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# 4 Questions - Page 13

That is fascinating Skidude,
Could you please give me an example of how two identical steering angle, both equal to 10 degrees for instance, generated in different ways of course would differ in their aportionmant to turning force and braking force?
Quote:
Originally Posted by Ghost

That is fascinating Skidude,
Could you please give me an example of how two identical steering angle, both equal to 10 degrees for instance, generated in different ways of course would differ in their aportionmant to turning force and braking force?

Yes.

If you generated the steering angle by primarily using rotary, you will get more breaking less turning... relative to.... if you just tipped the skis, and obtained the full 10 degrees of steering angle by ski bend, and no rotary at all.
Quote:
Originally Posted by Ghost

And they go through flexible ankles, boots, knees and hips too!

Still, if you consider all the forces acting on the skier and his skis, you get the actual acceleration of the skier and his skis.

I'll take 165 cm Fischer WC SL skis over blades on any slalom course you care to name.

I will agree that for most people I see on the slopes the turning force at the tips and tails is minimal compared to the turning force underfoot. The force at the tip depends on how well you are carving and on how much bend you get in the ski.  Skidding at low edge angles on a soft straight ski = not much effect.
Bending a good SL ski into a tight arc   = quite a bit.

The tip generates almost no force normal to the path of the CM when carving.  What normal force it generates is used in bending the ski and keeping it bent.  It is cutting a groove for the rest of the ski to follow.  It will create torque, but no normal force that will turn the skier.

The origin of this normal turning force is directly underfoot.

FIS limits the men's SL ski length to 165 or more, as shorter is much turnier -- the length of the ski does not get in the way of making a short arc.   The added length does not make it turn more.
BigE,
What you say is true about the tip guiding the rest of the ski and determining where the centre of the ski will end up and in what direction the normal force underfoot will be pointing when it gets there.  I still would not discount the effect of those forces at the tip and tail though on the skier's direction.

Here's a thought experiment for you BIGE.  Turn your ski upside down and place a short 4x4 post under it at boot centre and place a bathroom scale under the post. Now bend the tip and tail by pressing down on the ski's tip and tail. If you have a reasonably stiff ski it will take quite a bit to bend it. Notice that although this force is applied at the tip and tail, the scale in the middle registers the force you put on it!  All the forces put on the ski, any part of the ski, get transmitted through the ski to the boot and to the skier.

How much of the turning force comes from underfoot and how much from farther away depends on how stiff the ski is and how much it is bent.  Force additional to that required to bend the ski comes from underfoot.

Do you really think snowblades would have a better time in a SL course than SL skis, or do you think there might be an optimal length for slalom courses somewhere between 150 and 170 depending on weight of skier?
The example shows the ski holds the skier upright.

What the tip and tail will generate is torque, not turning forces.  The ski/boot act as a wrench, and apply torque to the skier, not turning forces.
The torque at the tip and the torque at the tail are fairly well balanced.
I do think that 100 lb reading on the bathroom scale is a force.

Edit: if the thought experiment isn't clear and you don't have the scale, just bend the ski over your knee.
You knee will surely feel the force you apply to the tips and tails.  You cannot apply a normal force at tip and tail to bend the ski without having that force transferred to the midsection.  It is the internal bending moment in the ski that transmits the force to the centre of the ski.  If you could apply a load at the ends of a beam without supporting that load, you wouldn't need columns to support beams, and all our buildings would be floating in the air.
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.............leg
Quote:
Originally Posted by Ghost

Edit: if the thought experiment isn't clear and you don't have the scale, just bend the ski over your knee.
You knee will surely feel the force you apply to the tips and tails.  You cannot apply a normal force at tip and tail to bend the ski without having that force transferred to the midsection.  It is the internal bending moment in the ski that transmits the force to the centre of the ski.  If you could apply a load at the ends of a beam without supporting that load, you wouldn't need columns to support beams, and all our buildings would be floating in the air.
..............|  |
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.............leg

Actually Ghost,...not quite.  Unlike you analogy, the ski touches the ground its whole lenght, not just at tip and tail as you suggest.  Hence your analogy does not apply.

Legs

Above would be more accurate...the only point is that the load distribution is not even.
Great work Skidude.  I guess for the majority of skiers, that is the case. Of course if you weigh very little and tip to a very big angle on a stiff ski on ice it might look like this (except that the line should be curved),
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Regardless of the relative loads at tip and tail, all the loads get to the foot and it the sum of all the loads that turn the skier.
Quote:
Originally Posted by Ghost

Great work Skidude.  I guess for the majority of skiers, that is the case. Of course if you weigh very little and tip to a very big angle on a stiff ski on ice it might look like this (except that the line should be curved),
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.............................||

Well, I know you are kidding...but of course that could only work if you had those old dynastar's with the weights in the ski tips, and those weights weighed more then you bindings, and you combined....

Quote:
Originally Posted by Ghost

Regardless of the relative loads at tip and tail, all the loads get to the foot and it the sum of all the loads that turn the skier.

Yes, I think that is true.

What are your thoughts on post #362?
Well, the load shown on the diagrams is coming from the snow in reaction to my loading the skis. Total snow load must equal total load put on the ski by me at the boot.  Even though the load is applied by me at the boots, it gets to the tips where the snow pushes back on the skis because the skis resist bending.  You don't need to tape a weight on the tips to weight the tips.

Quote:
Originally Posted by Skidude72

Yes, I think that is true.

What are your thoughts on post #362?

Quote:
Originally Posted by Skidude72

Yes.

If you generated the steering angle by primarily using rotary, you will get more breaking less turning... relative to.... if you just tipped the skis, and obtained the full 10 degrees of steering angle by ski bend, and no rotary at all.

I think the situation with as you put it "primarily using rotary" is much worse than that.  If you generated the steering angle by primarily using rotary you would not have an edge-locked arcing ski.  You would in fact have a 10 degree steering angle underfoot, a little higher angle at the tip and a little lower angle at the tail.  In the edge-locked carving you would have a 10 degree angle at the tip and a -10 degree angle at the tail with a zero degree angle in between near the boots.

Hey if anybody can't accept zero degree steering angle underfoot, don't blame me; I did not define steering angle; it's direction of CM travel versus direction edge is pointing.  Steering angles don't turn skiers; forces turn skiers.
Edited by Ghost - 11/22/09 at 11:13am
Quote:
Originally Posted by Ghost

Well, the load shown on the diagrams is coming from the snow in reaction to my loading the skis. Total snow load must equal total load put on the ski by me at the boot.  Even though the load is applied by me at the boots, it gets to the tips where the snow pushes back on the skis because the skis resist bending.  You don't need to tape a weight on the tips to weight the tips.

I think the situation with as you put it "primarily using rotary" is much worse than that.  If you generated the steering angle by primarily using rotary you would not have an edge-locked arcing ski.  You would in fact have a 10 degree steering angle underfoot, a little higher angle at the tip and a little lower angle at the tail.  In the edge-locked carving you would have a 10 degree angle at the tip and a -10 degree angle at the tail with a zero degree angle in between near the boots.

Hey if you (generic you, not skidude you) can't accept zero degree steering angle underfoot, don't blame me; I did not define steering angle; it's direction of CM travel versus direction edge is pointing.  Steering angles don't turn skiers; forces turn skiers.

doublepost
Don't know why the sofware keeps quoting my edits.
Quote:
Originally Posted by BigE

The example shows the ski holds the skier upright.

What the tip and tail will generate is torque, not turning forces.  The ski/boot act as a wrench, and apply torque to the skier, not turning forces.

As most things, its not always that all-or-nothing.

Taking Ghost's bathroom scale example.  If you  push on BOTH the tip and tail at the same time, then the full weight of your pushing ends up pushing on the scale.   If you only push one end of the ski, the ski will just teeter totter off the scale with barely a reading.  That's the torque effect you speak of BigE.  And that is what would happen too if there was any negative steering angle in the ski universe, which there is not.

When we ski, we load up the same side of the ski, like pushing down on both ends of it in the scale example.  So the pressure tip and tail, DEFINITELY can influence the way the skier is moved.  However the local steering angles of those two ends are what will determine how equally or unequally some torque happens.

Using Ghost's example again, think about if you are pushing down on both ends of the ski, onto a scale in the middle, but not quite equally with both arms.  Then one end will go down, the other not go down as fast (ie, torque), and some of the reading on the scale will be reduced as well, not entirely, but somewhat.  With enough difference in pressure between the two arms, one side might even come up a bit, but not all of the pressure is gone or else that side would just fly up in the air like a catapult.

Edited by borntoski683 - 11/22/09 at 1:20pm
Quote:
Originally Posted by Ghost
Steering angles don't turn skiers; forces turn skiers.

The magical force you dream of that turns a skier only occurs precisely because of steering angle.

Quote:
Originally Posted by Skidude72

If you generated the steering angle by primarily using rotary, you will get more breaking less turning... relative to.... if you just tipped the skis, and obtained the full 10 degrees of steering angle by ski bend, and no rotary at all.

.

Those are two methods to establish steering angle.  I'll add a third which is fore-aft balance.  By moving your weight to a different part of the ski, you obtain a different steering angle, due to the sidecut.

However, I'll add edge control to that topic about how, after a turn is initiated, that the same 10 degrees of steering angle could have different steering effects on the skier with the same steering angle.

Yes without a doubt you could have 10 degrees of steering angle (with skidding) and totally alter the steering effect of that 10 degree steering angle, simply by adjusting your edging.

You could also play with fore-aft, but changing fore-aft is actually changing the steering angle.

The methods we have discussed about how to establish steering angle are really just different approaches to get to the same thing.  If you twist your feet, you'll have steering angle, but it doesn't do much for you until you have sufficient edge angle.  If you crank on the edges at that point, you may go too far and either stop yourself or chatter.  That's why its difficult to enter an arcing turn this way.  If you smoothly enter into an arcing turn by tipping your ski into a bend, then essentially you are creating steering angle and edge angle at the same time.....so the more efficient use of steering angle is obtained without a need to guestimate how to change the edging to avoid braking.

Arcing is the ultimate form of steering angle because  you obtain the right amount of edge angle so that there is no skidding.  All the momentum of the skier is redirected according to the steering angle.  With such purity, much less steering angle is required, but its there just the same.

Further to that, most of us spend most of our time NOT in arc purity, which means that practically speaking in terms of skiing, the name of the game is to identify steering angle and use it.  Not pretend that it does not exist.

BTS,

once the ski is bent, it stays bent.  How then does the tip and tail contribute?  Only as a support providing edge hold and creating the groove.  Yes, there is a force there, but it's not the turning force.
The pressure has to go somewhere
Sure, and a few pounds of contribution from a flexible beam -- which can break if you stomp on it while supported between two chairs -- is not enough for it to turn the skier.

Yes I do understand the notion of vectors and summing all their infinitesimal contributions to turning the skier.   But that's not what was being argued.  The claim was that forces from the tip and tails will turn the skier.  They won't.  What little contributions they might have to actually making the skier tern are grossly overwhelmed by what is going on underfoot.  As far as turning the skier goes, their contributions are meaningless.
One of my WC SC skis can support all of my weight by the tip and tail and still only bend as far as I have seen it bent on the slopes.  Maybe to the grossly overweight, 165 lbs is a few lbs, but to me, that 1 g is not a meaningless nor an insignificant turning force.

Once the ski is bent it continues to stay bent only so long as tip and tail continue to be pressed into the snow and the snow continues to push back.

That magic force exists because we are pushing our feet sat 90 degrees sideways to our direction of motion against the decambered ski (very efficiently due to the 0 degree local steering angle).  If we did this with a straight unbent ski we would have to extend and move away from it and fall on our faces, but because the ski is curved into an arc, the ski moves forward and turns with us.  We and the ski move away from the platform the ski is riding on, but the ski moves forward onto the new platform that is being cut.  It is the local steering angle at the tip that causes the ski tip to turn and the rest of the ski follows it and keeps that underfoot portion right beside us at zero local steering angle.  By the time the midski gets to the point where the 10 degree (say) local steering angle at the tip was acting to cut the new groove, our direction of motion has changed by 10 degrees.
I don't think its totally "meaningless" BigE.  It would only become meaningless if the ski had limitless bend, which it does not.  Sooner or later it stops bending and stops pivoting like a pinwheel, and that is when the pressure from the tip and the tail are going to transmit into the skier body just as much as any other place on the ski.  They do not dissipate into thin air.
Ghost, it doesn't work that way.  You're confused.  Read LeMaster.
Quote:
Originally Posted by borntoski683

Ghost, it doesn't work that way.  You're confused.  Read LeMaster.
Just when I though you were coming around.

The definition of steering angle is dead simple; it is the angular difference between the direction the edge is pointing at any point on the edge and the current direction of the skier's momentum.

I am not confused.  I do not need to read LaMaster to understand that definition, nor read it to understand how the angle determines the split between turning forces and retarding forces.  I am only using the term steering angle, as defined by Lamaster. I am not trying to prove anything by appealing to his authority nor to use any of his other theories.
well be that as it may, you are confused about it.  Good luck.
Really?  What part of the definition of steering angle do you think I am confused about?
After you purchase and read Ron LeMaster's latest book "Ultimate Skiing", then I may consider engaging with you more, but until then all you are going to get from me is more and more of "ghost you don't know what you're talking about".  LeMaster is not the final authority, but he's certainly more of an authority then you and what he has to say does not line up with your incorrect thinking.  Worse I fear your incorrect thinking is being repeated here to the point that it may falsely lead readers/skiers down a false path of understanding.

Centripetal force does not provide quite enough juice to turn the object along a curved path.  What it does is push back with EQUAL force to the thing that is pushing out.  If Centrifugal force is pushing out so much, on a certain direction, then a perfectly edged ski will push back just enough to prevent the ski from skidding sideways.

Something else is required to pull the ski towards the direction of the turn.  Something has to deflect it.  That is steering angle my friend.  Trying to pretend it does not exist or going off on tangents about balls on the ends of strings is not helping anyone, including yourself.

Understanding and mastering your steering angle control is a huge part of the equation to high end skiing.
Edited by borntoski683 - 11/22/09 at 4:13pm
Sorry to disabuse you of your illusions but the centripetal force is exactly the amount of force required to maintain an object in a path of a given radius.  Don't take my word for it.  Isac Newton, who is a recognized authority in physics, even more so than LaMaster, showed this and it has been accepted by physicists all over the world for quite some time now.
Well I have you translating the words and concepts of Newton to apply towards skiing in a way that does not make sense to me, and I have Ron Lemaster translating the words of Newton towards skiing in a way that makes complete sense.  Hmm...who should I listen to?
You should think for yourself and base you opinion on logic.

If you can accept that a push exactly at 90 degrees to the direction of motion will cause the skiers path to change, then the choice is obvious.

Since you don't trust my physics, I suggest that you re-read LaMaster.  Particularly on page 40 where he says "the balls path is defined by the reaction force the bowl exerts on it, pushing it toward the center of the bowl." That is sound physics.  The bowl isn't pushing the ball at an infinitesmal angle behind the centre of the bowl.  It is pushing it right at the centre of the bowl.   I also suggest you rethink, "At every instant the ball is rolling into a surface that has a slight angle to it's path."  It is not there yet, but it is rolling into it.

Think for yourself.  Look at the ball in the bowl.  In that instant, the part of the bowl exactly right beside the ball, touching the ball is pushing the ball 90 degrees to the balls direction of travel, toward the centre.  The spot an infinitely small distance ahead has an infinitely small angle to the balls current path of travel, but the ball isn't there yet.  "In a pure carved turn the ski cuts a groove in the snow, which it then slides along, much like the ball rolls along the side of the bowl."

I think you are getting hung up on Lamaster's next statement, "At every moment the groove has an infinitesimal steering angle under the skier's foot, and so the turn is extremely efficient."

Think for youself.  All you have to do is look at the curve. The ski under the skiers foot is a finite length of ski, and averaged over that length there may very well be an infinitely small steering angle.  At one point under the foot (looks like to me, but the turning effect is still there whether zero or almost zero under foot), the local steering angle is zero it is an infinitesimally larger than zero slightly ahead of that point and infiniteslmally smaller behind that point.  Look at the curve.  You don't need non-zero steering angle in order to supply a turning force; you need non-90 degree steering angle in order to supply a turning force.

The local steering angle at the tip turns the ski tip.  The ski tip cuts a groove, the rest of the ski follows that groove.  All the pressure forces acting on the base of the ski act in whatever direction is normal to that surface at the point where they act.

Some people think it is necessary to pivot the ski to provide an initial steering angle under the boot before the ski can start to turn. Others know that simply tipping and decambered shape will provide the steering angle required at the tip to guide the tip in its groove cutting.  The ski follows along the cut groove through the arc.
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