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Open Questions to Bob Barnes and Todd Murchison

post #1 of 29
Thread Starter 
Hi guys,
I'm an instructor evaluating some deficiencies in my teaching/skiing knowledge.

How would you explain, to a student, why the main point of foot pressure changes from toe to "arch" to heel thoughout the arc of a purely carved turn (or from front to back foot for boarders)?

Why do my Elan monoblock scx parabolics J-hook the completion phase of a purely carved turn? Is this true of most parabolics? This does not happen when I am carving on radially-cut shapes.

I've been reading ski reviews that only give a ski's tip, waist and tail dimensions and mention nothing of its carve radius. Is there a formula to do this that would also tell if the ski is parabolic or radially cut? The last part may be impossible I realize.

Your help is appreciated.
post #2 of 29
welcome Doolio.
You found a great place. I can't answer that one (not an instructor so don't have the experience) but I do "think" I have an idea what is going on. I'll leave it up to the better technicians to explain it. I'm interested in learning from your question as well..
post #3 of 29
Just wanted to say Hi and welcome! I'll let the experts talk!
post #4 of 29
Hello Doolio, thats an excellent question - and the subjects touched on there are often debated among coaches/instructors. I think that you'll find good input from many other members than just Bob and I on this. Here are my initial thoughts on this:

>>How would you explain, to a student, why the main point of foot pressure changes from toe to "arch" to heel thoughout the arc of a purely carved turn (or from front to back foot for boarders)?<<

First I would say that in my experience its not a given that there *is* always a pressure change. You can stand neutrally (in the center) of the modern skis and carve a clean turn. However, to change the radius/shape of a turn you can choose to change where you are weighting your ski. Now there is a difference between simply shifting the weight under your foot - and pushing forward or pulling back against the boot cuff/shaft.

Weight distribution changes only controlled by direct pressure under the foot do make changes in turn shape, but they are more subtle than changes made by *leveraging* the tip or tail of a ski through use of the boot shaft.

It definately was a given on the older skis, unless you wanted to make a very long radius arc. To make shorter arcs one generally had to use selective leveraging of the ski through the turn.

Racing on the previous generation of skis I had my body well trained to initiate a turn on the tip of the ski(s), control the turn in the center, and finish on the tail. Then move your "center" (weight) aggresively forwards back to the tips of the skis. I found that for me it was most effective to think of "pushing" my feet forward through the whole turn - which resulted in a smooth rolling of the weight across the ski, and then to start a new turn 'falling' back diagonally across the fall-line to cross over to the other side of the skis and simultainously get my weight back on the tips of the skis to start the new turn.

When you watch top racers and free-skiers today you'll see that they spend a great deal of time in a more neutral position on the skis - unless they need to make an adjustment to their line. When we do need to make this adjustment we can shift our weight fore or aft or for more extreme changes leverage the boot forward or back. The new skis are much more responsive to this weight shift as well.

When I first tried the early prototypes of shaped skis - 10 years ago or so now, I remember that the skis seemed so reactive that they scared the hell out of me! Thats because I was still habituated to using a powerful leveraging to create the turn shape I wanted in a carved turn. The skis say "alright mofo' --- you want to turn that sharp . . . here!" It would catch me off guard and I couldn't keep my center up with the skis, and was constantly getting left behind.

Anyways - sorry about the long tech stuff, looking back your question was actually how to explain it to a student. I think that you obviously don't need to go into the kind of detail I just did, and it would in fact be counterproductive (unless they ask for such detail - which some advanced students will). For me its worked best to have them experiment with the extremes of weight changing and leveraging. I.E. -- doing turns where they are totally neutral. Turns where they are all the way forward for the entire turn, all the way back for the entire turn - and then mixing it up in turns. Their bodies will figure out the result of weight distribution changes much more quickly than most folks will be able to apply it with a technical explaination.

>>Why do my Elan monoblock scx parabolics J-hook the completion phase of a purely carved turn? Is this true of most parabolics? This does not happen when I am carving on radially-cut shapes.<<

You, like me, probably cut your ski teeth on skis with much less sidecut. So you have trained your body/mind to weight and leverage with intensity and timing optimized for that equipment. I definately had to learn to be much more subtle on my skis when I moved to extreme shapes.

>>I've been reading ski reviews that only give a ski's tip, waist and tail dimensions and mention nothing of its carve radius. Is there a formula to do this that would also tell if the ski is parabolic or radially cut? The last part may be impossible I realize.<<

You will usually find the turn radius if you search hard enough - perhaps on a review site/magazine if the manufacturer isn't advertising it. It is generally referred to as an __X__ meter radius sidecut or something similar. For example some of the more radical sidcut skis can pure carve a round turn as low as 8-9 meters radius. And of course if one only needs to carve part of the turn (like you'll in many of the turns in SL racing) the turn radius can be even shorter.

Good subject, I'll look forward to hearing what the other Bears have to say on this. Thanks for helping me get my brain warmed up for ski season!
post #5 of 29
thanks Doolio for a great question. and thanks Todd for the first shot.
Todd, your assessment confirms my thoughts about switching from straight to Shaped skis. I probably moved more gradually than most. I went from Dynamic vr17 FM GS to a vr27 SL then to salomon equip e9000 SL 3s then prolink equip 3s and now X Scream series. (I know not really shaped but much more so than my prolinks equips)

I'm still working on that "J" shaped turn. and Doolio, I am one of the students that would want the indepth explaination but I am probably not the typical student.

Great opening question... I look forward to more.<FONT size="1">

[This message has been edited by dchan (edited September 17, 2001).]</FONT>
post #6 of 29
> I've been reading ski reviews that only
> give a ski's tip, waist and tail
> dimensions and mention nothing of its
> carve radius. Is there a formula to do
> this that would also tell if the ski is
> parabolic or radially cut? The last part
> may be impossible I realize.

If you know for sure that the shape of the sidecut of a given ski is actually part of a circle, then there is a formula that fits a curve of circular shape through the 3 points you mentioned (the tip, mid and tail widths). The sidecut radius of curvature falls out of this formula naturally.

OTOH, if you know the ski is parabolic, there is a different formula that applies. The same is true for elliptical and hyperbolic shapes, with distinct formulas for each of them as well. However, only for the small set of shapes that I just mentioned do formulas involving just three measurements exist. That is because all of these curves are a member of a class called "second order curves". For more complicated shapes, (eg, 3rd and higher order curves), more than 3 measurements are needed (even assuming that the class of curve is know ahead of time).

With just three measurements, you simply can not distinguish between the various 2nd order curves. Fortunately, this is usually not a big deal because the variation in sidecut radius within this family of curves (for a given set of tip-mid-tail measurments) is not all that large, so you can just apply the formula for a circle and be pretty close.

One final factoid that might be of interest. If you happen to know just one more parameter, namely, the exact fore-aft position of the ski waist, with just the tip, mid and tail width information, since the edge is parallel to the ski centerline at the waist, you actually can fit separate second order curves to the forebody and aft sections of the ski. When this is done, and it is indeed rare, people usually approximate these two separate curves as parts of two different circles, and obtain distinct radii of curvature for the forebody and aft parts of the ski.

The formula for the simplest case (ie, assuming the sidecut is one continuous circular arc), and has been reproduced many times. A good reference book on such matters is: "The Physics of Skiing". I suspect that the same formula is probably also reproduced in Bob Barnes encyclopedia (unfortunately, I don't have a copy in front of me), in various threads in the archive of epicski.com, and on several websites, for example:
http://www.epic-ski.com/ubb/Forum1/HTML/000734.html
http://www.math.utah.edu/~eyre/rsbfaq/physics.html


Hope this helps,

Tom / PM


PS - Let me add my welcome to everyone else's.
post #7 of 29
On the subject of "shaped skis" I could be wrong but I believe only one company really makes a "Parabolic ski" and that is Elan? I think the rest are shaped but not Parabolic.
post #8 of 29
Are the Elan skis asymetric? I remember one series having varying sidecut so that you had a true right and left ski. If so, could that asymetry lead to hooking?
post #9 of 29
Yes - only Elan actually uses a Parabola shaped sidecut. Thats not saying they are any better (or worse) than the other manufacturers super sidecut skis - they just have trademark on that specific shape.

I remember Fisher doing some assymetrical skis, and The Ski did some as well. It never caught on and I don't think any are being manufactured anymore.

Oh - and Evolution made some as well.<FONT size="1">

[This message has been edited by Todd Murchison (edited September 18, 2001).]</FONT>
post #10 of 29
Todd,

I liked your response. It sure brought back
some memories. I, too, initially had a hard
time liking shaped skiis (10 or so years ago) because I did just what you described --overskied them. Movements today are much more centered and subtle on these wonderful
new boards.
post #11 of 29
Thread Starter 
Hi everyone,

Thank you for the nice welcome.

Todd, thank you for the detailed response. I'm an info-maniac and you can never give me too much technical information . I eat it up.

On the fore-aft foot pressure, you cleared that up for me. I believe that my movement is very subtle during my normal skiing. But now I plan on getting back on my Volkl P20s, as well as my shapes, and experimenting with leveraging.

As for my J-hook sensation, I believe there may be something else going on with the Elans because it doesn't happen with my Head Cybers. How about this? The resultant forces on the skis are greater after the fall-line than they are before it. The increased load bends the skis *slightly* more. But due to the extreme sidecut of the Elans, this slight difference in turn shape is more noticeable.

I'm not positive, but that is the best I can come up with. I didn't want to say that upfront so as not to "influence the audience" prior to their answers.


Thank you all for responding.
post #12 of 29
There are fine skiers who continue to move their center fore and aft throughout each turn. In the guys & gals standing on the podium however I think you'll most often see that this movement is reactive instead of proactive. When they maintain their planned line they tend to very accurately stay over the center of their skis, when they have to make an adjustment to their line (recovering from a tactical or physical error generally) they will move forwards or back on their skis. The exception has been generally in SL where starting every turn by leveraging the tips has been necessary - but now with the new generation of SL skis coming out, even SL racers are starting to stand more centered throughout the turn.
post #13 of 29
Good responses all. If it is truly a classic "J" turn, the tails are breaking away, indicating excessive forward leverage. But with the SCX, tipping with too forward a stance this is ecaserbated due to the radical tip width. The typical issue with the earlier SCXs was tail-hook...trying to get out of the turn. Most skis since have more tapered tails.
Welcome to the forum!
post #14 of 29
> Doolio: "...As for my J-hook sensation, I
> believe there may be something else going
> on with the Elans because it doesn't
> happen with my Head Cybers. How about
> this?

If I remember correctly the dimensions of the SCX were something like 112-60-110. This translates into a 52 mm tip minus waist sidecut, and a 50 mm tail minus waist sidecut. These are both huge numbers compared to current skis.

There might be the odd exception, but I don't think there is a single ski sold any more with a tail minus waist sidecut this large. Most are in the mid-30's to mid-20's, with the trend towards less aft sidecut.

I don't know which Head Cyber you have, but even the oddball (tail wider than tip) Solomon "Equipe 10 2v" (97/66/100) has a tail minus waist width difference of only 34 mm.

Such a deep aft sidecut on your SCX's could easily contribute to the sensation you described.

Tom / PM


PS - Hey Robin - It must be a "great minds think alike" thing. As I was composing this message, originally time stamped 8:38 AM, you must have been posting your message at 8:34 AM, both of us writing about the SCX's tail hook phenomena.
<FONT size="1">

[This message has been edited by PhysicsMan (edited September 19, 2001).]</FONT>
post #15 of 29
yuki,

>>"Are the Elan skis asymetric? I remember one series having varying sidecut so that you had a true right and left ski"<<

The only ski I remember being asymmetric off the top of my head was a Dynastar. Except for the Norweigian Nordic team racing on tele skis with sidecut on the inside edges only. I can't profess to having too much knowledge about Elan SCXs, though I skied several pairs of the various SCX models. I don't remember any of them being asymmetrical.


Doolio,
I didn't take the time to thoroughly read every post in here, but did anyone mention the tip-waist-tail width issue? In my memory the SCX has similar tail/tip widths. Even Elan quickly began making skis with a signficantly narrower tail which is significantly more user friendly.

What follows are two changes I had to make in transitining from more traditional skis to super-sidecuts:

Turn transitions take longer. The added width of the widepoints take a little longer to go through edge-change in relation to narrower skis, adjusting timing is necessary to get a smooth, clean start to turns. Balancing over the length of the ski as opposed to laterally helps here.

Hooking was a problem for me too. Mostly because I was so used to, like Todd describes, levering the front of the boots to get tip presure, something not needed on the super shapes. Staying in the middle, keeping contact with, but not levering against the front of the boots helped a lot.

I used to have problen with hooking even before supershapes, seems I was so into lateral movements that I would get too far inside (off balance)to control pressure, so it would all hit the skis at the bottoms of turns, crushing the skis into a suddenly deeper bend and fishhooking the majority of my turns. Learning to let that lateral angle develop from ski reaction instead of trying to force it helped.
post #16 of 29
I have a pair of SCX-RPs that I've skied the bottoms off. The tail is less than the tip but I don't know how much. The HCX is 100mm tip and tail and doesn't seem to hook.
My thought would be, think "seamless" turns. As one turn is ending the next has already begun. Energy flows smoothly from the downhill foot to the uphill(not necessarily all of it). It sounds like you may be hanging onto the old turn too long.("release")
post #17 of 29
Physicsman - I've got several pairs of various models of SCX's . . . and don't encounter that sensation on any of them.
post #18 of 29
Slatz - re your HCX's: I don't think they are 100 mm tip and tail. PeterKeelty.com lists them as 110/62/100, so the tail minus waist difference is 38 mm, considerably less than the 50 mm difference of the SCX (112/60/110).

BTW, there is one minor terminology issue. I'm trying to keep myself from calling the tail minus waist numerical difference "the aft sidecut" because the former really needs to be divided by 2 to get the latter.
---------

Todd - re your statement, "I never saw the effect" versus Robin's statement, "The typical issue with the earlier SCXs was tail-hook" and Roto's, "Even Elan quickly began making skis with a signficantly narrower tail which is significantly more user friendly":

Hummm ... I guess one possibility is that the deep aft sidecut could magnify minor errors in technique. From what he said, I think its probably safe to assume that you are a better technical skier than Doolio (and certainly most amateurs), hence the difference in experience.


Tom / PM

<FONT size="1">

[This message has been edited by PhysicsMan (edited September 20, 2001).]</FONT>
post #19 of 29
Thread Starter 
Ahhh..

I think you might have it Tom/PM, "that the deep aft sidecut could magnify minor errors in technique," as well as Todd being a better technical skier.

I am going to experiment this season by getting well-centered on a pair of skiboards and then ride my SCXs to see if the problem persists.

Bob, I didn't think you were ignoring me. I understand how people get busy. We all have to keep some semblance of order in our lives so that there is time and money for skiing. Thank you for the welcome.
post #20 of 29
Doolio - Another possibility is that since you are an instructor and the SCX's are probably at least a couple of years old, I'm guessing that you probably have logged a lot of miles on these skis.

Could it simply be that your tails have gotten soft (relative to the foreski) with heavy use?

Its been a long time since I last skied a pair of broken-down boards with the tails softer than the forebody, so I really don't remember how they felt, but, I suspect that all else being equal, soft tails coupled with a firm forebody could make skis more prone to hooking. Comments from anybody with more recent relevant experience?

Tom / PM


PS - FWIW, I also own s pair of old, extremely stiff, red SCX monoblocks (193 cm) that have not seen all that much use.

I'm certainly not an expert skier, but I don't have any problem with the ends of my turns on the SCX's J-hooking either. OTOH, I have noticed (like FamilyManSkier in another thread) that these skis aren't exactly ideal for skidding and other old-school techniques. My observation may be related to yours.

Tom
<FONT size="1">

[This message has been edited by PhysicsMan (edited September 20, 2001).]</FONT>
post #21 of 29
PhysicsMan
Checkout www.elanskis.com. The HCX is listed as 100-60-100. It was the same last year. I broke mine and sent them in for warranty so I can't put a caliper on them. I will measure the SCX-RPs and get back to you.
post #22 of 29
Thread Starter 
PhysicsMan,

I remember them doing this pretty much right out of the box. But I am a heavier, linebacker sort (215#) and they have always seemed a little flimsy. So this still could be another possibility.

Doolio/Patrick
post #23 of 29
Slatz: Thanks for the info. I double checked on Keelty's site,

http://www.peterkeelty.com/elanold.htm ,

and he definitely shows last year's HCX as 110/62/100, so somebody clearly typed in a number incorrectly, and I'll bet its not the mfgr - grin. If you measure them and Peter is indeed wrong, you probably should drop him an email (and let me know as well).

In any case, my argument about the tail minus waist width difference is still a possibility - 40 mm is still a good bit less aft sidecut than the 50 mm of the old SCX's.

----------------

Doolio: Given what you said about them being flimsy right out of the box, there were many, many variants of the SCX marketed over the years. The ones that I have are probably the stiffest skis in my quiver - right up there with my Stocklis and old straight skis, but Elan also made some very soft versions of the SCX. A partial list of the different versions of the SCX can be found at:
http://www.hyperski.com/articles/may...parabolics.htm

If your pair is one of the softer models, your 215 lbs adding in the same direction as centrifugal force at the end of turns could really put them into a deep flex and really shorten the turn radius. How do they feel when you flex them by hand, say, as compared to your Head Cybers? They might simply be too soft for your weight.


Tom / PM

<FONT size="1">

[This message has been edited by PhysicsMan (edited September 21, 2001).]</FONT>
post #24 of 29
Thread Starter 
Hi PhysicsMan,

My Elans have a red topskin and are not one of the softer models. I did find something in that review that confirms what I'm feeling:

"carving a turn with an angle greater than 360 degrees (not quite a circle, though; the arc got tighter as I went around so I ended up carving a spiral)."

I have noticed this too. It is probably a better explanation than I initially gave, calling it a "J-hook."

Doolio
post #25 of 29
Doolio
Your SCX don't have enough taper angle so the trail hooks up more at the end of the turn. Some of the older shaped skis were designed like this, with tips and tails nearly the same width. A lot of racers didn't like the early shape skis because they tended to weight the tail of the ski at the end of the turn and a ski with low taper angle doesn't feel like it will release from the turn.

Nord
post #26 of 29
I did get that feeling sometime on the HCXs in the bumps.
When carving a circle the momentum carries better if you don't tighten the arc. Besides it looks cool when the tracks run back into themselves.<FONT size="1">

[This message has been edited by SLATZ (edited September 21, 2001).]</FONT>
post #27 of 29
Hi All
Forgive me for researching through old threads. 
The theme coming out of this seems to be that to ski circular arcs, you need a radial side-cut ski and to ski parabolas, you need parabolic cut skis.
Does anyone still believe this?
D
post #28 of 29
Some people still believe the earth is flat, even though educated folk knew it was roughly a sphere and even calculated it's diameter thousands of years ago.
post #29 of 29
Davey,

I just read the entire thread and didn't get that theme-impression at all.  It seems to be a discussion of old vs. 'new' (circa 2001) ski design and the tendency to over-work the ski.

Didn't see any references to 'circular paths' of carving vs other paths.  They did talk about different sidecuts being circular vs. parabolic, but nowhere did anyone suggest any sidecut relationship with the shape of actual carved outcomes.

.ma
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