal force, meaning that if edge angles are built too quickly the amount of centripetal force able to be created to hold the skier up in the turn can be exceeded and you fall inside, something most of us have experienced at some point.

Rotational motion: rotation of a body about an axis passing through its COM. This is almost assuredly not happening to our skier. Remember load is evenly distributed along the length of a ski in a carve and this will resolve directly under the skier's COM. Gravity is acting directly through the skier's COM. There is a frictional force along the base of the ski that theoretically represents a moment about the COM but this is easily overcome by gravity in a turn and our bodies can generate sufficient internal forces so as to prevent our legs from moving separately from our upperbody/COM. To be clear this, force balance absolutely goes out the window when skidding is introduced and the ski is asymmetrically loaded.

This difference is why I think the torque Franko was talking about it is in reference to the ski itself bending, nothing to do with changes in angular velocity, ASSUMING he's talking about a carving ski not parallel or short turns. The ski itself is not generating any asymmetric torque on the skier to influence angular motion in either regime of motion.



Given all the above, I'm confused by what you're saying here. What mass is rotating about the COM? Are you talking about the skis/feet moving separately from the COM/upper body? For short/parallel turns I would agree with you certainly then that there is rotational motion occurring from the tips being loaded more initially which rotates the feet/skis about the COM causing the turn to occur much faster/sharper than is possible via centripetal force and gravity. The skier then naturally uses the tails a bit to arrest the rotation enough to begin the next turn.

This all is of course very grey in the real world where skidding/edging is blended, imperfect technique, the skier manipulating the motion of their COM through muscular effort instead of passively accepting forces, snow conditions etc.

You're neglecting the physics that describes how turns start, stop, or change. It's all in the textbooks, but you have to delve a little deeper than the basic intro to physics, or at least what most people might remember from intro to physics. Your graph shows the skier rotating around his COM, and it shows the skier rotating around a circle. These are facts. If there wasn't rotation around the COM in your graph, the skier's position would be the same at all points around the circle; skis to the left; head to the right, for example. The skier is like a tidally locked planet. A tidally locked planet is considered to rotate around its axis exactly once during its orbit around the sun, even though it's moving around the circle just like the skier. If you believe the skier isn't rotating around its COM, then you also have to believe that a tidally locked planet isn't rotating around its COM, and therefore you'd have to argue with the entire scientific community instead of just me, because it is universally accepted that the tidally locked planet rotates around its COM. It's also universally accepted that the skier rotates around its COM by physicists, but that one might be harder for you to look up and see such simple language confirming the rotation. Please don't tell me you're a physicist. This will destroy my faith in our educational institutions.

A planet doesn't have to be tidally locked. It can have any number of rotations around its COM as it travels around the sun. How is this determined? It all depends on the initial conditions which involve, guess what, torque. Skier rotates around the COM, this requires torque to get started. These are facts. Once someone understands this, then the next step is to identify where those torques come from (through the skis). Though, one important point about posts like these, no one really cares. Maybe Francois cares.
 

geepers

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Sounds to me that 1 means get forward, but whatever.

Fore/aft is not really meaningful unless we discuss what's happening in the feet. Before the fall line pressure roughly evenly along the whole the inside 'edge' of the outside foot. As the turn progresses into the fall line pressure builds more towards the heel. Wouldn't define that as "getting forward".

I don't think hip flexors play much of a role when why'skiing, they are pretty weak in most people

Most ppl can walk and most can kick a soccer ball. Must have some hip flexor capacity to do those. Don't really need more.

Maybe HK (Kitz '23) isn't using hip flexors here - don't really know as he doesn't make vids explaining his technique afaik. But if not, what is he using?

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markojp

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Sounds to me that 1 means get forward, but whatever.

I don't think hip flexors play much of a role when why'skiing, they are pretty weak in most people

Skis are slick. The coefficient of friction is low. It doesn't take much strength to make things happen.
 

JESinstr

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Found this in a search:
"Hip flexors are located on the front top part of your thigh in the pelvic area. It is because of the hip flexors that you can flex your hips and bend your knees to your hips. They are important to keep the posterior pelvic muscles in balance. The hip flexors are a group of muscles, the iliacus, psoas major muscles (also called the iliopsoas), and the rectus femoris, which is a part of your quadriceps. The quadriceps runs down from your hip joint to your knee joint."

IMO From this description, it appears that the hip flexors are integral to the function of the quads which are pretty powerful. When retraction is the goal, I would submit it is the activation of the quads that triggers the hip flexors. On the other hand, if the skier is skiing in the "Back Seat" that puts the ownness on the hip flexors to support balance of the upper body with the quads in resistance hence the thigh burn many feel. I may be wrong, but this seems logical to me.

This concept of fore and aft balance is very misleading. We ski centered and manage to the extremes of fore and aft.
 

James

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Nah, it’s the tibialis anterior that does all the work. Quads schmads, hamstrings heartstrings.
 

markojp

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Nah, it’s the tibialis anterior that does all the work. Quads schmads, hamstrings heartstrings.

But, but, but.... it's too weak to do anything! Jam your knees in the front of the boot! Now that's ski'in!
 
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Zirbl

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When you watch a baseball game, you see a wide range of mechanics based on individual strengths, preferences, and development over time. But all skiers do exactly the same thing?
 

David Chaus

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I’m amazed that people are concerned about which muscles are used for movements in skiing. Pressure the damn tips already, just not only the tips but the whole ski. The muscles will take care of themselves, but not by overthinking.
 

Scruffy

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I’m amazed that people are concerned about which muscles are used for movements in skiing. Pressure the damn tips already, just not only the tips but the whole ski. The muscles will take care of themselves, but not by overthinking.
Deconstructed food has nothing on us.
 

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