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Why Not Use Isotropic Surface Structures?

Fri, Feb  4, 2011 - By Mark Waechter

From Nordic Ultratune's February 2011 Newsletter (PDF). Thanks for sharing Mark!

I am occasionally asked why ski bases don't have random or non-directional structures. Typically the question is posed with the addition of "...like a golf ball!"

Structure a ski base with dimples like a golf ball?The idea of random isotropic surface structures seems attractive at first. Something simple, elegant, and maybe even something easy to apply to a ski. Imagine a surface finish on a ski that could be sprayed on! Perhaps smooth indentations on the ski base from a shot-peen gun, or cuts from an abrasive sandblasting apparatus. That seems simple and relatively inexpensive. But would it work?

Golf balls travel through the air, spinning, without a set orientation to the travel direction (imagine having to line up the golf ball just so on the tee...!).

Additionally, the axis of spin can translate while the ball is flying through the air. So a nondirectional isotropic arrangement of the dimples is best.

But a ski doesn't travel randomly; it doesn't behave the same in all directions. Cross country skis travel in a straight line, and always work through the snow tip-to-tail. For this reason, the base structures are optimized to maximize the benefits, and minimize any negatives. Structures and base finishes need to take into account the motion of the ski against the frictional interface.

A structured ski: Linear elements organized with an angled pattern.

Note that while skating there is typically a bit of torsional force and movement of the ski in the snow. Additionally, skate skis usually slide through less transformed snow than a ski in a set (classic) track. So skate skis and classic skis are sometimes treated differently.

Skate ski structures are tending toward broken patterns, which have easier separation and less stickiness in the ski release. Long, straight, continuous linear structures with continuous frets tend to track well but have a stickier release or separation from the snow. Broken structures with linear elements, but with angled interference patterns are proving to work very well on both skate and classic skis. (Photo at right: A structured ski - Linear elements organized with an angled pattern.)

So, skis don't get the simple isotropic base structures, but rely on lengthwise oriented patterns. The ski structure designs utilize depth, spacing, orientation, and pattern to optimize the ski speed through the snow in various conditions.