During the 2008/09 season, I decided I needed new touring skis. My 2004/05 Dynafits were getting a bit old, but the real problem was that they never performed well in powder/soft-snow. So surface area was a major consideration in choosing new ones.
Most manufacturers don’t seem to quote surface area, so I needed to come up with a way of comparing skis of different lengths, widths and side-cuts. Rather than doing it properly, I made a simple model, where a ski is shaped like this:
and the waist is bang in the middle. This allows you to break the ski down into a long rectangle (A in the diagram below), and four triangles, B1, B2 (of equal area, which together make a rectangle) and C1, C2 (likewise):
The area is of course: A + B1 + B2 + C1 + C2
or: l * w + l/2 * (t – w)/2 + l/2 * (p – w)/2 <=> l(w/2 + t/4 + p/4)
I ran this into a Google spreadsheet and calculated values for a my old skis, my Volkl Mantra freeride skis and the two touring skis I was considering (Volkl Snow Wolf and Dynastar Altitrail Powder).
I also did a rough calculation of ‘sidecut ratio’ to see which was the straighter ski (important for steep skiing – my passion – and icy slopes – unavoidable).
Dynastar came out top and along with this glowing recommendation on PisteHors.com, my mind was made up. It was a good choice as seen here.
A couple of weeks ago, I came across a report on a new carbon fibre ski that did quote surface areas. Based on this, my model was coming out about 5% too high, so I applied a fudge factor to the above calculation to get the final numbers you see in the spreadsheet.
Given that the actual surface area doesn’t matter, and it’s the relative values that are useful, perhaps this calculation is good enough – and there’s a limit to how accurate you can get without measuring the actual shape of the ski. That said, does anyone have a better model to replace this series of fudges and approximations?
