or Connect
New Posts  All Forums:Forum Nav:

# 4 Questions - Page 11

The implication of a rudder is that forces hitting the ski from the "negative side" would push the tail of the ski out, like the way that water passing under a boat pushes on the negative side of the rudder and pushes the tail of the boat out.

I do not think a ski works like that at all.  The reason the tail can fan out on a ski is because the pressure on the positive side of the ski is less at the tail then it is at the tip.  but make no mistake the pressure is on that side, not on the negative side.
Yup, in fact the rudder has a positive steering angle in the other direction and moves the stern in the other direction.

The tail of a carving ski at any instant in an edge-locked turn points to the right compared to the direction the skier is traveling when the skier is turning left, but it is also pushing the skier to the right. Pressure is on the base, unless you have caught an edge.

The rudder's a red herring, except that the tail of the ski is actually moving left when the boot is moving down and the tip is moving right.
Hey man, I really liked what you said about slingshot and stuff, but this boat thing goes a bit too far astray. Maybe if you ski in powder where edge-contact is not relevant.
Hmmm, actually ... I'm not sure if it's a bad analogy or not.  I guess it depends on what perspective you apply and what you're trying to get out of it.

What if there were a 'rudder' in the front of a ship as well as the back?  What if there were three rudders, front, back and middle?  Heck, what if there were 8 rudders all along the keel as below?  Looks a little bit like the image Ghost posted to me.

With our ship moving forward (based on prop power, or momentum alone) then we'd have pressure on the opposite side of those rearward-rudders.  But, what if the skip is sailing in a strong side wind coming from the left side of the image?  What if it's a really powerful wind?

Which side of those rear-rudders would our 'pressure' be on in that case?  (As I say, it's just a matter of perspective and context to see what we're looking for.)

.ma
ma,

In a boat, the water passes under a boat as it glides across the water.  By turning the rudder to the side the water that is passing underneath pushes on the INSIDE of that radius which is formed by the boat and rudder.  That pushes the tail of the boat out.

We don't experience pressure from the snow on that side of the tail when we ski.  Pressure from the snow pushes inwards from OUTSIDE the radius.

The ski is bent because of the fact that centrifugal force is pushing the ski into an outward bend.  Centripital force is pushing back from the snow, from the outside of the radius.

We're talking about forces thus far, not the direction of movement.

The ski is also sliding across the snow in some direction.  If its in an arc'd turn then the snow is passing under the ski directly along the length of the ski and there is no rudder effect.

if the ski is skidding a bit, you can be sure the tail will be skidding out away from the radius, not inward.  So the snow would be passing under the ski from the outside to the in.

In all cases that I have just described, there is no case where the snow pushing on the ski or passing under the ski does so from the inside of the radius towards the outside.
Well, golly BTS... since you insist on sinking my poor defenseless little ship - I've decided to take up Skateboarding on Ice.

Below is my brand new skateboard.  It's completely different than my ship, utterly, completely, wholly different (as you can see by the very different drawing ).

My new Skateboard has two individual skates on it, but both were bent at an angle when I bought it.    Both Skates are bent as shown (Black items).   When I try to slide straight forward, my danged new Skateboard always seems to turn (!).

1) Which side of each individual Skate is the 'pressure' on when sliding forward?
2) Does this matter in the least to the outcome, as it carves a given radius in the ice just as a ski carves in the snow?

Does it really matter which side the 'pressure' is on?  Isn't the directional angle of engagement far more important?

.ma
nope, a skateboard doesn't work like a ski either.  I've already explained this about 3 or 4 times, not going to try again.  Sorry if you still don't get it.

Yes it makes a big difference which side its on when we're discussing about steering angle and how to steer your skis.

have fun.
Well as I see it, the model clearly demonstrates that it really doesn't matter which side the pressure is on (for either skate) because the skateboard will slide along the exact same arc either way.

If we slide this skateboard forward with one hand while pushing firmly to the left with the other hand, the skateboard will arc to the left.  If we slide it forward the same way while pushing firmly to the right, it will still traverse that same exact arc to the left.

This matters to the carving because the same principles apply.  On hard ice our carving skier can twist the skis quite a lot of force without the skis breaking out of the rut and changing the arc.  That twisting-effort changes the direction of forces acting on our skis while not affecting the current arc.

For our Skateboard this is because Steering Angle of front and back skate define the arc.  For a ski it's the many Local Steering Angles all along the ski that define the arc and (directionally speaking) really doesn't matter against which side pressure occurs.

Pressure toward the base-side bends the ski into the given shape for our arc, but it's the arc of the ski itself that defines the radius it travels.  If our ski were to suddenly 'lock' in its current bend it wouldn't matter which side pressure came from - the traveling arc would remain the same.

This is to say that while lateral pressure does bend our ski into a particular shape, it's that resulting shape that actually determines our path of travel, not the lateral pressure itself.

.ma
In a turn, the skateboard is rotating.  Actually if the skateboard (or boat for that matter), is rotating quickly enough the tail end of it will be moving somewhat to the left in a right hand turn, just like the ski.  If you rudder too hard for the turn the board (or boat) is making the water pushes on the inside, but if you rudder not hard enough the pressure is on the outside.  Think of your car in a four-wheel drift, the pressure is pushing on the outside of the tires towards the centre of the turn, even if you have some rear wheel steering.

However I think the rudder is a poor example,  The ski's tails are never curved too far for the turn; internal stresses will only cause it to curve in the direction of the applied forces, and only so far as resistance permits.  If you suddenly start moving in such a fashion that the skis tail is pointing too far across your direction of motion, the ski will first straighten, and then catch an edge.
Agreed, it's not the best analogy but my post (and yours for that matter) tend to show that Steering Angle is it's own isolated thing, a inherent geometrical relationship with our object's current direction of travel.  It's the pattern that matters to me, not the specific context.

Examining a sailboat tacking into the wind is another extension of context where the pattern still applies despite non-intuitive outcomes.  How can a sailboat sail into the wind?  What is its Steering Angle vs. Forces applied to it?

.ma

michaelA, Im a naval architect and I dont really think your rudder analogy is that great.... sorry. But if you think about it, a modern sailing boat with a sencetive aft rudder and a deep fin keel will turn much the same way you pictured in your diagram. And that is called steering as well .

Yes, michaelA, the side on which the pressure is developed DOES matter.  It matters a lot, even on the skateboard.  Why? Because if you don't have the balance vector pointing inside the base of support, you will fall.

You have an interesting way of "explaining" something....you tend to discard important details to speak of a specific action, then you make a generalization from that fabricated reality.  But since the important details are absent the generalization is incorrect.

Nobody here does that better.
I'm sorry you feel that way BigE.

As I see it, the critical element in going from knowledge to comprehension of this (or any) topic is our ability to detect the underlying Patterns and being able to recognize those same basic Patterns in other not-so-similar contexts.

If we only understand something in a single context (or particular set of exactly similar contexts) then our understanding resides in that general context alone, but there's no comprehension of the underlying principles.   Personally, I'm interested in the underlying principles for all these things and not just a single simplistic application.

Context is important when trying to specify the exact inputs and exact outputs for a proposed application, but the inherent fundamental Patterns behind any proposed application may also exist in a variety of contexts.   That's the whole point of bringing up analogies: To highlight the inherent Pattern rather than the current context constraining comprehension.

.ma
ma, in a very abstract sense you may have a point, but you will only convince me if you can bring this all full circle and talk specifically about how any of your ideas can help us improve our skiing.
Quote:
Originally Posted by michaelA

I'm sorry you feel that way BigE.

As I see it, the critical element in going from knowledge to comprehension of this (or any) topic is our ability to detect the underlying Patterns and being able to recognize those same basic Patterns in other not-so-similar contexts.

If we only understand something in a single context (or particular set of exactly similar contexts) then our understanding resides in that general context alone, but there's no comprehension of the underlying principles.

Actually, recognition of patterns in various contexts does not require any comprehension of the underlying principles. The notion of the tail of a ski being rudder like is proof.

Personally, I'm interested in the underlying principles for all these things and not just a single simplistic application.

It is completely beyond me what "all these things" could possibly reference.

Context is important when trying to specify the exact inputs and exact outputs for a proposed application, but the inherent fundamental Patterns behind any proposed application may also exist in a variety of contexts.   That's the whole point of bringing up analogies: To highlight the inherent Pattern rather than the current context constraining comprehension.

Suppose a particular pattern exists precisely because of certain inputs.  To remove the inputs that create this pattern and then link that pattern to similar patterns from contexts that do not contain these defining inputs is flawed.  Such a practice will necessarily lead to incorrect conclusions when reflecting results from the second context back the first.

.ma

What your line of reasoning actually does is that it elevates the fallacy of suppressed evidence to a learning tool.  I know that the rudder and the ski turn with completely different physics, but I suppress the physical evidence to make a point.   I'd be unable to make that point if I did not suppress the evidence.

If I unknowingly drew the analogy between the rudder and the ski, then I am guilty of argument by spurious similarity -- mere resemblence (the pattern) is no proof of relationship.

The notion that detecting Patterns in dissimilar contexts reveals comprehension is nothing more than a lame attempt at justifying the construction of fallacious arguments to support your conclusions.  The idea that we can learn something by ignoring the inputs that have created the "Pattern" and seek out other contexts that have similar patterns is a formal statement that arguments by suppression of evidence and spurious similarity show comprehension.  They do nothing of the sort.
BigE,

I've long since come to expect arguments from you against most things I post.  Responding to your posts is always problematic as you seem work so hard to create the appearance of "Wrongness" in others over things you're unwilling to consider.   My posts above seem perfectly reasonable to me, my analogies seem reasonable and I'm perfectly comfortable with my own comprehension of the topic as presented.

I'm not trying to convince you of anything, merely stating my own perspectives and reasoning.  If it's not good enough for you, so be it.   If you now feel it important to have a try at discrediting me or my reasoning because it's not what you believe, that's OK too, as it long ago lost any impact since you do this so very often.

Years ago, when I first explained on these forums what a 'Fallacy' was and pointed everyone to pages listing a variety of them, I realized at least some people would only read up on them in order to use them as weapons to artificially discredit others through "Fallacy Charges" based on deliberate mis-applications of the definitions I pointed everyone to.   I've not been disappointed in that expectation.

As to Patterns, I've made a lifelong study of Patterns.  Communications patterns, writing patterns, thinking and logic patterns, evolutionary patterns, business and economic patterns, and so on.  I always look for the underlying patterns within context-constrained ideas.  Context is still important, but I'm primarily interested in the patterns that make that context (and possibly others) work.  It's not about "suppressing" elements of a context but rather seeing beyond rudimentary relationships to more fundamental relationships.

Had you not been so quick to be critical and judgmental, perhaps you'd have taken the time to investigate that last bit I posted on increasing comprehension via context independence.  Had you made an attempt to search on "Knowledge" or "Thinking Patterns" you'd probably have found diagrams similar to the following...

In my posts above I simply chose not to be limited in my thinking by the specific data and simplistic relationships being offered as "final".   Instead, I was seeking to explore the patterns and principles involved.   I'm sorry if that's not how you prefer to think about things.

.ma
The rudder and the sail boat relate to the ski steering angle in that they all develop a normal component of force on the rudder, sailboat, tracks on a skateboard or ski from the steering angle.  At a very fundamental level they are all the same; a normal force is developed on a plane surface due to motion or attempted motion through a medium.  Bravo Michael!

They differ significantly at the one-level-higher complexity, that takes account of the direction of that force.  The rudder in a typical application, develops a force on its front surface pushing the stern out, the wheels on the rear of a skateboard, like the tail of an edge-locked-carving ski develop a force on the rear surface.  Bravo BigE!

To get the sail boat to go into the wind you have to take the direction of the keel into account, again adding another degree or two of complexity.

How does all this help a skier?  Understanding the difference between an-edge-locked-carving ski where the force is spread along the bottom surface of a ski according to ski stiffness and thrust direction and always pushing in, like the surface of a runner's foot an a perfectly banked turn, helps I think to get the most out of your ski.  Whether that understanding needs to be cognizant or intuitive is another question, but we certainly don't want to have in our head that the local force at the tail is in the wrong direction, which is what blind application of the steering angle theory of a skidding ski would provide us.
Hey MA,

It is certainly appreciated that you try to bring up new points. Even though I think I may understand where you are coming from, this boat thing is just very far away from skiing. Water versus snow, the steering only coming from the rudder and being moveable, The boat never really changing its shape.
Whereas the ski does initiate a turn more or less throughout its sidecut when put on edge and you influence the outcome by bending it, and it does not work well if the weight is all the way on the tail.

Thanks.
Once a fallacious argument is constructed it can only be supported by more fallacious arguments.

Quote:
Originally Posted by michaelA

BigE,

I've long since come to expect arguments from you against most things I post.  Responding to your posts is always problematic as you seem work so hard to create the appearance of "Wrongness" in others over things you're unwilling to consider.   My posts above seem perfectly reasonable to me, my analogies seem reasonable and I'm perfectly comfortable with my own comprehension of the topic as presented.

I'm not alone in thinking that the analogy is bad.

I'm not trying to convince you of anything, merely stating my own perspectives and reasoning.  If it's not good enough for you, so be it.   If you now feel it important to have a try at discrediting me or my reasoning because it's not what you believe, that's OK too, as it long ago lost any impact since you do this so very often.

This is a vesion of the ad-hominem argument, called poisoning the well.  It's intent is to diminish the "impact" of my statements.

Years ago, when I first explained on these forums what a 'Fallacy' was and pointed everyone to pages listing a variety of them, I realized at least some people would only read up on them in order to use them as weapons to artificially discredit others through "Fallacy Charges" based on deliberate mis-applications of the definitions I pointed everyone to.   I've not been disappointed in that expectation.

This is incorrect.  I first became familiar with the tools of fallacious argument 30 years ago.  Your taking credit for revealing them to me is unwarranted and patronizing.  It leads to the fallacious argument by call to authority, which follows:

As to Patterns, I've made a lifelong study of Patterns.  Communications patterns, writing patterns, thinking and logic patterns, evolutionary patterns, business and economic patterns, and so on.  I always look for the underlying patterns within context-constrained ideas.  Context is still important, but I'm primarily interested in the patterns that make that context (and possibly others) work.  It's not about "suppressing" elements of a context but rather seeing beyond rudimentary relationships to more fundamental relationships.

The above  passage is the worst form of call to authority -- using one's self as the authority.  As a corollary, it allows ma to offer this "insight" on what I'd find had I taken time to investigate....  Please note the bolded part in the quote above.  It becomes very important....

Had you not been so quick to be critical and judgmental, perhaps you'd have taken the time to investigate that last bit I posted on increasing comprehension via context independence.  Had you made an attempt to search on "Knowledge" or "Thinking Patterns" you'd probably have found diagrams similar to the following...

In my posts above I simply chose not to be limited in my thinking by the specific data and simplistic relationships being offered as "final".   Instead, I was seeking to explore the patterns and principles involved.   I'm sorry if that's not how you prefer to think about things.

The "simplistic relationships" are the underlying physics of the systems that you chose to compare.  Physics is something that you must respect when discussing the mechanics of motion.  You cannot compare the path of a boat and path of a ski without regard to the physics that controls the activity.

.ma

The bolded section above is exactly what the drawing ma posted is talking about,  but the application by ma was faulty.

Looking past rudimentary relationships to more fundamental relationships requires that we "compare and contrast" the attributes of each relationship and abstract them from their individual contexts. However, that is what not what happened with the analogy of rudder and ski.

In the case of turning ski and rudder, it is clear that the underlying physics makes these two
events poor candidates for analogy.

The quest for higher understanding does not mean where ever we see turns we have licence to pronounce that they are fundamentally equivalent.  It means we first have to look at the patterns that define the relationships present in each context.  Only when we are satisfied that the patterns are similar, then we can begin moving to more "fundamental" relationships.

One cannot view only the geometric attributes of bodies in motion.  "Pattern" is much more than a visual quantity.  It also refers to the mechanics and physics of the bodies in motion.  Such "rudimentary" information cannot be discarded.  You will make poor generalizations.

\
Well BigE, I don't know what more to say.

It seems no matter what I say you'll find a way to interpret some form of "Wrongness" into it.

I've watched you do this many times to others and see that you're quite practiced at it by now.  Still, I just don't have an interest in continuing that game with you so you'll have to play on your own going forward.

I'll not respond to you further in this thread.

.ma
SimplyFast,

I'm glad for your effort at looking for the relationship I was trying to bring up.

While the ship itself doesn't deform - it's the combination of rudder and keel taken together that creates a 'new form', very  similar to the idea of a bending ski creating a 'new form'. The ski simply creates a more progressive new form along its entire length.   By showing a rudder at both bow and stern (or many rudders along the keel) I'd hoped to make the "deformation" of the *overall* ship object more apparent.

Just as a ski turns based on its overall interaction with the snow (solid form of water), a ship turns based on its own overall interaction with the water, not just the rudder acting alone.  If the rudder were acting alone, the ship would simply go right when the rudder swung to the left.  With the sailboat analogy I'd hoped to shine a brighter light on the keel's influence over that of the rudder in determining overall directional influence.

From there, I hoped to take the discussion into the middle ground of skiing powder - where we're actually moving through a semi-fluid medium (neither water nor solid) and our ski begins acting even more in the pattern of the ship analogy.

Of course, no analogy is perfect but if we look for similarity in patterns between original idea and analogy then analogies are useful as they highlight patterns hidden in the obfuscating detail.   If we focus only on the difference in details between original idea and the analogy then we can easily gut any analogy for not being exactly the same thing and therefore reject any similarity in core patterns might otherwise be shared between examples.

To be fair, I didn't spend a whole lot of time searching for the best possible analogy nor did I spend hours crafting a written description to ensure step by step understanding on where I was going.   Instead, I relied on good-faith efforts in readers/participants to do just what you did - trying to see the highlighted similarities suggested, rather than trying to find ways to highlight differences to show errors and wrongness.    (and Thank You for that...)

.ma
No, the ski never acts like that in power either.

Sorry ma, but you're just simply very wrong here.  I don't know why you insist on defending your point, but this thread has taken a very dark turn for the worst, with nonsense, and now bickering on whether you should be allowed to continue to spew nonsense.  Perhaps you can start a new thread entitled something like "skiing as a rudder" and go off on that subject matter over there; but this thread, which started out as a thread about steering is now so totally lost down the rabbit hole as to be useless.
Quote:
Originally Posted by borntoski683

this thread, which started out as a thread about steering is now so totally lost down the rabbit hole as to be useless.
this is page 11 after all
I've been thinking about this negative steering angle dilema a little bit.   We're talking about pure arc'd turns here where the steering angle under foot is just slightly above zero.

I believe the answer is that there is never any negative steering angle; but that in a pure arc'd turn the entire length of the ski has the SAME steering angle, something just a little bit bigger than zero.

The area around the feet is what most steers the skier, the other parts of the ski mostly pivot the ski through their local steering angle...but...in an arc'd turn....it doesn't pivot.  Rather every point along the ski is sliding forward and being redirected with approximately the same steering angle...assuming exactly center balanced, tail following tip.

In a skidded turn, there is VERY MUCH positive steering angle along the entire length of the ski, obviously no negative, and likely a pivoting effect in action, or the potential for it.  Fore-aft balance, edging, the actual steering angle, would all play a factor in determining the steering vs slowing and the ski's self-pivoting effect.

If you had said skid angle (direction of local motion versus direction of local edge pointing), I would have agreed with you, the difference between 0 and almost 0 not giving me much concern.
If a positive steering angle is deflecting you into the turn, then a negative steering is deflecting you in the opposite direction.

What part of the ski is doing that?
There has to be more than zero for the ski to turn.  I don't know why you are so obsessed on zero Ghost.  Anyway, that point is...there is no such thing as negative steering angle.  It does not exist anywhere on the ski.  Ghost if you want to imagine there is zero steering angle over the length of an arcing ski, knock yourself out.  Its certainly not negative at any point along the ski.
What I think Ghost is suggesting is that in the limit as the chord length from the point directly underfoot to another on the arc where the ski is under maximum flexion approaches tzero, the steering angle also approaches zero.

It's a mathematical notion that does not have a physical reality here -- the steering angle approaches zero, but since the chord length will never actually be zero, the steering angle cannot be zero either.

What could be a source of confusion is that the tangent to this point directly underfoot represents the zero steering angle -- it's the same direction as the CM would take if released from the arc.  It is clear that the ski is not straight here, so the steering angle will never be zero.
I refer you to figure 3.6 on page 23 of LeMaster's book.  He has defined the local steering angle at the tip in that figure.  Note how the local steering angle is the angle between direction of the skier's motion and the edge direction.

Please draw another line on that figure at halfway between the tip and the boot; the angle is lower.  Don't stop now!  Draw one at the middle of the ski.  Not zero yet?  Ok keep going back.  It goes through zero and gets negative.

For a skidding ski this isn't likely to be problem, it's unlikely you will get the ski bent into enough of a curve to run into this situation.  However  for a carving ski, it happens.