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Groomed turn- MA - Page 5

post #121 of 148
Page 122 of Ultimate Skiing
103 of The Skier's Edge
post #122 of 148

  Well there has to be some reason racers aren't on 105's underfoot!!biggrin.gif

 

   zenny

post #123 of 148
Quote:
Originally Posted by Atomicman View Post

I think you guys are missing the whole enchilada with skates and narrow skis, it has to do with the alignment of your foot to the edge.of the ski/skate and geometry. There is a chapter on this in Ron Lemaster's book. I will see if I can  find it!

 

Quote:
Originally Posted by cgeib View Post

Page 122 of Ultimate Skiing
103 of The Skier's Edge

 

Yup read it....torque.  FxD.  Ski width and riser height affects "D".

 

But still interested in JamTs thoughts.

post #124 of 148
Quote:
Originally Posted by cgeib View Post

OK, I misunderstood what you were getting at.
It would seem that as the ski gets wider not only does the torque "holding the ski on edge" decrease, it actually starts torquing the ski off of the edge …if we look at it in relation to the resultant force anyway. Would it make sense to say, when the resultant force is to the outside of the edge we are working against it and when it is inside of the edge we are working with it, to hold the ski on edge? I take Jamt's statement that you quoted above to mean that if there was sufficient torque to keep them on the edge, then the edge would hold. Is getting that torque efficient though? Would a racer be able to hold the ski on edge in their high G turns on a 130mm wide slalom ski?

I don't like my fat skis on hard surfaces and seem to have to lever the boot to get them to stay up on edge, since I'm actually lifting myself off the surface toward the inside of the turn in opposition to centrifugal force pulling to the outside of the turn. I don't like it on my knees and I don't feel it is efficient, but I doubt the ski cares as long as I find some way to keep it tipped.

I'm curious in Jamt's thoughts as well.

My thoughts exactley....

post #125 of 148

First let me just make clear that I am not trying to say that fat skis are better for ice grip, racing or whatever. I do however have a different view of why this is the case than what I think is the conventional belief. I made a picture to illustrate:

 

This is a simplified front view of a skier.

In picture 1 we have wide skis and an angulated skier, both knee and hip.

The force line from the edge passes through the CoM, which means the skier is in balance at this moment in time.

The distance between this force line and the interfaces is proportional to the moment the respective interface has to carry. The interfaces are instantiated by bindings, knee, boot etc.

In picture 2 everything is the same except that the ski is narrow.

In picture 2 the skier is no longer in balance, so the skier would have to incline LESS and use a somewhat smaller edge angle, as illustrated in picture 3. Smaller edge angle means less grip and less turning. This is one factor that makes wider skis have good ice grip, you can edge MORE!

If we compare the interface moments we see that in picture 1 the boot-binding moment is larger, but the knee joint carries LESS moment. If you are banking your turns this is not true, but when you are angulating the knee wants to track to the inside and wide skis reduce the moment. If you are experiencing excessive knee moment on wide skis all you have to do is angulate more. Find the angulation where the force line passes through the knee plane for zero moment.

This is all in theory, in picture one the moment around the boot-binding interface is much larger, and thus the give will be larger.

What I am claiming is that THIS is the major cause of bad grip with wider skis, it is not the ski itself. In particular since wide skis usually have bad bindings. Theoretically, with ideal material properties wide skis do not have bad ice grip. In practice they do have worse grip, but with good equipment (Plates, 20 DIN race bindings and good ski construction) the difference is not that big. I have not shown it in the figures, but it should be clear that binding plates will also reduce the bending moment in the boot-binding interface. (That is one reason why race bindings on fats make a big difference)

 

In general I'd say that the binding is the most undervalued component in the equipment. It is really important. I've seen bindings on rental skis that give several degrees with very little force. There is no way you get any good performance out of a ski with that kind of binding.

 

There are also other drawbacks with wider skis. You have to retract more, they are slower up on edge, the balancing is more difficult, they are not as torsionally stiff etc.

 

Edit: Banking, all moments are smaller with narrow skis.

 

post #126 of 148
Quote:
Originally Posted by Jamt View Post


In picture 2 the skier is no longer in balance, so the skier would have to incline LESS and use a somewhat smaller edge angle, as illustrated in picture 3. Smaller edge angle means less grip and less turning. This is one factor that makes wider skis have good ice grip, you can edge MORE!


 

 

 

Isnt that backwards?

 

I added the "effective degree of inclination" in red to your diagram.  I also made your "wide ski" wider just so the difference between the two "effective degree's of inclination" were obvious.  For simplicity for now, lets just assume angulation is not possible.

 

So what does this diagram tell us?  Well if the turn radius and speed were the same...one of them is out of balance. 

 

For both of these guys to be in balance (assumign same turn radius on skis), the narrow skidude must be going faster then the wide skidude.  Correct?

 

If you increase the edge angle of the wide ski...you will also decrease the turn radius.....so not really comparing apples to apples anymore.

 

If the narrow skidude was in balance for a given speed, then the wideski guy would really struggle to hold that edge angle...thus lose grip.

 

If the wide skidude was in balance for a given speed, then the narrow ski dude would be ok if he angulated.....the opposite cant be said for wide.

 

 

Correct?

post #127 of 148

Sorry I did not explain or draw the banking picture very well. In the angulating one I assumed that you had all pressure on one ski, but when you bank you need to keep weight on the inside so the force line is parallel to the leg as I drew, but I only showed on leg.

 

 

 

Quote:
If the narrow skidude was in balance for a given speed, then the wideski guy would really struggle to hold that edge angle...thus lose grip.

the wide ski guy would have a higher edge angle if inclining the same amount. And yes as you say that will shorten the radius so you can incline more. This results in higher edge angles and if you can manage that the grip is fine, but the balancing is more difficult.

 

 

 

Quote:
If the wide skidude was in balance for a given speed, then the narrow ski dude would be ok if he angulated.....the opposite cant be said for wide.

Yes, but will that angulation lead to higher edge angles? Not necessarily, if you angulate so that you have the same edge angle the location of the COM need to be at the same place relative the force vector. In the second skier in the first picture that would correspond to angulating so that the COM is aligned with the force vector.

In other words you have to angulate more to reach the same edge angle, and when you run out of RoM the wide-ski guy can continue to angulate more, i.e. higher edge angles.

 

Look at this extreme picture. Super wide skis, a lot of edge angle with Zero inclination (CoM directly above edge). You need some extra "leaning" just get up to the balancing point. Problem is how to get up to this edge angle and stay balanced there :-)

 

 

post #128 of 148
Quote:
Originally Posted by Jamt View Post

Sorry I did not explain or draw the banking picture very well. In the angulating one I assumed that you had all pressure on one ski, but when you bank you need to keep weight on the inside so the force line is parallel to the leg as I drew, but I only showed on leg.

 

 

 

the wide ski guy would have a higher edge angle if inclining the same amount. And yes as you say that will shorten the radius so you can incline more. This results in higher edge angles and if you can manage that the grip is fine, but the balancing is more difficult.

 

 

 

Yes, but will that angulation lead to higher edge angles? Not necessarily, if you angulate so that you have the same edge angle the location of the COM need to be at the same place relative the force vector. In the second skier in the first picture that would correspond to angulating so that the COM is aligned with the force vector.

In other words you have to angulate more to reach the same edge angle, and when you run out of RoM the wide-ski guy can continue to angulate more, i.e. higher edge angles.

 

Look at this extreme picture. Super wide skis, a lot of edge angle with Zero inclination (CoM directly above edge). You need some extra "leaning" just get up to the balancing point. Problem is how to get up to this edge angle and stay balanced there :-)

 

 


I understand what you are saying...but I dont think you are right.  You need to move from theory to reality.

 

For example in your above diagram....is that possible to turn/hold that edge angle with turning forces present? I say "no".

 

 

I think the biggest discrepency between us, now that I read more of your stuff is you suggest that a higher edge angle results in better grip.  I would argue that edge grip is maximised at the critical angle...and anything above that, simply reduces the turn radius...thus becomes another turn....which can be made on a narrow ski too.

 

Ultimatley I think my argument of higher speeds needed for narrower skis for a given radius is correct, and why racers et al use the narrower ski and risers.

 

However, I agree now, that a wide ski can grip as well as a narrower one, all things being equal....however, reality of biomechanics (torque on knees etc) means all things are not equal.

post #129 of 148
Quote:
Originally Posted by Skidude72 View Post

 


 

 

I think the biggest discrepency between us, now that I read more of your stuff is you suggest that a higher edge angle results in better grip.  I would argue that edge grip is maximised at the critical angle...and anything above that, simply reduces the turn radius...thus becomes another turn....which can be made on a narrow ski too.

 

Ultimatley I think my argument of higher speeds needed for narrower skis for a given radius is correct, and why racers et al use the narrower ski and risers.

 

However, I agree now, that a wide ski can grip as well as a narrower one, all things being equal....however, reality of biomechanics (torque on knees etc) means all things are not equal.

No, I don't suggest that higher egde angles means better grip. But what is relevant is the platform angle. The angle between the red line in your picture and the ski. The sharper this angle is the better grip you have. If I choose a longer sidecut on the wider ski I could draw a situation where the turn radius (lateral force) is the same but the platform angle is sharper.

 

However, Racing is 90% about balancing not reaching high edge angles, and doing this balancing is more difficult on wider skis. This and the fact that in reality the interfaces are not ideal.

One important factor on hard surfaces is ski chatter and that is quite related to how far off ideal the interfaces are. Chatter can worsen with wider skis because of the increased moment (chatter can also lessen due to bad interfaces but then you loose grip instead)

post #130 of 148
Quote:
Originally Posted by Jamt View Post

No, I don't suggest that higher egde angles means better grip. But what is relevant is the platform angle. The angle between the red line in your picture and the ski. The sharper this angle is the better grip you have. If I choose a longer sidecut on the wider ski I could draw a situation where the turn radius (lateral force) is the same but the platform angle is sharper.

 

 

Or put another way...you are saying, the more angulation you have, the better grip you have?  Alternatilvey you are suggesting that you could also ski in a squat stance, and improve your grip....really?

 

Not sure i buy that.  Whats the physics behind that? 

 

Or have I misinterpreted?

post #131 of 148

It is very simple really:

 

 

Here you have the same inclination and turn radius, i.e. the same lateral forces. (assuming for simplicity that the skis have different radius)

The left picture (e.g. banking with excessive weight on the inside skis) you have a platform angle that makes the ski skid.

The right picture (angulating) you have an aggressive platform and the ski wants to "dig" into the snow. 

You can also look up "platform angle" in Ultimate skiing.

post #132 of 148

A little side story, a member of our club was a fore-runner at a FIS-event. He is a recent national level skier and he had a better time than all the racers, On wide twintips! He made a pretty good job at holding the edges. 

 

Edit: Sorry Alex, pretty far from your MA now redface.gif

post #133 of 148
Quote:
Originally Posted by Jamt View Post

A little side story, a member of our club was a fore-runner at a FIS-event. He is a recent national level skier and he had a better time than all the racers, On wide twintips! He made a pretty good job at holding the edges. 

 

Edit: Sorry Alex, pretty far from your MA now redface.gif

  Seems somewhat relative though--of course Alex is not on ice but he is "carving". I'm struck by the idea of less torsional rigidity. If there is inherently less tip/tail pressure on a wide rocker and twisting out begins in the forebody, then it would seem that that right there is problem #1 on ice (at least to me. Ski deformation is something carvers would want to avoid. Problem #2 is Jamt's point about the sloppier/weaker interface, through which skier input and stability must surely be lost. Also decreased (as noted above somewhere) is the ability for fore/aft leveraging. On Ice getting the turn started early in the high c is usually a good idea--and it seems on wider rockers is something that will be more difficult to accomplish, so the skier in question will have to work much harder to achieve the same radius of turn--even if the radius  between the two skis is the same.

 

   Disclaimer: My wide ski as a full cambered 98, so what I said is all conjecture.

 

    zenny

post #134 of 148

True Zenny, but I could argue that the torsional rigidity is more important in soft conditions. When the ski bends more, more pressure is distributed towards the ski ends. remember the edging experiment in the livingroom? It does not take much force to push through the camber if the surface is flat and hard. Major part of the ice hold is under foot.

post #135 of 148
Quote:
Originally Posted by Jamt View Post

It is very simple really:

 

 

Here you have the same inclination and turn radius, i.e. the same lateral forces. (assuming for simplicity that the skis have different radius)

The left picture (e.g. banking with excessive weight on the inside skis) you have a platform angle that makes the ski skid.

The right picture (angulating) you have an aggressive platform and the ski wants to "dig" into the snow. 

You can also look up "platform angle" in Ultimate skiing.

 

Ok just looked it up.  Chapter 2, pages 19-21.  Doesnt say anything about ski width being a factor here. 

 

But I did draw the vector diagrams as he did:

 

 

 

If I got that right, then the fatter ski, with the less "effective inclination" results in greater "pentrative force", and less "slip force"...ergo fat skis grip better then skinny, from a pure physics point of view.

 

Does that align with your reasoning?

 

 

EDIT:

 

This diagram is the same as above, but takes into account "reality" with our COM naturally inside our outside ski....makes the 90 critical edge angle easier to see, but the outcome doesnt change.

 


Edited by Skidude72 - 4/12/13 at 6:42am
post #136 of 148
Quote:
Originally Posted by Jamt View Post

True Zenny, but I could argue that the torsional rigidity is more important in soft conditions. When the ski bends more, more pressure is distributed towards the ski ends. remember the edging experiment in the livingroom? It does not take much force to push through the camber if the surface is flat and hard. Major part of the ice hold is under foot.

  Ahh yes, the "dead farm animal" experimentwink.gif! But at the same time, softer snow yields greater penetration--a larger effective platform (in regards to width of the ski, not platform angle), thus supporting the ski more along it's length, making torsional deformation less likely right?

 

   zenster

post #137 of 148
edit: my last post should have ended with "torsional deformation" NOT "longitudinal" deformation. sorry for the confusion smile.gif

zenny

fixed it for you -Rusty
post #138 of 148
I think we are more or less in agreement now Skidude.
post #139 of 148
Quote:
Originally Posted by Jamt View Post

I think we are more or less in agreement now Skidude.


Um.....this is definatly the single biggest development in my understanding fo skiing ever on Epic, or anywhere....for about as long as I can remember.

 

We should start a new thread on this for sure, as the practical applications of this are massive. 

 

It doesnt change what we do....but provides the physics behind a number of "conventional wisdom" ideas on great skiing.

 

I am floored!

 

THANKS JAMT!!!!!!!!!!!!!!!!!!!!

post #140 of 148
SD, did you decide that a fat ski holds an edge better than a narrow ski?
post #141 of 148
Quote:
Originally Posted by Metaphor_ View Post

SD, did you decide that a fat ski holds an edge better than a narrow ski?


The physics of it, yes.  But there is other caveats of torque, balance/speed which means you will not likley ever see ski racers opting for a fat ski over a narrow one....although having said that, it does make an interesting case that the narrowest ski, might not necessarily be the best...ie is 66mm better then 68mm?  But that is another discussion.

 

 

Having said that, the real break through for me, is understanding the "fact" that the greater the accuteness (ie less) the angle between the platform and the line of "effective inclination" the better your grip!  This is massive.   As I wrote before...it wont change anything...but it provides the "why" or the "physics" behind why we do, alot of what we do in skiing.

 

For example it explains:

 

  • Why it is so critical to establish the platform in phase two, and only tip in as required for balance (this is such a huge issue for so many advanced/expert level skiers)
  • Why rushing the turn and or banking loses grip
  • Why edging should be seen as a "spice" and only used as little as required...why more than is needed is BAD.
  • Why angulation will never go out of style
  • That getting low...actually does improve grip (where do we need grip the most in a turn? where are we lowest?)
  • Why balancing over the outside ski is much more effective and efficient
  • Why leaning into the hill on steep stuff sends you sliding

 

And I am sure there is more...this just all popped into my head.

 

 

 

No wonder Lemaster puts this at the start of Chapter 2 in his book.  I read it before....but I obviously hadnt picked up all the implications until now.

post #142 of 148
Quote:
Originally Posted by Skidude72 View Post

Having said that, the real break through for me, is understanding the "fact" that the greater the accuteness (ie less) the angle between the platform and the line of "effective inclination" the better your grip!  This is massive.   As I wrote before...it wont change anything...but it provides the "why" or the "physics" behind why we do, alot of what we do in skiing.

 

For example it explains:

 

  • Why it is so critical to establish the platform in phase two, and only tip in as required for balance (this is such a huge issue for so many advanced/expert level skiers)
  • Why edging should be seen as a "spice" and only used as little as required...why more than is needed is BAD.

 

That's interesting--it runs counter to what I got out of the CSCF-2 course: edge early, progressively work up to high edges throughout the turn, move the CoM inside the turn (using extension, toppling, whatever it takes), and use the edged platform to impulse the ski. There wasn't a single skier in the group using a "minimal" edge, and transition between edges lasted a split second whether in GS or SL courses. I can't even conceptualize of what too much edge would look like, other than throwing the racer out of the course... 

 

How would you reconcile the CSCF model with the above? (recognizing that racing employs different tactics from all mountain skiing)

post #143 of 148
Quote:
Originally Posted by Metaphor_ View Post

 

That's interesting--it runs counter to what I got out of the CSCF-2 course: edge early, progressively work up to high edges throughout the turn, move the CoM inside the turn (using extension, toppling, whatever it takes), and use the edged platform to impulse the ski. There wasn't a single skier in the group using a "minimal" edge, and transition between edges lasted a split second whether in GS or SL courses. I can't even conceptualize of what too much edge would look like, other than throwing the racer out of the course... 

 

How would you reconcile the CSCF model with the above? (recognizing that racing employs different tactics from all mountain skiing)


What the CSCF is saying is exactley what I am saying.  This idea being expressed above by JamT is the "why".

 

For example...why did your coaches say do it progressivley?  Why not just dump all the edge on at once?  It wont work for the reasons JamT explained. 

 

Dont get me wrong...when I say "minimal" I mean as little as is necessary...no more....no less.

 

Conventional race wisdom:

 

"Edges are slow."

 

"Dont think of edging like a light bulb "on/off" switch...think of it like a "dimmer" switch"

 

You might hear good racers say: "I was too hard on my edges" when critiquing a poor run.

 

 

Above is the why. 

 

 

 

What does too much edge look like?  At worst it will look like the skis breaking away from under the skier, sending him or her sliding away on their hip.  At best, its "banking". 

post #144 of 148
Quote:
Originally Posted by Skidude72 View Post


What the CSCF is saying is exactley what I am saying.  This idea being expressed above by JamT is the "why".

 

For example...why did your coaches say do it progressivley?  Why not just dump all the edge on at once?  It wont work for the reasons JamT explained. 

 

Dont get me wrong...when I say "minimal" I mean as little as is necessary...no more....no less.

 

Ahh, I did get you wrong, and now I've got you right--skiers need to understand what minimal means. I think it's worth identifying a continuum of edging where "enough edge" is fast, a high edge is slower, a skid is far slower, and braking is slowest. (Truth be told, a clean high edge on a Whistler blue on a GS ski will still feel rocketly fast for a recreational skier--it's all relative). The trick then is to create minimal edge without so little that you're unable to get the turn radius you want. Throw pivoting in the mix and then it starts to get really complex to blend it all together for a rec skier...

 

When you say "the greater the accuteness (ie less) the angle between the platform and the line of "effective inclination" the better your grip!" - it may explain why most recreational skiers can comfortably ski ice on a green, but lose grip when facing ice on a blue (where higher edge angles are basically automatic due to the slope). Perhaps more side edge angle helps compensate for less acute angle (i.e. more tipping) between snow and ski. 

post #145 of 148
Quote:
Originally Posted by Metaphor_ View Post

 

-skiers need to understand what minimal means. I think it's worth identifying a continuum of edging where "enough edge" is fast, a high edge is slower, a skid is far slower, and braking is slowest.

Exacltey...too many people think "more is always better"....this is not true.  Too much, is as bad as too little....generally speaking.

post #146 of 148
skidude and jamt (and metaphor) the more i consider this the moe i cant wait for a new thread on this... fascinating! of course the "which ski stores more energy" may rear its head...biggrin.gif lol!
zenny
post #147 of 148
I knowz which ski haz more spritz: the left, always the left!!! Dont ask what experimentz I have done to prove that!! Hey, ma, where's my beer? And where's them katz at?

About the OP's issues - they are two big ones that I can see: stance is too wide and he rotates like that's how it's done.

So 1) exagerate the opposite and ski with the skis close together and get a feel for what it means to be INSIDE that arc. and 2) rotate against the skis, nit with the skis. I.e. counter-rotate.
post #148 of 148
Quote:
Originally Posted by Jamt View Post
 

First let me just make clear that I am not trying to say that fat skis are better for ice grip, racing or whatever. I do however have a different view of why this is the case than what I think is the conventional belief. I made a picture to illustrate:

 

This is a simplified front view of a skier.

In picture 1 we have wide skis and an angulated skier, both knee and hip.

The force line from the edge passes through the CoM, which means the skier is in balance at this moment in time.

The distance between this force line and the interfaces is proportional to the moment the respective interface has to carry. The interfaces are instantiated by bindings, knee, boot etc.

In picture 2 everything is the same except that the ski is narrow.

In picture 2 the skier is no longer in balance, so the skier would have to incline LESS and use a somewhat smaller edge angle, as illustrated in picture 3. Smaller edge angle means less grip and less turning. This is one factor that makes wider skis have good ice grip, you can edge MORE!

If we compare the interface moments we see that in picture 1 the boot-binding moment is larger, but the knee joint carries LESS moment. If you are banking your turns this is not true, but when you are angulating the knee wants to track to the inside and wide skis reduce the moment. If you are experiencing excessive knee moment on wide skis all you have to do is angulate more. Find the angulation where the force line passes through the knee plane for zero moment.

This is all in theory, in picture one the moment around the boot-binding interface is much larger, and thus the give will be larger.

What I am claiming is that THIS is the major cause of bad grip with wider skis, it is not the ski itself. In particular since wide skis usually have bad bindings. Theoretically, with ideal material properties wide skis do not have bad ice grip. In practice they do have worse grip, but with good equipment (Plates, 20 DIN race bindings and good ski construction) the difference is not that big. I have not shown it in the figures, but it should be clear that binding plates will also reduce the bending moment in the boot-binding interface. (That is one reason why race bindings on fats make a big difference)

 

In general I'd say that the binding is the most undervalued component in the equipment. It is really important. I've seen bindings on rental skis that give several degrees with very little force. There is no way you get any good performance out of a ski with that kind of binding.

 

There are also other drawbacks with wider skis. You have to retract more, they are slower up on edge, the balancing is more difficult, they are not as torsionally stiff etc.

 

Edit: Banking, all moments are smaller with narrow skis.

 

 

Great post Jamt, I learned a lot from it! Normally, in a smaller world should come to your door with many beers, but considering the situation, I salute you!

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