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 = 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 Wailer 105 178cm (spec, but supposed to be based upon actual production runs)
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.