Maximizing Your Calculatizing — Ski Surface:Weight Spreadsheet Fun
Lou put me up to this. No, really, he did — it wasn’t my idea! Although I suppose he knew I was the kind of person who would run with it (as opposed to run away from it).
So okay, what’s this all about? Well, some skis are light, some skis are fat, and some are kind of both. So how about a calculation for the ratio of surface area to weight?
I wish I could say that my home ski shop has some sort of scanner device that automatically determines the surface area of a ski. No, actually, I don’t wish that at all. But either way, I don’t have that (although admittedly I haven’t checked all the forgotten corners of my basement recently, so who knows).
But what I did do was set up a spreadsheet that calculates an estimate of a ski’s surface area using only the sidecut dimensions and length, then divides that by the weight, and scores the result as a percentage of the ski I’ve actually weighed with the best ratio.
Nitpicking on the particular approach I’ve implemented is easy pickings, but our goal is a calculation that requires no detailed measurements, and instead adopts some simplified ski geometry and a few simplifying assumptions. In other words, you could conceivably get results from the blurb in a catalog, though a “real world” weight and length measurement will do better.
Specifically, in this calculation I treat a ski as two isosceles trapezoids, joined together at the point of minimum waist width. Now what about those tips and tails? I exclude from the surface area calculation any portion of the tip and tail with more than a 1cm gap from ski-to-ski when the skis are placed edge-to-edge (i.e., with the bases on the floor).
Based on measuring some skis, I chose 7cm of tip and 4cm of tail as the portions to exclude for my generic values. Also based on some sample measurements, I chose 61 percent as the portion of the ski for the isosceles trapezoid from the tip to the waist, and then the remaining 39 percent for the portion of the ski from the waist to the tail. The calculator can of course be modified for model-specific values instead of the generic values.
This of course assumes that ski surface area in a rocker or slow-rise region is still part of the “running surface” for soft-snow flotation purposes.
One obvious weakness is that actual physical ski length is reported slightly differently by different manufacturers. That, combined with more or less sidecut and tip/tail shapes skews the results. Thus, the approach here is to create an ESTIMATE that’s accurate enough give each ski a weight score in relation to other skis. Please know this is NOT an attempt to arrive at an exact grams per unit surface area number. (And even such an exact calculation would still leave out many other factors that contribute to ski flotation. Okay, enough caveats for you yet?)
To check how well this works, I developed a more accurate method for a few pairs of skis I had lying around, then compared results to this streamlined method. The results were close enough for confidence that the formula below will yield a useful comparison between ski models in terms of how much running surface you’re really getting for the weight you are hauling. Just remember that differences of a few percent probably mean very little, or perhaps nothing. In other words, the idea is to place a given ski into a weight “class” by using the results as a scoring system.
To perform your own calculations, just copy this formula in a spreadsheet cell:
(D5-11)*(AVERAGE(E5:F5)*0.61+AVERAGE(F5:G5)*0.39)/H5/16.9879
… where:
D5 = ski length in cm (with 11 subtracted to represent 7cm in the tip and 4cm in the tail)
E5 = tip width in mm
F5 = waist width
G5 = tail
H5 = actual measured weight (which can very significantly from spec) in grams (for a single ski)
And the 16.9879 value? That’s the ratio (based on actual values instead of those 11 and 0.61/0.39 generic values) for a Movement Fish-X rando race ski. Some other model out there might be able to edge it out a bit, but realistically I think this has to represent an unattainable upper bound for touring skis, so that will be my elusive 100-percent score.
Based upon running the generic values through my calculator, some values I’ve derived include:
94% Goode Wasatch 169cm (weight based upon emails with Mr. Goode)
92% Movement Logic-X 168cm
73% Dynafit Manaslu 169cm (first generation)
73% Trab Duo Sint Aero 164cm
73% DPS Skis
71% Trab Volare 171cm
64% DPS Wailer 99 184cm
60% Black Diamond MegaWatt 188cm (2010-11 version)
Oh, and just for kicks, an alpine downhill slalom race ski (with plate removed), 48 percent.
Now, I’m not suggesting that you should choose your skis based solely on the score for the ratio of surface area to weight. (Or am I?) But this calculation does provide some interesting insights. For example, although the BD MegaWatt is nobody’s idea of a light ski (or is it?), given how much surface area it provides, its weight is actually quite respectable.
Keep in mind though all the previous caveats concerning the assumptions and approximations in my formula. (And remember that flotation is of course a function of far more than just surface area. Or did I say that already? Can I add any more caveats here…?) As a rough rule of thumb, I’d say that deviations in scores up to 5% are probably entirely meaningless given all the assumptions and approximations in my formula, so if you do care about this score, then probably only up around the high single digits do differences across scores have much meaning.
And yes, others have played around with this approach; we are not the first and make no claim to that. Just something fun that might be useful to those of you with the Wildsnow state of mind.
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Comments
50 Responses to “Maximizing Your Calculatizing — Ski Surface:Weight Spreadsheet Fun”
Just spend two hours calculating different skis. Thanks for decreasing my work productivity 50 fold.
Yes, even though WildSnow.com overall helps the ski industry, it certainly is a drag upon certain economic sectors!
One good way to categorize statistics is to place groups by deciles; ie top 10% bottom 10% etc. This bypasses the 5% deviations which you say might not make much difference.
Jim, good point, but then the problem is that the decile rankings would change over time depending on the total universal of skis measured, plus two skis just barely on opposite sides of the cusp would be put into two different deciles even though they just slightly differed from each other.
I think this is possibly the most valuable calculation to have when choosing a ski. More surface area under the foot will support the weight better than that at the tip and tail, so of course the area is not the be all and end all, but this is a very important consideration for someone like me (6’5″) who has trouble deciding if a shorter fatter ski can take my weight.
This just makes me smile on so many levels.
You guys that are out there calculating away, share your results! And don’t forget you can troll up some real world weights from our weight chart (see menu above for link).
Lou
Since this is a bc/at site, such a comparison should also include skin weight as a minimum, and possibly also aditional boot and binding weight.
It would be nice to get a floatation formula on some uniform surface (tub of honey) 🙂 and corelate it to skier’s weight and of course the surface area of a ski.
I just want to be able know which ski will provide the best opportunity for Face Shot photos. Glistening teeth must show through lightly dispersed sunlit flakes.
nerds
NICE! I did a similar thing earlier this year, only I divided the ski into thirds and called each third a rectangle. Also I listed the actual ski mass in its own column so that I could use Excel’s sort feature to look at both parameters. Unsurprisingly, some skis have a great surface area: weight ratio, but are probably more heavy than most of us want for a touring ride (see Ski Logik Depth Hoar as an example).
I love this idea and it would be awesome if WS kept a running database with these numbers!
On a slightly related note, I just happen to have a pair of Goode Wasatch 177cm in my hands as of last night. I also spent most of last spring on a pair of Goode Wasatch 177cm(different pair). More to come soon after a bit of ski time to refresh my memory on their performance.
Rob, the proper term is geeks, meaning smart nerds who rule the world.
Bob, where/how did you even know these exist? Besides a Backcountry mag review of the women’s version (Three Sisters), the new Goode bc line-up seems to have very stealthy marketing.
And actual weight of the 177cm Wasatch?
ummmm…..
theyve had a rough version of this over at TGR for years? 😛
we should probably start by stealing that and make it better?
😀
Exactly where is this other calculator located?
not a calculator
just a compiled list of ski length/weight ratios to compare actual weight
a lot simpler than this idea
Ima try and find it
Jonathan, my history with Goode goes back to their very first carbon poles which is god only knows how many years ago and predated any skis. I skied on their early protoypes of skis and on the Carbon 95 as my AT ski for many years. I can give you lots more background, history and current status though I think it is far more than anyone on Wildsnow would care to digest unless they are geeks like us. Please contact Lou directly for my email and cell# and let’s talk. That said, I will post a full review including weights as soon as I get a chance to spend more time on the skis.
TGR has everything, and had it first, we should cancel Wildsnow.com.
lou, dont be like that now… 🙁
I posted this in support of the idea, Im really excited about this.
I think looking at it would save time…
why not have a look at what others have done before first?
Alpin Magazine actually arranges all their skis in their reviews by weight:length ratio. See http://www.wildsnow.com/4658/ski-reviews-touring-backcountry/
But again, without a surface area ratio it’s not that useful of a number, though it does tell you what the plain old really light skis are.
We’re publishing this year’s online English version of the Alpin reviews soon, perhaps even some tomorrow.
As for TGR, you can’t blame me, we start having fun with something and half the time someone chimes in with no contribution but a link to some rowdy or even unpleasant comment thread over there. Buzz kill. And a bit insulting, as anyone who even knows what Google is can find that stuff. Doing so is not rocket science.
If it’s something cool and useful, no problem linking. But yes, I get touchy. Reminds me of the DPS comments we got for a while, when it seemed like every time we’d do a ski review, an obvious shill would chime in with a DPS link. Or worse, the Quiver Killer vs Binding Freedom cat fight.
Ok Lou, I understand.
But I posted because they WERE actually calculating surface area ratio there too.
And I really do believe those threads are very relevant.
Ok, sorry about that. Like I said, I get tired of the folks dropping by with messages that have no more content than “come on over to the other party.”
For what it’s worth, here is a Google Search that leads to other weight threads.
https://www.google.com/search?q=ski+weight+thread&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
Incidentally, I was interested to see some water skiing posts related to this subject…
Going only by weight:length ratio, I should be on my xc race skis all the time.
That TGR thread got off to a good start in 2007, but unfortunately the original poster never updated his spreadsheet, or explained his calculations (as far as I can tell), or even made the presentation more, well, presentable. Plus almost all of his skis are quite heavy models, so why bother with such skis?
Since then it’s just been posts with weights mainly for the type of gear whose weight nobody generally cares about all that much. And no further surface area calcs as far as I can see (unless I’m missing something?).
Lance Riek in Backcountry Magazine for Issue # 29 for 2001 (month not specified, perhaps to avoid any issues with publication timing and shipment expectations?) reviewed five skis for which he calculated the pounds for an equivalent three-tenths of a square meter. He didn’t provide his calculations, but based on his benchmark dimensions for 0.3m^2, I got 0.2875, so we’re reasonable close to each other.
Running those skis through my calculator, 59% for Atomic Tour Guide Superlight and 63% for Dynafit Lite Tech, both with current rando race dimensions and performance, yet pretty heavy compared to the 100% target for a current rando race ski, so shows how far we’ve come since then for those types of skis.
The Tua Helium & Hydrogen were more respectable at 69% & 70% (respectively), although I recall they had breakage issues?
And the Goode Extra Versatile with a 91mm waist scored 73% — ten years ago! (Bob, were you ever on that model?)
Whoops, correction: the 2007 TGR spreadsheet does explain the surface area calculations (very similar to mine).
Jonathan, yes, both I and my partner in the cat skiing biz in Aspen had the Extra Versatile. He still has a pair sitting in the corner of the office collecting dust. Not to mention the handful of garages in Aspen I could go to tomorrow and grab barely used pairs still hanging on the walls. It wasn’t a particularly good ski from a performance or durability perspective but the concept had to start somewhere to get where it is today. Which is to say light years from ten years ago. Of course, if we are going down memory lane then let’s revive the DuraFiber!
This is a stat that is totally worth geeking out over. I mean, what will be more fun, a 177 mantra (95mm waist) ~2kg/ski vs something similar in weight but longer and wider (Justice, Charger)? or something with similar dims to the mantra, but 0.5kg lighter (Manaslu)? A fun ski that makes efficient use of surface area (in terms of weight) is desireable
The calculation of ski weight to surface area will be invaluable next time my skis want to go skiing by themselves. They will know exactly how deeply they will sink in, assuming that they also have acquired instantaneous telemetry capabilities to measure the density of the snow under their bases.
Meanwhile, I’m more interested in the ratio of running surface to my weight. At last measurement it looks like I need a ski 2.201 times as big as my girlfriend’s, but I’m having trouble finding any lightweight 250cm skis.
Wildsnow has the best parties.
That being said, here is a link to a chart with sortable columns which includes a density calculation of some sort and it includes current ski models:
http://www.skitour.fr/matos/comparatif-skis.html?tri=largeur
JonM, nice find with that sortable chart, thanks!
They put five other race skis (plus a few unobtanium touring models) ahead of the Fish-X, but the “Densité” column matches up very well with the intermediate calculation I have in my spreadsheet, so nice to have that verification.
Have been out on the Atomic Charter which is basically a fat (100mm) reverse camber ski with a straight side wall camber under foot. Never imagined i’d buy into the reverse camber concept, but these actually ski incredibly well on hard snow and no comparison in powder and crud; they are unbelievable. It also comes in a 186 which adds weight to a light ski, but it adds so much to performance it’s worth it. It’s about 10cm’s longer than what i have been on, but I bet in a shorter length it would be really good too. I have no idea where the ski would fit into the equation in the story, but am sold on the movement towards manufacturing skis that go well with oversized tennis rackets and golf clubs. I just turned 47, so on the band wagon I ride. ha.
Thanks for the Charter review. Been very interested in this as the Access is quite a ski, but the Charter is hard to find and even harder to demo…
Jonathan: If the unobtanium models you are refering to are the alp controls, I feel it should be mentioned that these seem to be the real thing. The french magazine “Montagne” gave Alp Control top scores, well ahead of x-logic that was next best.
OK. I’m a damn engineer. Here’s your sign. This will waste the rest of the day.
Great stuff Lou and you guys have a nice turkey day.
http://www.math.utah.edu/~eyre/rsbfaq/physics.html
Wow, fun stuff here – thanks for posting.
The weight/surface area is a good metric, but for me the hard to quantify factors of FLEX (distribution) and CAMBER/ROCKER still make an on-snow demo worth the effort before a purchase decision. I’ve tried out lots of lightweight skis, including fat ones, that really don’t do it for me when I’m coming down.
Happy Thanksgiving, all!
Glad to hear that the AlpControl skis are legit — I’d always wondering about them. But oddly enough, when I run their specs through my calculator, I get results between 86% and 90% (depending on model/length), which although very impressive is still slightly below the 92% Logic-X.
Bottomline though is that I think the 16.9879 value still provides a reasonable “100%” standard. And also as previously stated that small differences (e.g., between what I calculated for AlpControl v Logic-X) are probably meaningless. (And of course as many have commented that what makes a good backcountry for both the up & down goes way beyond a simple number like this — well, except for the upcoming WSFF, i.e., WildSnow.com Fun Factor that will even take into account the opportunity cost of what you would have spent the money on otherwise.)
Going down is very subjective indeed. Montagne loved alp control and x-Logic, were mediocre towards The mustagh ata. Fri flyt ( norwegian magazine) and fremch ski had The mustagh ata as winner and were mediocre towards x-logic…
@JonM
@jonathan
just be aware the skitour.fr chart uses PUBLISHED weights not real world from what I can see
some of them are 10% off
Thanks everybody for all this work and input.
It seems to me that the 7cm that you subtract from the tip and 4cm you subtracted from the tail depend on the type of snow you’re on. For instance, if you are in 30cm of fresh snow, the surface area measurement that you are going to want to use will not subtract any of the ski’s entire running length at all, because the entire ski’s surface area makes contact with the snow. Whereas if you are on harder snow, you probably wouldn’t want to calculate tip and tail rockered portions of the ski. Maybe this would boost the BD Megawatt in the standings, because you are using all of that big rockered shovel to help you float in pow.
Just what I need, another speadsheet of cycling/skiing stuff. Seems like I should just use skating skis for everything, based on this. 🙂
re xc skate skis, I just calculated a Rossignol X-ium Skating WCS 1 at only 79%.
^ Atomic’s World Cup skate skis are alleged to weigh 900g/pair, so for one ski (IIRC 41/44/44mm) x 184cm I get 98%, and there are other skate skis close to this weight and width. I’m not quite sure if the Dynafit bindins would fit on the topsheet, given the width, but no doubt a lightweight riser/adapter plate could be made.
Note: I’m not saying this is a sensible idea(!), but you did say “Going only by weight:length ratio, I should be on my xc race skis all the time,” and this started me thinking 🙂
On Nov 22, 2011, Bob Perlmutter mentioned that he had a pair of Goode Wasatch skis that he was trying out and planned to review. With our poor season, has Bob had a chance to test these? They are a ski in which I am interested but am having trouble getting thorough, unbiased information on. I intend to use them, or similar, for BC skiing mainly in the Wasatch. I am 51 years old, 75 kg and 6 feet tall, a good but not great skiier – more strength than technique. I am researching a few but am impressed by the favourable weight to surface area ratio of these.
Thanks.
John S; I have been looking for that review as well! We have been using Goode skis at our house for awhile, and, are ready to replace our current skis.
Bar and all, we like the super light weight of the Goode skis, Lisa has two pair she uses mostly for fitness uphilling, and we have another friend who’s an excellent skier who does the same thing. In our experience, they’re all only average in downhill performance, hence we’ve not put much energy into reviews since we generally tend to just review stuff we like for backcountry skiing. Since you guys are asking, however, perhaps we can pressure Bob P. into leaving an extensive comment on his Goode experiences. He’s tested them much more than I have. Bob?
Forthcoming. Major parenting duties taking priority but will respond in full soon.
OK, I have resurfaced and back to the subject of skiing, specifically the Goode Wasatch. Goode has totally revamped their entire ski line this year. All new dimensions, profiles and constructions with production moved from China to Ogden, UT. Most of the skis are made in two versions, a wood core alpine construction and a “hellraiser”/kevlar core backcountry construction. The obvious advantage to the BC construction is lighter weight.
I had a pair of loaner 177cm Wasatch w/ Fritschi Freeride Pro through early Jan. of this season. Unfortunately no significant snowfall came to Colo. until mid Jan. That said, I used them for uphilling at the ski areas and some low to moderate angle side country in the cat skiing terrain where I work. The basic lowdown are dimensions of 135/98/125 R=18.5 and an unpublished weight of 2865gr. The weight was provided to me by Goode’s ski engineer. They claim the Wasatch has a slight tip rocker but if that is the case than I need a microscope to discern it.
My AT ski of choice for a number of years was the Goode Carbon 95. The performance of the Wasatch and all of the new Goode skis is vastly improved over anything in the past. The Wasatch has greatly increased torsional stiffness, tracking, stability, dampness and responsiveness. It makes for a very good one ski quiver for the backcountry. Well rounded but not flashy in any one aspect. They exhibit the combination of smoothness and liveliness at a light weight that is unique to all carbon construction. That said, the BC construction does have a speed limit and if freeride is part of your equation then you might look elsewhere. My only wish would be for a true slightly rockered tip to improve soft snow performance. Towards the end of my brief affair I had the Wasatch tuned and it made a huge difference. Turn initiation greatly improved(increased bevel?) and all of the other characteristics were enhanced. I even thought what a great all mountain front side ski the Wasatch would make short of truly hard snow. I also had a pair of the Goode Fire on hand, the Wasatch wood core alpine counterpart. Take my comments about torsion, tracking, stability, dampness and responsiveness and ramp it up a notch or two and you have the wood core version. Some people might choose to fore go the weight savings of the BC version and go for the ultra performance.
The real question is not how the Wasatch stacks up against Goode skis from years past but against current models of similar dimension and weight from other manufactures. The two skis that come to mind are the DPS Wailer 99 Pure and Ski Trab Volare. Skis like the Goode that combine high tech with high performance. Do not fear as I have a Volare on hand for what remains of the soon to be forgotten dismal season in Colo. Luckily I am headed to Norway in May for a week of skiing and will offer a detailed comparison soon.
thanks Bob!
I would kill for an exact real world Wasatch weight not from Goode HQ.
Their claimed weights are known to be about -10% off.
I was told of an instance in which Dave Goode supposedly provided someone with a much different and lighter weight for the same ski. That tends to make me think the weight I was given is more accurate. If one is going to be inaccurate it seems the incentive is towards the lighter side rather than the heavier side. It would be nice to be able to verify the real world weight.
Heck
I have seen there weights off 20% or more.