What does the term "Platform Angle" mean to you?

[Mike King]

I've found that working with different snow types - softer conditions to harder - helps. Angulation for grip is much more critical on the harder surface. Tom Gellie calls it "honest" snow.



Kind of wish that hadn't been posted uncaptioned in a thread about platform angle:

• It isn't platform angle
• As an indication of "angulation angle" it doesn't taken into account her separation and that the torso is leaning forward and out of plane. Nor that there angulation of her head at her neck and other places
• One side is drawn on the vertical by randomly starting in right hip. Would you say the person below is standing with a lean just because I drew a line passing through one side of the hip? If we drew that line from middle of pelvis to middle of neck we'd get a better idea.

View attachment 104333

I have a suspicion that these vertical lines are one of the reasons we get this "inclination is bad" stuff. Probably the only thing to keep vertical is the head as it then provides a stable level base for eyes and inner ear.

Plenty of inclination here. Plenty of angulation (shown in red) at multiple joints. A wild guess at CoM locaation and platform angle (shown in blue).
View attachment 104334
ANGULATION or INCLINATION??...What's the point?

It was not intended to say anything about platform angle -- only that angulation reduces the amount of inclination required to create a desire edge angle (and hence radius of turn). I've edited the original post to make that clear.

Inclination is not inherently bad, nor is angulation inherently good. Inclination is mandatory in a turn -- without moving the CoM inside the turn (resulting in a deviation of the CoM from vertical, e.g. inclination) the force of the turn would cause you to topple to the outside and fall. And it is possible to over-angulate.

Paul's article is quite good.

Mike

 

[Guy in Shorts]

--For skiers whose stance is much more upright than Mikaela's, who typically ski on flatter skis, who want to use platform angle for grip, will need to work on keeping the inside ski up under the body. Flex the inside leg and roll its knee/ankle down towards the snow, keeping that foot up under its hip, instead of sending it passively out while pushing the outside foot/ski outward. Avoid enacting a hip drop or leaning the whole body into the turn. Keep torso as upright as possible while flexing the inside leg and rolling its ankle/knee, and by default the angulation will happen and platform angle can be small enough for the skis to grip firm snow.

A few years ago @surfsnowgirl asked what I called the type of skiing she saw my buddy and I practicing. Had a hard time describing the short radius turn based style where the platform angle is minimized. Pretty sure that @LiquidFeet has put into words the answer to the question. My new answer is it's Platform Angle Based skiing.
Over the years somehow I have taken on a few friends that wish to learn how to ski the mountain in this local style. Being a self taught skier the solutions that I have reached are my own. Fearing that I have wasted the over 10,000 hours that I have put in out on the slopes without mastering any skill is now put to rest in my mind. I have mastered the ability to control my speed under all 3D conditions using platform angle. My new goal is to see if this Platform Angle Based skiing is the Fountain of Youth that will allow me to enjoy the next 10,000 hours.

 

[LiquidFeet]

....My new goal is to see if this Platform Angle Based skiing is the Fountain of Youth that will allow me to enjoy the next 10,000 hours.

There are those who say it is.

 

[Steve]

@Skisailor may she Rest In Peace. Her motto was "The laziest skier on the mountain." She and Ursula, her mentor, taught efficient skiing.

 

[markojp]

If the skis don’t hold, assuming sufficient edge sharpness, it’s because angulation was insufficient?

You can still angulate, be back and inside, and lose platform angle. Odd how that works. Nothing matters unless you're truly over your outside ski, i.e., having platform angle and commensurately greater success with edge grip and then have access to play with the DIRT icing on the centripetal cake.

 

[GlacierNovesia]

Hi all,

this is a very interesting discussion. We had a similiar discussion on a german forum a couple of months ago and I like to talk about the theoretical physics of skiing.

It is the angle between the plane of the snow platform created by the ski's base and the net force exerted by the snow on the ski/skier. This ground reaction force must be must be 90 degrees to the platform, or pointing more up than sideways than a 90 degree angle in order for the ski to not slide sideways.

I believe that this conclusion based on a misunderstanding. When you have a look at this picture

you will see that you have to put your hips outside the turn and / or tilt your ankles to the outer edge (Fig. C1). But this shows more or less a very static situation, like the difference between drifting or ski holds when you stop at the hill.

I have used a similar drawing in the past:

ψ is the angle between the applied force and the ski.

Fp points out of the turn if ψ is less than 90°. But for this you have to put your weight on the outer edge. Either you move your hips outside the turn and / or tilt your ankles to the outer edges. Do you normally ski in this way? I don't think so. There is only one situation, when you put your weight on the outer edge: When you finished the current turn and you initiate the next turn.

But under normal conditions every experienced skier move his hips towards the center of the turn and / or tilts his ankles to the inner side. But even if two skiers move in the same way, one may slip while the other can carve.

So there must be other variables that have an influence. For example:

- the correct timing
- the correct movement
- the quality and constitution of the snow
- the weight of the skier
- the sharpness of the edges
- the quality of the edge setup
- ......

Thats the reason why professionals take so much care on their ski setup.

So an angle ψ of equal or more than 90° is a mandatory condition, but it's not the only one that decides if you carve or slip. And actually, an angle ψ of equal or more than 90 ° is a matter of course when skiing on the slopes.

Peter

 

[LiquidFeet]

@GlacierNovesia, welcome to PugSki. It's good to see new skiers who are interested in technical talk.

 

[karlo]

I have used a similar drawing in the past:

I don’t understand the drawing.

 

[François Pugh]

Hi all,

this is a very interesting discussion. We had a similiar discussion on a german forum a couple of months ago and I like to talk about the theoretical physics of skiing.



I believe that this conclusion based on a misunderstanding. When you have a look at this picture

you will see that you have to put your hips outside the turn and / or tilt your ankles to the outer edge (Fig. C1). But this shows more or less a very static situation, like the difference between drifting or ski holds when you stop at the hill.

I have used a similar drawing in the past:

View attachment 106945


ψ is the angle between the applied force and the ski.

Fp points out of the turn if ψ is less than 90°. But for this you have to put your weight on the outer edge. Either you move your hips outside the turn and / or tilt your ankles to the outer edges. Do you normally ski in this way? I don't think so. There is only one situation, when you put your weight on the outer edge: When you finished the current turn and you initiate the next turn.

But under normal conditions every experienced skier move his hips towards the center of the turn and / or tilts his ankles to the inner side. But even if two skiers move in the same way, one may slip while the other can carve.

So there must be other variables that have an influence. For example:

- the correct timing
- the correct movement
- the quality and constitution of the snow
- the weight of the skier
- the sharpness of the edges
- the quality of the edge setup
- ......

Thats the reason why professionals take so much care on their ski setup.

So an angle ψ of equal or more than 90° is a mandatory condition, but it's not the only one that decides if you carve or slip. And actually, an angle ψ of equal or more than 90 ° is a matter of course when skiing on the slopes.

Peter

Although I don't have time now to fully confirm your drawing, I believe we are actually in agreement. Your drawing shows the force exerted by the skier, and I am posting about the angle of the ground reaction force exerted by the snow (which is equal in magnitude, but opposite in direction. Same result: 90 degrees to platform holds; more vertical holds; more sideways slides.

 

[geepers]

Hi all,

this is a very interesting discussion. We had a similiar discussion on a german forum a couple of months ago and I like to talk about the theoretical physics of skiing.



I believe that this conclusion based on a misunderstanding. When you have a look at this picture

you will see that you have to put your hips outside the turn and / or tilt your ankles to the outer edge (Fig. C1). But this shows more or less a very static situation, like the difference between drifting or ski holds when you stop at the hill.

I have used a similar drawing in the past:

View attachment 106945


ψ is the angle between the applied force and the ski.

Fp points out of the turn if ψ is less than 90°. But for this you have to put your weight on the outer edge. Either you move your hips outside the turn and / or tilt your ankles to the outer edges. Do you normally ski in this way? I don't think so. There is only one situation, when you put your weight on the outer edge: When you finished the current turn and you initiate the next turn.

But under normal conditions every experienced skier move his hips towards the center of the turn and / or tilts his ankles to the inner side. But even if two skiers move in the same way, one may slip while the other can carve.

So there must be other variables that have an influence. For example:

- the correct timing
- the correct movement
- the quality and constitution of the snow
- the weight of the skier
- the sharpness of the edges
- the quality of the edge setup
- ......

Thats the reason why professionals take so much care on their ski setup.

So an angle ψ of equal or more than 90° is a mandatory condition, but it's not the only one that decides if you carve or slip. And actually, an angle ψ of equal or more than 90 ° is a matter of course when skiing on the slopes.

Peter

Welcome to the forum.

If you mean: is platform angle less than 90 degrees the only necessary condition for the ski to hold then no, it is not. There are other factors.
The snow must have enough strength to withstand the forces being imposed and the ski edges must be sharp enough to cut into the snow to create a groove. Assuming the snow has enough strength then the general rule is that it will be it is more difficult to retain grip (noslipping) on harder snow than on softer snow.
A very common issue not directly listed is correct fore/aft balance. To hold without slip it is necessary that there be no imbalance of forces acting on the ski that would cause the skier to rotate around their own vertical axis. If the skier has their weight too far forward on the ski then the rear of the ski will be less engaged in the snow - more reaction force towards the tip and less towards the tail will result in such a rotation - the tail will slip out regardless of platform angle.

 

[GlacierNovesia]

^^^ Yes, I agree with you. There are other factors beside the platform angle that makes the difference between sliping and carving. Only when you stop at the hill you can ignore most of them (except the snow strenght and others). But when we assume that other conditions are fullfilled we can reduce the question of slipping or stopping to the plattform angle.

I don’t understand the drawing.

Well, the drawing shows the parallelogram of forces for an plattform angle of less than 90°. For simplification I show only one ski. You will find similar drawing in the internet, e.g. in this article:

Physics of skiing (Chapter Preventing Ski From Slipping On Snow )

My drawing is an extension regaring to the necessary movement of the upper body and the position of the mass center. This extension makes clear that you normally never have an plattform angle of less than 90° within a turn.

 

[karlo]

^^^ Yes, I agree with you. There are other factors beside the platform angle that makes the difference between sliping and carving. Only when you stop at the hill you can ignore most of them (except the snow strenght and others). But when we assume that other conditions are fullfilled we can reduce the question of slipping or stopping to the plattform angle.



Well, the drawing shows the parallelogram of forces for an plattform angle of less than 90°. For simplification I show only one ski. You will find similar drawing in the internet, e.g. in this article:

Physics of skiing (Chapter Preventing Ski From Slipping On Snow )

My drawing is an extension regaring to the necessary movement of the upper body and the position of the mass center. This extension makes clear that you normally never have an plattform angle of less than 90° within a turn.

In your drawing, the skier is very inclined and the ski slides out?

 

[GlacierNovesia]

In your drawing, the skier is very inclined and the ski slides out?

Yes, I should have explained it in a proper way....

 

[karlo]

Yes, I should have explained it in a proper way....

Thanks

 

[cantunamunch]

So there must be other variables that have an influence.

One of my open questions for almost 20 years now, first posted on Epicski in relation to a similar picture: When the CoM is outside the body, does that necessarily involve more muscular effort - and therefore necessitate greater fitness?

 

[Chris V.]

One of my open questions for almost 20 years now, first posted on Epicski in relation to a similar picture: When the CoM is outside the body, does that necessarily involve more muscular effort - and therefore necessitate greater fitness?

Less stacked = stronger muscular engagement required, I should think. You ought to be able to get insight on this just by putting yourself into an angulated position in street shoes. Or better yet, going through cycles of angulation movements. The body angles will be similar to what you experience while skiing. What will be lacking will be the added inclination that you need to balance against centripetal force while skiing. Also, resisting that centripetal force while angulated is going to increase the muscular engagement required, compared to what you feel while in static angulation in street shoes. Just my non-expert observations.

 

[cantunamunch]

The problem with street shoe scenarios is that it is brutally difficult to get one's CoM far outside the actual body. Angulation-type movements won't do it - the base of support is too small and you will fall over long before it happens.

It's easier to do it in the front/back (sagittal) plane on skis- the base of support is huge - and yes there is muscle strain involved.

My open question can therefore be rephrased as "does having that additional centripetal force to balance against reduce the necessary muscle strain of having the CoM outside the body - in cases such as the poster above drew".

PSIman is, of course, a data point . When his CoM is outside the body he falls over.

 

[Chris V.]

The problem with street shoe scenarios is that it is brutally difficult to get one's CoM far outside the actual body. Angulation-type movements won't do it - the base of support is too small and you will fall over long before it happens.

I just don't think that's true. Standing up, it's enough to put your COM outside your body just to bend over deeply at the waist. (Unless your belly is huge, LOL.) For that matter, you could even do it lying on the floor on your side--no balancing required. Doing proper angulation in street shoes, you can readily tip your feet over to a substantial angle and remain standing. You just have to counterbalance properly with the upper body. We're all accustomed to everyday movement using a narrow BOS--two feet, close together.

 

[François Pugh]

Hi all,

this is a very interesting discussion. We had a similiar discussion on a german forum a couple of months ago and I like to talk about the theoretical physics of skiing.



I believe that this conclusion based on a misunderstanding. When you have a look at this picture

you will see that you have to put your hips outside the turn and / or tilt your ankles to the outer edge (Fig. C1). But this shows more or less a very static situation, like the difference between drifting or ski holds when you stop at the hill.

I have used a similar drawing in the past:

View attachment 106945


ψ is the angle between the applied force and the ski.

Fp points out of the turn if ψ is less than 90°. But for this you have to put your weight on the outer edge. Either you move your hips outside the turn and / or tilt your ankles to the outer edges. Do you normally ski in this way? I don't think so. There is only one situation, when you put your weight on the outer edge: When you finished the current turn and you initiate the next turn.

But under normal conditions every experienced skier move his hips towards the center of the turn and / or tilts his ankles to the inner side. But even if two skiers move in the same way, one may slip while the other can carve.

So there must be other variables that have an influence. For example:

- the correct timing
- the correct movement
- the quality and constitution of the snow
- the weight of the skier
- the sharpness of the edges
- the quality of the edge setup
- ......

Thats the reason why professionals take so much care on their ski setup.

So an angle ψ of equal or more than 90° is a mandatory condition, but it's not the only one that decides if you carve or slip. And actually, an angle ψ of equal or more than 90 ° is a matter of course when skiing on the slopes.

Peter

About that 2nd diagram....
It's pretty good, but the mass centre should be a little more to the left and up; the point of a centre of mass is that no matter how you cut it there is the same amount of mass on one side as on the other (given uniform density-otherwise it's same amount of mass times distance from centre). That being said, moving it a bit would not change the substance of the diagram.

To help understanding the diagram: The net force acting on the snow (exerted by the skier and his ski equipment) or if it's easier for you to understand, pushing the ski is shown as Fr (I assume r for resultant force meaning the result of summing all the vector forces). Fr can be split into any components you like, and in this case it was split into Fs which is perpendicular to the platform and Fp perpendicular to Fs and parallel to the platform. It was split this way so as to understand: if the angle is 90 degrees, then Fp = 0 and the ski does not slide out; if the angle is less than 90 (more sideways) Fp is positive and the ski slides out; and, if the angle is more than 90 (more vertical) Fp is negative (points the other way) and holds the ski in place.

Another component choice (just to help understand the forces and where they come from) is to split the resultant force into Fg which is always pointing to the centre of the earth and equal to the skier's mass times acceleration due to gravity (aka skiers weight), and the turning force Fc. The turning force Fc is equal to the turn acceleration times the skier's mass. The turn acceleration depends directly on the speed squared and inversely on the radius (a=v^2/R, Fc=mv^2/R). Points to keep: in mind Fg is constant and down, but Fc is always changing; it increases a lot with increasing speed and increases as 1/radius, so the angle depends on speed and turn shape and points to the outside of the turn.

 

[cantunamunch]

I just don't think that's true. Standing up, it's enough to put your COM outside your body just to bend over deeply at the waist.

Now stand with your bum and back against the wall and bend over to a similar angle.

The only reason you could do it before is you were compensating by putting your hips back - which also pulls your CoM back over your feet.

Doing proper angulation in street shoes, you can readily tip your feet over to a substantial angle and remain standing. You just have to counterbalance properly with the upper body. We're all accustomed to everyday movement using a narrow BOS--two feet, close together.

Again, you're not getting your CoM outside the body there either. You're compensating with the hip opposite the bend - if you try to do it with both your foot and hip against the wall, you will fall over again.