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# A simple self-assessment test - Page 21

Quote:
Originally Posted by MrGolfAnalogy

I think what may be missing is the concept that the speed is increases at the point of the transition, not on the way up. Obviously you will always loose speed going up, but that speed has already been gained passing through the transition. The orientation of the transition doesn't matter either, it could be up, down or horizontal. You go higher up the wall because you enter it faster from the transition.

To put it another way, by pumping, you leave a transition with more speed than you entered it, regardless of it's orientation or the grade around it.

Sure, we can say that, but it is only in the Y direction that you can increase the speed. I think that was discussed before when I explained the difference between speed and velocity.

However, this effect is much smaller than the raising of the CoM. This is the same speed increase you get when you jump the terrain feature I showed before. A small increase due to the new Y component while the X component stays the same. In the example of pushing at the beginning of a curved ramp however, the x component will decrease quite rapidly.

You could say that the major reason you get higher is that the COM is higher when you enter the ramp, not because you are going faster.

Quote:
Originally Posted by Jamt

Leaning forward would lower the COM, but it would not change it horizontally except for the small pumping effect.

I agrees, a pumping effect is there but just small.

Quote:
Originally Posted by Jamt

Just a small point about conservation of angular momentum that may be causing some of the confusion in this thread.

Conservation of angular momentum means that unless there is an external torque the angular momentum around a fixed point is constant.

The bolded part is important. Say that you are skiing a certain track and that you push yourself towards the center of the turn. Before your CoM revolves around this imaginary point "center of the turn". However, when you push you increase the centripetal force, and thus the center of the turn moves closer to you because its location is given by the centripetal force (your skis may be going the same way, but your CoM is not). Now at this point in time, infinitesimally after the start of the push, the angular momentum around the original center has not changed, but around the new center of rotation the angular momentum different.

Slightly after this point in time the force against you does not pass through the original fixed point anymore and thus there is a torque. The conservation does not hold any more.

In addition, I agree that decomposing the velocity in vector components makes for less confusing terminology, so..... if the velocity component is large due to rotational energy and how much larger does it have to be to overcome the contribution of  the torque and other factors?

Lind derives the change in rotational energy to be 2 times the change in R divided by R, where R is distance between the mass and the center. By having a terrain where the change in R is large, would this overcome the torques as the mass moves to another area where rotational energy can be leverage?

Quote:
Originally Posted by jack97

I agrees, a pumping effect is there but just small.

In addition, I agree that decomposing the velocity in vector components makes for less confusing terminology, so..... if the velocity component is large due to rotational energy and how much larger does it have to be to overcome the contribution of  the torque and other factors?

Lind derives the change in rotational energy to be 2 times the change in R divided by R, where R is distance between the mass and the center. By having a terrain where the change in R is large, would this overcome the torques as the mass moves to another area where rotational energy can be leverage?

I don't have the Lind book so I don't know what his model and assumptions are. Perhaps BTS knows.

Quote:
Originally Posted by Jamt

Sure, we can say that, but it is only in the Y direction that you can increase the speed. I think that was discussed before when I explained the difference between speed and velocity.

However, this effect is much smaller than the raising of the CoM. This is the same speed increase you get when you jump the terrain feature I showed before. A small increase due to the new Y component while the X component stays the same. In the example of pushing at the beginning of a curved ramp however, the x component will decrease quite rapidly.

You could say that the major reason you get higher is that the COM is higher when you enter the ramp, not because you are going faster.

Actually, when I think a bit more about it, you have to be jumping to make it faster to the top. if the speed has a Y component as you enter the ramp, it means you are jumping. If you just pump and stay connected to the ground you will be going slower.

Quote:
Originally Posted by fatoldman

....

He may be a true believer in the crap he throws around but that doesn't mean he isn't a troll. Almost single handedly he has turned a three page thread that was reaching its conclusion into a twenty page monster...

What's striking is that, since my last post, there have been over 40 posts, including several that you can lump under the heading of simply personal attack posts -- so I clearly haven't been extending this thread singlehandedly, at all.  It's noteworthy that the first few pages of the thread actually have Zentune and others attacking on a personal level someone other than me -- and that someone happens to be one of the most knowledgeable, capable people on here.  The attack culture overall is pretty clear.

As for "crap," well, that may be SOME people's definition of my doing things like posting, accurately, about some of the basics of dirt jumping or pumping on a bike, or absorption on skis.  This does remind me quite a bit of when lots of people got completely frosted that I'd accurately said that airbag packs were effective as safety gear, and didn't carry a 10 pound weight penalty.  In the real world, the only thing weird about my stating that, or stating the basics of dirt jumping in this thread, is that other, counterfactual posts made me feel the need to state it.  On here, yes, there's a further problem that some posters are committed to counterfactual positions, but it's not my problem.

Quote:
Originally Posted by zentune

What about the anti-pumping (which apparently is the opposite of pumping) video(s)--of skiers--that I and others have asked for? We were assured anti-pump is useful in a steep, narrow chute. That's how this all started Jack...

zenny

^^^^^

P.s. Josh and I have since made amends CT....

zenny

Quote:
Originally Posted by CTKook

What's striking is that, since my last post, there have been over 40 posts, including several that you can lump under the heading of simply personal attack posts -- so I clearly haven't been extending this thread singlehandedly, at all.  It's noteworthy that the first few pages of the thread actually have Zentune and others attacking on a personal level someone other than me -- and that someone happens to be one of the most knowledgeable, capable people on here.  The attack culture overall is pretty clear.

As for "crap," well, that may be SOME people's definition of my doing things like posting, accurately, about some of the basics of dirt jumping or pumping on a bike, or absorption on skis.  This does remind me quite a bit of when lots of people got completely frosted that I'd accurately said that airbag packs were effective as safety gear, and didn't carry a 10 pound weight penalty.  In the real world, the only thing weird about my stating that, or stating the basics of dirt jumping in this thread, is that other, counterfactual posts made me feel the need to state it.  On here, yes, there's a further problem that some posters are committed to counterfactual positions, but it's not my problem.

I think there is only one person left with a counterfactual position now.

Quote:
Originally Posted by zentune

What about the anti-pumping (which apparently is the opposite of pumping) video(s)--of skiers--that I and others have asked for? We were assured anti-pump is useful in a steep, narrow chute. That's how this all started Jack...

zenny

Using the exact same motions and forces we see in Ghost's backwards skating video (pushing heel out rolling alternating side to side) would most likely produce one footed snowplows alternating right and left if done on skis at a high rate of speed.  That certainly would help shave some speed while straightlining a steep, narrow chute. If was done with enough downward force so as not to skid out snowplows it would provide some resistance and momentum reduction.. but that doesn't seam feasible for the average skier, results would be negligible..  Trying it on really short skis might help, but on anything steep the rider would likely go over the handlebars if pressed too hard.

Hence my position that it is possible to use these anti pumping forces while turning on skis, but not very practical compared to other available methods to control speed.

Edited by crgildart - 9/12/13 at 8:10am
Quote:
Originally Posted by mdf

...

Think of a bike trick video,  where the rider balances on his rear wheel and then pops up onto the top of a fence post.  He gets forward motion somehow.  And he doesnt seem to have to work hard to get the height.  The whole thing looks like it violates the laws of physics, but I'm pretty sure it isn't supernatural.

There are a couple different ways to do what you're describing, and yes you can even do it chainless (i.e., without any force from the pedals via the chain).

Part of what you're alluding to is in a way cultural.  It may surprise lots of people reading this to hear that there's a growing sport of chainless bmx racing, for instance.  That has obvious implications for this discussion (and no, I didn't say the sport was going to become more than a niche, when I said it was growing).   Even for lift-served mtb, a good drill is to do runs chainless.

On-snow, taking runs where you absorb and suck up speed wherever you can, and runs where you pump everything you can, can both be good learning experiences.

Another tick for self-assessment:  inventory your favorite piece of woods, gully, whatever you like in terms of varied terrain.  What's your current average turn shape through it?  Can you ski it using pumping as a significant part of a run?  Can you ski it using absorption as a significant part of a run?  Are you using all available terrain features -- here, I'm not saying do you ski it like a professional freeskier, but would you say you make intelligent and aesthetic use of the terrain?

But, to get there, it first helps to know what pumping the face of a bump, or a swale of snow wedged into deadfall, etc., actually does.

Edited by CTKook - 9/12/13 at 8:06am

CT,

There was no personal attack. I simply pointed out that you are employing the tactics of a troll and that because of that I would classify you as a troll. Kind of like "It quacks, it waddles when it walks, it flies, it swims, you know I think its a duck." if you give a damn about what I think (I know you don't) then tell me something. Why the hell do you bother to post on this forum?

The crap comment was not directed at you it was directed at the faulty information you are trying to spread. Just because you believe what you say makes no difference. I grew up on a small farm/ranch and got into the habit of calling crap crap.

Hopefully, this exchange may prompt Cirquerider to lock this.

fom

Quote:
Originally Posted by fatoldman

CT,

There was no personal attack. I simply pointed out that you are employing the tactics of a troll and that because of that I would classify you as a troll. Kind of like "It quacks, it waddles when it walks, it flies, it swims, you know I think its a duck." if you give a damn about what I think (I know you don't) then tell me something. Why the hell do you bother to post on this forum?

The crap comment was not directed at you it was directed at the faulty information you are trying to spread. Just because you believe what you say makes no difference. I grew up on a small farm/ranch and got into the habit of calling crap crap.

Hopefully, this exchange may prompt Cirquerider to lock this. [emphasis added]

fom

Yes, part of the game on for some on here is in fact the part I just bolded.

The fact that many posters do have problems with real-world info is clear on here, be it the basic mechanics of dirt-jumping, or airbag packs.  In the thing known as the real world, that doesn't change that info, or turn it into crap.  You still take dirtjump lines a certain way, and airbag packs are still effective at mitigating the effects of avy entrainment, no matter how much smoke gets blown on here and how much moderator intervention may happen to and in threads.  The desire to shout over that good info is a striking and curious thing.

CT,

Who's shouting? You call it 'good information' , I call it 'crap', simple difference of opinion. Yes, 'crap' is a negative term but haven't you used negative terms to refer to the information presented by those you disagree with.

fom

Quote:
Originally Posted by fatoldman

CT,

Who's shouting? You call it 'good information' , I call it 'crap', simple difference of opinion. Yes, 'crap' is a negative term but haven't you used negative terms to refer to the information presented by those you disagree with.

fom

In the real world, saying that one approaches a dirt jump line a certain way, or that airbag packs are effective, ain't crap.  Real simple.

CT,

Never said anything about how to approach a dirt jump line nor disputed that there is a best way to do that depending on the outcome you want. Pretty much everyone on here understands pumping, absorption, etc. and can perform those activities fairly well. It's your theories about how these things work 'in the real world' that are being disputed. You just don't understand the 'reality' of 'real world' physics and obviously have no desire to.

As has been stated before, an understanding of what is happening from a physics standpoint is not necessary to performance. In fact, a faulty view can even lead to one attaining superior performance. For example, 'sucking up speed' as you have presented the idea is just nonsense when viewed from the physics perspective of how the 'real world' works but it may very well aid someone in learning/refining the use of flexion and extension in whatever activity they are involved in.

Have no idea what avi-lungs have to do with all this.

fom

Quote:
Originally Posted by fatoldman

Have no idea what avi-lungs have to do with all this.

They have nothing to do with all this, it's just CTKook being a troll. I thought you understood that.
Quote:
Originally Posted by fatoldman
...

Have no idea what avi-lungs have to do with all this.

fom

I'm sure you don't.  AvaLungs AREN"T  airbag packs, and haven't been referred to by me in this thread.  They are two very different things.

Again, it's ok to not know the difference, but you and several other posters on here don't even seem to know what you don't know, in a variety of contexts.

When discussing things like pumping, this can have the effect of things like the inversion of mechanics that some have been putting forwards, with some examples like the inversion of dirtjumping being real stark.

Quote:
Originally Posted by CTKook

I'm sure you don't.  AvaLungs AREN"T  airbag packs, and haven't been referred to by me in this thread.  They are two very different things.

CT,

Ok I don't know the difference and because it has no effect on me I don't care, but a red herring is a red herring and using red herrings is a tactic of trolls.

Did you know that if you could actually demonstrate what you say in the last paragraph of that post you would overturn the entire science of physics and become famous throughout the world and be remembered for the rest of human history. And imagine the ego boost when you prove you are right and the entire community of scientists are wrong. Looking forward to that proof. So far all I have seen is jumping up and down arms being waved and the same things being said over and over.

fom

Quote:
Originally Posted by CTKook

...

As for "crap," well, that may be SOME people's definition of my doing things like posting, accurately, about some of the basics of dirt jumping or pumping on a bike, or absorption on skis.  This does remind me quite a bit of when lots of people got completely frosted that I'd accurately said that airbag packs were effective as safety gear, and didn't carry a 10 pound weight penalty.  In the real world, the only thing weird about my stating that, or stating the basics of dirt jumping in this thread, is that other, counterfactual posts made me feel the need to state it.  On here, yes, there's a further problem that some posters are committed to counterfactual positions, but it's not my problem.

This is why it's not a red herring.  Posters on here can be oddly drawn to counterfactual positions, and then oddly nasty when someone makes simple real-world statements.  So, my mentioning how to take a dirt-jump line becomes your "crap," just as some people on here just recently were saying that my mentioning airbag packs and their effectiveness was not just wrong, but out of line.

Meantime, this statement from me a few posts up,

"Even for lift-served mtb, a good drill is to do runs chainless.

On-snow, taking runs where you absorb and suck up speed wherever you can, and runs where you pump everything you can, can both be good learning experiences.

Another tick for self-assessment:  inventory your favorite piece of woods, gully, whatever you like in terms of varied terrain.  What's your current average turn shape through it?  Can you ski it using pumping as a significant part of a run?  Can you ski it using absorption as a significant part of a run?  Are you using all available terrain features -- here, I'm not saying do you ski it like a professional freeskier, but would you say you make intelligent and aesthetic use of the terrain?"

has gotten zero responses, but there have been several attack posts.  Using terms like crap is one way to try to obscure or derail meaningful discussion, and FOM has explicitly said he hopes to get the thread locked.  It is what it is.

Quote:
Originally Posted by CTKook

This is why it's not a red herring.  Posters on here can be oddly drawn to counterfactual positions, and then oddly nasty when someone makes simple real-world statements.  So, my mentioning how to take a dirt-jump line becomes your "crap," just as some people on here just recently were saying that my mentioning airbag packs and their effectiveness was not just wrong, but out of line.

Meantime, this statement from me a few posts up,

"Even for lift-served mtb, a good drill is to do runs chainless.

On-snow, taking runs where you absorb and suck up speed wherever you can, and runs where you pump everything you can, can both be good learning experiences.

Another tick for self-assessment:  inventory your favorite piece of woods, gully, whatever you like in terms of varied terrain.  What's your current average turn shape through it?  Can you ski it using pumping as a significant part of a run?  Can you ski it using absorption as a significant part of a run?  Are you using all available terrain features -- here, I'm not saying do you ski it like a professional freeskier, but would you say you make intelligent and aesthetic use of the terrain?"

has gotten zero responses, but there have been several attack posts.  Using terms like crap is one way to try to obscure or derail meaningful discussion, and FOM has explicitly said he hopes to get the thread locked.  It is what it is.

CT,

Let's see, bringing in bringing up airbag packs (see I learned something) isn't a red herring because it shows how everyone on this site is out to get you. You really must thrive on the abuse that you know you will receive to keep posting here.

You complain about not getting a response to a statement that everyone here probably agrees with, so...Great post CT if everyone followed your suggestions they would become better riders/skiers. There, feel better now.

What attack posts?

Crap is a perfectly good way to label crap and any meaningful discussion involving you in this thread ended long ago. Now don't take the preceding as an attack its just my observation that nothing meaningful will come from further discussion with you on the subject at hand nor has anything meaningful been produced for from that source for many, many pages.

fom

Quote:
Originally Posted by crgildart

cr,

I did say back there somewhere that about all anyone could get out of this thread at this point was a certain level of entertainment.

fom

Quote:
Originally Posted by Jamt

Actually, when I think a bit more about it, you have to be jumping to make it faster to the top. if the speed has a Y component as you enter the ramp, it means you are jumping. If you just pump and stay connected to the ground you will be going slower.

Yes!  Pump on the flat bottom of the trough to help fight against gravity.  Once you have finished the pump, gravity is slowing you back down again.  If the ramp is high enough you will come to a stop  no matter how hard you pump or how fast the pump makes you go.  Gravity will win eventually.  You might have enough momentum to clear the top with or without a pump, you might need a pump to help.

But the pump on the flat bottom of the trough can only be Y component, you can't have a forward X component because of the lack of friction.  It might help you jump if the ramp is short enough, but it won't speed up your forward direction of travel.

Now if you can pump against a surface that is tilted with an orientation such that the Y component of that slope is angled towards the direction of travel, then you can increase forward momentum.

The frame montages I sent from the bike video are pretty clear.  The guy extends down the backside and creates pressures.  On the face he absorbs and reduces pressures.

If we relate this all back to skiing (I know what a concept), the claim was made that absorbing bumps slows you down.  Yet this biker is speeding up by absorbing the bumps.

All of this discussion about about how pressures create accelerations (and decelerations), which is ultimately how speed can be manipulated and direction change.  When talking about skiing we need to understand how we are manipulating pressures to change or direction or speed.  Absorbing bumps does not slow you down.  Pressuring against the face of them does.  There is no possible way that anyone has yet described for how you can anti-pump to slow down a ski turn either.

Quote:
Originally Posted by MrGolfAnalogy

I think your challenge may be possible if all the variables like approach speed, transition angle, ramp angle, ramp length were set up perfectly. Maybe a gentle grade up a short ramp, with good speed? If the height differencial burns up less energy than your pump generates, then you will have a net gain in speed at the top.

It would be fun to see if some one had the time, money, physics and resources to set this challenge up and prove it. Sounds like a job for Myth Busters.

Even if you jump the entire transition -- that is, don't touch the ramp at all -- you won't speed up in the horizontal direction.  At least not by 'pumping' like in a halfpipe -- maybe you could do something funky with a bunny hop plus a back-to-front weight shift on a bike, etc., but that wasn't what was being claimed.

Quote:
Originally Posted by MrGolfAnalogy

I think what may be missing is the concept that the speed is increases at the point of the transition, not on the way up. Obviously you will always loose speed going up, but that speed has already been gained passing through the transition. The orientation of the transition doesn't matter either, it could be up, down or horizontal. You go higher up the wall because you enter it faster from the transition.

To put it another way, by pumping, you leave a transition with more speed than you entered it, regardless of it's orientation or the grade around it.

This is not an accurate description of what is happening.  If you could "leave a transition with more speed than you entered it, regardless of it's orientation or the grade around it" then you could accelerate on level ground or while going uphill by "pumping" without turning.  But that obviously doesn't work.

You can gain speed in the vertical direction if you press hard enough, but not horizontally.  Watch one of those videos of a skater pumping in a halfpipe -- you can see that on the 'downhill' pump they accelerate as they extend, but on the 'uphill' one they don't gain that burst of speed horizontally.

Quote:
Originally Posted by CTKook
When discussing things like pumping, this can have the effect of things like the inversion of mechanics that some have been putting forwards, with some examples like the inversion of dirtjumping being real stark.

Feel free to take up my ramp acceleration challenge if you think I'm wrong.  Clearly everyone but you (including the bike riders in various videos; see post #593 for detailed breakdown) has it backwards.

It should be trivial to show that more pressure on an uphill transition speeds you up coming out of it and less slows you down, right?  A quick \$50 and you get to embarass all the physics geeks!

It is abundantly clear just which of the abundant posters above has actual experience and true knowledge about pumping a half pipe and which ones (90% of the above) have zero actual experience doing it yet claim to understand what happens where in the process..There are just some things that you can't fully learn and understand from a book.

Quote:
Originally Posted by crgildart

It is abundantly clear just which of the abundant posters above has actual experience and true knowledge about pumping a half pipe and which ones (90% of the above) have zero actual experience doing it yet claim to understand what happens where in the process..There are just some things that you can't fully learn and understand from a book.

It is really really easy to pump a half-pipe so I don't know what you are going on about.

About pumping, isn't it strange that not a single skier in the olympic ski cross final seems to know that they shouldn't suck up the feet before the terrain features in CTs world, it is supposed to slow them down. CT would clearly win the olympics in his real world.

What the best racers do is suck up the bumps by jumping BEFORE them.  That makes them light as they pass over them and causes them to catch less air and less momentum disruption.  It is pretty much the same principle as Kook discusses.  And, any experienced downhiller will tell you it is a crucial skill to success in the event.

so you weren't trolling?

Quote:
Originally Posted by crgildart

What the best racers do is suck up the bumps by jumping BEFORE them.  That makes them light as they pass over them and causes them to catch less air and less momentum disruption.  It is pretty much the same principle as Kook discusses.  And, any experienced downhiller will tell you it is a crucial skill to success in the event.

WHAT? How could you miss pages of discussion where CT and others claim that you gain speed by pumping into the simple terrain feature I drew, while I and others have explained that the only way not to lose speed is to jump before the feature, like you describe above. I'm glad for you that you understand this. I guess that makes you one of your 90%. Go to the skate-park :-)

Quote:
Originally Posted by crgildart

What the best racers do is suck up the bumps by jumping BEFORE them.  That makes them light as they pass over them and causes them to catch less air and less momentum disruption.  It is pretty much the same principle as Kook discusses.  And, any experienced downhiller will tell you it is a crucial skill to success in the event.

Yeah, in skier x or boarder x, it's not too different from racing bmx in that you want both the straightest line to the finish, other things being equal, and only the right amount of air.  Extending too hard and getting too much pop off the lip of a kicker can both cause too rounded a line through the air, and cause someone to overshoot the landing.  It's similar to, on a dirtbike, grabbing too much throttle and accelerating the bike too much -- substitute "pump control" for "throttle control," and you have the same general idea.  (On EpicSki, it's possible that if you grab too much throttle on a dirtbike, the bike slows and loses energy on the kicker, but in the real world, it accelerates, and even with that the flatter line is generally faster.)  For racing, smooth is often better.

This is again a case where some posters seem to not have any idea what they don't know.

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