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Sidecut parabole or round

post #1 of 20
Thread Starter 

Do carving skis very in the shape (dont mean size) of the side cut i.e., would some form a parabolic arc while others form the shape of part of a circle & if so what are the advantages & disadvantages of each. Would a parababolic shape make more of a spiraled shaped turn while a ski with a round curve make more eliptical or round turn? Thanks

post #2 of 20

Yes.

 

It's complicated. The basics are: a ski, in use, behaves in three dimensions not two. The ski is tipped at an angle and deformed (bent) during a turn, so a simple section of a circle is a bit too 'simple' to really work well. In fact the first really popular shaped ski was the Elan SCX which used a parabolic arc for the sidecut shape... the term 'Parabolic' became a generic term for 'exaggerated sidecut' for a while, much like xerox in the photo copy world.

 

There are tons of factors that effect the way a ski turns, sidecut is just one of many factors at play, all of which need to be looked at as a system to get a feel for the skis performance. Sorry if that seems like a vague answer, but it's a realistic one.

post #3 of 20

Although some manufacturers experimented with noncircular sidecuts (and some people still refer to shaped skis as "parabolics"), the standard nowadays is constant-radius (circular) sidecut. 

 

The only significant difference I've seen is the Fischer Progressor, which has a 'dual-radius' sidecut where the front is ~18m and the back is ~14m -- but each is a constant radius, not progressive.  I'm not sure about some of those reverse-sidecut boards, though.

 

I think a true parabolic sidecut would tend to rush you through the beginning and end of the turn and extend the middle.  Probably not ideal for most people.

post #4 of 20
Thread Starter 

Thanks guys

 

 If the sidecuts are circular then 2 differnt ski models or brands that are the same length should they not ever have only 2 of the sidecut measurements being the same. If 2 side cuts measurements are the same & the 3rd one not in the same length of ski. Would it not mean the binding is not being mounted in the ideal location for both or one of the skis ? Wondering if this happens often. Of course one would have to measure ski length same way.

 

          

post #5 of 20

Yikes...think about this, guys. Regardless of the shape of a ski's sidecut, do you really think it will "want" to carve a different arc in different parts of the turn, based on its sidecut shape? The ski won't care (or know) what part of a turn it's in. The only thing it will respond to is pressure and edge angle. If you tip it or pressure it to different degrees, or in different places (ie. forward, center, back), it will affect the carving radius, but that's up to you, not the ski.

 

As the Elan designers explained it to me, the point of the old "parabolic" sidecut was not to change the turn shape, but to make the ski bend into a "perfect" circular arc on the snow with controlled pressure tip to tail when tipped and pressured on the sweet spot. It was thought that a truly circular sidecut would not bend into the ideal shape. I don't know how much actual testing there was for these ideas.

 

On the other hand, preceding the parabolic sidecuts, does anyone remember the Elan "cycloid" sidecuts? These were, indeed, marketed to create a cycloid-shaped arc--theoretically the fastest line around a gate. (Cycloid is the fastest shape for, for example, an owl's path as it swoops off a tree branch to capture a mouse on the ground--straighter at first, then radius increasingly tightening.) Those skis had a tighter radius sidecut in the tip, becoming increasingly straight toward the tail. It was a short-lived experiment due, I suspect, to the fact that once again, the ski couldn't care less which part of a turn it's in.

 

Best regards,

Bob

post #6 of 20

 

Quote:
Yikes...think about this, guys. Regardless of the shape of a ski's sidecut, do you really think it will "want" to carve a different arc in different parts of the turn, based on its sidecut shape? The ski won't care (or know) what part of a turn it's in. The only thing it will respond to is pressure and edge angle. If you tip it or pressure it to different degrees, or in different places (ie. forward, center, back), it will affect the carving radius, but that's up to you, not the ski.

 

Maybe I'm not thinking about the physics of this the right way, but if the ski has a non-constant radius (particularly the front of it, due to momentum), wouldn't that tend to force the ski to follow a non-circular path when it's riding the sidecut?  Obviously you can 'override' that as you see fit by adjusting the edge angle dynamically and/or skidding, but if the edge is engaged, wouldn't it tend to carve along the shape of the sidecut?

post #7 of 20
Quote:
Originally Posted by Matthias99 View Post

 

  Obviously you can 'override' that as you see fit by adjusting the edge angle dynamically and/or skidding, but if the edge is engaged, wouldn't it tend to carve along the shape of the sidecut?


No.  

 

It would tend to carve along the shape of the flex, and the sidecut is there to help the skier achieve that flex given a certain set of input pressures and input pressure distributions.
 

 

Imagine a ski  sidecut that was a perfect single-radius projection except for one spot that is slightly wider than that projection requires.    The ski would still ride approximately the same arc but would  prefer to flex more in that one spot.     We have therefore, in that one spot,  reduced the pressure required for the skier to properly bend the ski.

 

If you apply that principle to the ski  in more spots than just one, you will begin to see that sidecut is just a *tool*  designed to achieve a ski bend of particular shape.

post #8 of 20

 

Quote:
It would tend to carve along the shape of the flex, and the sidecut is there to help the skier achieve that flex given a certain set of input pressures and input pressure distributions.

 

Yeah, but isn't the "shape of the flex" determined (or at least influenced) by the sidecut when the snow surface is hard enough?  Wouldn't a parabolic sidecut give a progressively deeper flex for the same amount of edge angle, forcing a progressively tighter turn compared to a circular sidecut?

 

I must not be visualizing it right.  I might have to play around with some cardboard cutouts or something later...

post #9 of 20


 

Quote:
Originally Posted by Matthias99 View Post

 

 

Yeah, but isn't the "shape of the flex" determined (or at least influenced) by the sidecut when the snow surface is hard enough?

 

"Determined", no.   

 

"Influenced by", to a point, yes.    The two are distinct enough, and so many factors determine the shape of the flex,  that we can reliably say the ski is /not/ riding the shape of the sidecut.   

 

  Wouldn't a parabolic sidecut give a progressively deeper flex for the same amount of edge angle, forcing a progressively tighter turn compared to a circular sidecut?

 

 

Why would it?    

 

Given a parabolic sidecut and a true-circular sidecut with the same sidecut depth and the same chord length ,  the parabolic ski would just tend to flex less at the ends, given the same edging pressure.  

 

In other words, lets say you're hand-flexing two skis in the shop.   Same sidecut depth, same chord length, same flex as far as your hand feels.   The tips and tails of the parabolic one would feel "stiffer" or "less sensitive" on snow.

 


Edited by comprex - 3/29/11 at 11:59am
post #10 of 20
If variations in sidecut radius mattered a great deal to the actual turn radius then how might these skis operate on a firm surface? smile.gif

.ma
post #11 of 20

.ma, that's just cruel, poor M99. biggrin.gif

post #12 of 20

Quote:

Originally Posted by Matthias99 View Post

 

 

if the ski has a non-constant radius (particularly the front of it, due to momentum), wouldn't that tend to force the ski to follow a non-circular path when it's riding the sidecut?


Matt--consider that, whatever shape the ski's sidecut may be, it's the same shape at every point in the turn. 

 

For a ski with a sidecut like the old Elan Cycloids that I described above (tighter radius at the tip, increasingly straight at the tail), you could manipulate the turn radius somewhat by levering (pressuring) the front, mid-section, or tail of the ski at different parts of the turn. But again, the sidecut is the same throughout the turn, so all else being equal, the radius it carves will be the same throughout the turn, resulting in a round turn shape. 

 

Best regards,

Bob

 

 

post #13 of 20

I thought that many (more than just 1 or 2)  skis currently in circulation had dual radius sidecut.

post #14 of 20
Quote:
Originally Posted by tromano View Post

I thought that many (more than just 1 or 2)  skis currently in circulation had dual radius sidecut.



Quite a few Line skis have multi-radius sidecuts. For example, the P-100 had something like four or five (sharp radius at the tip, medium below that, long under foot, medium again, and maybe something different just at the tail). Their site used to show diagrams of their sidecuts, but now it just says the ski has P-cut or G-cut shape.

post #15 of 20

Maybe for race skis the shape of the sidecut may have some noticeable effect, but if a race ski maker had a shape he knew was faster he wouldn't tell you what it was.  For skis designed for anything except race courses, it's hard to believe that the shape (as opposed to the average overall radius) is critical.  There's just not enough continuous edge engagement over the length of the ski in soft or variable conditions to make that a critical variable.

That's why the new half-rocker skis work.  You put a short stiff section in the middle and it skis like a short ski in hard snow because the long ends of the ski don't engage much, but the stiff section doesn't cause much problem in soft snow either.

 

BK

 

 

post #16 of 20
Thread Starter 

The sidecut shape I think would make a difference. If the ski is bent more then just putting it on edge & letting it bend because of the sidecut by for aft presure along the ski edge. If the tail of the ski is flaired out @ the end the leverage it creates takes a lot of time & energy to over come by bending the tail more @ the end of the turn. I think this is one of the reasons shape skis are so slow from edge to edge & not the best tool for lightening quick turns in the bumps. Shaped skis are a great tool for larger turns racers make.

post #17 of 20
Quote:
Originally Posted by Bob Barnes View Post

Matt--consider that, whatever shape the ski's sidecut may be, it's the same shape at every point in the turn. 

 


Hmmm. How can that be, given that we're not exerting equal force throughout the turn? Can't; our F and a are changing. (OK, it's angular, but keep things simple). So IMO the sidecut's shape is constantly being redefined. Imagine a slice at an angle through a cone. Now imagine that you squash the cone slightly, but keep changing the pressure. The real edge path in space will always be 3D, because the ski is always moving in both planes, and the edge itself is neither a semi-circle nor a parabola, but something weirder and less constant. Because the section of the deformed cone that describes the shape of the edge is changing as our pressure changes the deformation in a non linear manner (for instance, thicker parts of the ski will deform less, our CM is changing through the turn, our F is changing, the ski is actually undulating in waves). I suspect what the makers' CAD's do is actually predict the shape as the ski is loaded and in motion, then they fine tune with testers. Whiteroom's description is actually the closest here...

 

Oh, and for the record, there are a bunch of dual radius designs besides Fischer: Kastle, Scott, 4FRNT come to mind also. But I doubt they're as simple as two arcs off circles spliced together, so the name is misleading.

post #18 of 20
I believe that dual radius/sidecut skis have a different radius in the tip section than the tail section, presuming that at turn initiation the skier is more fore-biased, and is more heavily pressuring the tip section of the ski so that radius has more effect on the overall turn shape. Then, after the middle of the turn the skier becomes more aft-biased, and is more heavily pressuring the tail section of the ski so that radius has more effect on the overall turn shape. Tail stiffness is also used to "spring" the skier forward into the next turn. The difference between fore-biased and aft-biased is a subtle positional/balance change within the sweet spot used to affect/regulate pressure.
post #19 of 20

 

Quote:
Hmmm. How can that be, given that we're not exerting equal force throughout the turn?

 

Even taking that into account, I'm still not entirely convinced that a ski has to bend into a perfectly circular arc, even if it bends the same way throughout the turn.

 

I would, however, buy the argument that with practical geometry and construction (stiffness, etc.), a 'parabolic' sidecut ski probably does bend into something very close to a purely circular arc when tipped and pressured.

post #20 of 20
Quote:
Originally Posted by beyond View Post




Hmmm. How can that be, given that we're not exerting equal force throughout the turn? Can't; our F and a are changing. (OK, it's angular, but keep things simple). So IMO the sidecut's shape is constantly being redefined. 


Hi Beyond--you are, of course, right that the forces that affect the shape a ski bends into change constantly throughout turns, and from turn to turn. But sidecut does not refer to the shape the ski bends into. Sidecut is a design parameter, and as such does not change. A ski's sidecut is the same at every point in a turn, meaning that any change in the turn's shape (or the ski's reverse camber bent shape) from beginning to end is due to other factors, some of which you have described. There is no doubt that a skilled skier can shape a turn any way he or she wants, and can exploit sidecut and other ski design parameters in the process. But again, the ski itself is the same ski throughout the turn. 

 

A ski with a "creatively shaped" sidecut may or may not carve a clean turn (ie., bend into a round arc). And clearly, it would be possible to design a ski with non-round sidecut as a feature that you could exploit with various movements and technical finessing (particularly, pressuring different parts of the ski to engage portions with different sidecut). But as I've said, all else being equal, the ski does not know or care--or respond differently to--which part of the turn it's in.

 

Best regards,

Bob

 

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