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# Atomic 9.12 stats

I'm from the "pocket protector-slide rule" generation.

Looking for techinical information on dimensions and the radius caused by the sidecut of the different sizes of 9.12s. Example, 160 cm would have a different radius than 150cm....but what is it? They have the same sidecut, but create different arcs.

Saw the tech manual on K2, but can't seem to get a hand on the Atomic version of the tech manual. The web site has no contact information.

Any techies out there? Any store owners with a copy?

Thanks.

Oh, I use this stuff when I teach math. The recent problem I gave my students was how to find the angles on the support legs of an Adirondack chair made of skiis. (trig!!!) [img]smile.gif[/img] [img]smile.gif[/img]
I believe I can answer this one, but it would be nice if someone could confirm my info.

I think the actual dimensions of all the lengths are the same (except for the length itself). I do know though that the turning radius is different for each length. The 170 has a turning radius of 12 meters, and I think the 160 has a turning radius of 11 meters, which is what I ski. And It goes down 1 meter for each proceeding length.

I'm not sure of this though.
KeeTov:

These should be pretty close for 9.12 dimension of 115mm Tip, 65mm Waist and 100mm Tail:
150cm R=9.5m
155cm R=10.3m
160cm R=11.1m
165cm R=11.9m
170cm R=12.7m
Thanks, now I can figure out the mathematic relationships to determine any arc if given the length of the chord.

I'm sure my students will also thank you(???).
Is all this "natural turn radius" stuff calculated on a dead flat ski? That is, by the numbers, on a a ski that is not cambered positive or negative.

That would be a tuff condition to control "in the field". If I have my precepts correct, as soon as I start (initiate) a turn, the tip engages the snow to develope negative (sign conventions?) camber.

I can understand the comparative relation between a greater or lesser turn radius as applied to general response, just not the exactitude.

Can someone shed light for me?

CalG
> ...Is all this "natural turn radius" stuff calculated on a dead flat ski? ...

CalG - Good observation.

There is a terminology problem here. They are actually talking about "sidecut radius", NOT "natural turn radius". The former is a simple geometric quantity and is totally independent of whether or not the ski is edged, is in reverse camber, etc.

Unfortunately, people get sloppy and tend to use the first term interchangeably with the second, but as you pointed out, the "natural turn radius" depends dramatically on the ammt of edging, camber, etc.

If you make a considerable number of assumptions, you can come up with a formula to estimate the "natural" turn radius from the sidecut radius and edging angle, but its not terribly exact because of the assumptions made. If I remember correctly (I don't have access to my notes), for small edging angles, the natural_turn_radius is approximately equal to the sidecut_radius times the cosine of the edging angle. Note that by "edging angle" I simply mean the angle the base of the ski makes with the snow, not any more complicated related quantity such as "critical edging angle"..

KeeTov: With respect to your class, its obviously no big deal to derive the formula to go from 3 width measurements to an estimated sidecut radius, but in case you are interested, it has been published in many places. There were a few threads about it a year or so on Epic, but given that the search function doesn't seem to work, the easiest way to find a derivation may be to look in the book, "Physics of Skiing":

http://www.amazon.com/exec/obidos/AS...232137-8425647

Hope this helps.

Tom / PM
Physicsman,

Thanks, forgot I had the book!! I'll spend the weekend taking another look at it.
Norefjell,
Where did you get your stats? I know for a fact that the 170 has a turning radius of 12 meters (hence the name 9.12). There are no 165's and 155's either. I am assuming you used some sort of equation.
PhysicsMan,
The radius of a skis sidecut is measured when you allow the ski to form a complete circle. It is fixed and will always remain constant and it affects turn size. The sidecut radius is mathematically calculated from the skis geometry.

The turning radius is the size and shape of the turn and is determined by the skiers efforts on snow - steering, pressure and edge angle.

I believe the one that we see listed in magazines is the sidecut radius, not the turning radius (which can very greatly within a certain model).

I hope this helps.
9.12 - umm ...

How is what you just said different from what I said in my earlier post in this thread (timestamped May 29, 2002 10:04 AM)?

I'd be more than happy to start a debate on something, but I don't think we have any differences of opinion. [img]smile.gif[/img]

Tom / PM
PhysicsMan,
Sorry about that. I was just trying to elaborate more on what the sidecut was, and how it was measured. Didn't mean to cause any problems.

-9.12

BTW, where do you ski? I ski/race for Ski Roundtop. I'm sure you heard of that place, unless you live in western Maryland.
9.12

My response was based on plugging in the tip, waist and tail dimensions for the 9.12 ski into my spreadsheet. The lengths were just input and may not reflect correctly what the actual production ski lenghts are. The formulas I had derived agree with the book referenced by Physicsman. This book also has formulas that give you the turning radius as a function of the edge angle. The side cut radius is the turn radius as the ski is pressed flat and barely edged. Assumptions for the equations for the side cut radius is that curved part of the edge follows the arc of a circle. A few years ago I created my spreadsheet that I have used to compare a lot of skis. I also have a copy of the FIS method of calculating side cut radius which is slightly different than the method I used. The spreadsheet is an Excel file. PM me if you want a copy.
Here are the dimensions for the 2002/03 Atomic Race 9.12

length - tip-waist-tail (mm) - turn radii

140cm - 107-65-92 - 10m
150cm - 111-65-96 - 10m
160cm - 115-65-100 - 11m
170cn - 119-65-104 - 12m

These are from my technical manual.
Quote:
 Originally posted by Norefjell:... I also have a copy of the FIS method of calculating side cut radius which is slightly different than the method I used. The spreadsheet is an Excel file. PM me if you want a copy.
That's interesting. I did exactly the same thing a couple of years ago, and found that to get the best agreement between the radii resulting from my calculation and published radii, I had to:

(a) estimate the correction needed to convert the advertised "length" (presumably chord length) into contact length; and,

(b) put in a few percent ad hoc correction factor on top of that.

My calculation is also in Excel, so if you would like to exchange and compare spreadsheets, I'll dig mine up from my old hard disk and PM you with my email adr. KeeTov, you are obviously welcome to a copy of mine as well -- let me know. BTW, I obviously don't guarantee the accuracy of any of the data in my spreadsheet, so if you find errors, pls let me know.

Another interesting calculation is to calculate the forebody and aft radii separately. To do this, you make use of the fact that at the ski waist, the edges are exactly parallel to the long axis of the ski, and you must know the exact fore-aft position of the waist.

Since one rarely knows the latter on skis that you don't own, and since mfgrs rarely publish the fore and aft radii separately, this calculation isn't of tremendous use, but it is most interesting because it points out that for many skis, because the waist is located to the rear of the center of the contact length, even if the tail of the ski is much narrower than the tip, often the aft sidecut radius is much less than the forebody radius. Essentially, what this is telling us is that the sidecut of many skis are not well-approximated by a simple circular arc.

Cheers,

Tom / PM

[ May 29, 2002, 10:40 PM: Message edited by: PhysicsMan ]
Quote:
 Originally posted by 9.12 skier:PhysicsMan, Sorry about that. I was just trying to elaborate more on what the sidecut was, and how it was measured. Didn't mean to cause any problems.
No problem! I was just uncertain if you were agreeing and restating things, or if you were trying to point out some subtle difference that I wasn't picking up on.

Yup, I certainly do know Ski Roundtop, altho, living near DC, I almost always go to Whitetail on day trips, and bypass Roundtop if I'm heading north on I-83 to either the Poconos or New England. Its probably been a decade since I last skied Roundtop, but I keep saying to myself that I've got to give it a try again.

I haven't participated in the threads on colleges and racing that you were active in, but have enjoyed reading your posts. Let me also offer you a belated welcome to EpicSki. BTW, during the season, you should give www.dcski.com a look. They're almost totally inactive right now, but during the ski season, they have quite a bit of info and chatter specific to the mid-Atlantic areas that you won't find on Epic.

Cheers,

Tom / PM
PhysicsMan,
Thanks for the welcome. It's great to have someone in the forum who is familiar with the mid atlantic area. You should definaltly give roundtop a try. Whitetail is a great mountain, the lodge and base area of roundtop doesnt even compare with whitetail's, but it faces south. The snow quality at roundtop is much better (it faces directly north). Roundtop may not have the vert of whitetail, but the snow is better. [img]smile.gif[/img]
Posted above:
"… Example, 160 cm would have a different radius than 150cm....but what is it? They have the same sidecut, but create different arcs”

As Beta Racer points out differing lengths of the same model can also have different sidecut specs beyond length only related radii differences further complicating things.
9.12:
> ...The snow quality at roundtop is much better (it faces directly north)...

Yup, and, lasts longer!

Lostboy:
>...differing lengths of the same model can also have different sidecut specs...

Absolutely true. A very informative exercise is to measure the actual sidecut widths on skis you own, and compare them with the published specs. Even if you know the length that the published specs pertain to, there are often several mm of difference.

Tom / PM
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