Ski radius: what does it really imply ?

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Ski radius: what does it really imply ?

Postby tommy » Sun Mar 07, 2004 12:14 pm

In many conversations with fellow skiers, I've often heard the phrase "the built in *turn* radius of my skis is x meters". I've always been somewhat puzzled by the implicit message contained here that somehow the ski would have a "default", built-in *turn* radius that corresponds to the *shape* radius given in the specs...?

To my understanding, the given radius for a ski is a measure of the sidecut, i.e. the theoretical arc that the ski manufacturer used to "cut" the ski, but it doesn't, at least directly, correspond to the radii of the turns that you can make with the ski.

As far as I can understand, the way a ski (shaped or not) does a carved turn is by bending; that is, if you want the ski to turn without skidding, you will have to bend it. The sharper a turn you want to make, the more you will have to bend the ski. Even if you have a radically shaped modern ski, with really small "factory" radius, and put it 90 degrees on edge against the snow, if it's completely flat, it will go straight (with maybe minor steering caused by the tip).

So, how come a modern shaped ski is easier to do a carved small radius turn on, than older, straight skis ? My guess is that what the "shapedness" really does, is to allow the ski to be bent with much less force than a straight ski: since a radically shaped ski, when put onto edge, seems to "ride" more on the wide tip & tail, with the waist having less snow pressure against it, it's easier to bend into an arc. The more shape, the more the waist is "off" the snow, and the easier it is to bend the ski to a smaller arc (everything else, e.g ski stiffness, being equal).

Cheers,
Tommy
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selecting skis...

Postby tommy » Mon Mar 08, 2004 2:39 am

Having spent some more thought on the topic of how a ski carves a turn, I came to think about how to choose a ski that matches one's ability:

If it in fact is the amount of bending that defines the turn radius, and that bending is caused by the force generated by the skier, then you could argue that an intermediate skier, like me, should probably not choose a stiff "top-of-the-line" (race) ski. I should instead go with a longitudinally softer ski, which doesn't require as much force to bend.

The factors that build up the force that bends the ski should include:
* body weight & strength
* speed
* edging ability
* technique (e.g. ability to move CM to inside)

So, if I'm a small (light) person, who doesn't like to carry a load of speed, with less than adequate technique, then a stiff top-of-the line race ski might actually inhibit my ability to practice carving, because I will not be able to bend it enough. Obviously, a softer ski will have some drawbacks, like less stability at speed, less rebound and energy, but since (by definition!) an intermediate should not run with huge speeds anyway, a softer ski should be a better choise.

As an aside, when shopping for skis, the sales person often seems to select a ski based on the individuals technical ability and body *length*; but I wonder if it wouldn't be more correct to substitute length for weight, i.e. a tall but light person should probably have a relatively speaking shorter ski...?

Cheers,
Tommy
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Postby milesb » Mon Mar 08, 2004 7:27 am

While the sidecut is the most important factor, overall design of the ski plays a surprisingly large part in the carving radius of a ski. If you try different skis with about the same sidecut, you will find vast differences in the size of turns the ski will carve. So, really the only way to really determine the "carving radius" is to actually use the ski. Hope that helps muddy the waters even more for you!
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Postby Guest » Mon Mar 08, 2004 8:21 am

I googled around and came up with the following:

http://www.math.utah.edu/~eyre/rsbfaq/physics.html

Some PSIA hits came up, as well, and are interesting reading.

Also, apparently "In the early 1980s, the Head Ski Company pioneered the identification of sidecut radius using the NTR (natural turn radius) identification for models in meters."

The Head web site has so much Flash crap running that I can't search around for references to NTR.
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Postby Jeff Markham » Mon Mar 08, 2004 8:22 am

Sorry for the anonymous post. That was me above with the URLs, etc.
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Postby Hobbit » Mon Mar 08, 2004 8:33 am

Hi Tommy,

It might be just the opposite. I think that the factors you've mentioned make it easier for a relatively shorter turn radius under the same circumstances. I have two pairs of Atomics R9 and R11. I wish I could compare R9 and C11 but I believe the conclusion would be similar. The softer ski is so much more forgiving and the stiff R11 has a very small ?sweet spot?. It is very sensitive to alt/for balance and will tell you if you have a problem right away. With the softer ski you have a risk of balancing issues getting into your technique and the lack of the sufficient ?feedback? from the ski. So after reaching some skill level it?s better to practice on the stiffer skis. The best option in my opinion is to have both types of skis so that you can choose a better tool for a particular practice.

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Postby tommy » Tue Mar 09, 2004 4:47 am

Jeff,

thanks for the URL, I found it quite interesting! Some time ago I'played a bit with the mathematics of how much you'd need to bend a ski of certain length to achieve an x-radius turn. I was quite amazed to see that the "formula" I came up with, exactly corresponds to the sidecut formula presented in the link! If this is correct, it means that the radius designation for a ski should be interpreted as "the radius of the arc that you will carve if you bend the ski the same amount as the skis sidecut size", where the sidecut size is the orhogonal distance from the waist of the ski to an imaginary line running from tip to tail.

So after reaching some skill level it’s better to practice on the stiffer skis


Hobbit,
good point! I guess that for those of us, who *really* want to improve our skiing, and are willing to go through any pains to improve, your position is a good one. Whereas for those people who might ski only a few days/season, who are not that interested in investing time, money & blod, sweat and tears to enhance their skills, but still would like to experience the sensation of carving, a softer ski might be a better choise.
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Re: selecting skis...

Postby BigE » Fri Mar 12, 2004 2:55 pm

tommy wrote:As an aside, when shopping for skis, the sales person often seems to select a ski based on the individuals technical ability and body *length*; but I wonder if it wouldn't be more correct to substitute length for weight, i.e. a tall but light person should probably have a relatively speaking shorter ski...?

Cheers,
Tommy


No. In general, a tall light person should be on a long and soft ski. The ski for a tall light novice would be very soft. The ski for a tall light expert would be long and still relatively soft, unless the speeds at which they ski warrant the added stiffness. eg. racers or other speed demons.

In general, a tall heavy person would be long and stiff. Beginner a bit softer, and expert stiffer.

The notion of length getting shorter for lightness is not correct: a long base is needed to stabilize a tall item. A tall person will feel like they can fly over the tips of a too short ski, if they lean too far forwards.

I'm 6'2" and felt that I could easily trip over the fronts of an 168 cm Elan 662 I was demoing, if I was too far forwards. Why? Leverage. This never happens on my 176 cm Salomon 10 3V's.
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Postby BigE » Fri Mar 26, 2004 8:45 am

tommy wrote:
So after reaching some skill level it?s better to practice on the stiffer skis


Hobbit,
good point! I guess that for those of us, who *really* want to improve our skiing, and are willing to go through any pains to improve, your position is a good one.


Beware: stiffer skis will demand higher speeds to work well, and they usually have more rebound. These tend to be more performance/speed oriented, so binding mount points tend to be farther back as well, making it very easy to get into the back seat. Back seat skiing increases likelihood of loss of control and crashing, especially given the increased rebound. Hopefully the crash is not into someone or something. In short, students may not be able to manage stiff skis safely. Going too stiff could harm others as well as themselves.

I'd suggest stiffer skis are appropriate only if you are encountering problems at the speeds and energy levels at which you normally ski. Otherwise, it is unlikely that the added stiffness will be of any benefit, may hinder the learning process, and may actually be unsafe.
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Re: Ski radius: what does it really imply ?

Postby Iamaguest » Tue Mar 30, 2004 10:59 am

tommy wrote:..........................
As far as I can understand, the way a ski (shaped or not) does a carved turn is by bending; that is, if you want the ski to turn without skidding, you will have to bend it. The sharper a turn you want to make, the more you will have to bend the ski. Even if you have a radically shaped modern ski, with really small "factory" radius, and put it 90 degrees on edge against the snow, if it's completely flat, it will go straight (with maybe minor steering caused by the tip).

.........................


The turn radius specified by the manufacturer is essentially the continuation of the ski's sidecut when the ski is laid flat. Any carved turned ON A SUFFICIENTLY HARD SURFACE will be of a smaller radii. At around 60 degrees angulation if the ski is carving on a sufficiently hard surface the turn radii is one half to up to one third of the ski's *built in turn*.

A ski with no sidecut and there is no such alpine ski, ("shaped" skis just have much more sidecut then "straight" skis) would not bend when put on edge on a hard surface. On a soft surface (e.g. powder) a ski with no sidecut turns because the shovel bends the ski and the middle of the ski 'sinks' because that's where most of the weight is. This is why powder skis do not need much sidecut.

As you said 'The sharper a turn you want to make, the more you will have to bend the ski' and this is what the sidecut will help the ski do. Your last paragraph explains this. It is not necessarily true or relevant that the waist of the modern shaped ski would have less pressure to achieve a tighter turn.

The reason skis with similar turn radii seem to make different size turn lies a lot with the interaction with longitudinal flex and torsion. Skis with softer flex tend to also be softer in torsion which lets the tip and tails twist more when the ski is put on edge. This reduces the gap/space between the ski and the snow surface that occurs when the ski is edged. The ski does not bend as much so the turn radius becomes wider.

Your last paragraph also touches on the reasons why early on when "shaped" skis appeared some critics believed they would not work or be successful. At that time many skiing manufacturers believed that it was increasing the longitudinal flex while manipulating torsion was the way to get a ski to carve shorter radius turns.

There are many other details that are involved in all of this. Hopefully my post is clear. I'm not the best at communicating by writing.
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