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Stiffness as a Function of Length

post #1 of 21
Thread Starter 
This is something I have always been curious about. Clearly if the same ski was built the same way regardless of length, the longer ski when acting as a beam would bend more under the same load. Hence longer skis would bend more easily. Do the majority of manufacturers adjust the construction, thickness of the materials or strength of material to compensate so as to result in an equivalent stiffness for every length or is the longer ski made differently to appeal to different level/types of skiers or are longer skis generally softer?
post #2 of 21
Hoooo boyyyy.......you're gonna git me started here.

Each size should be individually engineered to a particular spec. Generally speaking they are not. Sad but true...they are not.

What usually happens is the "design length" is carefully engineered and tested. This is an extensive/expensive project. Then the the other sizes are spec'd off of the design length. Sometimes this is very effective, sometimes not.

The oft vilified Rossignol company has always been very diligent about engineering the longer skis within a model run to be appropriately stiffer than the shorter sizes. Many other companies make a cursory attempt or just.....................don't.

This subject has cheesed me off for 25 years........I need a beer.

post #3 of 21
Over the years there have often been classic skis that achieved perfection, but only in one particular length of a model.
post #4 of 21
Originally Posted by mudfoot View Post
Over the years there have often been classic skis that achieved perfection, but only in one particular length of a model.
190 Chubbs.
post #5 of 21
Thread Starter 
Well, S.J.'s comment could explain allot. I have often wondered how skiers or testers comment or impressions of a particular ski can be so "all over the map". I could cite specific examples but I am sure you all know what I mean. One may find a ski soft the other stiff, one say great in short turns the other in long turns etc. Mogul performance in particular varies tremendously in different testers opinions. Of course it is all quite subjective and dependent on skier weight and style. But you would think that at least "professional" testers should be more consistent in their opinions. So I wonder if the characteristics of a particular length ski are strongly at play here.
post #6 of 21
Are movie reviewers consistant? How about food critics? or Dog Show judges?

subjective opinion + human nature = wild difference.
post #7 of 21
Thread Starter 
Perhaps I did not make the statement clearly enough when I said: "it is all quite subjective" but I don't accept simply comparing a manufactured item to a movie. Furthermore, when talking about magazine reviews, I assume we are talking about the collective experience of a panel of "experts".
post #8 of 21
This is a very cool topic; skis aren't built the same way at different lengths. There was a great thread about this in which an engineer/physics type - think it was either 219 or Physicsman - corrected my thinking on this. The real issue, turns out, is not length but mass/unit of SA. Meaning that a longer ski SHOULD be a bit softer all else equal because as a ski gets bigger, there's more surface area to deal with your same weight. If you maintained a constant absolute flex, the ski would actually get stiffer and stiffer relative to a constant load.

Here's an example using PM's calculator: Take a typical midfat, 120/82/110. At 170 cm, it's 1581 sq cm. At 177, it's 1646 sq cm. Now if a 160 lb skier uses the 170, the wt/SA is about .10. If he uses the 177, it'll drop a bit, .96 lbs/sq cm. So for him, the longer ski will be stiffer (less weight per unit SA). Unless you make it flex a bit softer, in which case, it'll still have the same flex. Of course, he can just grin and ski it faster, which is a more interesting way to bend stuff than gaining weight.

But with apologies to the engineers/physics guys here, I don't think longer skis are particularly softer. Why? Cuz companies assume that longer skis will have bigger skiers attached to them. So let's say a 185 lb guy straps on the same 177. He'll be at .11 lbs/sq cm, so the 177 will be a little softer for him than the 170 was for for the 160 lb guy. Maybe he'll need to go even longer, or the company may make the 177 actually a touch stiffer to allow for the probability that bigger folks will ski it. Like Sierra Jim says, each company has proprietary formulas for this. I've tried repeatedly, for instance, to get a clear answer from Volkl about their strategies, nada. But my guess is that any prediction model for skier weight to SA at any length is non-linear, meaning that the longest and shortest skis are disproportionately stiff or soft to allow for very big or small skiers. And at the top ends, very fast skiers.

These are big enough diffs to have an impact on ski feel. OTOH where it really shows, and where I think 219 et al. are right on, is in width, since SA is more affected by that than length. Let's say our skiers lust for a fatter version of the same ski: 130/88/120. OK, now the 177 cm length will be at .10 lbs/sq cm for our 185 lb skier, or about the same as the 170 82 mm was for the light skier. Put another way, wider skis need to be disproportionately softer relative to narrower skis to achieve the same flex. Which is why TGR guys who ski pow at 60 mph are always complaining about noodles. If you left 100+ skis with the same build as 88's, they'd all ski like the Supermojo 103. :

You'll be tested on this tomorrow.
post #9 of 21
You'll be tested on this tomorrow.
Nope....I'm gonna be sick that day.

post #10 of 21
Originally Posted by beyond View Post
I don't think longer skis are particularly softer. Why? Cuz companies assume that longer skis will have bigger skiers attached to them.
Because of my size and ability I generally end up skiing the longest length in a particular model ski. I have been lamenting the fact that many ski models now stop in the low 180s, but that's another issue. I have found 3 types of longer skis. Many acutally feel softer than the shorter length, which is what I look for and prefer to ski. They design the ski in a particular length and then keep the materials the same, which results in a ski that is easier to bend in a longer length so it effectly skis softer. [This is the same with most boots where there are "softer" for bigger skiers and stiffer for smaller ones because they don't change the thickness of the material with different sizes.]

Sometimes the longest length of a model is a completely different ski because it is designed for the company pros to use. An example of this was the Rossi XXX, which was a fantastic ski in the shorter lengths but the 195s were about twice as stiff as the 185s, even though they looked exactly the same. Another example is the original Stockli Storm Riders that had an extra layer of titanal in the two longest lengths (198 & 204) making them considerably stiffer and useless at low speed.

The other category is where they acutally change the ski to make it stiffer as they get longer resulting in it effectly skiing the same for all skiers, provided they are on the proper length for their weight. This is what ski companies lead you to believe they are doing, but I don't think it is really that common.

From my experience I think you need to be careful. If you demo a ski in a particular length and then buy a different length, you may be getting a ski that feels a whole lot more different than you expect.
post #11 of 21
Thread Starter 

Re. Surface Area

If you are talking about wider skis that are supported by deeper snow then what you are saying is no doubt relevant. However, if we are talking about a carving ski, up on edge, then it is pretty well acting s a beam and surface area is irrelevant.
post #12 of 21
Bingo! The level arms get longer, so, all other things being equal, the ski would become much easier to flex at the longer lengths. The lever arms being between you and the tip/tail.
post #13 of 21
Ideally,the ski is acting like an upside down beam, supported in the middle, with a uniformly distributed snow load along it's length (or some manufacturers might have a greater load nearer the midsection, but lets use a UDL as a first cut). It's been too long since I last designed a beam, so the beam deflection equation isn't at the tip of my tongue so to speak....Young's modulus is stress over strain, bending moment as in the integral of the load times distance from the post integrated along the post must equal bending moment of stress in the beam integrated across the crossection..... Go ahead and resolve it if you want to. I will approximate.

A longer ski, ideally is designed for a heavier person, hence a greater load pushing up on the support post, and a greater total load pushing down along the entire length. Again, ideally you have the maximum load the snow can support, bending the beam into the turn radius. So for a carving ski the load per unit length of ski is the same, and you want the deflection to be the same IF and only if you want the same radius. Typically though, the longer ski also has a longer turn radius. For longer skis that have a longer turn radius, you will have stiffer ski.

Bottom line, most skis I've come across have a longer turn radius and are stiffer in the longer length, but there are exceptions.
post #14 of 21
Has anyone asked BrooklynTracy about this?
post #15 of 21
Question, then: Do you think ski flex INITIALLY is designed around a beam model or is the whole ski taken into account? More to the point, does a "mid-fat" ski represent a compromise between two models, one beam, one optimized for running flat in deeper snow? Seems to me that a lot of the actual differences between brands that we attribute to flex ("stiffness") may distill down to whether their model more weights being on edge or being supported by snow.
post #16 of 21
A ski IS a beam, so the beam deflection equation must apply. There is no way around that. A beam has width, and the ski width affects the amount of load at that particular distance form the ski when we are dealing with softsnow, wider tips deflect the beam more and stiffer material cause it to deflect less. The way material and stiffness is distributed along the ski must be considered in conjunction with width.

However I think I understand what your trying to ask, should you optimize including the effect of width or just assume the ski's edge will govern without having to adjust for width as on hardpack. There is definitely a compromise there. Most skis I've had the pleasure of testing seem to work better when the ski is able to get enough of the tip and tail into the snow that ski width factors into it. My guess is that they are optimized for width being included, with some compromise for hardpack specialty skis. The compromise means that the ski is a little too stiff for the turn radius when tipped on edge, but not burried in the snow and is trying to put more of the load on tips and tails than ideal for the edge grip available.
post #17 of 21
Thanks Ghost, your comment clarified things for me.
post #18 of 21
I thought it was common knowledge that stiffer is longer.
post #19 of 21
One thing alluded to must be remembered...only compare ski lengths between other lengths in that ski brand and model. Other models of that brand might compare fairly, and might not. Other brands might compare fairly, and might not.

Years ago Salomon tried to get away from marking ski length on their skis. They marked "Performance Factor" on the skis...I skied well on PF8 skis. PF9 were too demanding, and PF7 were too whimpy. That was a good system that didn't sell. Ski shop clerks picked up a PF8, or whatever, to show to a customer and said, "here's the 197 you asked for."

Length doesn't really matter...in skis. The feel of the ski matters. The ski feel relates to length, but also relates to all the other parameters so clearly expressed in the posts above.
post #20 of 21
Originally Posted by epic View Post
Has anyone asked BrooklynTracy about this?
Brklyn skis are very soft, like really soft. Have you skied them?
post #21 of 21
Originally Posted by BushwackerinPA View Post
Brklyn skis are very soft, like really soft. Have you skied them?
I guess you missed her post in the lounge.
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