You can easily tighten a turn but can you do the opposite?

ghost, I understand that but even then he's suggesting no control over the inward acting forces. Which simply isn't true. He's stuck because he believes he can't do anything about being stuck. That was the line of thinking about not being able to run a sub four minute mile. Once it was done everyone began thinking it wasn't so impossible and now a four munute mile is considered slow.
To review what was said earlier about the movements we use to create more inward acting forces:

•  tipping the skis to a higher edge angle,
• pressuring the skis so the bend into a tighter arc,
• or steering the skis into the turn.

By reducing any of these the turn radius will get bigger immediately. Sidecut alone rarely defines the raduis of the turn and it isn't a limiting factor, hell I routinely make carved twenty five meter turns on my 14 meter slalom skis. It's a combination of many many factors...
Hope this helps.
ski well my friends...

It is offcourse a very simplified diagram of the forces in the lateral plane yes. Feel free to make a more detailed diagram. I think the diagram is sufficient for prooving my point. I dont think the forward motion changes much of anything. Also, the centrifugal force is a populistic expression for the force pulling you out of the turn. The forces are also sums. Gravity and the centrifugal force as well as the normal force and friction and wind drag are spread over the whole mass. Thats why an arm can make a lot of difference yes.

So how do you reduce tipping/ski bending/steering in the high C part of the turn? IMO by reducing any of these you will get pulled to the ground immediately. You have to balance the forces in the lateral plane. See my diagrams.

Until you expand you thinking to include forward movement and momentum you are missing more than a third of the story. And that my friend is why your getting stuck, you think it's inevitable. Like Dorothy in the Wizard of Oz, you always have the power to change things,  You just don't believe you do. All I ask is for you to go out and play with what I suggested before dismissing my suggestions. I think you'll change your mind. Has it occured to you that all the guys who usually take opposite sides of debates are all saying the same thing. You don't have to get stuck, we've also all suggested how to get un-stuck when you want to open the radius of your turns.

I think tdk is perfectly capable of untipping his skis and changing their path.  He is just thinking of a situation where his cm is so far inside the turn that he is limited to having the skis catch him up if he doesn't want to hit the ground.  It happens.  In fact it is sometimes fun to make it happen.

Thanks for the input. Yes, you are right. The centrifugal force is wrong. It should offcourse be in the direction fo the slope. But its very short in the high C because I wanted to show a frame very early when skis were not turning much. Thats why gravity pulls so strongly, gray arrow. Maybe I should change that. Anyway the resultant of both forces act more in the same direction in the bottom half of the turn.

Thanks. Yes, thats exaclty what it is all about.

I got that ghost, the first time you said it. It's not impossible even in that extreme situation. I was doing my best snowboard impression of big fast knuckle draggers when a buddy on a board came from out of nowhere and I has to open my turn radius and eventually pull my skis off the snow so he could pass beneath me. i was mid turn when my ass was almost scraping the snow, I might add. So I know it can be done, I've done it. Although not nearly as well as some of my friends over in Aspen could probably do it. Or as well as some park and pipe guys I've seen do the same thing. I was probably going fourty and he went by me at at least fifty so we were lucky that we didn't kill each other that day.The idea here is that neither of us crashed or fell inward. We both continued to lay down arcs with the exception of the short retracting of my landing gear. I wish someone had filmed that near collision so I could post it.

I dont think Im narrowminded. This issue came up in annother thread and I thaught it would be interesting for people to discuss it. The discussion has been good. First denial but then a swing in the opposite direction. I dont think anybody that has taken part in this discussion and read all postings dissagrees on this issue anymore. Im very openminded. I listen to all sorts of interesting consepts of which many have changed the way I look at skiing.

Since you think Im missing out valuable force vectors in my diagram I took a quick search in our racing manual and came up with a diagram of the same kind of lateral forces. If its good enough for WC skiing its ok with me. Usually very simplified diagrams are the best but that is my personal view. I try to simplify things and Im glad you dont. That way I can learn lots of stuff you have to say. I was looking at my diagram and wondering how to redo it so that the forward vectors would show..... in 5min.....

TDK it's too bad we're so far apart we could go out and experiment together. That where the real proof lies.

Im not saying you are wrong but theory doesent support your experiance. I have a similair experiance where a ski school group was lined at the rim of the slope and watching a woman wedge down in the fall line some distance apart. I came on 30+ GS skis full trottle and chose to pass between the lineup and the woman. All of a sudden the woman turned across the slope and I opened up my line and took some air and passed over her skis inches away from her body in a tuck. The thing is that I was not committed to the turn yet but I had no place to go so I did not incline as much at initiation and could make a much wider turn. That is not the same as when I carved 8 track with a buddy of mine and all of a sudden we headed straight at each other. He made a quick impulsive pivot to the inside of the turn even though he was supposed to pass below me but I was quicker and managed to carve above with a J turn. Note that I never said that its impossible to open up the turn, just that its not as easy as tightening it. Like most people claim.

I could not agree more but if you look at the bright side if we did not have the net and this forum we would not be sharing thaughts at all.

JASP & Tdk6,

The context of that rock at the end of a sling helps describe centrifugal/centripetal force but doesn't really describe the nature of a 'balanced' skier in a turn because there isn't any BoS vs. CM relationship to consider.  For skiing we're stuck evaluating the forces acting on the CM separately from those forces acting on the BoS.  

Tdk seems to have created a specific (manufactured) context from which to propose his questions. It's not an artificial context as it happens to all of us at times, but it's not really 'normal' when considering typical skiing technique.   If we accept his constraint (being too far inside, aka: overly committed) then we trip the exclusion I deliberately slipped into my earlier post (about NOT being too far...).

So, supposing our skier launches their CM down the slope while their skis are going in an uncomplimentary direction... what can they do? 

a) Pivot their skidding skis more across their current direction of travel so that friction increases rapidly and 'props up' the skier (laterally).
b) Rapidly tip their carving skis more to both increase friction (as above) and sharply shorten turn radius.
c) Energetically retract both legs while reorienting both body and skis in mid-air to realign things before making contact with the snow again.
d) Transfer all support to the inside-ski and motor-on ... hoping it's enough.
e) Pucker up and brace for impact.

.ma

 tdk6 what do they call those 2 gate vertical combinations set in a race course over on your side of the pond?  Here we call them a hairpin.  They require you to end your turn and transition into a new one when your skis are in the falline. If it was impossible to end a turn without the help of gravity to pull you across your skis, as you have suggested, making the exit turn out of a hairpin would be impossible.  You'd just keep turning until you got out of the falline and ski right away from the course.  The reason that doesn't happen is because centrifugal force is the one we need to facilitate a transition, and it exists at any point in the turn, regardless of our orientation to the falline or gravity.  

The important thing to understand about this topic is that we can not reduce an edge angle to 0 in the blink of an eye when engaged in a high edge angle turn.  It takes time for the centrifugal forces we exploit to do their thing.  This is true in a normal transition, just as it is in an emergency exit.  When doing a normal transition we begin making the moves that will bring our turn to an end well before we've finished turning as far as we desire, because we know once we begin our transition our skis will continue to turn for the time it takes them to tip back up to edge angle neutral.  That can result in quite a few additional degrees of turning after we begin our transition.

The same thing will occur in emergency exit situations.  When we see the danger and launch our escape, it's going to take some time for the skis to tip back off edge.  That's probably the "I can't do it" sensation you're experiencing,,,  that built in lag time.  In a hairpin you know the turn is coming, so you begin your transition before you get to the falline. That lag time exists in the emergency situations to, it's just that we may not be able to afford to have it, so it becomes more noticeable.  

   

Tdk6,

There is also the example of skiers who maintain their CM's momentum across the slope instead of believing the upper body "must always" be moving down the slope.

As this skier finishes a turn they deliberately maintain support from that old set of edges long enough to progressively redirect their CM's momentum across the slope.  With this across-the-slope momentum they can easily engage the new edges and 'feel pressure' against those new edges right away - rather than having to wait until later in the turn for pressure to develop.


In the image above only one ski is shown (for clarity).  At the moment shown our skier's CM is traveling very much across the slope while the ski travels a very similar path.  This means our skier and ski are close to being "in balance" with respect to the given moment.  They'll probably need to topple further inside the upcoming turn to maintain lateral balance - which actually helps edge engagement. 

This is because as the CM 'drops' downward (assisted by Gravity) a portion of that 'pulling downward force' is directed against the ski which provides partial off-center support.  This is like standing on a ladder leaning against a wall where some force is directed straight down but another portion is trying to drive the ladder base to slip out away from the wall. This lateral component helps pressure the ski-edge against the snow enabling early edge pressure.


In this second image we have the same scenario except that our skier is allowing their CM to 'flow' more directly down the slope.  This time, the CM is not on a trajectory anywhere near the trajectory of the ski at the moment shown.  Here, our skier better be very flexed and have long legs to extend - or they will quickly lose pressure on the ski because the CM vs. BoS paths are diverging rapidly. 

With little pressure on the ski's edge, it will not hook up and come around (it will tend to go straight rather than following the green line drawn).  This skier will probably need to quickly twist that ski in order to re-align the two paths so they remain in lateral balance over the ski.

.ma

Bingo! Centrifugal forces unopposed move your entire body outward. that's the letting go part of the rock and string part of the experiment. The simple act of not resisting that force, or resisting it less at any point in any turn, still has the same result at any point. You go flying off on a tangent. All in all it still comes down to what keeps us off the snow when we get as far inside as TDK say he does. the simple answer is Momentum and all the outward fleeing forces that accelerate us outward. TDK you are correct that In the first half of a turn Gravity is a motive force that acts to pull us into the turn. So we really can't expect to use Gravity to pull us to the outside of the turn. That doesn't mean all the rest of the forces that act to pull us away from the turn have gone away though and it is those other forces that keep our butts off the snow at that point. If you lose edge purchase you might slide across the snow but the direction of that slide will not be towards the middle of the turn. So just like the rock your body will return to linear motion away ona tangent to the arc, not towards the inside of the arc. If that's happening you're going very slow, too slow for that much inclination to be a valid tactical choice.   
So while I agree with you Michael about a rock acting like a particle and the body being able to change it's shape and thus change the location of the CoM, The CoM is still being accelerated inward or outward and when that point moves, the rest of the body must follow. Aligning the CoM along a line of force, or a line of a system of forces and through the BoS is exactly the dynamic balance I mentioned in my first post. Something that was dismissed as wishful thinking.

To be clearer, and from a physics standpoint: Centrifugal force is an inward force, which directs the momentum of an object towards the center of an arc or circle. Centripetal force does not exist. When one studies physics, there is NO outward force being exerted upon an object travelling on an arc or in a circle. The outward sensation one feels in a turn is the natural tendency of the object (namely you) to travel in a straight line. The force being exerted is to keep you in the arc, which is the force you exert upon the lever of your skis. 

Since physicsman isn't here, I guess I'll step in.

CentriFugal force pushes away from the centre the meaning is literally "Centre-Fleeing"
Centripetal seeks the centre.

When you are driving your car along a curve in the road, you will notice (if you drive like I do) that you get squashed against the drivers side door if it's a right turn with a left hand drive car.   You are not traveling in a straight line; your velocity is changing in that your direction of travel is being changed by a force pushing you toward the centre of the arc described by the curve.  The force that is pushing you (acting between you and the door, being applied on you by the door) is being directed toward the centre of the arc.  It is a centripetal force.

Your momentum makes you want to go straight and you will go straight unless an outside force changes that.
The force that pushes your skis and you around a turn is a centripetal force. 

It is often convenient to use a frame of reference that travels with the skier in a turn.  If we do so, we must include a centrifugal force or the motion of the skier cannot be explained using Newton's laws of motion.

Centrifugal is just an adjective that means moving away from the centre, so it can be applied to any force that points in that direction.  However there is a special centrifugal force that is needed in accelerated frames of reference (I will try to explain without giving you a nosebleed.)

For example if you were playing pool on a pool table in the back of a moving van and sank the 8 ball in the corner pocket, you might say the ball traveled 2 mph at a 45 degree angle to the table, using the table and van as a frame of reference whether the truck was traveling straight down the road at 70 mph or stopped.  In the back of a moving van, you only have the floor walls and ceiling of the van for reference.  If you had a glass floor you could use the road for reference.  If you were to use the road under the truck, you would have to calculate the velocity of the ball and truck compared to the ground and the path would change.

If the truck were accelerating because the driver downshifted and floored the gas pedal, the ball would seem to be pulled to the rear of the van.  You could miss your shot because of it.  From the point of view that uses the road as a frame of reference the ball just tried to stay moving along at a constant speed, and the truck increased it's speed, moving forward faster. Someone moved the table while you were taking your shot.   From the point of view of the folks in the van with nothing outside the van to refer to, everything somehow got pulled to the back of the van.  The table resisted the pull, the balls did not.

Newton codified a certain way of thinking of motion and forces that is mostly expounded by three laws.  I've already used one above (objects remain in a steady state of motion unless acted upon by an external force). One of his most famous laws states that the force applied to an object is equal to the product of its mass and its acceleration.  F=ma.  These laws must work in any frame of reference. They must work for the people in the van.  In order to have F=ma work for the people in the van, they must have a force causing the acceleration of the ball toward the rear of the van. 

If the truck were to go around a corner at constant speed, the ball on the table would seem to be pulled to the side of the the table.  In order to have F=ma work for the people in the van they must have a force acting on those balls to make them move sideways.  This force is the centrifugal force.  The math works out and all of Newton's wonderful mechanics can be applied in any frame of reference, so long as you include a body force to account for the acceleration of objects those frames that are accelerating.

The trippy thing is there is no absolute frames of reference, all frames of reference are accelerating with respect to something.   The road is accelerating because Earth is rotating and also revolving around the sun which is in an arm of the milky way revolving around...

Centripetal force
The term centripetal force comes from the Latin words centrum ("center") and petere ("tend towards", "aim at"), signifying that the force is directed inward toward the center ofcurvature of the path. Isaac Newton's description was: "A centripetal force is that by which bodies are drawn or impelled, or in any way tend, towards a point as to a center." (Wikipedia)

Centrifugal force
Centrifugal force (from Latin centrum "center" and fugere "to flee") represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation. In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertial reference frame or a reaction force corresponding to a centripetal force. (Wikipedia)

Nice post Ghost. I use a plane in a similar example because it allows me to include a stronger focus on the forward movements. We can stop the car without much consquence but stop the plane and it falls out of the sky. If we aren't moving forward the extremely inclinated stance would only cause us to topple over. It also allows us to discuss the axis of balance and how that changes relative to the turn size and the speed were traveling.

The reality is that at least 50% of what we feel when we ski is the centrifugal force idea, which is why its generally more useful to discuss skiing from that perspective.  Its often convenient to discuss physics in terms of centripetal, as is so often pointed out by the physics professors on this site.  You can look at it either way and debate all day long about where the force originates from, but from our perspective as skiers, we create this force by motion of our body towards a constrained radius (like the car door analogy).  It feels most like Centrifugal and in addition its most effective to think of it that way when establishing your balance on the outside ski.

I said 50% because there are times when the thought process is reversed and our perception is one of our skis being pushed in towards the middle, like the whole rebound concept.

In general I feel its not a good idea to get too locked into one viewpoint or the other, centrifugal vs centripetal.  They are really  just two different ways to discuss the same opposing forces.  In some cases its more effective for skiing to concentrate on how your moving body will effect the centrifugal force.  And other times its more effect to think about how centripetal forces are going to push your skis towards the center.  I tend to feel the latter is mostly applicable at the very end of turns.  But most of the turn I feel its more effective to think about what my CoM is doing and how it will effect centrifugal, and how to keep it balanced over the BoS.

Freeski919,

While both Ghost and Nolo have provided plenty of material on the subject, back in August, 2005 PhysicsMan (a genuine physics professor) decided to post some comments on Centrifugal/Centripetal force because the terms were being used so creatively back then.  It's also worth a read and can be found here.

Centripetal / Centrifugal Forces arise from the context.  This matters greatly because that's exactly what I exploit to more easily engage my edges at the top of a turn. 

As shown in the drawing I posted above, by deliberately finishing the turn further across the slope I deliberately create a context where Centripetal Force will exist so that I may use it to my own purpose (as indicated in the first drawing).

If I allow my body/CM to maintain a primarily down-slope momentum (as in Drawing 2) then centripetal force is not available to prop me up laterally at the top of the turn and I have the problem Tdk6 is proposing.

.ma

So to be clear here centripital forces coming back up through the skis exists in all parts of the turn. Not to mention Gravity acting as a centripital force and pulling us into the next turn. So what about the cetrifugal forces needed to balance those forces. Where do they come from? 

Both Centripetal and Centrifugal forces exist in exactly equal measure through all phases of the turn.  The question is whether you want to think about it in your head as being a centrifugal force pulling you out or a centripetal force pushing you in.  One does not exist without the other in terms of turn forces.  They are really just two different ways to look at the same interaction of your CoM's momentum being blocked by the engagement of your edges on the snow.

In my view, it depends on whether are of paying close attention to what you're doing with your CoM or if you are paying closer attention to movements with your feet.  Balancing on your BoS and focusing your mind on your CoM will lead you to the inevitable feeling of a force pulling you out.  On the other hand, when your CoM seems to become more stabilized and your feet do more of a cross-under, then you will distinctly feel Centripetal force in action.

 This is why for quite a while I just referred to that force we feel as momentum.  it avoided a lot of grief.  

As far as I know, we humans can't feel 'Momentum' so I'm not sure I'd use that term in place of a legitimate description. 

.ma

I tend to avoid all the math but if you want to render all of this down to the most simple concepts, it really comes down to acceleration. To accelerate in any direction there must be a sufficiently strong unbalanced sum of forces to overcome inertia. The resulting motion is linear along the line of the sum of forces. For a change in direction to occur there also needs to be an unbalanced lateral sum of forces. Ignoring either forward or lateral forces for the sake of simplicity doesn't work if for no other reason than if it was only opposing forces along one axis we would only move along that one axis. A curved path requires more than that though. 
So to return to the top of the turn we have an unbalanced sum of forces in at least two planes. So in each plane the weaker of the two opposing forces is overwhelmed by the stronger force and we accelerate in the same direction of the stronger force. The weaker forces don't go away though and in the example of the first third of the turn there is enough outward fleeing force to keep us from falling over to the inside of the turn but not enough to prevent us from turning in that direction. Which again is why we can find a balance axis and remain erect even though we so happen to be inclined inside the turn. Change the balance of forces though and that balance axis changes immediately. We do that by using a combination of body movements and the only time we get stuck is when we quit moving.
That's the key here and why I said you have the solution in your grasp all along. You may choose to move too far into the turn which said another way means you moved beyond the balance axis for the intended turn. Why you want to do that suggests either you are seeking to tighten the turn by increasing the edge angle, or you overshot the balance axis. In either case to open up the turn you just need to move your CoM back over the skis a bit. You have three options to accomplish that and a fourth option of doing nothing. The fourth option is when you get stuck though...
Edited by justanotherskipro - 11/8/09 at 4:28am

If it was impossible to reduce the inclination between the CoM and the Bos, how does it become possible later? Assigning that task to Gravity is a mistake since it ignores the rest of the outward fleeing forces involved that never went away and also our ability to change the shape of our body to increase or decrease the egde angle.I want to point out that Angulation is only one of many options.

•  A flexing of the support leg also changes the angle of inclination between the snow, BoS, and CoM.
• The aft stance shift, if not excessive, takes pressure off the tip so the steering angle is reduced and the radius gets bigger. That's how we used to open up the radius on old school equipment. Tail carving is what we called it back in the day.
• Steering the feet away from the turn will also reduce tip pressure since the skis are tipped up on edge and in effect we are redistributing some of our weight towards the tail. Which is the opposite of loading the tip with an inward steering of the skis. 

The assumption that these movements only work in the last half of the turn is the biggest problem I see. If you tell yourself you can't open the radius of a turn before the fall line then it's more than likely that you won't even try. I see that as a mentally imposed parameter that at some point you will discover is what is keeping you "stuck" in the turn.  

Yea JASP I agree.  I think this thread just exposes one of the reasons why its important not to huck into in the center of the next turn and wait for your skis to catch up at the fall line.  You sacrifice much control that way.  

When  you start a turn, the turn forces are not very big yet.  You shouldn't dive onto the new edges to lock yourself in and then park-n-ride to the end.  Rather, you should progressively tip and progressively inclinate as the turn forces build. In those early stages, providing you haven't hucked yourself, you have a lot of options to vary your tipping, vary angulation, vary steering/pivoting, etc.  The turn is developing.  You are still determining what the radius will be.  You can control how fast or slow you progress towards the edge angles.  Basically, you can guide your skis to the desired turn radius during the upper half.  

I made an analogy on another thread, I'll repeat it again here, its like throwing a dart at a dartboard vs walking up to the dartboard and pushing the dart into the bullseye.