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# Mantra, Gotama, and Sumo similar radii explained

I noted on Volkl's website that these three have nearly the same radii at 175-177 cm. Couldn't believe it, ran the dimensions through PM's wonderful spreadsheet, got very different radii. So I asked Volkl if they, ah, made a mistake. Thought some of you might enjoy this reply:

"We feel the given values are correct.

To properly discuss radius, we need to properly define what these values represent. These numbers are simply circular approximations for non-circular (Volkl sidecuts are elliptical) sidecuts on a new, untuned, unflexed ski when it leaves the factory.

The term "turning radius" implies that a certain ski will naturally make a certain sized turn in the snow, and the truth is that there is no math to support that, so we try to stay away from that terminology.

As for the direct answer to your question...the values are correct because the key to determining sidecut radius is knowing the distance from the widest part of the tip to the narrowest point at the waist, and the distance from the waist to the widest part of the tail. Total ski length can only confuse this issue because the measurement includes tips and tails - parts of skis that don't figure into the computation of a radius. This effect is worsened by twin tips like the Gotama and Sumo.

So a 175 Sumo or 176 Gotama can have a similar radius (remember that ski radii are only approximations anyway) to a 177 Mantra, even though the sidecut does not appear to be as deep, because the sidecut has a much shorter effective running length than a 177 Mantra.

Hope this helps."

I'll guess that that eliptical or more complexly shaped sidecuts, together with mildly flipped tails, may explain why some of PM's radius numbers can be in disagreement with various mfr websites. Also, as Volkl points out, the experienced turning radius - not to mention the ease of turning - cannot be quantified. As we've discussed elsewhere, I'd guess the flex and skier style have more to do with those than radius or even width - thus the PR/1080 Gun lineage are weirdly good in bumps and trees, even compared to some narrower, more sidecut skis from stiffer lineages.
That's the biggest load of horse hockey I've read in some time. I agree that based on the dimensions (and my tweaked version of PM's sidecut calculator) there's no way that Volkl is publishing good numbers. I found the same thing with my Elan's and wrote to them last season. They claimed to be calculating it correctly, but they clearly we're just printing a marketing number on the ski.
Yeah, definitely smells funny. OTOH, the point about non-circular radii will have an impact. Just not 8m, which is the diff between PM's sheet and the website numbers for the Sumo. I also noticed that the differences were greater as the skis got fatter. Pretty close for AC3 etc. Unclear why. Maybe they figure people will be freaked out by skis with 30m+ radii, or maybe the turned tails really affect their calculation of running length.
I really don't know the answer, but I will offer a couple of points to the discussion.

1) PM's calculator makes the same assumptions for effective edge length for twin tips as it does for non twins. I would submit that there is a noticeable difference between the effective edge length of a 177 Mantra and a 176 Gotama.

2) We'll never know the real answer until somebody measures the actual effective edge length of both and plugs those values into the equation.
Quote:
 Originally Posted by troutman 2) We'll never know the real answer until somebody measures the actual effective edge length of both and plugs those values into the equation.
Exactly. PM's calculator is based on effective edge length, but it calculates it automatically by subtracting a fixed amount from the length of the ski. That amount WILL vary. The calculator works well for many skis, but is not foolproof.
Hi Guys - Those are very interesting discrepancies you have found. In this particular case, I'm not sure the reason which is most applicable, but, FYI, the following old posts discuss these sorts of discrepancies:

http://forums.epicski.com/showpost.p...1&postcount=19
http://forums.epicski.com/showpost.p...2&postcount=26
http://forums.epicski.com/showpost.p...95&postcount=1
http://forums.epicski.com/showpost.p...27&postcount=6

Anyway, please keep me informed on what you discover.

Sincerely,

Tom / PM

PS - Also, don't forget that the original purpose of the calculator was simply to be a quick and dirty way for people to get a rough estimate of the sidecut of skis that didn't list this number. It was never intended to be the definitive reference that could be used by FIS officials (or ski geeks like us). . For the latter crowd, I recommend the method I describe in the 2nd post I cite above.
Quote:
 Originally Posted by U.P. Racer Exactly. PM's calculator is based on effective edge length, but it calculates it automatically by subtracting a fixed amount from the length of the ski. That amount WILL vary. The calculator works well for many skis, but is not foolproof.
This is exactly the part of the formula that I "tweaked" for my calculations. Based on my real-world measurements of tons of skis, I have modified the "assumption" and now use .90 (or 90% of the advertised length). In general non-twin tip skis have an effective edge length of about 90% of the advertised length and twin tips can be anywhere from 86%-88%.
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EpicSki › The Barking Bear Forums › On the Snow (Skiing Forums) › Ski Gear Discussion › Mantra, Gotama, and Sumo similar radii explained