Dorsiflexion and its role in skiing

[LiquidFeet]

Right. Do it with tipping. Skiers need to stop "pressing" harder, pushing, jumping on it. Just tip higher.
On hard snow, that is.

 

[geepers]

@Mike King I agree that if you pressure the tip only that the tail will skid out, but if you are pressuring both tip and tail, but increase the amount of pressure to the tip while keeping pressure on the tail (change the ratio of pressure) why do you think that the ski stops carving? Instead of describing part of a circle, the skis are describing part of an oval - still able to be edge locked. At least that's how I imagine it.

Maybe not.


In real life.




And a big shout out to @Loki1 for rebooting Ski School

 

[Steve]

?

 

[geepers]

I was a "stand in the center of the ski" advocate, but now I believe that pressure needs to be manipulated with intent to achieve a desired outcome.

Another take-away from listening/watching Tom... the "center" is not a static place. For example the slope angle changes as we turn into and out of the fall line.

 

[geepers]

?

Ok, kind of thought those vids were self-explanatory.

1st vid... if pressure on tip increased then the back of the ski will have less engagement along its edge. And vice versa.

2nd vid... Hendrik's outside ski tip flexes and releases many times in that turn. The rest of the ski (and Hendrik) follow a less oscillating path.

 

[Steve]

2nd video wasn't available to play for me.
1st video was far from self-explanatory -- to me anyway.

Never did like people answering a question by posting a video with no textual explanation of what they were trying to say. (Think 2 Bakersfield Doctors for example.)

 

[LiquidFeet]

....And a big shout out to @Loki1 for rebooting Ski School

Ditto.

 

[geepers]

2nd video wasn't available to play for me.
1st video was far from self-explanatory -- to me anyway.

Never did like people answering a question by posting a video with no textual explanation of what they were trying to say. (Think 2 Bakersfield Doctors for example.)

Fair enough.
On 1st vid... still more explanation?

The second vid is an extract of one turn from this vid - slomo run from 2:40 onwards. The exact turn posted was the turn to the left at 4 minutes. As you can see the front of the ski is very dynamic.

 

[Loki1]

The problem with Tom's video is that is not how a ski works in the real world. The ski bends through deflection of the tip as it moves forward through the snow. While sidecut is one part of the equation so is longitudinal and torsional flex. The other thing is the ski will actually bend more on a hard surface than a soft surface. Where he is correct though is in the idea that the more we tip the ski the more it will bend through tip deflection as long as we are balanced on that ski. As well as once we are in a certain arc the only way to change that arc radius is through tipping of the ski, either more or less. Edging a ski while balanced upon it creates the pressure many are looking for by trying to pressure through other movements.

 

[Mike King]

@Loki, do you have some evidence to support your assertion of that the ski bends as a result of tip deflection? Note that the other video provided by @geepers, the one that actually measures the bend of the ski, is by Jirij Franko, the designer of the first shaped ski for Elan, and the conclusion is the same one reached by Tom. In fact, when Tom was in Aspen, he used Jirij's video as evidence that the ski cannot bend more than the edge angle allows on hard snow.

BTW, Tom Gellie does actually talk a fair bit about both torsional and longitudinal flex. That flexing and releasing in Henrik's GS run is actually the result of torsional flex in the ski that is releasing the edge angle, according to Tom.

Mike

 

[Steve]

For some reason videos often show as unavailable on Safari, I opened the page in Chrome and could watch the video.
I don't see how that video proves that a ski that e.g. has more pressure (weight distribution) on the tip than tail stops carving. I also don't see that from the other video.

So you guys are saying that fore/aft balance if it's even a little more fore than aft, stops a ski from carving and skids the tail? Just 10-20% more weight towards the front of the ski and the tail is disengaged and skidding?

Can you explain how the first video proves that? I'm honestly not saying I disagree, I just need more help in understanding how the video proves this. Thanks!

 

[Mike King]

Yeah, @Steve, I've been having issues with some of the videos here in Safari. When that happens, if you reload the page, then you can click on the "copy link" icon on the video and paste the copied url into a new tab -- with the result you can watch the video.

The Jirij video is the one that really shows the result. There's two things that go on. First is that loading one end of the ski more than the other causes a gap to arise between the edge on the opposing section of the ski. If you watch from 1:30 in the video, you will see that even at 10 kg of loading, the gap is arising under the front binding with a gap all the way to the tail. The second effect is that the ski develops a twist -- the edge angle at the tip is less than the edge angle at both the mid-section and tail of the ski. So, that's what causes a difference in ski snow interaction between the tip and tail of the ski and why the tail follows a different path that the tip when the load is moved forward or aft of center.

As I said above, the degree of difference of the path obviously can be great. And personally I do not know how much deviation from center is required before the paths start to diverge.

Mike

 

[François Pugh]

@Loki what I disagree with in that comment is the notion that when you pressure the tips, the ski bends more, you shorten the radius, and the skis still carve. By pressuring the tips, you disengage the tail and the result can be a shortening of the radius of the turn, but the mechanism is no longer a carved (edge locked) turn. Instead, the tail of the ski is displaced and turning in a wider arc than the tip.

As to whether you can bend a ski more (on hard snow) once the edge contacts the snow, here's a video by Tom Gellie explaining that you cannot.

Cool video. I still remember having this aspect of ski design explained to me in a ski store when buying my first pair of SG skis. Prior to that I had very little idea of how skis worked. Much like the motorcycle rider who never took a course and didn't know how a motorcycle turned (PSA push the inside handlebar grip forward to tighten the turn), I was just fortunate not to have ended up disassembled via crashing out in the trees at ludicrous speed. While it is true that a ski will bend into an approximate radius equal to the side-cut radius times the cosine of it's tipping angle when tipped on hard snow, there's more to the story of how a ski works (see next reply below).

Maybe not.


In real life.




And a big shout out to @Loki1 for rebooting Ski School

Take away from first video - if you do not have sufficient force applied to the ski to bend it fully (force available is the vector sum of your mass times acceleration due to gravity pointing towards the centre of the Earth, and your mass times your speed squared divided by the turn radius pointing to the outside of the arc you are scribing in the snow, plus dynamic loads available only very temporarily), moving the mass forward bends the tip more and varying the load placement changes the maximum displacement of the ski (so what?).

Take away from the second video in real life on real snow things are not always so smooth and skis tend to deviate from the ideal theoretical path.

IMHO @Loki1 is on the right path to understanding. You have to think in four dimensions - the fourth one being time. When in a typical turn in arc to arc skiing the turn radius decreases on the way to the apex and increases on its way out of the apex. However, the ski, once locked into a carve can only change its shape by moving forward (in time and space). It is the tip that first adopts the tighter radius on its way to the apex, and the rest of the ski follows in the groove cut by the tip. The ski is flexible and has some give to it - the angles at the tip and the angle at the base, the angle at the edge on the snow and the angle at the edge in the air, are all not exactly the same. The ski is deformed by the acting stress. Placing more force on the tip increases the stresses and makes the tip want (for lack of a better term) to turn more as the tip moves forward.

Also please note that skis have taper. The approximated circle that matches the curve of the front of the skis (tip to mid-binding) is typically shorter than that in the back of the skis (mid-binding to tail). The ski deforms so that the exact shape of the ski matches the line made on the hard snow (think wooden floor) when you press that shape into the floor - with some give for bumps, weight distribution oscillations, etc.)

Also being an engineer who is pro-science, the real proof of the pudding is in the tasting. My experience shows me that I can make tighter arc-2-arc turns when pressuring the tips more, and learned decades ago not to over-do it and lose the tails' grip.

 

[Tony S]

My experience shows me that I can make tighter arc-2-arc turns when pressuring the tips more, and learned decades ago not to over-do it and lose the tails' grip.

Also may be worth noting that we think and talk more about pressure on the tips more than we talk about pressure on the middle or tail of the ski not because it's so much more important to a turn from a pure physics point of view, but because it's physiologically and - especially - psychologically more difficult to master for us apes. If you're walking downhill and step on a banana peel, the chances of landing on your face vs. your butt are almost zero.

 

[Loki1]

Ok. Aboutthe first video. So it says the ski is hits maximum bending at something like 20-25kg. From what i can tell that is an adult ski. At 25 kg that equals about 55lbs. Not many adults I know that weogh less than 55lbs. So if we can agree that most adults would have the weight to deform the ski completely, we can say that just by standing on the ski there is enough pressure to flatten its camber. So if the camber is flattened then the entire edge will be in contact with the snow, even at 0 edge angle. So if I now tip the ski, with the edge already fully engaged with the snow. How does it then bend? It bends through the tip deflecting as it moves forward through the snow. This also comes into play when you move the pressure for/aft. While moving it too far will elicite the reactions some have pointed out here, it won't have the same effect that it shows in the video.

 

[Uke]

Loki1,

If you tip a ski from flat on a very firm surface the center of the ski is raised from the surface and the pressure on that center will cause it to bend until the edge is again in contact with the surface. The tip of the ski encountering the surface has little or nothing to do with this flexing, it does have a lot to do with the self pivoting property of a ski which is why forward pressure will generally tighten the arc of the skis path.

uke

 

[François Pugh]

Loki1,

If you tip a ski from flat on a very firm surface the center of the ski is raised from the surface and the pressure on that center will cause it to bend until the edge is again in contact with the surface. The tip of the ski encountering the surface has little or nothing to do with this flexing, it does have a lot to do with the self pivoting property of a ski which is why forward pressure will generally tighten the arc of the skis path.

uke

If you tip the ski while keeping it pressed down on the floor not moving it forwards the middle of the ski slides sideways. If you tip the ski while keeping it pressed into the snow and moving it forwards, the tip of the ski cuts to the inside of the turn and nothing slides sideways (if done correctly for pure carving intention).

 

[François Pugh]

And now for something else, breaking it down into partial components, some folk will see a connection to skiing: if you push a flexible yardstick into the ground, it will bend- how much depends on the force and the yardsticks stiffness and slenderness ratio.

 

[Mike King]

IMHO @Loki1 is on the right path to understanding. You have to think in four dimensions - the fourth one being time. When in a typical turn in arc to arc skiing the turn radius decreases on the way to the apex and increases on its way out of the apex. However, the ski, once locked into a carve can only change its shape by moving forward (in time and space). It is the tip that first adopts the tighter radius on its way to the apex, and the rest of the ski follows in the groove cut by the tip. The ski is flexible and has some give to it - the angles at the tip and the angle at the base, the angle at the edge on the snow and the angle at the edge in the air, are all not exactly the same. The ski is deformed by the acting stress. Placing more force on the tip increases the stresses and makes the tip want (for lack of a better term) to turn more as the tip moves forward.

A couple of things. Why does the radius shorten as you approach the apex and lengthen as you depart it? My answer would be because you build edge angle to the apex and decrease it as you depart.

Second, look at the effect of pressuring the tip in Jirij's video -- namely the torsional loss of edge angle at the tip. When you pressure the tip, you are actually causing it to lose edge angle. And the result can be seen in Henrik's GS turns where he is forward on the skis.

So why do we move forward on the ski? There are two reasons, in my opinion. First, as the skis turn down the hill into the fall line, the skis are accelerating. You have to move forward to keep up with the skis. And since the slope on the path that the skis experience is increasing as they turn down the hill to the fall line increases, you have to move forward to stay over the center of the ski. In effect, the body has to move more quickly than the fee to stay over the center of the ski. You can see this in an inline skater dropping into a half pipe:


There's a couple of things to take away from this video. Note that the skater is stopping the movement of their feet by pivoting on the lip of the half pipe with the center of the skate -- as the body tips forward sufficiently for the wheel to engage, the feet are released. Also, the feet are offset fore and aft to provide a larger base of support and to compensate for any errors in aligning the body perpendicular to the slope angle (the center). Finally, as the feet decelerate as the slope levels out and starts inclining on the other wall, the body has to move aft to stay in a perpendicular relationship to the slope.

The reason to move the CoM forward of the BoS is to shorten the radius of the turn. You can see this in WC skiing. Here's a turn by Alice Robinson at Kranjanska Gora (she won).


Notice how she is on the tail of the ski at the end of the turn to get the tail to bite and provide grip in the end of the turn. She then moves forward to create a steering angle at the gate, but once she has the angle, she is lined up on the center of the ski to get the maximum push across the hill, then moves to the tail for grip. Etc.

If you tip the ski while keeping it pressed down on the floor not moving it forwards the middle of the ski slides sideways. If you tip the ski while keeping it pressed into the snow and moving it forwards, the tip of the ski cuts to the inside of the turn and nothing slides sideways (if done correctly for pure carving intention).

By your analogy, a WC skier would never move any direction but laterally on pressure injected snow since the edge won't create a grove in that surface. In my opinion, the reason the ski moves forward instead of laterally is because of the difference in friction, not any grove.

Mike

 

[François Pugh]

A couple of things. Why does the radius shorten as you approach the apex and lengthen as you depart it? My answer would be because you build edge angle to the apex and decrease it as you depart.

Second, look at the effect of pressuring the tip in Jirij's video -- namely the torsional loss of edge angle at the tip. When you pressure the tip, you are actually causing it to lose edge angle. And the result can be seen in Henrik's GS turns where he is forward on the skis.

So why do we move forward on the ski? There are two reasons, in my opinion. First, as the skis turn down the hill into the fall line, the skis are accelerating. You have to move forward to keep up with the skis. And since the slope on the path that the skis experience is increasing as they turn down the hill to the fall line increases, you have to move forward to stay over the center of the ski. In effect, the body has to move more quickly than the fee to stay over the center of the ski. You can see this in an inline skater dropping into a half pipe:


There's a couple of things to take away from this video. Note that the skater is stopping the movement of their feet by pivoting on the lip of the half pipe with the center of the skate -- as the body tips forward sufficiently for the wheel to engage, the feet are released. Also, the feet are offset fore and aft to provide a larger base of support and to compensate for any errors in aligning the body perpendicular to the slope angle (the center). Finally, as the feet decelerate as the slope levels out and starts inclining on the other wall, the body has to move aft to stay in a perpendicular relationship to the slope.

The reason to move the CoM forward of the BoS is to shorten the radius of the turn. You can see this in WC skiing. Here's a turn by Alice Robinson at Kranjanska Gora (she won).


Notice how she is on the tail of the ski at the end of the turn to get the tail to bite and provide grip in the end of the turn. She then moves forward to create a steering angle at the gate, but once she has the angle, she is lined up on the center of the ski to get the maximum push across the hill, then moves to the tail for grip. Etc.



By your analogy, a WC skier would never move any direction but laterally on pressure injected snow since the edge won't create a grove in that surface. In my opinion, the reason the ski moves forward instead of laterally is because of the difference in friction, not any grove.

Mike

Why does the radius shorten as you approach the apex and lengthen as you depart it?
Answer: Because I am in control of the turn and I choose to increase my tipping angle as I approach the apex and decrease it as I leave the apex.

"By your analogy, a WC skier would never move any direction but laterally on pressure injected snow since the edge won't create a grove in that surface. In my opinion, the reason the ski moves forward instead of laterally is because of the difference in friction, not any grove." Clearly you misunderstood me. I agree with the shifting the load to the part of the ski where it can best accomplish your goals. My point is not why the ski moves forward (gravity), but what it does when it is tipped as it moves forward as opposed to what it does when it is tipped while not moving forward.