Scaling Construction and Dimensions in Ski Design

[Dakine]

Well for a purely carved turn on hard snow.

You can bend the front more , and the rear more in sliding somewhat while on edge. Most common example- on steep 3-d snow, soft skis with an aggressive skier will “fold up” or bend too much causing a dramatic slowing. I guess the same can happen in carving mode, where the bending is not just proprtional to the edge angle.

Noodle skis are slower, and will force you to ski slower, even if carving. Often you don’t realize it if you get used to a soft flexing ski, till switching to a stiffer ski with similar sidecut. Why?

Skiing my Blossom SC's has reopened my eyes to what soft, torsionally rigid skis can do.
A friend who was an Austrian National Team member told me that the softest ski you can work through a course without overloading it will be the fastest.
That being said, if you overpower a soft ski either through lack of finesse or too demanding conditions, things get ugly fast.
The SC's are impossible in cut up crud when my Whiteouts will blast on through.

 

[DanoT]

When Head came out with their woman's line of Joy skis, they nailed it with the various model's performance as well as names: Total Joy, Absolute Joy, Great Joy, Wild Joy.

When they came out with the light weight Kore skis, it was great that the skis' width at the tip, tail, and under foot varied in dimension with ski length, theoretically this meant that the ski feels the same for big and small skiers. Great, but the Kore 93 name does not then really fit.

Head could have chosen a name like Hard Kore, Soft Kore, Hollow Kore, Rotten to the Kore, Kore-Rect or gone with the old song: Korina, Korina, (I love you so). Ok these are all horrible names that I have come up with but it just goes to show that I could have had a marketing career in the ski industry because they often do just as poor a job with ski names.

 

[James]

A friend who was an Austrian National Team member told me that the softest ski you can work through a course without overloading it will be the fastest.
That being said, if you overpower a soft ski either through lack of finesse or too demanding conditions, things get ugly fast.

Bit like Bode saying, I think discussing downhill, you want the edges as dull as possible but sharp enough to do the job.

It’s all relative though.

 

[Seldomski]

I am a little confused about the "Size Scaling: Dimensions" label. How much and which dimensions need to change for this to be flagged in the ski selector? Is this still a work in progress and the ski reviews are still being updated?

For instance, the Fischer RC One GT 86 has different tip, waist, and tail widths corresponding with different lengths, but the "Size Scaling" is flagged as "none." The Head Kore 93 gets "constructions and dimensions" label. Head Total Joy gets the "dimensions" label.

2022 Fischer RC One 86 GT

Philpug: The Fischer RC One GT 86 has a mouthful of a name; an accomplished skier could make it down the better part of some ski trails before saying the whole thing. There is no question that we have been impressed with Fischer's offerings in the past few years, especially those that say "RC"...

www.skitalk.com

2022 Head Kore 93

Philpug: Some say Head is taking a big risk changing the immensely popular Kore 93Ti; I look at it the other way, Head would be taking a bigger risk resting on its laurels another year. The Kore 93 was one of the reference skis in this category, and deservedly so. It was light, playful, easy to...

www.skitalk.com

2022 Head Total Joy

AmyPJ: Did they take the Joy series and make it even more Joyful? I do believe they did! The sidecut on this ski makes it ridiculously easy to enter into a turn. Yet, I could feather the edges and drift and skid and basically do whatever I wanted on it. I did take it into the soft, deep chop...

www.skitalk.com

 

[Philpug]

I am a little confused about the "Size Scaling: Dimensions" label. How much and which dimensions need to change for this to be flagged in the ski selector? Is this still a work in progress and the ski reviews are still being updated?

For instance, the Fischer RC One GT 86 has different tip, waist, and tail widths corresponding with different lengths, but the "Size Scaling" is flagged as "none." The Head Kore 93 gets "constructions and dimensions" label. Head Total Joy gets the "dimensions" label.

2022 Fischer RC One 86 GT

Philpug: The Fischer RC One GT 86 has a mouthful of a name; an accomplished skier could make it down the better part of some ski trails before saying the whole thing. There is no question that we have been impressed with Fischer's offerings in the past few years, especially those that say "RC"...

www.skitalk.com

2022 Head Kore 93

Philpug: Some say Head is taking a big risk changing the immensely popular Kore 93Ti; I look at it the other way, Head would be taking a bigger risk resting on its laurels another year. The Kore 93 was one of the reference skis in this category, and deservedly so. It was light, playful, easy to...

www.skitalk.com

2022 Head Total Joy

AmyPJ: Did they take the Joy series and make it even more Joyful? I do believe they did! The sidecut on this ski makes it ridiculously easy to enter into a turn. Yet, I could feather the edges and drift and skid and basically do whatever I wanted on it. I did take it into the soft, deep chop...

www.skitalk.com

This year has been one of the tougher ones for getting information. I did make the change to the Fischer 86. So, yes, this is a work in progress and very well I will be making changes as the information is available to us. Needless to say, not all of these specifications are always promoted and published by the brands. One of the reasons we are doing this. If anyone finds a product that we missed or is incorrect, please no not hesitate to PM me or use the "report" feature.

 

[geepers]

Ever try to tip a ski without adding weight to it? Of course it bends as soon as you click into the bindings , it starts by decambering. then, as you tip to an edge, you are putting energy into the ski, its bending as the tip engages and bites the snow, as you come across the turn, the rest of the ski is bending as energy and forces build. when you release the edges and you COM changes direction, the ski energy releases, and flexes again. this exactly why I like cambered skis.

Yep, understand that. It's just that the forces and energy required to bend a ski are much smaller than the forces and energy available. From Ron le Master "Ultimate Skiing"
Contrary to common belief, the skis themselves don’t contribute appreciably to rebound through a trampoline effect. Compared to a skier’s weight, the skis are simply not stiff enough to store and return much energy. In addition, the skis are supported by the snow directly underneath the skier’s feet, unlike a trampoline, which is supported only along its edge.

And as @James and @François Pugh posted, their concern was more with some skis bending too much.

Appreciate your point on the effect of size on how some skis feel. Only had a few times to have the chance to try different lengths of the same model ski. With some of them it seems a small difference. (Early model [2012?] Rossi Exp 84 felt very little different 160/170.) A couple it felt like a totally different ski. (Rossi 88 and Blizzard Bushwacker 88). But if we stood on those skis in the garage would there be all that much difference in amount of bend for the same weight between the different sizes? Maybe some-one with access to enough skis can try it...

Interested in what makes the difference. If it's a question of pressure (force spread over a bigger area) then why not a marked difference for all skis? Is it the extra length making them harder to pivot or contacting different parts of bumps? (That can be noticeable in all of them.) Does the extra length bridge between different parts of bumps? Is an extra bit of fore/aft movement required to work tip and tail appropriately through the turn?

 

[DanoT]

Don't overlook torsional stiffness when assessing a skis turning characteristics because in many ways torsional stiffness is more important than longitudinal stiffness; stability at speed and tracking come to mind.

 

[Tony S]

Cool thread.

This is where I agree with scaling.... a 93mm ski for someone 5'3" is like a 98mm ski for a full sized adult.

People who are 5' 3" ARE full sized adults, Phil. They're just not Phil sized adults.

We have concluded that the metric system is an arbitrary unit of measurement in the ski industry.

Ahhh, but it is not, it is a standardized unit of measurement. 1cm = 1cm.

"It was decided that the new system would be based on a natural physical unit to ensure immutability. The academy settled on the length of 1/10,000,000 of a quadrant of a great circle of Earth, measured around the poles of the meridian passing through Paris. An arduous six-year survey led by such luminaries as Jean Delambre, Jacques-Dominique Cassini, Pierre Mechain, Adrien-Marie Legendre, and others to determine the arc of the meridian from Barcelona, Spain, to Dunkirk, France, eventually yielded a value of 39.37008 inches for the new unit to be called the metre, from Greek metron, meaning “measure.”

Mfrs mislabeling skis is a whole different issue.

Nope. As @dbostedo pointed out, that is exactly the issue. Measurements are used by the marketers to indicate something about a ski that is not borne out by objective measurements. If you're going to use measurements, make them accurate or lose my faith in you and your product.

Augment's claim that they can custom build a ski to ten stiffness specifications on order.

We learned very late in the game, from Ron, bless him, that in fact they're just indicating where a given pair's stiffness falls in the natural variation of a production run. Any company could do the same ... and xc companies routinely do. They're not intentionally producing specific stiffnesses.

When Head came out with their woman's line of Joy skis, they nailed it with the various model's performance as well as names: Total Joy, Absolute Joy, Great Joy, Wild Joy.

When they came out with the light weight Kore skis, it was great that the skis' width at the tip, tail, and under foot varied in dimension with ski length, theoretically this meant that the ski feels the same for big and small skiers. Great, but the Kore 93 name does not then really fit.

Head could have chosen a name like Hard Kore, Soft Kore, Hollow Kore, Rotten to the Kore, Kore-Rect or gone with the old song: Korina, Korina, (I love you so). Ok these are all horrible names that I have come up with but it just goes to show that I could have had a marketing career in the ski industry because they often do just as poor a job with ski names.

Dano for mayor!

 

[Wilhelmson]

Wait until you pay for ai. Huh?

 

[Dakine]

"We learned very late in the game, from Ron, bless him, that in fact they're just indicating where a given pair's stiffness falls in the natural variation of a production run. Any company could do the same ... and xc companies routinely do. They're not intentionally producing specific stiffnesses."

I came to that conclusion six months ago.
Mentioned it in a post but nobody picked up on it.
Atomic has been marking race skis for stiffness for years but nobody has cracked their code.

 

[oldschoolskier]

Scaling designs are not simple as material properties themselves don't scale, you try and design the scaling around them. Engineering 3rd year.

 

[AlexisLD]

The stiffnesses of pretty much all skis is scaled with respect to their length. However, it is not rare to see that two lengths of a model, out of the 4-5 different ones offered, have the same stiffnesses. That means that they are probably using the exact same laminate and core thickness. I am not sure why that is, maybe it is what should be done, maybe it is time, maybe it is complexity, maybe it is money... Often, but not always, you also see that the longest length is way stiffer than the other sizes. The longest length is kind of a "pro-model" or a "you want a lot, we will give you as much as we can".

It is a pretty hard to say if a scaling produces a "consistant" feel across length. How would I be able to tell if a 160 cm ski give the same on-snow feel to a smaller/lighter person than the 180 cm ski that I am using? It is also pretty hard to say if enough scaling has historically been used. Very little user testing is done on the shorter and longer skis.

Regarding geometry, scaling has always existed. Length was always different (obviously... however you could ask yourself why that is the case if we are trying to do the same turns on snow), rocker length is different, binding area is different, tip height is different, etc. Sidecut radius and width scaling was also always present for different length. I.e., when you change the length of a ski, and let say that you want to keep the same waist width, you have to choose if you want to maintain tip/tail width or the sidecut radius. You can't have both!

There seems to be the new buzz about changing waist width with length, more than before. I hope that it is based on user testings, because it creates some weird things. One model out there has sidecut radius going from 13 to 21 m for their 164 vs 186 cm long skis respectively. I could be wrong, but I highly doubt these are producing the same on-snow feels... They are different beast. Should they have the same name? Is it confusing or helpful?

When you scale skis for different person, you have to think about what you want to keep constant, or what you want to change. Is it the effective turn radius? Is it the surface area of the ski to the skier mass? Is it the surface area underfoot? Is it the pressure distribution shape under the ski? Is it weight?

What else do you guys think we should scale a ski for?

 

[oldschoolskier]

Part of the reason why radius changes, it is scaled. This SL skis have about a 11-13m radius at 165cm yet GS (few years back) had a turn radius of 23(ish)m at 186cm.

Think what would happen if you skied a 13m radius at 186cm at speed and just edges a bit and caught a rut. All I can think of is pain, in more ways than one.

Again scaling is not as easy as it sounds because it depends on what you trying to achieve. It was one of my most difficult courses in engineering as scaling physical material properties vs physical dimensions vs reactions and results are not similar nor proportional.

 

[François Pugh]

Yeah, it's complicated. There are several factors that scale differently, but the model can only be truely accurate to one of them.

As to ski scaling, if you're buying a SL ski and going from 155 to 170 cm, you want all the skis to be SL skis, and they should scale that way. Another approach is to not have the different lengths to be the same type of ski. For example the Fischer Curve series, the shorter ones are designed to act like SL skis and the longer ones are designed to act more like GS skis. Combining both types of skis into one model does prevent more accurate scalling for other factors (such as skier weight).

Not going to address model scaling, I'm not going into detail, if your more interested you can read all about it elsewhere; my old "Theory of Dimensions and Models" professor has written a few books about it.

Also not going into detailed beam design, but to give you an (simplistic) idea of what's going on with skis, consider a uniformly loaded simple beam supported only at its center
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
........................................|.............................................
The load (a) farther from the center has more leverage (think of a wrench), and requires more resistance from the beam, so it has to be stiffer. If you were to take the same load and spread it over a smaller area,
AAAAAAAAAAAAA
.................|.....................
even though the load per area is bigger (same total load, less area), the lever arm of the load is closer to the support. Less resistance required. Try pushing a door near the hinge and far from the hinge.

 

[Dakine]

Yeah, it's complicated. There are several factors that scale differently, but the model can only be truely accurate to one of them.

As to ski scaling, if you're buying a SL ski and going from 155 to 170 cm, you want all the skis to be SL skis, and they should scale that way. Another approach is to not have the different lengths to be the same type of ski. For example the Fischer Curve series, the shorter ones are designed to act like SL skis and the longer ones are designed to act more like GS skis. Combining both types of skis into one model does prevent more accurate scalling for other factors (such as skier weight).

Not going to address model scaling, I'm not going into detail, if your more interested you can read about all about it elsewhere; my old "Theory of Dimensions and Models" professor has written a few books about it.

Also not going into detailed beam design, but to give you an (simplistic) idea of what's going on with skis, consider a uniformly loaded simple beam supported only at its center
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
........................................|.............................................
The load (a) farther from the center has more leverage (think of a wrench), and requires more resistance from the beam, so it has to be stiffer. If you were to take the same load and spread it over a smaller area,
AAAAAAAAAAAAA
.................|.....................
even though the load per area is bigger (same total load, less area), the lever arm of the load is closer to the support. Less resistance required. Try pushing a door near the hinge and far from the hinge.

It is very complex if not impossible to scale both bending, torsional and vibratory responses in the same beam.
But skis are not space station parts and so some compromise is perfectly acceptable.
What I find particularly questionable is fixing the waist width in a given series of skis and offering a series of lengths in that waist width.
The width to length or slenderness ratio is the first variable I would concentrate on if I was scaling a series of skis that were supposed to ski similarly.
Oh but the mold costs......

 

[AlexisLD]

It is very complex if not impossible to scale both bending, torsional and vibratory responses in the same beam.
But skis are not space station parts and so some compromise is perfectly acceptable.
What I find particularly questionable is fixing the waist width in a given series of skis and offering a series of lengths in that waist width.
The width to length or slenderness ratio is the first variable I would concentrate on if I was scaling a series of skis that were supposed to ski similarly.
Oh but the mold costs......

Why do you think that the slenderness ratio is important? Do you think that it is more important than waist width or sidecut radius?

How does keeping the same waist width allows someone to save on mold cost? I don't think manufacturer are reusing molds for different lengths, are they?

The main raison that I think waist width is so important is that it is how people categorize skis. This is one of the first number that people will look at. People are told that if they want a piste ski they need to look under a certain width, that a powder ski should be more than a certain width, etc. Nobody consider your mass/height when making these recommendations. So it is what people are looking for and that is also what companies must provide...

I think that there are good reasons to scale the waist width. For example, I think that we should scale powder skis so that the float-to-skier mass is kept constant. That involves scaling the surface area of the ski (maybe through the waist width, but length also play a role) but also to the binding location (a -8 mount point doesn't mean the same thing for all ski lengths) and other factors. Furthermore, the ratio of tip/tail surface area to waist area is also important to determine how a powder ski flex in soft snow and thus, how it floats.

However, I still find it a little weird to scale the waist width of skis that will touch hard snow and getting sidecut radius that change a lot from that. Look at the Blackops Sender (https://www.rossignol.com/ca_en/rajmj01-000.html), its sidecut radius changes from 13 to 21m. Do you think these skis give the same on-snow feels? Do you think that a review of the 186 cm length will be informative to someone interested in the 164 or 172 version? Why do you think these skis are so different?

Changing sidecut radius from 13 to 21 m seems like a big change to me. However, maybe the thing that skiers are feeling more on the snow is the sidecut distance (e.g., 33 mm in a 139-106-129 ski), not the radius. Any thoughts on that?

 

[Dakine]

This shows how hard it is to "scale" a ski model.
The slenderness ratio should mostly affect the longitudinal stability of the ski when it is off edge.
Your point about scaling by effective surface area is clearly important for soft snow skis.
For carving skis the slenderness ratio is immaterial when on edge and the waist width affects the quickness from edge to edge.
If I was in charge, skis would be specified by effective sidecut radius as the primary variable.
After you look at the radius, then the width and shape become important in selection.
It all depends on what is important to your skiing but ski marketing has focused on waist width as the primary selling point.
This gets a bit strange when you see a 100mm ski in 160 length and it looks like a snowshoe when compared to a 190.
No way are those extremes going to ski with similar characteristics.

My speculation about mold costs comes from my suspicion of how manufacturers use plugs in their molds to make them adaptable to more than one ski length.

 

[AlexisLD]

The slenderness ratio should mostly affect the longitudinal stability of the ski when it is off edge.

"Off edge" like in if the ski laying flat on a surface? In that case, how is the ski width important?

If I was in charge, skis would be specified by effective sidecut radius as the primary variable.
After you look at the radius, then the width and shape become important in selection.

It is a little early to share this (i.e., no 2021-22 skis yet), but I think you might like it:
https://compareskis.shinyapps.io/compare (the comparison tool)
https://soothski.com/compare-skis-with-measures/2021/ski-comparison-application-guide/ (the how-to guide)

You can search for skis with measured sidecut radius and add any other criteria you like. If you add enough criteria, you will end up with a short list of ski to consider out of the 2000+ skis in the database. Each ski has been individually measured. You can also compare how various models are scaling their parameters up/down with length by either selecting the various lengths of that model and/or using the custom graphs to see how one parameter of the full database varies with respect to another one.

It all depends on what is important to your skiing but ski marketing has focused on waist width as the primary selling point.

I think the 90s were the years of sidecut. The last 20 years have been the years of shape (i.e., waist width, reverse camber and rocker). If you can only say a few things, it makes a lot of sense to talk about sidecut radius for a slalom ski. However, it makes more sense to talk about waist width for a powder ski.

This gets a bit strange when you see a 100mm ski in 160 length and it looks like a snowshoe when compared to a 190.

I think this is kind of happening already. It is just that you have to switch model. That is, most models are only available in 3 lengths and they rarely cover the full 160 to 190 cm spectrum (and there are shorter/longer skis out there). If you geometrically scale a 120 mm wide powder ski that is 190 cm long into a 160 cm long version, you would get a 100 mm wide ski. Data from the link above show that it is rare to see skis wider than 100 mm in the 160 cm length (i.e., women ski).
(but of course, that is just the data from the skis that we have measured so far... you need to be careful before drawing conclusions)

 

[Dakine]

""Off edge" like in if the ski laying flat on a surface? In that case, how is the ski width important?"

It may not be on a hard surface but in softer conditions a long skinny boat tracks better than a wider boat of the same length.
Every time I ski my old Kastle National Team WC super g 209 straight skis with a groove down the center I get a refresher course in how well a ski can track.
Going straight in a tuck on a pair of wide shaped skis is scary, the Kastles make it so easy you want to go faster.

 

[Tony S]

Every time I ski my old Kastle National Team WC super g 209 straight skis with a groove down the center I get a refresher course in how well a ski can track.

Omg, you and François need to go on a date.