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# A simple self-assessment test - Page 24

thanks mga...as an ex racer i am certainly familiar with pumping rollers on the flats, its the anti pump i was not familiar with. agreed on what you said about coaches...a lot of athletes arent sure either

zenny
Quote:
Originally Posted by jack97

If you think I'm blowing smoke, go re-read Lind, he sometimes breaks down the downhill path using this vector component. Maybe ski instructors should be mathematicians.

Jack I would make the same request to you.  If you think there are some particular paragraphs that may lend clarity to this discussion then please make reference.

I would like to refer you to pp 135-136 and figure 6.3:   "Pumping to Increase Velocity"

The confusion some of you are having is by confusing two different concepts.  One is conservation of momentum by changing the moment of inertia.  The other is actually changing momentum by adding energy to the system.

You guys have repeatedly expressed the idea that you think if you lower your CoM, to move it outwards on the radius, then by principles of conservation, and changing the moment of inertia, the rotational speed would slow down.

Lind does not make this claim never once that I can find in his book.  He does talk about pumping to speed up, he never talks about about anything remotely resembling anti-pumping to slow down or manipulation of the moment of inertia in some way to do so.

He is clear to state here (and also in the later Technote 10 we discussed earlier) that a skier in a half pipe or someone on a swing, increases their momentum  (not conservation) by adding energy to the system on each lap up and down the two sides of the half pipe.

In his view this is best accomplished by pushing against centrifugal forces, which are largest at the bottom of the swing (not the sides of the wall).  At this point, by pushing down on the bottom of your feet to push your CoM up against centrifugal forces, you add energy to the system.  He also talks about combining the pump with rocking of the CoM relative to the feet, in order to optimize the pump.

He says a pump is possible on an open slope, but fractionally effective....as many of us have argued for this entire thread.

Note that pushing to create more energy in the system is completely different then moving the moment of inertia to change the rotational velocity.  The inverse which you have staked your anti-pump assumption on, would be that the inverse change to the moment of inertia would have the opposite effect.  However Lind does not ever talk about moment of inertia or manipulating it, he only talks about changing momentum by adding energy to the system and specifically he talks about increasing speed.  If you wanted the inverse of what he proposes, you would add energy in an opposite direction in order to slow down, in other words change momentum with work being added somehow.

Lind does not support the anti-pump conclusions being presented here by some of you.  He does support pumping, but not for the physical reasons being put forth by you guys.  The reason you are confused about anti-pumping is because you do not really appear to understand how pumping works either.

Quote:
Originally Posted by MrGolfAnalogy

anti pump. The only time I ever use it is getting snaked (some one gets in you line) entering a transition; if it's too late to turn, you can lesson you momentum by collapsing into the ramp.

You can slow done but collapsing into the face of a mogul, but again why would you with so many other, better options available. And the effect of the "anti pump" is relatively minor, you are not going to see some one come to a screeching halt on to of a mogul after ripping the zipper line. At best, you might be able to ditch just enough speed to make a turn in better control.

Now we are getting somewhere. Do you realize that by collapsing into something that has increasing steepness you increase the pressure when the obstacle is steeper and that slows you down.

That is what we have been saying all along. That is not the same thing as absorbing or sucking the feet over the obstacle. You also lower the CoM which means you will not make it as high.

Maybe this is one reason for the disagreement in this thread.

Quote:
Originally Posted by borntoski683

Jack I would make the same request to you.  If you think there are some particular paragraphs that may lend clarity to this discussion then please make reference.

I would like to refer you to pp 135-136 and figure 6.3:   "Pumping to Increase Velocity"

The confusion some of you are having is by confusing two different concepts.  One is conservation of momentum by changing the moment of inertia.  The other is actually changing momentum by adding energy to the system.

You guys have repeatedly expressed the idea that you think if you lower your CoM, to move it outwards on the radius, then by principles of conservation, and changing the moment of inertia, the rotational speed would slow down.

Lind does not make this claim never once that I can find in his book.  He does talk about pumping to speed up, he never talks about about anything remotely resembling anti-pumping to slow down or manipulation of the moment of inertia in some way to do so.

He is clear to state here (and also in the later Technote 10 we discussed earlier) that a skier in a half pipe or someone on a swing, increases their momentum  (not conservation) by adding energy to the system on each lap up and down the two sides of the half pipe.

In his view this is best accomplished by pushing against centrifugal forces, which are largest at the bottom of the swing (not the sides of the wall).  At this point, by pushing down on the bottom of your feet to push your CoM up against centrifugal forces, you add energy to the system.  He also talks about combining the pump with rocking of the CoM relative to the feet, in order to optimize the pump.

He says a pump is possible on an open slope, but fractionally effective....as many of us have argued for this entire thread.

Note that pushing to create more energy in the system is completely different then moving the moment of inertia to change the rotational velocity.  The inverse which you have staked your anti-pump assumption on, would be that the inverse change to the moment of inertia would have the opposite effect.  However Lind does not ever talk about moment of inertia or manipulating it, he only talks about changing momentum by adding energy to the system and specifically he talks about increasing speed.  If you wanted the inverse of what he proposes, you would add energy in an opposite direction in order to slow down, in other words change momentum with work being added somehow.

Lind does not support the anti-pump conclusions being presented here by some of you.  He does support pumping, but not for the physical reasons being put forth by you guys.  The reason you are confused about anti-pumping is because you do not really appear to understand how pumping works either.

Hmmm... called it done but can't let go and opts to salvo a condescending reply.

ok, I'll say something new.  In a technical program, when a physical science or engineering prof tells me to re read the section, they mean to understand the concepts within. You almost cited verbatim Lind's sections but failed to realize the concept. Technote10 is clear to me of an example angular conservation, specifically equation T10.2. Derivation of this equation assumes this, I would suggest you try to derive this as well.

Edited by jack97 - 9/15/13 at 12:38am
Quote:
Originally Posted by Jamt

Do you realize that by collapsing into something that has increasing steepness you increase the pressure when the obstacle is steeper and that slows you down.

That is what we have been saying all along. That is not the same thing as absorbing or sucking the feet over the obstacle. You also lower the CoM which means you will not make it as high.

yep.

I think there are several things that has caused confusion and disagreement in this thread. I'll try to summarize.

Speed in Moguls was compared to pumping in ramp/ half-pipe. You CAN pump to increase your speed in a half-pipe, BOTH going into the ramp and coming down. You get higher and higher and the speed increases more and more.

However, this does not carry over to moguls, because speed in moguls is primarily the horizontal speed if we compare to the pipe. The best way to maintain this speed is to absorb the bump completely. If you have pressure when the mogul is steep this horizontal speed is decreased because you have a force pushing you backwards.

I can understand that if you come from the skate/board/bmx world where you have learned that you can pump speed and then you hear someone saying that if you extend/pressure in bumps you slow down. But it is not really the same thing. There is an important difference of timing where you pressure also. The steeper the face of the bump is the more you slow down. This is different from the half pipe where you extend before you go into the ramp and continue it in the very bottom part. In moguls this would be the equivalent of extending in the bottom, i.e. more or less jumping over the mogul, i.e. max speed.

So, you can both increase or decrease speed by extending/maximizing pressure into a bump. If you do it early you increase speed (compare jumping over mogul). If you do it later you slow down.

Similarly you can decrease or increase speed by flexing/sucking/absorbing up. If you suck up your legs early you will hit the bump lower and with later pressure and the speed will decrease. If you suck up your legs later when the natural pressure grows, the speed will not decrease. (Decrease/increase is in relative terms here)

That is why I showed the simplified terrain feature. You cannot pump any speed in this feature, you can just minimize the loss. You can pump it to loose less speed than if you just ride it. You can even have a higher speed than the horizontal speed you come in with early in the ramp, so I guess you can argue that you can pump speed, but you cannot have higher speed at 3 than at 1. The reason you can pump the speed up higher than the one you come in with is that you add a vertical component (this is the same reason you can pump a bit going into the half-pipe), but you cannot increase the horizontal one. At 3 this vertical component is gone, and the best you could do is to jump from 1 to 3 or absorb the ramp completely.

Another thing that has caused confusion is the one about conservation of angular momentum. I won't go into details about this one, just rephrase what I said before that the conservation is around a FIXED point, and we don't have many of those in skiing or skating/boarding/bmx. It is very easy to fool yourself. If I have time I might draw a picture to illustrate how pumping for speed works, without involving angular momentum.

I hope I have clarified a bit. Is there even the slight possibility that we can we agree now?

Quote:
Originally Posted by Jamt

I hope I have clarified a bit. Is there even the slight possibility that we can we agree now?

Not until we break 700

Quote:
Originally Posted by jack97

Not until we break 700

Working on it, only 2 to go :-D

This is the third longest thread in the instructor forum. Only MA of Highway star and Max's BPSRT thread are longer.

http://www.epicski.com/f/9/ski-instruction-coaching/sort/replies/dir/desc

Quote:
Originally Posted by Jamt

..... because speed in moguls is primarily the horizontal speed if we compare to the pipe. The best way to maintain this speed is to absorb the bump completely. If you have pressure when the mogul is steep this horizontal speed is decreased because you have a force pushing you backwards.

So, you can both increase or decrease speed by extending/maximizing pressure into a bump. If you do it early you increase speed (compare jumping over mogul). If you do it later you slow down.

Similarly you can decrease or increase speed by flexing/sucking/absorbing up. If you suck up your legs early you will hit the bump lower and with later pressure and the speed will decrease. If you suck up your legs later when the natural pressure grows, the speed will not decrease. (Decrease/increase is in relative terms here)

Another thing that has caused confusion is the one about conservation of angular momentum. I won't go into details about this one, just rephrase what I said before that the conservation is around a FIXED point, and we don't have many of those in skiing or skating/boarding/bmx. It is very easy to fool yourself. If I have time I might draw a picture to illustrate how pumping for speed works, without involving angular momentum.

Just finished walking my dog. Joking aside, Lind uses a similar analysis where by the external forces pushes or pulls the skiers away from or toward the terrain. By absorbing and extending at the right times, the speed would have to decrease due to the direction of the external force. Lind does not show is how much absorption or extension would change this speed via using the external force approach, he only does this with angular momentum, mainly because there is variable to express this in the latter.

Old school bumpers required maximum absorption due to how the course and moguls were formed ..... so how much speed reduction is obtained is just as important.

edit: needed a second cup of coffee

Edited by jack97 - 9/15/13 at 7:31am

Maybe I have to get the Lind book, can you recommend it?

700!!

# A simple self-assessment test

In case anyone has forgotten.  Underlined and highlighted the key words.

24 pages and 700 posts later....

Quote:
Originally Posted by Jamt

Maybe I have to get the Lind book, can you recommend it?

700!!

He got his PhD from Cal Tech and is listed as a professor emeritus at CU. The school has joint research with National Institute of Standard and Technology. He and the school are not slouches, I would recommend the book. IMO, its targeted to people with an advance degree to a minimum with a good back ground in classic mechanics. He tries to simplify things, hence in some areas, he isn't strict with vector notation and does not fully derive some equations. Perhaps he feels the motivated students would derive these themselves.

You might find a lower price book in Amazon used book sections.

Edited by jack97 - 9/15/13 at 7:33am
Quote:
Originally Posted by L&AirC

# A simple self-assessment test

In case anyone has forgotten.  Underlined and highlighted the key words.

24 pages and 700 posts later....

The test is simple: Can you ski the same slope at different speeds (fast and slow) without resorting to slamming on the brakes when you want to go slow?  If you can you're good.  If you can do the same in steep moguls you're better, and if you can do the same in steep icy moguls you're even better. (even though you may still suck at moguls).

Just because the test is simple doesn't mean the technique is simple.

Quote:
Originally Posted by Ghost

The test is simple: Can you ski the same slope at different speeds (fast and slow) without resorting to slamming on the brakes when you want to go slow?  If you can you're good.  If you can do the same in steep moguls you're better, and if you can do the same in steep icy moguls you're even better. (even though you may still suck at moguls).

Just because the test is simple doesn't mean the technique is simple.

Nor the way it is explained...... really. Jamt did a great job under the context of this thread. I was tempted to do the same a couple of days ago but would have converted the terminology and notation borrowed from Lind. Frankly, I wasn't sure if I should have spent my time doing this given how the tone of this thread was going.

It always comes down to pressure control--the when/where of it. This discussion is not so different than many others, other than it's length. Now....no one else post so mine can be the last

zenny

Quote:
Originally Posted by L&AirC

# A simple self-assessment test

In case anyone has forgotten.  Underlined and highlighted the key words.

24 pages and 700 posts later....

To be fair, I did try to start this subject as it's own thread when it first came up. I think it died after seven posts and this one made it to seven hundred.

Quote:
Originally Posted by Jamt

Now we are getting somewhere. Do you realize that by collapsing into something that has increasing steepness you increase the pressure when the obstacle is steeper and that slows you down.

That is what we have been saying all along. That is not the same thing as absorbing or sucking the feet over the obstacle. You also lower the CoM which means you will not make it as high.

Maybe this is one reason for the disagreement in this thread.

Thanks, and nice summary.

Edited to say, sorry Zenny, I was writing my post as you were posting yours. Please go again and have the last word.

Quote:
Originally Posted by MrGolfAnalogy

Edited to say, sorry Zenny, I was writing my post as you were posting yours. Please go again and have the last word.

No, that's cool...go ahead

zenny

Quote:
Originally Posted by jack97

Just finished walking my dog. Joking aside, Lind uses a similar analysis where by the external forces pushes or pulls the skiers away from or toward the terrain. By absorbing and extending at the right times, the speed would have to decrease due to the direction of the external force. Lind does not show is how much absorption or extension would change this speed via using the external force approach, he only does this with angular momentum, mainly because there is variable to express this in the latter.

edit: needed a second cup of coffee

Jack, how about some page references?  I've been looking all through there looking for any place that he implies absorption can slow you down the way you and others have presented.  I don't find it.  If you can provide references it would be helpful.

Speaking of pressure control (and it is the very foundation of this thread IMO), how about the virtual bump? What parallels can we draw between managing it's effects and the effects of real bumps/rolls that we have discussed here over the last 20 pages or so?

For instance, typically in moguls we try to time our A&E so that we don't crumple on the face, get thrown off line, or even launched. Do not we also take these very same factors into consideration when dealing with the forces encountered during an across the hill release? I think we do. Consider the following image:

Sorry for the huge size of this. Now notice how the green portions of this diagram align with a groomed run turn where pressuring is lightest high in the c and as we move towards the red zone and through the apex how pressure increases until the moment where edge release occurs before/along the crest of the VB. This variance in pressuring is at least partly due to the ever changing effective slope angles we encounter even on a smoothly groomed run. We move progressively from "flat" (across the fall line) to steep (in the fall line) and back to "flat" again.

We have a few options for dealing with the forces of VB during finishiation; on one end of the spectrum we can flex and absorb which allows our CM's to move inside the new turn sooner thus getting us on edge quicker. During this vault is lessened and we are able to take a more direct route down the slope. On the other end of the spectrum we can resist the forces at the bottom of the turn (see where I'm going with this?) as we near the crest of VB during which we have a more pronounced rising of the CM, further distance and more time spent across the fall line, increased vault, and subsequently a delayed reengagement (all of which is fine, IF we have the room or the time to do so).

By absorbing VB as much as possible we spend less time across the fall line and by resisting (no pushing or we can pop up or even launch!) we spend more time across it.

To me at least, the above has clear implications in the moguls--from a tactical perspective--not quite the same thing here but not altogether different either. As I said earlier, it's more about pressure control rather than speed control and which tactics we employ have a direct influence on the outcome. Thoughts?

zenny

Edited by zentune - 9/15/13 at 1:07pm

Yup,

Just like moguls (real bumps), if you apply too much pressure (don't allow flex to release some vertical pressure) you will have trouble applying pressure on the down side of  the virtual bump.  Also pulling back the feet works similarly in both.

Seems obvious I know, but I just wanted to tie the whole thing together with the original intent of this thread in case people ever read this thing all the way through . Speed control via turn shape- a turn shape partially dictated by where/when/how we choose to flex or not (among a host of other things of course).

zenny

Quote:
Originally Posted by borntoski683

Jack, how about some page references?  I've been looking all through there looking for any place that he implies absorption can slow you down the way you and others have presented.  I don't find it.  If you can provide references it would be helpful.

If you haven't found it already, chap 5 "Skiing in Non Planar Surface. Key point is that the external force (snow's reaction force) accelerate the skier down the hill. Skier's inertia is changed by extending or retracting on this type surface. If com is held to a straight line, then the external force is made constant. The later implies that the com can be moved to make external force non constant.

I have more deadlines to make at work so I'm out. But if you still don' t see it, jamt's key phrase (at least to me is this)....

If you have pressure when the mogul is steep this horizontal speed is decreased because you have a force pushing you backwards.

I would suggest you keep this in mind while reading Lind. However Lind using describes it from using the normal component of the external force.

Quote:
Originally Posted by jack97

If you haven't found it already, chap 5 "Skiing in Non Planar Surface. Key point is that the external force (snow's reaction force) accelerate the skier down the hill. Skier's inertia is changed by extending or retracting on this type surface. If com is held to a straight line, then the external force is made constant. The later implies that the com can be moved to make external force non constant.

I have more deadlines to make at work so I'm out. But if you still don' t see it, jamt's key phrase (at least to me is this)....

If you have pressure when the mogul is steep this horizontal speed is decreased because you have a force pushing you backwards.

I would suggest you keep this in mind while reading Lind. However Lind using describes it from using the normal component of the external force.

It is not the movement of the COM, it is the acceleration. Important difference.

MOvement corresponds to distance, rate of movment corresponds to speed, rate of change of speed is acceleration.

Consider an extreme example. COM moving on a straight line and you run into a brick wall, The CoM did not move up and down but the force will be huge.

Quote:
Originally Posted by MrGolfAnalogy

To be fair, I did try to start this subject as it's own thread when it first came up. I think it died after seven posts and this one made it to seven hundred.

I posted in that thread, along with MGA, and it still only came to 7-8 posts.  Attempts by some posters to get me blocked from this thread by saying that I've single-handedly extended it are all the more obvious and amusing in context, particularly since there yet again have been nearly another 40 posts since my last one.

MGA, you'd made a good general observation that, to paraphrase, pumping as a general matter may carry more priority than absoprtion in skating and, say, bike riding, while absorption carries more priority for skiing.  I think that that's generally a good point (though, both as a drill and for enjoyment, trying to see different places that can be pumped on skis can be a fun thing to do).

I also played around with what seems to be the conventional wisdom on here for dirt jumping -- only pump the landing, absorb the lip of the next jump to keep speed -- this weekend.  On the beginner dirt-jump line at the local park, I couldn't even make it over the first jump if I fully absorbed.  Now, if I pumped that jump -- which, on here, I'm told slows me down -- it set me up for the whole line of jumps quite nicely.  The implications for bump skiing among other things should be obvious.

Why would you think I would try to get you blocked from this thread? I said I would like to see it locked and that I think you are a type of troll. But try to get anyone blocked from a thread? Nope, sounds like you are a bit paranoid

Quote:
Originally Posted by fatoldman

Why would you think I would try to get you blocked from this thread? I said I would like to see it locked and that I think you are a type of troll. But try to get anyone blocked from a thread? Nope, sounds like you are a bit paranoid

Nope, simply aware of the way moderation occasionally gets done around here, and aware of how some posters try to game that process.

Quote:
Originally Posted by CTKook

I posted in that thread, along with MGA, and it still only came to 7-8 posts.  Attempts by some posters to get me blocked from this thread by saying that I've single-handedly extended it are all the more obvious and amusing in context, particularly since there yet again have been nearly another 40 posts since my last one.

MGA, you'd made a good general observation that, to paraphrase, pumping as a general matter may carry more priority than absoprtion in skating and, say, bike riding, while absorption carries more priority for skiing.  I think that that's generally a good point (though, both as a drill and for enjoyment, trying to see different places that can be pumped on skis can be a fun thing to do).

I also played around with what seems to be the conventional wisdom on here for dirt jumping -- only pump the landing, absorb the lip of the next jump to keep speed -- this weekend.  On the beginner dirt-jump line at the local park, I couldn't even make it over the first jump if I fully absorbed.  Now, if I pumped that jump -- which, on here, I'm told slows me down -- it set me up for the whole line of jumps quite nicely.  The implications for bump skiing among other things should be obvious.

Depends on the timing CT, pumping can speed you up or slow you down. Read my post 696.

Quote:
Originally Posted by CTKook

Nope, simply aware of the way moderation occasionally gets done around here, and aware of how some posters try to game that process.

I'm pretty straight up. When you're really annoying, I just push the red flag. I'm sure you've returned the favor! That's what it's there for.

(no red flags from me on this thread at all... kind of entertaining in a train wreck sort of way... there's been some good rescues!

Quote:
Originally Posted by CTKook

Nope, simply aware of the way moderation occasionally gets done around here, and aware of how some posters try to game that process.

Are you aware that this is the statement that a paranoid person would make?

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