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# Dan Dipiro's Mogul Book - Page 31

Quote:
Originally Posted by Uncle Louie

Quote:
Originally Posted by mdf

A couple of points to note:

The analysis in the recent posts by Borntoski and jamt is generally correct.

However there is no difference between extending on the flat bottom and the lower part of the curved side.

When we stand up on a curved surface we do NOT actually speed up. The angular rate (theta dot = d(theta)/dt) goes up, but the speed is (radius)*(theta dot) and that stays the same.

So far we have not done any front foot/back foot analysis. The math had been for a single point of support in line with the center of mass. The front-back effects are a very interesting question. My hunch is that it provides many additional ways to pump, but that is only a hunch.
Any thoughts on accelerating off the tail of the ski ?  I don't seem to have seen that mentioned in the course of the thread.

I think that if you accelerate forward by pressing more on the tails, and then bring the skis back under you unweighted, you get a pumping cycle with a net energy input.
(I hope you aren't asking about using the tails in real turns. 3 dimensions and edging are a way more complicated ball of wax.)
Quote:
Originally Posted by mdf

However there is no difference between extending on the flat bottom and the lower part of the curved side.

When we stand up on a curved surface we do NOT actually speed up. The angular rate (theta dot = d(theta)/dt) goes up, but the speed is (radius)*(theta dot) and that stays the same.

Are you sure about that? More work is required by the legs to extend against a larger force. When gravity and centripetal forces line up in the same direction, there is more force then gravity alone, and an opportunity to use muscles to create work against that extra force. More energy injected into the system.
Hmmm. Maybe. I'll have to think about it more.
Quote:
Originally Posted by mdf

Hmmm. Maybe. I'll have to think about it more.

I'm pretty sure it increases. If we take the angular momentum it is

L=I w, and if we assume a point mass I=r^2m and w=v/r, which gives

L=rmv

If we assume conservation of angular momentum we see that v must increase if r decreases.

However, conservation of angular momentum requires that all forces pass through the center of rotation and this is only true exactly at the bottom, otherwise the gravity force does not pass through the center.

If we assume a short push close to the bottom it is a sufficiently good approximation to use conservation.

Another way of thinking about it is forces. When we move closer to the center the force acting on the point mass is no longer perpendicular to the direction of travel (because the travel direction is not perpendicular to the original center of rotation), and thus the point mass must be accelerating its speed.

Perhaps the simplest is to look at energy. Since we add more energy than just the mgh the speed must have increased if we have point mass, because the rotational energy and the linear energy is the same for a point mass.

Edited by Jamt - 5/10/15 at 9:37am

Jamt, put into your simulator an end point just slightly into the halfpipe.  Will the oscillations grow?  No. Why not?  Now put in a curved bottom.  Will the oscillations grow?  Yes.  Then deep in your heart you will understand the mistakes you made about not understanding how parametric oscillation related to swings and halfpipes is entirely about angular momentum, you'll understand that this entire flat bottom discussion is a completely irrelevant and incorrect trolling tactic by you just to try to put me in my place.  You'll understand, maybe, but no one else will, so this forum will never know how clueless you really were.

Quote:
Jamt, put into your simulator an end point just slightly into the halfpipe.  Will the oscillations grow?  No. Why not?  Now put in a curved bottom.  Will the oscillations grow?  Yes.  Then deep in your heart you will understand the mistakes you made about not understanding how parametric oscillation related to swings and halfpipes is entirely about angular momentum, you'll understand that this entire flat bottom discussion is a completely irrelevant and incorrect trolling tactic by you just to try to put me in my place.  You'll understand, maybe, but no one else will, so this forum will never know how clueless you really were.

What I mean by an end point is where you retract your legs.

Whoa. Holy Smokes! I've been seeing this thread in my "subscriptions" list getting a lot of attention, so I decided finally to catch up on it--reading the last 473 posts, I believe it was. I'll admit that I didn't read every post 100% carefully, but I think I caught the jist of them all. 473 posts, and I have to say, very little to show for it, unfortunately.

Are people really arguing whether it's harder to walk, climb, skate, or pump uphill than downhill? The so-called physics arguments are unbelievable--good grief, how complicated does it need to be? The personal attacks, some outright, most snivelingly cowardly and underhanded, the "strawman" misrepresentations of others' arguments simply to assert ones own "superiority," and the attempts to "argue" and establish credibility simply by announcing your so-called "credentials" and proclaiming that, by virtue of your credentials and education, you must therefore be right.... Wow. We've seen these things before, of course. If nothing else, this thread has knocked the stuffing out of any past, present, or future arguments that an advanced degree makes one infallible (it's been so argued, a number of times over the years), since we've seen several people with, presumably, equally advanced degrees in the same subject not only disagree, but disagree vehemently over some very basic points.

My father once reminded not to forget that some people are "educated beyond their intelligence." Such people don't tend to last very long around here, and I suggest that that is not necessarily a bad thing. Who was the wise person who said that we should seek wisdom from anyone earnestly seeking the truth, but flee from anyone who claims to have found it (or something to that effect)?

---

Alas, although I had plenty of thoughts while reading through the thread, I cannot imagine that any of them really matter much in the grand scheme of how to ski moguls--much less that they have much to do with Dan DiPiro's book. Certainly, I've already written most of what I'm willing to say about these things.

But let's cut to one little point that, while it may well bear very little importance to a discussion of mogul technique, seems to have been a point of contention and heated discussion for several of the last pages of this thread. Here's Jamt's simple diagram again:

Just to make sure I'm not misrepresenting or misunderstanding the point, what I see is Jamt's elegantly simple explanation of how a skier can ultimately gain, or lose, speed in a half pipe, through optimally timed flexion and extension movements. The diagram shows the path of the feet, in black (following the contour of the half pipe), and the path of the body (center of mass), in blue, and suggests that the skier starting flexed low at A extends (gets taller, with muscular effort to extend the legs) between B and C, and then flexes low again at D, ready to repeat the cycle in the other direction. By doing this, he ends up higher on the right wall than he started on the left wall, and thus would continuously gain speed (and height) as he repeats the cycle in both directions. Conversely, if he were to do it in reverse--starting on the right and then flexing lower between C and B, he would shed speed and end up significantly lower on the left wall than where he started on the right wall. Jamt--if I'm misrepresenting this, please let me know (my next point is based on this interpretation).

Several people, it appears, have tried to refute either Jamt's conclusion (that we can speed up or slow down in a half pipe with flexion-extension movements) or the physics behind his conclusion-. Arguments have run the gamut from calculus to statements of Newton's Laws of Motion, complex assertions involving angular and linear momentum, and references to the impossibility of gaining speed because that would violate the Law of Conservation of Momentum (or of energy). I really have to think it's much simpler than all that! I'd like to look at the physics here in a way that a non-physicist should understand. If picture work better than words for you, please just skip to the revised diagram below.

First of all, pretty much anyone who has ever dropped into a half pipe knows that it's true--you can "pump" to gain gain height and, therefore, speed. Any physicist (or just a freshman student of mechanics) would recognize that going higher adds potential energy, which can be converted to increased speed (kinetic energy) as you drop back down. So let's not try to argue about why it "cannot happen" when the evidence clearly suggests that it can, and does.

No, it cannot violate the principle of Conservation of Energy (which states that energy can be neither created nor destroyed, but merely converted from one form to another, or transferred from one object to another). And it doesn't. Jamt clearly pointed out (several pages back, don't remember which post) that energy is added to the system between B and C. Where does it come from? It obviously takes energy (converted from chemical energy to muscular force) to extend the legs and "push" the center of mass up between B and C. The added energy is easy to explain--it did not come out of nowhere.

And what did that added energy do? Actively (muscularly) extending the legs pushes (applies a force) against (perpendicular to) the ground. The ground, of course, returns the favor and pushes back (applies a force) equally hard and in the opposite direction (Newton's Third Law of Motion). Since this action-reaction takes place on the flat (level) bottom of the pipe in Jamt's diagram, the force the ground applies to the body (center of mass) is directly UP (perpendicular to the level ground).

What does an upward force do? It causes (unless countered by other forces in the opposite direction) acceleration, in the same direction as the force (Newton's Second Law of Motion). In other words, Jamt's forceful leg extension between B and C produces an upward force from the earth that pushes the skier up, resulting in him ending up higher on the pipe (beyond D). Higher means greater potential energy which, when he comes back down, converts to greater kinetic energy--which means more speed. Simple.

Why does Jamt's skier go higher (and faster)? Simple--there's an external force pushing him up! Yellow arrows ("vectors") indicate the upward force from the ground on the skier's center of mass, as a result of (equal and opposite reaction to) his extending his legs and pushing down against the snow. The force is straight up (because the ground is level in this section), causing acceleration up, which sends him higher in (or out of) the pipe, and ultimately results in increasing speed as he repeats the cycle.

No need to bring angular momentum, complex discussions of the mechanics of "pumping," "harmonic oscillation," and such into this. There's a very simple upward force that pushes the skier higher on the pipe wall with each repeating cycle, resulting in ever higher speeds as he drops back down from ever higher heights.

Some have argued that, while the skier will end up higher when he extends as shown, it would only be due to, and as much as, his increased height when extended. On the contrary, I suggest that the amount of height he gains will be proportional to the strength (magnitude) of the upward force from the ground. In other words, just as in a high jump, how high he gets will depend on how vigorously he extends, not just how tall he is.

And, of course, if the skier wants to reduce speed instead, that's equally easy. Reversing the timing of his flexion-extension movements will reduce the upward force from the ground, minimizing how high he goes up the wall. Again, pretty simple.

Carry on! (Better yet, go out and ski some bumps. Your body already knows all this stuff.)

Best regards,

Bob

As I look at my revised diagram and read my description above, I realize that there is one good objection to be made. The increase in upward force from the ground between B and C would be followed by a decreased upward force when the skier reaches his maximum extension (technically, as he begins to slow the extension). Ideally, the timing of the extension movement would need to continue a moment longer, up the transition--past C, for the maximum effect. Ultimately, the total upward impulse (force x time) must increase to cause the skier to go higher. Any way you look at it, the added force (impulse) comes from the effort of the skier, pushing down on the ground, and the resulting reaction force from the ground pushing up. The overall potential and/or kinetic energy of the skier system increases as chemical energy is converted to impulse.

What's your take on this, Jamt? Engineer? MDF? Anyone else?

Best regards,

Bob

Quote:
Originally Posted by Bob Barnes

Are people really arguing whether it's harder to walk, climb, skate, or pump uphill than downhill? The so-called physics arguments are unbelievable--good grief, how complicated does it need to be? The personal attacks, some outright, most snivelingly cowardly and underhanded, the "strawman" misrepresentations of others' arguments simply to assert ones own "superiority," and the attempts to "argue" and establish credibility simply by announcing your so-called "credentials" and proclaiming that, by virtue of your credentials and education, you must therefore be right.... Wow. We've seen these things before, of course. If nothing else, this thread has knocked the stuffing out of any past, present, or future arguments that an advanced degree makes one infallible (it's been so argued, a number of times over the years), since we've seen several people with, presumably, equally advanced degrees in the same subject not only disagree, but disagree vehemently over some very basic points.

I agree with this entirely.  This is why I don't tell people I have a Ph.D. in engineering unless other people try to claim superior arguments by claiming a greater education.

Bob, the rest of the argument is not really an argument.  Nobody is disputing any of it.  Merely, when I described the physics behind pumping to gain speed in a half pipe, Jamt felt it was extremely necessary to contradict me, right or wrong. (though wrong).  In his words, he thought it would be fun to point out the inconsistencies in my arguments, because I claimed to have an understanding of physics.  And, all the rest of the discussion has been about who's the better physicist.  I tried to stay out of the physics arguments once it was going nowhere, but after I graciously admitted to a tiny irrelevant misunderstanding, Jamt double downed on his superiority which really ticked me off.  Everybody else regardless of the arguments has picked their side which is mostly about who they like, because no one really understands the physics.  Parametric resonance with swings and half pipes is a well known phenomenon resulting from angular momentum, but because of Jamt's obstructionist tactics we had to argue this well known fact.  The whole bit was a complete waste of time other than to observe fascinating social dynamics of irrational forum behavior.  The truth is that with that flat bottom and friction you can hardly pump to pick up speed.  The real power comes from a curved bottom, and that's where the discussion needs to be for relevance, but this whole flat bottom part is just an irrelevant side show pushed for ego.

Edited by The Engineer - 5/11/15 at 9:14pm

I really don't care, TE. I am more interested in your take on my simplified analysis of the half pipe example.

I will say, though, regarding the "social dynamics" thing, that Jamt has a long posting history here that has earned him considerable respect from the community. He earned it the hard way--by posting high-quality, well-considered, often insightful descriptions and explanations. You came as a newcomer and, essentially, demanded respect by virtue of your education--going so far as to request that people address you and think of you as a professor. Sorry--I'm sure that you know that respect is hard won, rarely freely given, and even great professors have to earn the respect of their students. That's surely even more true on the Internet, where anyone can claim to be anyone he or she chooses, and "no one knows you're a dog."

Really, you have posted enough in this thread to have earned some respect, and I hope you continue to do so. No one is infallible, but your perspective and expertise can certainly add to a good discussion of skiing mechanics. But do not expect that it's just a matter of raising your flag with your college degree and expecting the masses to salute it!

Best regards,

Bob

Correct, on the last point BB, but still energy is injected into the system and the CoM is raised to a place of more potential energy, regardless if net force between gravity and ground reaction force over time = 0.

On the curved transition there is an opportunity to inject a bit more energy I believe, due to centripetal reactionary forces which are aligned with the direction of gravity for the first part of the transition, which provides an opportunity for the legs to have more to push against and therefore deliver more work and more energy from the legs.  So on the flat is not as effective of a place to "pump" as on the transition and to someone pumping there it would really not feel like a "pump" either compared to what it would feel like pumping on the transition with a little G force boost there to pump against.  Extending on the flat would feel more like raising yourself or standing taller, and a lot less like actually pumping, though the end result is similar of injecting energy into the system, its just not as much.

Also my right brain finds it significant that on the transition is where momentum is converted into reactionary force to pump against.  On the flat, you are just pumping against gravity, which as you said is like net force zero...  energy is adding but it doesn't feel like an extra pump so to speak.  When you hit the transition, some of the momentum is converted into centripetal forces and then you can pump against that.  So it feels different and more then gravity.

IMHO

Hah, Bob dropping into the pipe?

I was just about to post this on half pipe shapes. While I suppose there may be some with round bottoms, the usual for larger ones is flattish. Certainly for skateboard/bike pipes it's flat.

Don't see a lot of pumping here. ?

http://www.nytimes.com/interactive/2014/02/11/sports/olympics/mens-halfpipe.html

The new this year Double Pipe at Buttermilk. They removed the X Games Pipe and built this. Well Red Bull was paying I guess.

g

Greg Bretz

http://image3.redbull.com/rbcom/010/2014-05-01/1331648493238_2/0010/1/600/400/2/greg-bretz-lays-down-a-backside-720.jpg

Quote:
Originally Posted by Bob Barnes

I really don't care, TE. I am more interested in your take on my simplified analysis of the half pipe example.

I will say, though, regarding the "social dynamics" thing, that Jamt has a long posting history here that has earned him considerable respect from the community. He earned it the hard way--by posting high-quality, well-considered, often insightful descriptions and explanations. You came as a newcomer and, essentially, demanded respect by virtue of your education--going so far as to request that people address you and think of you as a professor. Sorry--I'm sure that you know that respect is hard won, rarely freely given, and even great professors have to earn the respect of their students. That's surely even more true on the Internet, where anyone can claim to be anyone he or she chooses, and "no one knows you're a dog."

Really, you have posted enough in this thread to have earned some respect, and I hope you continue to do so. No one is infallible, but your perspective and expertise can certainly add to a good discussion of skiing mechanics. But do not expect that it's just a matter of raising your flag with your college degree and expecting the masses to salute it!

Best regards,

Bob

I never once told anyone to listen to me, because I have a degree.  I never once told anyone to listen to me, because I'm a physics professor.  All I said was to treat me with the same respect and careful consideration that you would give a physics professor (don't treat me as if I know nothing).  I give that respect to everyone until I'm not given it back (with an occasional mistake).  Just like we shouldn't give extra credit for an education, likewise we shouldn't give extra credit for how well received many posts have been among like minded people.  Don't you see that pulling the many posts card is just like pulling the education card?

Bob, I really don't care about that half pipe example.  It's been dug into the dirt, and nearly irrelevant.  It was just an ego challenge by Jamt to try to prove I'm wrong about something, anything, that he could possibly find, just because I asked to be treated with the respect that we all deserve.

Yes, it's mostly academic, isn't it, Tog. In reality, there are other ways to gain speed and kinetic energy in a half pipe--since they tend to be set on a hill, rather than level ground. And the "flat bottom" is an important part of most half pipes, as it allows the skier/boarder to adjust speed significantly simply by choosing which direction (down, across, or up-pipe) he goes across the bottom.

So, what does this have to do with bump skiing? Yes, there are "pump-based" techniques that can add or reduce speed through well-timed flexion and extension movements on the front and backsides of the bumps. But really, most speed management comes either from braking (skidding) or controlling line (going uphill to slow down), or both. And flexion-extension-absorption is most critical for managing pressure throughout all phases of the turn, whether the intent is braking or direction control. That, to me, is and always has been the crux--regardless of whether you want to ski the zipperline, or any variation on it, or any other line or tactic.

Best regards,

Bob

Quote:
Originally Posted by Bob Barnes

Yes, it's mostly academic, isn't it, Tog. In reality, there are other ways to gain speed and kinetic energy in a half pipe--since they tend to be set on a hill, rather than level ground. And the "flat bottom" is an important part of most half pipes, as it allows the skier/boarder to adjust speed significantly simply by choosing which direction (down, across, or up-pipe) he goes across the bottom.

So, what does this have to do with bump skiing? Yes, there are "pump-based" techniques that can add or reduce speed through well-timed flexion and extension movements on the front and backsides of the bumps. But really, most speed management comes either from braking (skidding) or controlling line (going uphill to slow down), or both. And flexion-extension-absorption is most critical for managing pressure throughout all phases of the turn, whether the intent is braking or direction control. That, to me, is and always has been the crux--regardless of whether you want to ski the zipperline, or any variation on it, or any other line or tactic.

Best regards,

Bob

So this was the discussion.  How much does compressing on a curved surface in the moguls provide speed control through angular momentum effects?  The irony is that most players in this argument are in agreement.  A flat bottom is irrelevant to that discussion, because a curved surface, where the centrifugal forces are in line with gravity, create the most power.  Some would like to know exactly how much speed control can happen.  The truth is some.  I would say not much.  But, we were never able to get to the answer of how much because we were derailed by "the flat bottom".

About the question of whether you speed up (in addition to adding potential energy) by pumping on the curved surface vs on the flat...
Yes, I made a blunder. The perils of jumping to conclusions and posting quickly.

On the curved surface, the skiers legs exert a centrl force so they cannot change the angular momentum (with the origin in the obvious place in the cneter of the pipe). But the angular momentum is position cross velocity, which is r SQUARED times theta dot, not r times theta dot. So the linear speed does go up. Since r^2*thetadot=constant, linear velocity = r*thetadot goes like one over radius.

(Or Icould have just looked at my own results Iposted previously.)

Oops.
Quote:
Originally Posted by mdf

About the question of whether you speed up (in addition to adding potential energy) by pumping on the curved surface vs on the flat...
Yes, I made a blunder. The perils of jumping to conclusions and posting quickly.

On the curved surface, the skiers legs exert a centrl force so they cannot change the angular momentum (with the origin in the obvious place in the cneter of the pipe). But the angular momentum is position cross velocity, which is r SQUARED times theta dot, not r times theta dot. So the linear speed does go up. Since r^2*thetadot=constant, linear velocity = r*thetadot goes like one over radius.

(Or Icould have just looked at my own results Iposted previously.)

Oops.

The greater the force the greater the energy moving through that force.  At the bottom of a curved pipe, you will have the most angular momentum, because gravity is accelerating you.  Pushing at the moment of greatest force provides the greatest result.  Once you add in that flat bottom, the forces drop tremendously and you have less force to push through.

We would have to plug in some reasonable numbers to see how much more pumping you get on the curved surface rather than the flat on the bottom.

Also, I am not sure that this simplified radial pumping is even the dominant effect in the real world, where the spread out base of support allows the skier to generate tangential as well as radial forces. Add in the dimension down the axis of the pipe (turning! What a concept) and there is an even wider variety of ways to pump.
Quote:
Originally Posted by mdf

We would have to plug in some reasonable numbers to see how much more pumping you get on the curved surface rather than the flat on the bottom.

Also, I am not sure that this simplified radial pumping is even the dominant effect in the real world, where the spread out base of support allows the skier to generate tangential as well as radial forces. Add in the dimension down the axis of the pipe (turning! What a concept) and there is an even wider variety of ways to pump.

Jamt said a factor of three.  But, it will depend on the size and shape of the half pipe.  That's why I asked for the radius to be included in that calculation for slope and half pipe size.  The greater the angular velocity, the greater the centrifugal force, the greater the energy you can add.  When pushing on a curved bottom there is no limit to how much power you can add.  You can always create larger forces by increasing the angular velocity.  So, to answer CTkook's question, the size and shape of the moguls are extremely important.

Edited by The Engineer - 5/16/15 at 5:00am
Quote:
Originally Posted by mdf

We would have to plug in some reasonable numbers to see how much more pumping you get on the curved surface rather than the flat on the bottom.

Also, I am not sure that this simplified radial pumping is even the dominant effect in the real world, where the spread out base of support allows the skier to generate tangential as well as radial forces. Add in the dimension down the axis of the pipe (turning! What a concept) and there is an even wider variety of ways to pump.

Those equations you did do have it all.  I didn't double check them, but the term on the right represents mgh potential energy.  The cos term shows that gravity is reduced as we move off of the bottom.  The term on the left represents energy from centrifugal force.  The second derivative shows that the faster we are moving around the circle, the greater the power.  The squared radius wasn't immediately obvious to me, but shows the power potential with larger half pipes for a given angular velocity.  Right?

Edited by The Engineer - 5/16/15 at 4:21am

angular momentum is irrelevant in the bumps.  When we ski the bumps we are generally trying to keep our CoM quiet and traveling on a linear line with the Com itself having linear momentum.  While there may be a very brief curved section at the base of the face of the bump, many times it will not be curved at all and notwithstanding any of that, we are generally flexing our legs like crazy and allowing the CoM to travel on a linear path.  So the CoM doesn't really ahve any angular momentum.  The BoS might have a very small amount for a very brief amount of time.

Here's an interesting thought for the half pipe discussion.  If you are cruising across the flat section and your skis run into the transition, but you flex your legs in such a way that your CoM is sufficiently disconnected from the deflection of the BoS that it continues in a straight line, even while the BoS is traveling up the curved transition.  In that case does the CoM actually have any angular momentum?  I say no.  Yet the skier will slow down doing that.  So if the CoM doesn't have angular momentum, then lowering it towards the snow and slowing down has nothing at all to do with angular anything.

TE, JAMT's flat bottom example was in response to claims by others that the ONLY way to gain height in the pipe, is through angular momentum principles.  He was just trying to show that its possible to gain height and energy in the half pipe without using anything related to angular momentum.  It was a perfectly reasonable response to earlier claims.

Quote:
Originally Posted by Bob Barnes

So, what does this have to do with bump skiing? Yes, there are "pump-based" techniques that can add or reduce speed through well-timed flexion and extension movements on the front and backsides of the bumps. But really, most speed management comes either from braking (skidding) or controlling line (going uphill to slow down), or both. And flexion-extension-absorption is most critical for managing pressure throughout all phases of the turn, whether the intent is braking or direction control. That, to me, is and always has been the crux--regardless of whether you want to ski the zipperline, or any variation on it, or any other line or tactic.

Best regards,

Bob

The underlined is my feeling about this whole thing.  We want pressure pretty much always.  Flexion and extension are used to try to maintain a stable amount of pressure.  A little on the face, a little on the backside and everywhere in between.  If we eliminate pressure entirely on the face, we can't use our edges there either, not to mention that speed reduction happens from energy basically being transferred to the snow through pressure against the face.

I believe just about everyone here except for possibly Nail, believes we should flex our legs on the face and extend on the backside, which generally speaking will result in evened out pressure.  However, that is a simplified description and there is more nuance to that discussion for those seeking more finesse.  If you flex too aggressively on the face and lose too much pressure, then speed control is compromised until pressure can be established again.  Pressure allows edge engagement, and on the face allows transfer of energy into the snow.  If you compromise your pressure then you're floating and not reducing speed anymore.

If you have too much pressure on the face then the face will launch you airborne, so there is a sweet spot amount of pressure where some speed reduction and edge engagement can take place on the face through pressure, while also absorbing enough of that pressure such that the skis will stay on the snow, or have a minimal disengagement going onto the backside, in order to continue using pressure for edge engagement and further speed control.

If you overdo it on the face with too much pressure, you won't get speed control on the backside.  Conversely if you overdo it on the face with not even pressure, then while you may get excellent engagement on the backside, you will have missed speed control opportunities on the face and will be going that much faster by the time you hit the backside.  So basically you want to find that sweet spot of just the right amount of pressure on the face in order to have edge engagement there, and to also resist against the face of the bump, but not so much as to end up getting air off the bump.  That area of nuance is where the finesse is.

This discussion is somewhat academic, but there is a reason we're having it.  This side track notion of sucking up the legs and using the mythical angular momentum super hero force to suck up speed is a flawed point of view that does not recognize this small amount of pressure that is needed for speed control on the face of the bump.  And that is a critical factor.

I would like to go back to an earlier discussion here as well, regarding the effect of "absorption" on speed. Once again, barring some fairly minimal pump-like effects, and as BornToSki among others has vehemently argued, absorption alone, in and of itself, does not slow you down. Two things can slow us down--increasing resistance to sliding ("friction") and going uphill ("direction"). The former typically involves braking with intentional skidding, scrubbing off speed with the edges, but it can also include falling down, running into trees, and things like hitting a patch of springtime "glue snow." Not all of these are equally recommended!

These two categories are pretty much mutually exclusive--the more you skid, the less you can change direction, and the cleaner your skis track and hold your chosen line (that is, carve), the less braking there is. Great skiers mix and match and blend the two as needed, but I've long held that the greatest skiers invariably prefer speed control from direction when they can, and brake (speed control from friction) only when, and only as much as, necessary.

That's true in bumps as much as anywhere, although your tactical options in bumps are magnified (especially if you aren't a slave to the "zipper line" exclusively). On a smooth, flat, steep run, the only way to control speed with "direction" is to finish turns, perhaps even as far as going back uphill with each turn. In a half pipe or a gully, all you have to do is go straight up the other side. In a bump run, every bump presents an opportunity both to brake hard when you collide with it, or to slow down by gliding up it.

And this is the point I wanted to revisit. It has been said--I don't recall who the proponents were--that "going up the bump" to slow down is mostly a fallacy because, essentially, what goes up must come down, and any speed lost by going up will return when you glide back down. I suggest that this is only partially true and that, again, the key lies in the flexion-extension movements of "pressure control." The real key is that gravity only accelerates us down the hill when we're in contact with the slope, and have some pressure on our skis. When airborne, or without pressure on the slope, gravity pulls entirely straight down, toward the center of the earth. It's only when the slope "resolves" the force of gravity into a force parallel to the slope and another force perpendicular to the slope that the parallel-to-the-slope component pulls us down the hill. If this is not yet clear, imagine launching off a jump going directly across the hill. Your trajectory through the air will not curve downhill, but will simply bring you back to earth on a continuous line straight across the hill. THEN, if you release your edges, your path will start to curve downhill, as the slope "resolves" the force of gravity and creates a component force pulling downhill.

So, flexion-extension movements (absorption movements) that apply pressure on the uphill part of the bump (and don't forget that, with good directional control, we can come into the bump from the side, or even from below, as necessary or desired, in order to go uphill) will slow us down. Pressure on the downhill part of the bump will speed us up. "Complete absorption," by which I mean flexion-extension movements that maintain the same constant pressure on every side of the bump and through the trough, even if possible, would eliminate the bump entirely as a speed controller, as it would mean that the body (center of mass) goes neither up nor down on the bump. But partial absorption, which allows some increase in pressure when going up the bump (pushing the center of mass "up") and reduces the pressure or eliminates it entirely as the center of mass freefalls down the other side of the bump, will have an overall slowing effect. Again, that's because when there is no pressure on the skis, there is no component of gravity pulling parallel to the slope.

So, while what goes up clearly must come down, if we manage the pressure accurately during the "up" and "down" phases, we can slow down on the "up" phase without gaining speed on the "down" phase. In this way, it is really not a "zero sum" game.

"Pumping" to gain speed is largely the opposite of this "partial absorption," as we use flexion and (particularly) extension movements to increase pressure on the downhill side of the bump and decrease or eliminate pressure on the uphill side. Yes, there is surely some element of working with angular (rotational) motion fore and aft, as we do on a swing, to enhance the pumping effect, but in my opinion, this is generally minimal at best. And, of course, it is rare indeed in the first place for most of us to be trying to gain speed in bumps! Even for a mogul competitor trying to go as fast as you can, "pumping," like skating for an alpine racer, has very little effect at already high speeds.

Again, though, in the real world of bump skiing, I submit that these considerations are of minimal importance. More globally, pressure management is a critical part of all turns--as well as all braking movements. Absorption movements (by which I include both flexion and extension) provide this vital tool in bumps (and elsewhere, as forces in turns can vary dramatically even on the flats), enhancing control in every respect. As always, pressure control in isolation--in and of itself--isn't worth much, and to suggest that we manage speed largely through absorption alone is an oversimplification, at best. It is the interaction of pressure management with edging and rotary skills that ultimately provides control--of both direction and speed.

Best regards,

Bob

The curvature is not really that critical. It just makes the math easier, and provides an obvious origin (reference point that angular momentum is defined around).
A flat inclined plane at an angle to the skiers initial direction of travel will show similar, but not identical, effects.

MDF, inclined plane will definitely provide speed control, but in that case there is indisputably, ZERO angular motion of any kind.  Curvature is needed to support the various angular momentum theories being tossed around.

One of the cool things about late spring skiing is that you get large soft bumps on relatively flat stretches of trails between the steeper bits. THat happened last weekend at Killington. The steeper parts required the techniques Bob talks about in his last post. But the flatter parts let you go straight and rely 100% on absorbtion, which is not something I, at least, can normally do. Fun to have that variety.
Quote:
Originally Posted by borntoski683

MDF, inclined plane will definitely provide speed control, but in that case there is indisputably, ZERO angular motion of any kind.  Curvature is needed to support the various angular momentum theories being tossed around.
You do actually have angular momentum, its just not as obvious and the math is messier.

I can piece together flat sections to approximate a curved half pipe. The ride will be bumpier but the energy balance will be similar.

Or I could lay out the equations, but you don't seem to be an equation kind of guy.

I beg to differ.  You have linear momentum being deflected once or several times in a row.  Is there more reaction force?  Yes.  some of the energy is transferred to the face of the ramp, but centripetal forces and angular momentum is non existent.  if you had a polygon with many of those deflections very close together then perhaps the complicated math of many small linear deflections would happen to work out to be the same as a circle of the same size....but certainly a single angle, one ramp..there is nothing centripetal about it.

Quote:
Originally Posted by borntoski683

I beg to differ.  You have linear momentum being deflected once or several times in a row.  Is there more reaction force?  Yes.  some of the energy is transferred to the face of the ramp, but centripetal forces and angular momentum is non existent.  if you had a polygon with many of those deflections very close together then perhaps the complicated math of many small linear deflections would happen to work out to be the same as a circle of the same size....but certainly a single angle, one ramp..there is nothing centripetal about it.

Angular momentum is around some fixed point, so in that sense MDF is correct as long as that fixed point is not on the CoM path. However, the point is that since the forces do not pass through this fixed point, wherever it is, you cannot use conservation of angular momentum as a calculation tool.

Also it is often quite useful to divide the angular momentum into one component corresponding to a point mass, plus the angular momentum around the CoM, i.e. rotation of the actual body itself. The latter is zero on a flat plane. The latter has also been largely ignored but I think it actually helps prove you point that there isn't a lot of useful angular momentum in bump. It would be very hard to switch the polarity of a significant around-CoM angular momentum twice per bump.

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