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Is steering/guiding without pivoting real? - Page 4

post #91 of 107
Did you know...
The extensor muscle from the back leg of an adult female locust (Schistocerca gregaria) can develop a force of up to 1.4 kg. This means that the muscle from one back leg of a grasshopper (admitedly quite a large one) can lift almost a bag-and-a-half of supermarket sugar!

I've never seen a locust that weighed anywhere near 1.4kg, so how can it create that force - accroding to you it is impossible!

http://www.st-andrews.ac.uk/~wjh/jumping/legwrk.htm
http://www.st-andrews.ac.uk/~wjh/jumping/forcespd.htm
post #92 of 107
...and how could ET ride a bike in zero gravity? Hmmmmmm? :



post #93 of 107
Seriously, the bottom line is that muscles add force to the equation. They are an "outside force" discussed in Newtonian physics. Proof of this is my ability to throw a ball or twist a ski. In midair without any part of my body touching the ground. The total sum of all forces will always be zero, of course, but that doesn't change the truth of this. The force applied to the ball, for example, is out of all proportion to its mass or mine. Think of a baseball player throwing to first for a double-play after tagging second. He's in mid-air, but can still throw the ball at 50+ mph. How can he do that if there must be the ground pushing up at a certain force?
post #94 of 107
Quote:
Originally Posted by ssh
He's in mid-air, but can still throw the ball at 50+ mph. How can he do that if there must be the ground pushing up at a certain force?

...it's all done with counter...
post #95 of 107
Thread Starter 
MichaelA, good summary.

I too agree the title of this thread, posed as a question, has been answered. Like any title, it was only meant to be a starter, not the whole thing. We've gotten a lot of answers here but I think some questions will remain a matter of opinion and individual perception.

The relative efficacy of steering (applying a torque in the plane of a ski while pressuring a ski) vs. tipping (applying a torque around the long axis of the ski) is not clear cut. Your personal impressions and comments on this much earlier were right on target to this core issue.

The circumstances under which the ski shovel can be selectively bent further via steering without producing some level of release at the tail are still unclear. This may also relate to the first question in that I suspect that as the tipping angle increases the amount of steering force that can be generated is reduced.

The degree of confusion between "active" steering and "passive" steering remains something I wonder about. Increased tipping involves a considerable amount of hip rotation. Hip rotation can readily give the perception of an "active" steering force even when it may be producing only active tipping.

It seems to me that these questions would be best answered in an instrumented testing environment. Like any experiment, however, I suspect that what may seem like a simple experiment to design might be much more complex, especially in terms for controlling all the variables.

I am very thankful for people's participation in this thread in that it has giving me things to think about in my own skiing as well as an improved ability to interpret and understand the comments of others. I also think that this topic is a "primary" issue in better understanding differences in approach between some teaching systems. I hope that it has perhaps helped clarify things for others as well.
post #96 of 107
Quote:
Originally Posted by Wear The Fox Hat
So, if you are purely using your body weight (and no additional force) to ride the bike, then how can someone ride a recumbent bike? - You know the ones where their legs are out in front of them. Because as I see it, they are able to generate a force to push the pedals, and it has NOTHING to do with weight... and it also has a reaction force pushing them into the seat back - the forces are horizontal, not vertical, and unless a recumbent bike defies gravity, your theory of only being able to produce a force as a result of your weight kinda falls apart.
Or perhaps a mining engineer (inside engineering joke).

Your body weight pushes down. If you were to put a scale under both tires of the recumbent bike, the total force in the downward direction would add up to the weight of the bike and rider.

There is also a resultant force from the back of the seat. Considered as a whole the net force in the vertical direction is zero, and the net force acting in the horizontal direction results in the acceleration of the bike and rider.

With the book If you draw a freebody diagram around just you, the book pushes you down, the ground pushes you up, gravity pushes you down. Net force is zero UNLESS YOU ARE ACCELERATING.

You can push the floor with more than your weight and the weight of the book. You WILL ACCELERATE in the upward direction if you do this.
post #97 of 107
Quote:
Originally Posted by Ghost
Or perhaps a mining engineer (inside engineering joke).

Your body weight pushes down. If you were to put a scale under both tires of the recumbent bike, the total force in the downward direction would add up to the weight of the bike and rider.

There is also a resultant force from the back of the seat. Considered as a whole the net force in the vertical direction is zero, and the net force acting in the horizontal direction results in the acceleration of the bike and rider.

With the book If you draw a freebody diagram around just you, the book pushes you down, the ground pushes you up, gravity pushes you down. Net force is zero UNLESS YOU ARE ACCELERATING.

You can push the floor with more than your weight and the weight of the book. You WILL ACCELERATE in the upward direction if you do this.
But are you prepared to concede that muscles can actually create a force that has nothing to do with weight?

Do you also accept that skiing is not a constant velocity sport, so there is not a net zero on the forces?

If you are skiing and moving forward down the slope, as well as the acceleration due to your weight, you also have the acceleration (positive and negative) that comes from the forces your muscles generate.

Also, skiing is not done (much) on flat, horizontal surfaces, so to use your analogy of putting scales under the skier, the perpendicular "weight" will be less than the actual "weight" because of the angle.

Can't you see that?
post #98 of 107
Quote:
Originally Posted by Ghost
Well said Michael. Your lack of higher education doesn't seem to have affected your understanding of physics nor your ability to communicate . The "muscular tension" was used in response to WTFHs use of this wording. I think your post sums it up pretty much for a 1st order theory.

Now lets get dynamic. You can apply force to the front without taking pressure off the rear by accelerating your CM to the rear. This is a temporary effect. It is usually followed by a forward acceleration of the CM, which can be accomplished with an added pressure to the rear that doesn't not depressurize the front. When your "in tune", you make beautiful powerful bouncy linked turns linked turns just like tigger. You just have to find the right frequency.

BTW, You must have had Squidward for a physics teacher; I would have given you an A.
See above. In practise you rise a little as a result of the added pressure on the front, and drop before pushing off the tails in proper Tigger skiing style.
post #99 of 107
Si, Thanks for the opportunity to discuss this. Like almost every scientific skiing explanation we can describe a maneuver much easier without all of the physics. Joulbert wrote a great book on the subject but not many skiers have the dedication to muddle through it. For me the key is the bending ski and the deforming snow. Additionally it really does not matter how we describe it mathematically as long as we can perform the maneuver. I wish we had a chance to ski when you were out here working with Wiggs, the move is really easy.
JASP
post #100 of 107
Thread Starter 
JASP,

Hopefully we will be able to meet this coming year. I have the same conference in Snowmass. We'll have to arrange to meet with Wigs and crew at the Dragon for sure. Perhaps we'll all be able to get out one early morning before line up. I think George has Wigs and crew focusing on things other than steering, though! My main goal for this year would be to not fly off of an unseen cat track in flat light, especially since I have a new hip on the left to match the one on the right.
post #101 of 107
Si,
That would be fun! I work with another set of coaches so I am not sure what George and the boys are working on either. It all leads to the same point though because Katy insists that we are all within a specific range.
post #102 of 107
My original follow-up post attempts to this thread were nuked by the EpicEditor. Guess it randomly censors input. Oh well. Maybe it needed re-write anyway…

On the comment by SSH regarding ‘Outside Force’: I think placement of Muscular Force should be in the realm of Internal Force rather than Outside Force.

Any muscular force we create while skiing is Internal to the System we’re trying to analyze. Internal Forces can only affect the system studied (with respect to the outside world) in terms of orientation (like a cat’s ability to ‘right itself’ in midair - a very real example).

An Internal Force can't affect its External surroundings unless that force is somehow translated/transferred into an External Force. In skiing we do this transfer via collisions - like when we extend a Leg - Which extends our ski; which then smacks into the snow (a collision). Even air friction slowing us down is a collision - or rather many collisions with gobs and gobs of air molecules.

------
On the debate between WTFH & Ghost: It’s not so much Apples and Oranges I see them debating as it is Oranges and Orangutans. Both seem ‘right’ and I suspect both know exactly what the other is saying - each was just fishing for acquiescence from the other, right?

On the bike thing: As mentioned above Internal Muscular forces of the System interact with the outside world via collisions. The bicyclist pushing/pulling on a pedal (gotta remember those toe-straps) transfers mechanical energy to the pedal: Which transfers mechanical energy to the chain; Then to the hub sprocket; To the spokes; To the rim; To the tire rubber - where mechanical movement of the rubber collides with the road surface, driving the cyclist ahead.


Quote:
Originally Posted by Si
The circumstances under which the ski shovel can be selectively bent further via steering without producing some level of release at the tail are still unclear.
Google up Collisions and try to find a page dedicated to explaining the nature of elastic vs. inelastic collisions. In many ways this whole discussion is a discussion of collisions. It’s certainly not a topic for the faint of heart but does provide great insights into all things skiing. Wish I could find a synopsis out there to link in, but haven’t the time to search for a good one.


Quote:
Originally Posted by Si
This may also relate to the first question in that I suspect that as the tipping angle increases the amount of steering force that can be generated is reduced.
As the tipping angle increases the amount of Steering Force that can be generated is *increased* (if by Steering Force you mean the amount of centripetal force generated at the ski tip). At some degree of extreme edge-angle I’m sure it starts to decline though I’m not sure where. Not enough free time to mess with spreadsheets on it.

By the way, “Steering” by definition means: controlling the course and nothing more. Sorta like “Redirection” in the other thread; doesn’t really convey anything meaningful about precise mechanical movements. Tipping the ski creates a ‘steering force’ all by itself.


Quote:
Originally Posted by Si
The degree of confusion between "active" steering and "passive" steering remains something I wonder about. Increased tipping involves a considerable amount of hip rotation. Hip rotation can readily give the perception of an "active" steering force even when it may be producing only active tipping.
Not sure what you mean by “Passive” steering. To me, any kind of‘Steering would be Active. Park & Ride is actually a good example of *Not Steering*. The Existing Bend in the ski continues to turn the skier on a fixed course so the skier need not add any input. No new pressure, no new tipping, no new rotation = no steering (in my view).

Some degree of Increased Tipping can be accomplished by exclusive banking with no hip rotation. We can also angulate at the hips & waist without any hip rotation (in terms of femur rotation - not sure if people count adduction/abduction as a form of hip-joint “rotation” - which it is, just in a different plane.)

Quite agree with you that many people undoubtedly use knee-angulation rather than Independent-Leg-Steering (another term I’m not fond of) and may well misinterpret what they’re really doing. That said I think the participants of *this thread* know exactly the difference in the mechanics applied - their textual input demonstrates it.

---
One more noteworthy thing to consider is rut formation.

Only the tip (front) of the ski floats up as the ski moves forward. As a ski moves forward the tip climbs up on top of the snow surface. ('up' meaning relative to the surface)

As the ski continues to move forward, the snow is increasingly compressed (in all conditions - even ice). This surface-compression is maxed-out when the section of the ski underfoot passes over that point because that’s where the highest pounds-per-square-inch is (see a weight-distribution diagram of a ski). As the ski progresses still further forward the tail (having less pressure per square inch) cannot compress the snow any further - it just slides along against the bottom of the pre-existing compression.

Sometimes people envision a scenario as though the ski were on a Foam Pad but the foam pad analogy is inappropriate as applied to skiing.

If on a foam pad, when we move our weight forward on the ski, the tail rises up. It does so because the foam is Elastic. With the pressure reduced on the tail the foam is able to push the ski tail upward. The snow surface is Inelastic and does not rise up again.

Levering forward to create more pressure on the Tip simply moves the point of highest Pounds-per-square-inch (under the ski) further forward - it doesn’t Rock the ski Forward as it does on the foam pad nor will it pivot a ski if it’s on edge.

Of course, in Bottomless Powder that same ski might just pivot forward 'cause we're skiing on more of a Fluid than a Surface at that point.

.ma
post #103 of 107
Thread Starter 
Quote:
Originally Posted by michaelA
... (if by Steering Force you mean the amount of centripetal force generated at the ski tip). At some degree of extreme edge-angle I’m sure it starts to decline though I’m not sure where. Not enough free time to mess with spreadsheets on it.

By the way, “Steering” by definition means: controlling the course and nothing more. Sorta like “Redirection” in the other thread; doesn’t really convey anything meaningful about precise mechanical movements. Tipping the ski creates a ‘steering force’ all by itself.

Not sure what you mean by “Passive” steering. To me, any kind of‘Steering would be Active. Park & Ride is actually a good example of *Not Steering*. The Existing Bend in the ski continues to turn the skier on a fixed course so the skier need not add any input. No new pressure, no new tipping, no new rotation = no steering (in my view).

Some degree of Increased Tipping can be accomplished by exclusive banking with no hip rotation. We can also angulate at the hips & waist without any hip rotation (in terms of femur rotation - not sure if people count adduction/abduction as a form of hip-joint “rotation” - which it is, just in a different plane.)

Quite agree with you that many people undoubtedly use knee-angulation rather than Independent-Leg-Steering (another term I’m not fond of) and may well misinterpret what they’re really doing. That said I think the participants of *this thread* know exactly the difference in the mechanics applied - their textual input demonstrates it.
Thanks michaelA, let me clarify a bit. I have referred to steering as the application of a torque in the plane of the ski. I understand that some people use a much broader definition but when that happens I think it becomes pretty meaningless. I have tried to be pretty careful about repeatedly defining what I was talking about. Thus, my comment about steering can be translated into:

"I suspect that as the tipping angle increases the amount of torque that can be applied in the plane of the ski is reduced."

I say this because as angulation is increased I think it becomes more difficult to independently rotate the hip.

In terms of "passive" and "active" hip rotation I was trying to differentiate between hip rotation that occurs when purely tipping vs. that which produces a torque in the plane of the ski. I was suggesting that some may interpret hip rotation as an application of torque in the plane of the ski even when it may not be the case.
post #104 of 107
Quote:
Originally Posted by Si
"I suspect that as the tipping angle increases the amount of torque that can be applied in the plane of the ski is reduced."
Quote:
Originally Posted by Si

I say this because as angulation is increased I think it becomes more difficult to independently rotate the hip.
Gotcha.
I think your second sentence is more accurate as we can apply any amount of torque we wish regardless how much the ski is tipped. As the ski is tipped more, that torque just results in more pressure under the ski-tip. If tipped less, eventually the torque will be sufficient to overcome constraints that hold the ski in its rut and will be translated into actual rotation of the ski.

On the passive/active thing, I get the idea. Guess it depends on exactly which muscles we recruit when making the move.

.ma
post #105 of 107
Quote:
Originally Posted by michaelA
(in terms of femur rotation - not sure if people count adduction/abduction as a form of hip-joint “rotation” - which it is, just in a different plane.)
This is a confusing statement Michael. Joint movement is categorized by actions in the three planes of referenence. The fact that the hip can move in all three planes simultaneously (circumduction) does not mean that we can say that the movement in one is called the same thing as movement in another. there are differences in the muscles rectuited, the way specific muscles are recruited, and this affects the outcome. Hip/leg ad/abduction is a movement that happens parallel to the frontal plane, hip/leg rotation is a movement that happens parallel to the horizontal transverse plane. IN skiing of course we need to have a blending of our movements (skills) so we use the hip in multiple planes at the same time (circumduction), like skiing into counter (femur rotation) while we move lateraly inside the turn (ad/abduction) ect. But the movements are different, are categorized differently for a reason, and done exclusively will result in different outcomes while skiing. Sorry to nit pick, but,,,, Later, RicB.
post #106 of 107
Thanks RicB. Wasn't sure how others typically think of it when they rotate that joint inward or outward. Got to thinking that most regular posters automatically differentiate movement in the two planes, but what of the many lurkers out there?

Poked around Epic but didn't see an official glossary of terms anywhere. I know we often quote the 'BB Encyclopeida' but maybe it would be worthwhile to have a local reference we can just link to in the future?

You know, kinda like when Webpage authors attach a glossary-link to a term rather than having to explain it time and time again? If we accumulate a bunch of commonly misused/misunderstood/fuzzy terms in one (easily linkable) location perhaps we can avoid some of the ... less fortunate ... paths these threads sometimes take?

Dunno. Whadaya'll think? We don't need a zillion terms, just the fuzzy ones we often struggle with. Like when PhysicsMan put out the clarification on Centripetal vs. Centrifugal thing.

.ma
post #107 of 107
Quote:
Originally Posted by michaelA
Thanks RicB. Wasn't sure how others typically think of it when they rotate that joint inward or outward. Got to thinking that most regular posters automatically differentiate movement in the two planes, but what of the many lurkers out there?

Poked around Epic but didn't see an official glossary of terms anywhere. I know we often quote the 'BB Encyclopeida' but maybe it would be worthwhile to have a local reference we can just link to in the future?

You know, kinda like when Webpage authors attach a glossary-link to a term rather than having to explain it time and time again? If we accumulate a bunch of commonly misused/misunderstood/fuzzy terms in one (easily linkable) location perhaps we can avoid some of the ... less fortunate ... paths these threads sometimes take?

Dunno. Whadaya'll think? We don't need a zillion terms, just the fuzzy ones we often struggle with. Like when PhysicsMan put out the clarification on Centripetal vs. Centrifugal thing.

.ma
It is basic physiology. The psia core concepts manual gives us the three planes but doesn't go any further to differentiate how our movements relate to the three planes. I think this is a failing of psia in IMHO. My personal opinion is that the ski profession should not invent new terms to replace ones that are universaly used in the physiology and kinesoelogy world. But, this isn't always the case, which is why many don't understand the terms. If psia would just add another page to the core concepts manual it would be a non issue. there is really nothing fuzzy about the terms unless someone hasn't done the required reading, they are universal. Don't mean that to sound judgemental, the info readily available if anyone cares to look. Later, RicB.
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