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How he achieve theses turns? - Page 3

post #61 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
I think you have missunderstood the meaning of the word "inclination". At least as far as I have understood it inclination is how much we lean towards the inside of the turn. It really does not have anything to do with banking or angulation. Here is how you create inclination; stand up on your roller scates and stand with your feet slightly apart. Now flex one leg or lift it up. You start falling in that direction. As you fall you pass through 0-90deg inclination.
Thanks you for clarification. That's what I understand too.
post #62 of 114

Oooh...Ahhhh

I want to introduce my new pet technical term for the season. It refers to a sudden change in momentum. It is impulse. The skier in the video was effectively changing direction while maintaining momentum by directing his mass to crossover the skis creating the impulse to carry momentum into each new turn.
post #63 of 114
Thread Starter 
Quote:
Originally Posted by dogonjon View Post
I want to introduce my new pet technical term for the season. It refers to a sudden change in momentum. It is impulse. The skier in the video was effectively changing direction while maintaining momentum by directing his mass to crossover the skis creating the impulse to carry momentum into each new turn.
Thanks for contribution. May I ask where this impulse comes from, and how to create impulse?
post #64 of 114
As I understand it the CoM and the skis move in different direction and the impuls comes from the interaction between the two. Feel free to correct me if Im wrong.

Anyway, there has been a lot of feedback in this thread allredy but what it all boils down to is something very simple: brilliant skiing. Why do balley dancers look so light on their feet? Because they know how to efficiently move in order to minimize the effect of gravity. Its the same gravity for all of us but with the right body movements and perfect timing we can turn it arround and use it to our advantage. Join forces.

If you look very closely at the video you can see that the skier uses very short pressure. He is not carving. He is dancing. He is light on his feet. If you want to achieve those turns you need to start working with your feet and feel the spring effect. Like when you are jumping on a trampoline. Sudden pressure and then soring through the air. In many sports like in rowing, martial arts, tennis, boxing etc you need to be able to use poly rhytms and separate body movements from each other. With great timing you make them interact in a most efficient way. Im a part time musician and I like to think of many things we do in terms of rhythms. Insted of counting 121212 you should be counting 123123123 or 12341234 with the emphasis on 1. If one represents the pressure phase, notisse how much shorter it is in 123123 when compared to 121212. In 12341234 it is even shorter. When you do this right you can feel the impuls.
post #65 of 114
Quote:
Originally Posted by carver_hk View Post
Thanks for contribution. May I ask where this impulse comes from, and how to create impulse?
Impulse is force (a push) times Time. You can push hard for a short time, or push lightly for a long time. The impulse will be the same, and will be equal to the change in momentum.

For physics students: Force is ma, and has units of mass times meters divided by seconds squared. Multiplying by time gives units of mass times meters per second, which is mass times velocity (momentum). Force velocity acceleration and momentum have a direction associated with them.
post #66 of 114

Change of momentum

Acceleration is a change in momentum. A curve is an acceleration as momentum is constantly changing as the path of the mass is being deflected by the tool. (I'm still thinking this out). The greatest change in momentum or impulse occurs during the transition between turns when the body's mass moves forward along the length of the ski and laterally towards the apex of the upcoming turn (gate). Crossover and cross under (retraction) are both demonstrated by this skier, tending to cross under more when he was momentarily bucked to the back seat (evidenced by the lifting of the tip of the inside ski 53" +55"). None the less the line was held and the rearward leverage may well have been a tactic given the longer vertical displacement and slightly more open turns at the end of the course. Although common sense would make you think the slight lifting of the inside ski and rearward leverage would interfere with the impulse, reduce efficiency and result in a slower time.
post #67 of 114
Acceleration is a change in velocity divided by time. It is a change in momentum divided by the time it takes to change (also divided by the mass, but if the mass isn't changing we can ignore this factor for now).

The greatest change in momentum takes place when the greatest force is applied, at the apex of the turn. At transition there is no force other than gravity acting, and maybe some normal surface force.

The skis and the skiers centre of mass do not follow the exact same path. If you plot both paths, you will see where they cross.

Ideally the impulse is across the hill to stay on line, with an attempt to maximixe down the hill when available, but minimize up the hill, unless slower speed is called for.
post #68 of 114
Thread Starter 
Quote:
Originally Posted by dogonjon View Post
Acceleration is a change in momentum. A curve is an acceleration as momentum is constantly changing as the path of the mass is being deflected by the tool. (I'm still thinking this out).
If I look from energy point of view I get a different result. If I take deflection means the skis carving the upper & the lower C.

Loss of energy:
1. in compacting the snow
2. bending the skis
3. energy burnt by fiction.

Energy gain:
4. potential converted to kinetic energy from gravity
5. energy produced by skier pushing uphill at power phrase
6. energy release by rebound of the skis.

So when there are energy gain > energy lost the skier goes faster. The greatest gain in energy seems happen when the skier finish making the short push(again using mostly gravity) because gravity is uniform . The energy increased will increase the speed and therefore the momentum?
post #69 of 114
Quote:
Originally Posted by dogonjon View Post
Crossover and cross under (retraction) are both demonstrated by this skier, tending to cross under more when he was momentarily bucked to the back seat (evidenced by the lifting of the tip of the inside ski 53" +55"). None the less the line was held and the rearward leverage may well have been a tactic given the longer vertical displacement and slightly more open turns at the end of the course. Although common sense would make you think the slight lifting of the inside ski and rearward leverage would interfere with the impulse, reduce efficiency and result in a slower time.
In SL you need to quickly get your skis onto new edges and carve arround gates to eather side of the centerline. For efficiency and a smooth ride just like on cars, you need to keep the mooving mass as small as possible. Thats why you try to keep your upper body as stationary as possible and keep as much of the movement as possible in the legs. Your feet are your shock absorbers. This allso means that your CoM should move as little horisintally to the sides and vertically up and down. This is not allways possible so your CoM will move up and down and sideways but you need to minimize it. Its a form of optimizing. At apex your CoM is as low as it gets and at transition your CoM is as high as it gets. Higher edge angles mean lower CoM. Lower CoM at apex means more lift of CoM at transition. The trick here is to stay as low as possible at transition. That you do by retracting your leggs through transition. Retracting your leggs means due to how our body and bones are constructed you need flex at your knees. Flexing at your knees brings your lower leggs, shins and feet forward. In bumps Bob called it "back pedalling". Its the exact same thing in the gates. If you look at WC skiers from the side, seldome viewed, they are far in the back seat during transition but at apex they are centered over their skis. Byggmark is maybe the most radical. Boots and skis are pushed forward and then pulled back.
post #70 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
Sudden pressure and then soring through the air. ....
Im a part time musician and I like to think of many things we do in terms of rhythms..
Glad to learn that you are also a musician. I think ultimately rhythms play an important role just like what you said. Regarding the sudden pressure you like to tell the direction of the pressure, how you get it and at which moment?
post #71 of 114
Quote:
Originally Posted by carver_hk View Post
Glad to learn that you are also a musician. I think ultimately rhythms play an important role just like what you said. Regarding the sudden pressure you like to tell the direction of the pressure, how you get it and at which moment?
I play the guitar, whats your instrument?

Anyway, when you pressure your skis that are on edge you need to be in total controll of your fore aft balance. It doesent matter if the pressure phase is long or short but if its short it becomes more critical.
post #72 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
I play the guitar, whats your instrument?
Oh! I merely means you are a great skier and also a musician.

Quote:
Originally Posted by tdk6 View Post
Anyway, when you pressure your skis that are on edge you need to be in total controll of your fore aft balance. It doesent matter if the pressure phase is long or short but if its short it becomes more critical.
Yes, totally agree with your view on fore aft balance. But I am still not very sure when you like to have the short pressure. At pressure phrase start or at release. how to get it etc.
post #73 of 114
Quote:
Originally Posted by dogonjon View Post
Acceleration is a change in momentum.
Technically, it is change in velocity over time. Velocity has both speed and direction. If you change direction, you change velocity.

Quote:
Originally Posted by dogonjon
A curve is an acceleration as momentum is constantly changing as the path of the mass is being deflected by the tool. (I'm still thinking this out).
The skis deflect the mass in a turn, thus changing it's velocity, and it's momentum. So, when turning, you are changing momentum.

Quote:
Originally Posted by dogonjon
The greatest change in momentum or impulse occurs during the transition between turns when the body's mass moves forward along the length of the ski and laterally towards the apex of the upcoming turn (gate).
NO. This is where the LEAST change in momentum occurs, as the skis have stopped deflecting the mass of the skier. Velocity does not change as the mass moves into the new turn.

It is where the most change in the deflecting force occurs -- it is where the deflecting force is removed.

You are talking about "release". In particular, releasing the CM from it's arc. Releasing the CM from it's arc is not impulse.

Hope this helps.
post #74 of 114
Oct 9, 2008

Hi BigE and dogonjon:

Acceleration = instantaneous rate of change of Velocity. Not just change in Velocity.

Acceleration = d(Velocity)/dt

Change in Momentum is related to the concept of Force i.e. Force = instantaneous rate of change of Momentum.

Force = d(Momentum)/dt = d(mass*Velocity)/dt

Note that the above equation holds even at "relativistic" Velocities i.e. Velocities whose magnitude approaches the speed of light. For everyday skiing speeds, we can rewrite the Force equation in the regular Newtonian version which we are all familiar with from high school physics:

Force = mass*d(Velocity)/dt = mass*Acceleration

That's about as much physics as I can remember. Also, excuse me for going off thread.

CP

PS:the above physical quantities, vector quantities start with an upper case letter while scalar quantities start with lower case.
post #75 of 114
Thread Starter 

When is the rebound start and for how long?

Referring to the original vid, If we draw a clock face on the slope to take from 0(12) O'clock to 6 O'clock represent neutral to neutral, at what time position is rebound starts to release its energy and when is it fully released? And, most importantly how is it used? Thanks for any insight in advance.
post #76 of 114
Quote:
Originally Posted by carver_hk View Post
Referring to the original vid, If we draw a clock face on the slope to take from 0(12) O'clock to 6 O'clock represent neutral to neutral, at what time position is rebound starts to release its energy and when is it fully released? And, most importantly how is it used? Thanks for any insight in advance.
Look at the skier and when there is snow spraying from his skis. The more pressure the more snow spraying. Typically from 4-5 would be the time for maximum pressure. Note that at 6 the skier should be flexed at transition. The relece starts the unweighting and should happen at about 5. A turn with early pressure could start with pressure at 2 but the most pressure would still be past apex. Also, note that we usually never turn across the hill.
post #77 of 114
Agreed, the release move occurs at about 4:30-5 and 7:30-7 o'clock. This is what I would call "turn completion". It marks the moment that the CM is released from it's arc.
post #78 of 114
Thread Starter 
BigE & tdk - thanks for the very precise response. It helps a lot to my understanding.
post #79 of 114
Thread Starter 

what happen between the snow & the skis @ rebound?

second though. lets assume rebound always made happen at the time position for effectiveness or efficiency. What actually happen between the snow & the skis @ rebound? I ll tend to think in several dimensions: duration, direction, magnitude. I have seen people challenge the magnitude thingy for its significance. More input?
post #80 of 114
During rebound the pressure that has been built up during the pressure phase is reduced and the force is releced upward.

Duration
The duration can be long or short. If you have a very short duration then you might in combination with a very strong rebound get a lot of pressure reduction. Even down to cero and beyond. Airborne.

Direction
The whole purpose with up-unweighting is to reduce pressure on skis in order to pivot them into a skidd. Here in lies the whole secret to the non carving dimension of skiing. Bumps, powder, park etc. The problem with todays downhill skiing is that with modern skis you dont have to skidd in order to turn. You only need to tip and carve. And if you need to skidd you simply slack on some movements to reduce edge hold and drift off into a skidd. Works but not always. You might ask me why Im talking about skidding in a thread about WC skiing. Rightfully so, but you have more options with reduced pressure at transition. You can choose if you go for a pivot steer turn entry or if you aim for running along your edges in a pure carve/arc. Here in also lies the danger of always or accidentally steering your skis simply because you are used to doing it (not you but skiers that skied for generations) so you better know what you are dooing. In other words, rebound serves two purposes: pivot/steer and smooth and efficient CoM management.

Magnitude
Not sure about this word but I think it means how much. Well, enough for your intent I would say. In Austria snow contact is what is being thaught in ski school and coaching but racers do whatever they need to at any given time. Basic rule is not to pick your ski off the snow so if your skis kick back real strong simply absorb it with your legs. In powder this is called pumping. You dont jump, you pump.
post #81 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
During rebound the pressure that has been built up during the pressure phase is reduced and the force is releced upward.
Would that depends on the intention of the skier?
If racing skis is used the skier can well position the pressure nearer the tail and release some of the energy to propel the skis along its edge. So some energy helps the skier to retract his legs, some propel the legs to the other side?
post #82 of 114
I've been trying to understand notion that dogonjon was trying to explain I think it's something like this: the loading phase stores the power in the ski. To do this, the skier is aligned/stacked to resist turn forces.

As the skier completes the turn. The Cm begins to move outside.

This unstacks the skier, so that the push back from the ski (no longer in equilibrium) can push the skis/legs across the hill. The deflecting force now has a "different" effect; an impulse that pushes the skis quickly under the body.

Is that about right?

I'm done for the weekend. Post Monday.
post #83 of 114
Thread Starter 
Quote:
Originally Posted by BigE View Post
I've been trying to understand notion that dogonjon was trying to explain I think it's something like this: the loading phase stores the power in the ski. To do this, the skier is aligned/stacked to resist turn forces.

As the skier completes the turn. The Cm begins to move outside.

This unstacks the skier, so that the push back from the ski (no longer in equilibrium) can push the skis/legs across the hill. The deflecting force now has a "different" effect; an impulse that pushes the skis quickly under the body..
Thanks BigE. It adds a lots to my understanding.
post #84 of 114

Don't change the subject

Quote:
Originally Posted by tdk6 View Post
I play the guitar, whats your instrument?

Anyway, when you pressure your skis that are on edge you need to be in total controll of your fore aft balance. It doesent matter if the pressure phase is long or short but if its short it becomes more critical.
Hey, let's stick to the issue. I play guitar too. I'm in the only legal band in Mammoth. The town is taxing all public performances and if you get so much as a free beer for playing you owe them $60.00. We paid our dues and are playing a benefit for the High school music program 10/18 at Whiskey Creek tickets $10.00 at the door.
You can be wildly out of balance and still edge your skis. Park and ride is traditionally in the back seat.
post #85 of 114
Quote:
Originally Posted by dogonjon View Post
Hey, let's stick to the issue. I play guitar too. I'm in the only legal band in Mammoth. The town is taxing all public performances and if you get so much as a free beer for playing you owe them $60.00. We paid our dues and are playing a benefit for the High school music program 10/18 at Whiskey Creek tickets $10.00 at the door.
You can be wildly out of balance and still edge your skis. Park and ride is traditionally in the back seat.
Thats insane! 60 bucks! Not a very musician friendly place.....

Yes, you can be wildly out of balance and still on your edges but not if you want to ski properly.
post #86 of 114
Quote:
Originally Posted by carver_hk View Post
Would that depends on the intention of the skier?
If racing skis is used the skier can well position the pressure nearer the tail and release some of the energy to propel the skis along its edge. So some energy helps the skier to retract his legs, some propel the legs to the other side?
It doesent really matter if you have race skis or not. Good skiers can ski with any kind of skis.

Im not really sure what you are getting at but check out cross country scating style for maximizing forward motion.
post #87 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
It doesent really matter if you have race skis or not. Good skiers can ski with any kind of skis. .
I was told racing ski tails snaps into the snow because of its design. Non-racing skis's tail don't snap into snow and therefore pressure releases into both tail & tips.
post #88 of 114
Quote:
Originally Posted by carver_hk View Post
I was told racing ski tails snaps into the snow because of its design. Non-racing skis's tail don't snap into snow and therefore pressure releases into both tail & tips.
Racing skis are better for racing thats for sure. I dont know about the snapping though. Rigid stiff skis need more speed. Who told you they were better? You can ski perfectly well on Supershapes or similair skis.
post #89 of 114
Thread Starter 
Quote:
Originally Posted by tdk6 View Post
Racing skis are better for racing thats for sure. I dont know about the snapping though. Rigid stiff skis need more speed. Who told you they were better? You can ski perfectly well on Supershapes or similair skis.
its not just about rigidity but the shape of the tail. A friend who been some serious race told me the tail does make the difference.
post #90 of 114
A racing ski has a stiffer tail. A stiffer tail snaps back quicker and harder is all.
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