Fore/Aft Balance and Physics

[Mike King]

At the Aspen Ski School, we have an excellent training program. I recently had the opportunity to review a 3 part lecture on the physics of skiing by one of our instructors, John Fahey. It was amazing as it tied together a lot of things in my understanding of skiing that were not undermined by my own understanding of mechanics. Now they are.

So, I thought I might write this note to discuss fore/aft balance. There's been tons of discussions about fore/aft; many seem to discuss the subject without referring to the underlying mechanics. So in this note, I'll try to relate what I now understand about fore/aft and the objectives we ought to have in our skiing.

First, let's start with discussing the effect of fore/aft pressure on a ski. If the pressure distribution is in the center of the ski, the resulting shape of the ski, when edged, will be to bend the ski into an arc of a circle. Why is this circular shape important? Because a semi-circle is the only shape where as the ski moves forward, the tail can follow the same path as the tip. Any other shape (parabola, ellipse, etc.) results in a path where the tail will be displaced from the tip and that path will result in drag, or friction. So, the most efficient bend pattern to obtain in the ski is a semi-circle, and that pattern results from having the pressure point of the skis be at their center.

So, what about those folk who coach moving forward to engage the shovels of the ski? If one really moves the CoM forward of the center of the ski, then the ski will be bending more in the front than the tail and that's an inefficient bend pattern -- the tail will be displaced relative to the tip and create drag.

But does that mean that one should not "move forward" from the initiation of the turn into the shaping phase of the turn? Nope. But here's the reason why: as the skis turn down the hill and enter the fall line, they are accelerating. 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.

I've come to see this issue of acceleration as being the factor that reconciles what I here a lot of race coaches coach (get forward) and what a lot of ski instructs coach (stay in the center of the ski) that has appeared to be contradictory. It also is why you cannot look at a still image and judge whether a skier is forward or aft -- there's no information about the acceleration of the skis in a still image.

So, I invite comments on the above -- let me know what you think!

Mike

 

[RuleMiHa]

This is something I've been thinking a lot about, and is near and dear to my heart. I've really come to feel like there are two separate types of skiing and separate types of skies which respond to different styles. There is a fair amount of discussion about skis that ski well from the center and skis that do better being pressured from the front. So part of this discussion has to acknowledge the different style issues.

It could be that the point that needs to be pressured to make the optimal turn shape is in a different location depending on the ski, or a skiers individual anatomy plus binding mount point affects what he has to do to get to that point, and that determines whether he is skiing from the center or pressuring the front. Depending on the ski construction that point may not be midpoint. Basically the variables of ski, binding location, and skier could all contribute to the equation.

For me to carve a traverse, leaving two lines in the snow, there is a spot forward on my skis I must reach, otherwise my skis don't bend properly. If I do reach that point, they behave. This has been key to understanding what I need to do and what my failures are and has been key to developing my setup.

The other issue is that I don't think the shape of the skis stays the same throughout the turn. Yes, if you bent it into a static ellipse or parabola it wouldn't ski well but the shape changes and is fluid, so making comparisons about static shapes doesn't make sense, right?

I also think that skiing the center of a ski is a distinct style which carries its own pluses and minuses. The Skia sweet spot trainer is a device that trains you to be perfectly centered on a spot in the middle of your boots. I found that to be horribly detrimental to my skiing, but many people swear by it.

 

[Started at 53]

At the Aspen Ski School, we have an excellent training program. I recently had the opportunity to review a 3 part lecture on the physics of skiing by one of our instructors, John Fahey. It was amazing as it tied together a lot of things in my understanding of skiing that were not undermined by my own understanding of mechanics. Now they are.

So, I thought I might write this note to discuss fore/aft balance. There's been tons of discussions about fore/aft; many seem to discuss the subject without referring to the underlying mechanics. So in this note, I'll try to relate what I now understand about fore/aft and the objectives we ought to have in our skiing.

First, let's start with discussing the effect of fore/aft pressure on a ski. If the pressure distribution is in the center of the ski, the resulting shape of the ski, when edged, will be to bend the ski into an arc of a circle. Why is this circular shape important? Because a semi-circle is the only shape where as the ski moves forward, the tail can follow the same path as the tip. Any other shape (parabola, ellipse, etc.) results in a path where the tail will be displaced from the tip and that path will result in drag, or friction. So, the most efficient bend pattern to obtain in the ski is a semi-circle, and that pattern results from having the pressure point of the skis be at their center.

So, what about those folk who coach moving forward to engage the shovels of the ski? If one really moves the CoM forward of the center of the ski, then the ski will be bending more in the front than the tail and that's an inefficient bend pattern -- the tail will be displaced relative to the tip and create drag.

But does that mean that one should not "move forward" from the initiation of the turn into the shaping phase of the turn? Nope. But here's the reason why: as the skis turn down the hill and enter the fall line, they are accelerating. 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.

I've come to see this issue of acceleration as being the factor that reconciles what I here a lot of race coaches coach (get forward) and what a lot of ski instructs coach (stay in the center of the ski) that has appeared to be contradictory. It also is why you cannot look at a still image and judge whether a skier is forward or aft -- there's no information about the acceleration of the skis in a still image.

So, I invite comments on the above -- let me know what you think!

Mike

Love it!! I am sure the +2’ers will bring up all kinds of “skiing style” arguments.

I just sit back and laugh at most.... I won’t tag anyone tho

Thanks for the great post.

 

[Chris V.]

All valid as far as I know. But sometimes we want to create a turn that's tighter than the "natural" radius of the ski. Isn't moving forward to bend the tips more a valid way to do this, albeit one that sacrifices the ideal carve? To be done in combination with other movements like adjusting the right-left ski weight distribution, and adjusting the amount of angulation.

 

[Josh Matta]

there really isnt all kinds of skiing styles, physics is physics.

Its an unpopular fact, but I balance on my heels with my toes lifts(toes are still on bottom of boot) and move forward to the apex from there, and when at apex I start letting go of ankle flex.

I absolute hate the "Forward forward forward" advice given and also find stuff like standing on the balls of your feet, and crushing the front of your boot counter productive to good skiing.

IMO for and aft balance is actually the hardest balance to get correct so I cheat when skiing at high speeds in choppy terrain and use long stiff skis, and stiff boots. On smaller soft skis, and or soft boots I find it impossible to stay balanced in that plane and just end up having to slow down.

 

[Mike King]

there really isnt all kinds of skiing styles, physics is physics.

Its an unpopular fact, but I balance on my heels with my toes lifts(toes are still on bottom of boot) and move forward to the apex from there, and when at apex I start letting go of ankle flex.

I absolute hate the "Forward forward forward" advice given and also find stuff like standing on the balls of your feet, and crushing the front of your boot counter productive to good skiing.

IMO for and aft balance is actually the hardest balance to get correct so I cheat when skiing at high speeds in choppy terrain and use long stiff skis, and stiff boots. On smaller soft skis, and or soft boots I find it impossible to stay balanced in that plane and just end up having to slow down.

Josh, I suspect the effect of your fore/aft movements effectively balances you over the center of the ski give its acceleration at any moment.

 

[LiquidFeet]

Great OP, @Mike King. Thanks for taking the time to write that out. I want to add four factors to what you've said about the skis moving faster/slower through the turn, and the dynamic nature of staying centered or moving fore-aft over the skis relative to this issue.

Bindings are not centered on the skis.
The binding on an alpine ski is not centered. So center of the ski is under the ball-of-foot.
This complicates staying "centered" over the middle of the ski while it moves faster/slower through a turn. A line can be drawn upward from the center of the running surface of the skis, perpendicular to their length. How much of the skier's body is in front of and behind that line determines whether the skier is aft, centered, or forward over the skis... at that moment in time.

The tall boot cuff can add leverage to the ski.
There is a tall somewhat stiff cuff going up the skier's lower leg. When the lower leg stays in contact with the front of the cuff and the body above it is hovering over the front of the ski, that hovering weight can lever the shovel of the ski downward through the tongue-shin contact. The skier's option of levering the cuff adds more complication to how one stays "centered" as that ski speeds up and slows down through a turn.

There are two uses of the word forward.
Forward over the skis is the most common usage.
One "forward" is relative to the ski; forward in this case means pressure is directed to the shovel of the ski by the skier hovering the body over the front of the ski to some extent.
Forward on the hill is also used and is just as important.
The other "forward" is relative to the hill. When the skis are moving across the fall line, the skier can move the body downhill across the skis laterally. This is "forward" on the hill, because it means the body goes downhill ahead of the skis. The skis will catch up as they come around. The body gets there first. Conversely, leaning uphill as the skis move across the fall line is "aft" on the hill.
These two usages get mixed up.
Sometimes people are missing each other's points because one is talking about forward over the skis and the other is talking about forward on the hill. And this applies to "centered" and "aft" as well.

Skier's personal perceptions of centered/forward/aft vary.
Then there's the issue of words and their perceived meaning to individuals. When considering self-evaluation, one person's "forward" is another person's "centered." This perception matters in conversations online. To eliminate personal perception, I draw a perpendicular line upward from the center of the running surface of the ski to see how much of the skier's body is in front of that line and how much is behind it to see if the skier is centered, forward, or aft. The same goes for forward/centered/aft on the hill. A line can be drawn perpendicular to the ski laterally, and how much of the skier's body is downhill and uphill of that line can be seen in a video frameshot. This eliminates the need to rely on skier perception. But video is not always available. So personal perception will always play a role in discussions.

 

[RuleMiHa]

All valid as far as I know. But sometimes we want to create a turn that's tighter than the "natural" radius of the ski. Isn't moving forward to bend the tips more a valid way to do this, albeit one that sacrifices the ideal carve? To be done in combination with other movements like adjusting the right-left ski weight distribution, and adjusting the amount of angulation.

I've had people tell me that this is accomplished by moving forward to bend the tips and others say this is done by increasing the tipping angle of the ski. But even the tippers are advocating being forward over the skis.

 

[Mike King]

This is something I've been thinking a lot about, and is near and dear to my heart. I've really come to feel like there are two separate types of skiing and separate types of skies which respond to different styles. There is a fair amount of discussion about skis that ski well from the center and skis that do better being pressured from the front. So part of this discussion has to acknowledge the different style issues.

It could be that the point that needs to be pressured to make the optimal turn shape is in a different location depending on the ski, or a skiers individual anatomy plus binding mount point affects what he has to do to get to that point, and that determines whether he is skiing from the center or pressuring the front. Depending on the ski construction that point may not be midpoint. Basically the variables of ski, binding location, and skier could all contribute to the equation.

For me to carve a traverse, leaving two lines in the snow, there is a spot forward on my skis I must reach, otherwise my skis don't bend properly. If I do reach that point, they behave. This has been key to understanding what I need to do and what my failures are and has been key to developing my setup.

The other issue is that I don't think the shape of the skis stays the same throughout the turn. Yes, if you bent it into a static ellipse or parabola it wouldn't ski well but the shape changes and is fluid, so making comparisons about static shapes doesn't make sense, right?

I also think that skiing the center of a ski is a distinct style which carries its own pluses and minuses. The Skia sweet spot trainer is a device that trains you to be perfectly centered on a spot in the middle of your boots. I found that to be horribly detrimental to my skiing, but many people swear by it.

The first part of your post is not in conflict with my post — it simply suggests that the optimal pressure point depends on anatomy and equipment and may move forward or aft, but that doesn’t change with turn shape, speed, or terrain. The flex pattern of a ski also doesn’t change as a function of its bend as far as I understand, so I don’t think your shape argument holds water, but perhaps my understanding is wrong.

As to the Skia, it’s a static trainer so not of much relevance to balancing on an accelerating object.

 

[razie]

First, let's start with discussing the effect of fore/aft pressure on a ski. If the pressure distribution is in the center of the ski, the resulting shape of the ski, when edged, will be to bend the ski into an arc of a circle. Why is this circular shape important? Because a semi-circle is the only shape whereas the ski moves forward, the tail can follow the same path as the tip. Any other shape (parabola, ellipse, etc.) results in a path where the tail will be displaced from the tip and that path will result in drag, or friction. So, the most efficient bend pattern to obtain in the ski is a semi-circle, and that pattern results from having the pressure point of the skis be at their center.
Mike

There's a lot to disagree with. For one - I would not want to be caught skiing with constant pressure, in circles - that's just a gross oversimplification that a physicist that can't really ski could do...

My pressure generally goes to zero when the ski is flat, then goes to MAX around the apex, and then is reduced back to zero. My pressure is not an on/off switch. Thus I cannot ski in circles . I also have a nasty habit of not staying quiet in the middle of the ski the entire time and I tend to try to use all the parts of the ski - maybe that's my ADD...

A lot of other likely disadvantages would include dizziness and boredom.

But ok - so, to improve, I have to constantly pressure the ski at the given angle X and stay put in the middle of the ski. I should stop moving up and down to not affect the constant pressure and then I will fit in the... "Aspen model" ?

 

[Josh Matta]

I've had people tell me that this is accomplished by moving forward to bend the tips and others say this is done by increasing the tipping angle of the ski. But even the tippers are advocating being forward over the skis.

The can of worms is unless your going straight you can not just talk about for and aft balance by it self. If ski is being tipped(from the feet up with the tipping motion mostly end the femur joint) and assuming the balance is on the outside ski you will get pulled into forward though centrifugal forces.

@razie I dont think Mike King is saying have constant pressure or we should always be dead center.

 

[Mike King]

There's a lot to disagree with. For one - I would not want to be caught skiing with constant pressure, in circles - that's just a gross oversimplifications that a physicist that can't really ski could do...

My pressure generally goes to zero when the ski is flat, then goes to MAX around the apex, and then is reduced back to zero. My pressure is not an on/off switch. Thus I cannot ski in circles . I also have a nasty habit of not staying quiet in the middle of the ski the entire time and I tend to try to use all the parts of the ski - maybe that's my ADD...

A lot of other likely disadvantages would include dizziness and boredom.

Perhaps you are just being satirical, but in case you are not, I never suggested one is skiing with constant pressure nor that the objective is to ski in circles. That being said, if your objective is to minimize the drag induced by the ski, then you want to have the ski move over the snow along it's length -- that is, avoid the friction induced by presenting the edge across the direction of travel, and for that to work, you want to bend the ski into a semicircle.

BTW, bending the ski into a semicircle is using all of the parts of the ski...

 

[Mike King]

@razie I dont think Mike King is saying have constant pressure or we should always be dead center.

It's a good question of what is dead center? Think about being on an airport shuttle train. As the train accelerates, you have to move forward to stay centered over your foot. As it comes to a stop (decelerates), you have to move aft for a similar result.

In skiing, it seems to me that we ought to be aiming for a similar result. Of course, if you wish to have the tails diverge from the tips, one could always try to get in front of the foot...

Mike

 

[Josh Matta]

I would contend there is no such thing as dead center and no one ever will be dead center, we just keep moving and correcting constantly to be where we need to be, where ever that may be.

 

[Mike King]

I would contend there is no such thing as dead center and no one ever will be dead center, we just keep moving and correcting constantly to be where we need to be, where ever that may be.

True that!

 

[Mike King]

Just to emphasize how dynamic balance requires moving fore/aft as the feet (skis) accelerate, watch this video of a rollerblader in a halfpipe -- there a lot of adjustment that has to be made as the feet acerbate/decelerate.

 

[Josh Matta]

On inline skate on ramps you typically cause a tip lead to get more for and aft BOS. I know I was quite good at one point in time. WE also use to wear our skate super loose, so that we could accomplish different rail tricks.

 

[RuleMiHa]

It's a good question of what is dead center? Think about being on an airport shuttle train. As the train accelerates, you have to move forward to stay centered over your foot. As it comes to a stop (decelerates), you have to move aft for a similar result.

In skiing, it seems to me that we ought to be aiming for a similar result. Of course, if you wish to have the tails diverge from the tips, one could always try to get in front of the foot...

Mike

I don't think acceleration changes dead center, it's that different body tissues and substances are more flexible and less anchored so move at a different rate. Think about a caricature of a very wrinkled person accelerating in a train in a straight line. The skeleton and bones move at the rate of the train but the wrinkled skin stretches back, because the attachments are not as secure and it has more lag time before the forward motion translates to it. Basically the recentering you are talking about in a straight line has a lot to do with the motion variables of our fat, soft tissue and organs.

In skiing, we are dealing with centripetal forces, so I think it's different. Those forces are displacing us in an angular fashion.

 

[RuleMiHa]

Its an unpopular fact, but I balance on my heels with my toes lifts(toes are still on bottom of boot) and move forward to the apex from there, and when at apex I start letting go of ankle flex.

I absolute hate the "Forward forward forward" advice given and also find stuff like standing on the balls of your feet, and crushing the front of your boot counter productive to good skiing.

IMO for and aft balance is actually the hardest balance to get correct so I cheat when skiing at high speeds in choppy terrain and use long stiff skis, and stiff boots. On smaller soft skis, and or soft boots I find it impossible to stay balanced in that plane and just end up having to slow down.

I'm not entirely sure about what you mean here but your previous descriptions of how you allow your stiff boots to support you did more to help me with my for/aft issues than anything I've done in years.

Could you explain the difference between allowing the front of stiff boots to support you and crushing the front of the boot?

 

[Mike King]

I don't think acceleration changes dead center, it's that different body tissues and substances are more flexible and less anchored so move at a different rate. Think about a caricature of a very wrinkled person accelerating in a train in a straight line. The skeleton and bones move at the rate of the train but the wrinkled skin stretches back, because the attachments are not as secure and it has more lag time before the forward motion translates to it. Basically the recentering you are talking about in a straight line has a lot to do with the motion variables of our fat, soft tissue and organs.

In skiing, we are dealing with centripetal forces, so I think it's different. Those forces are displacing us in an angular fashion.

True, but that is not the issue. Acceleration doesn't change dead center, but it does affect where the CoM has to be in relationship to the point of balance. And you can do it in Newtonian (describe the motion of the CoM) or Eulerian (describe the motion around the CoM) space.