2 footed release - turning forces?

[h.harb]

Up dates for the two footed crowd

http://www.harbskisystems.com/hblog/hblogindex.html

[skinoob]

If you stand on the center of the skis, (or slightly forward) as both skis come flat, the tips will go downhill.

Wondering if the position of binding makes much difference in TFR drill. In other words, if the binding is mounted backward, will it become harder
for the tip to drop into the fall line ? Is it more effective to adjust it to be more forward for the drill ?

[h.harb]

Not unless someone made a big mounting error.

[Max_501]

Not unless someone made a big mounting error.

Don't use the manufacturer's suggesting mounting point as an excuse for improper technique.

[milesb]

Here is a TFR I did for this ridiculous challenge thread over on Epic. Maybe it helped someone

http://www.youtube.com/watch?v=cvv3mRwy ... 089&page=9

[polecat]

Well, no snow in sight around here. So I’m digging through old threads to see what trouble I can get myself into.

I stumbled into this old thread and figured I’d have a go at hacker’s question.

…
Can someone explain how "gravity and balance pull the skis into the falline".
A simple force diagram would help. What are the rotational forces turning the skier/skis?
I don't believe gravity by itself will 'turn' the skis as gravity will be pulling on all of the skis and the skier.
I am thinking some opposing forces are in the mix?

We’ll look at a single ski, but the physics is basically the same for two. So you can look at these diagrams as either a one-footed release or half of a two-footed release.

Let’s start from the start, before we release, standing on the snow, skis sideways to the slope, edges dug in.



In the above diagram the skier’s weight is directly over the center of the ski (as indicated by the round checkerboard center of gravity symbol).

The force of gravity pulls the skier down, effectively at their CG.

Since the edges are set, the snow is pushing back with a force equal and opposite to the force of gravity. Of course the snow is pushing all along the length of ski, but it behaves the same as a single force applied at the average point (in this case the center). This force is labeled as Ffs (force of static friction).

The forces cancel each other out so there is no motion.


Next, we release the edge, flattening the ski on the snow.



Now that the edge is released the ski begins to move because the friction force exerted by the snow on the ski’s base (Ffd, the force of dynamic friction) is smaller than the force exerted by the skier’s weight.

Note that even though the ski is slipping sideways down the hill it is not turning. There is no rotational force in this scenario because the opposing forces are aligned.

This is the situation in the exercise “Release to Sideslip” (ACBES1, section 4-1 or PMTS Instructor Manual, section GB-1).

With some practice you can sideslip all the way down the hill without turning at all.


Next, instead of releasing with our weight directly over the center of the ski we move our weight forward, so that our CG is in front of the center of the ski.



Here we have the same forces at work (sliding friction on the base and our weight), with the same magnitude as in the last case but the gravitational force is in a different location.

The two forces, now separated by a distance, create a torque.

This, hacker, is the turning force that you were looking for. It causes the ski to rotate, in this case clockwise, as it slides down the hill.




The force of gravity pulls the ski down the hill. The force of friction is always opposite the direction of motion. As the angle of the ski approaches parallel alignment with the forces, the distance between the centerlines of the forces (moment arm) gets smaller and smaller. Once the distance becomes zero the torque goes to zero as well. The ski now heads straight down the fall line.

If there were some overshoot or other angular disturbance, the angle would generate a restoring torque and the ski would tend to realign with the fall line.

The ski will continue straight down until we use edging to change direction, run out of hill or find a tree.


The important thing about the about the above example is that even though it’s a highly simplified case, is still shows a fundamental characteristic of good skiing, that you’re making your speed and direction changes purely by balance and tipping. You do not apply any twisting or turning forces to the ski!

All turning forces occur at the ski/snow interface. Your job is to tip it and balance on it.



But I have to disagree just a little bit with Harald on one statement:

To those who are confused about the TFR, this is not rocket science.....

The physics here is really similar to that of at fin-stabilized ballistic projectile, i.e. a rocket (or an arrow or a dart or…).

In the case of the rocket, we balance the CG against the forces of aerodynamic pressure, rather than snow friction. And the rocket acts in three dimensions rather than two. But the directional stability principles are the same.


pc.

[hacker]

Thanks Polecat, that makes sense.

So the essential thing to do is move your CG forward of the center of the ski while the skis are flat on the snow.

I didn't quite get that originally, I will try it next time on the slopes.

[GregM]

So the essential thing to do is move your CG forward of the center of the ski while the skis are flat on the snow.

There is an important semantics which have been mentioned many times. Read for example the intro chapter of Essentials where this is described in detail. If you mention "move your CG forward" to an average person they would just lean forward and this is the opposite of what's needed. Leaning forward might work in release from the static position (bad habit anyway) but does not work well in skiing. Instead, you position your CG forward by pooling your feet back under you.

[arothafel]

Good call, Greg

Leaning forward might work in release from the static position (bad habit anyway) but does not work well in skiing. Instead, you position your CG forward by pooling your feet back under you.

I am a good example of misinterpreting the meaning of "getting forward" as I am constantly bent over at the waist. I used to think this was getting forward. Pulling both feet back underneath makes a huge difference. I'm still working on it.

[MonsterMan]

In the case of the rocket, we balance the CG against the forces of aerodynamic pressure, rather than snow friction. And the rocket acts in three dimensions rather than two.

So what you really mean is,

To those who are confused about the TFR, this is not rocket science.....

[jclayton]

Hi Art ,
been doing your Pilates Ab excercises lately ? Contract lower abdomen , nuetral lumbar spine , just bend from thorax up , tension in the pelvic floor and feel tall up through the cervical spine . Feel the separation between lower and upper abs , not a concertina effect . ( have yet to try this myself , out with the carvers !! )
cheers ,
Jeremy

[arothafel]

Hey Jeremy...

Good to see you on the forum! I'll try your exercise. I've been working on almost nothing but core to protect my lower back. It's been good for 3 years in row!

I'm out on the Carvers as soon as it cools down. It's over 98 degrees today. Been in the hundreds last week.

I'm still working on my arms/hands as well as my bending at the waist stuff. Perhaps that's where your exercise will come into play.

I'll be posting video soon!

Are you coming to ABasin this year? Bev and I are both enrolled. It would be great to ski with you again.

Art