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# Why, oh why....

I think this topic has been debated previously(few years ago), but, even so:
why are ski dimensions and not turn radius kept constant throught the whole line of lengths? I know that some models do change the dimensions, but for the most part, you can have a 3-4 meter difference between the radius of the 164 length and the 182 length.
So there is really little resemblance between the two. Why is this a good idea?
Because little skis are for little people making little turns, and big skis are for big people making big turns?
Quote:
 Originally Posted by psy I think this topic has been debated previously(few years ago), but, even so: why are ski dimensions and not turn radius kept constant throught the whole line of lengths? I know that some models do change the dimensions, but for the most part, you can have a 3-4 meter difference between the radius of the 164 length and the 182 length. So there is really little resemblance between the two. Why is this a good idea?
Some manufacturers will adjust accordingly as a ski goes up and down in size, but it is not common.
Big skis are for big people who weigh more and exert a higher load on the skis at the same speeds. It's easier to make the ski have a higher turn radius and have the big people tip them more than it is to make stiffer skis. If you take a 3-dimensional look at a longer radius ski that is tipped more to achieve a turn radius, you will see that it is decambered more also (closer to 90 degrees = closer match to exact curve of ski on snow) Of course the problem with that is most of the skiing public do not tip their skis enough.
What Ghost said +Big skis are also for people of all sizes going faster and thus generating more Gs mid turn due to the extra speed. Having a longer ski gives more surface contact and thus better hold on the surface at higher speeds.
Quote:
 Originally Posted by psy So there is really little resemblance between the two. Why is this a good idea?
Keep development and mold costs down.
The radius of the turn carved by a ski is function of the sidecut of the ski, how much it is flexed, and how much it is tipped on edge. Increasing any component results in a tighter turn. Bigger and faster skiers flex the ski more, and better skiers tip it on edge more, and since they are generally on longer skis with less sidecut, it all more or less evens out.
If you hold the tip/tail/waist widths constant and vary the length, the radius has to change. The other option would be to hold the radius constant and vary the length, and then some combination of the tip/tail/waist width would have to be changed.

I'm not sure the latter would be any less confusing. And a significantly wider or narrower ski with the same radius would be just as "different" as one with the same widths and different radius, maybe even more so. Certainly the torsional stiffness could be dramatically different, whereas that probably doesn't change as much if you increase the length and turn radius proportionally but keep the same dimensions.

### What they said...

...and if you want to take the example of race skis, this is the rationale behind the change in FIS regulations. A few years ago, there weren't much of any regulations on lengths and sidecuts. Now for men on the World Cup, for GS, the minimum length is 185 and the minimum sidecut is 27 meters. Let's say that, for the sake of argument, the minimum length was still 185 but there were no sidecut regs. Some racers might be tempted to go with a turnier ski, say in the 18 meter range, in an attempt to carve more of the turn on what are increasingly turnier courses. The FIS says this is a bad idea, and I have to say I finally agree with them. If you have a long ski with a very short turn radius moving fast and you jump on it a little too hard, it's probably going to hook up radically and buck you off...maybe even torque your knee and blow out your ACL. To make a really extreme example, what if you had a 217 DH with a 12 meter radius, which is what you see on SL skis? That would be a really scary ski, and you'd never see me on it.

It is not totally true, BTW, that for a given manufacturer, there is one sidecut for a given length. My current GS ski, for example, is a 186 GS12 with a 27 meter radius. But you can get Atomic all mountain skis in a similar length that have a turn radius of 20 or less meters...
Quote:
 It is not totally true, BTW, that for a given manufacturer, there is one sidecut for a given length. My current GS ski, for example, is a 186 GS12 with a 27 meter radius. But you can get Atomic all mountain skis in a similar length that have a turn radius of 20 or less meters...
Obviously. But the all-mountain skis will be wider at the tip and tail, and/or narrower at the waist. Put another way, the GS skis are a "smaller" section (in degrees) of that 27-meter circle.

If you fix the tip/tail/waist widths and pick a length, there's basically only one radius that "fits". If you pick a length and a radius, it fairly tightly constrains the widths you can use. You can play around with it a bit by using things like variable-radius sidecuts, but that will only get you so far.

If you make anything too extreme, you'll start running into all sorts of physics-related problems -- the skis will be too grabby, or too stiff, or that materials will break under the strain. A 217-cm ski with a 12m radius would be ridiculously wide at the tip and tail.
Quote:
 Originally Posted by Matthias99 A 217-cm ski with a 12m radius would be ridiculously wide at the tip and tail.
~117-35-99, I can almost picture it, too.

### I think we're in violent agreement...

Quote:
 Originally Posted by Matthias99 Obviously. But the all-mountain skis will be wider at the tip and tail, and/or narrower at the waist. Put another way, the GS skis are a "smaller" section (in degrees) of that 27-meter circle. If you fix the tip/tail/waist widths and pick a length, there's basically only one radius that "fits". If you pick a length and a radius, it fairly tightly constrains the widths you can use. You can play around with it a bit by using things like variable-radius sidecuts, but that will only get you so far. If you make anything too extreme, you'll start running into all sorts of physics-related problems -- the skis will be too grabby, or too stiff, or that materials will break under the strain. A 217-cm ski with a 12m radius would be ridiculously wide at the tip and tail.
...because of what you said, yes, as soon as you pick a length, you've got a fairly narrow range of sidecuts due to the width issues you discuss. This is a more accurate description of why lengths and sidecuts are interdependant than the one I came up with...
Y'all jess' skeered of rly narrow waists,'s'all.
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