Tom’s Musings | Stonegrind 301
- Posted on
- By Tom Novak
- Posted in Stonegrinding
Structuring the base of your skis is the final stonegrinding step. The 3rd detail of grinding after #1 flattening/removing sealed polyethylene and #2 polishing.
Ski speed (influencers being structure, ski flex and wax) is determined by friction between the ski base and snow. Micro mechanical friction found in cold dry abrasive snow and water adhesion found in warm snow. We will focus on the part structure plays to help mitigate these effects. A very lightly structured ski base in wet snow or a roughly textured ski base in cold dry snow - respectively, neither structure would be successful creating speed. In fact, those grinds being applied in the wrong snow conditions greatly inhibits ski speed. Your skis glide best on a thin moisture film beneath – studies show 0.6 micron to 8 micron (0.0006 – 0.008mm) (compare to human hair thickness 50 – 70 microns) - easily present in warm snow through friction or already existing and a moving target in cold snow. However, too much moisture creates capillary drag (adhesion) – water molecules sticking to each other and other surfaces. Now you aren’t just skiing over a water layer but you are dragging it with you. Wet snow requires a structure management approach that efficiently channels excessive moisture from beneath your skis mitigating adhesion.
Cold dry pointy chunky abrasive snow – a sandpaper like effect – results in micro mechanical friction. This stuff wants to grab your base and create drag. In the absence of warm/wet snow and the accompanying lubrication, cold dry snow crystal compaction, abrasion, sheer and tumble characterizes mechanical friction happening under your skis. You’ll never glide as easily in cold snow as wet snow. In these conditions, creating enough friction to produce an effective moisture film relies on a variety of variables to perfectly line up. A tall order. Simply, friction has a hard time maintaining enough heat to produce effective snow melt lubrication. Instead, snow crystal tumble at the snowpack surface and snow crystal abrasion and sheer as the ski base glides over the snowpack, aids glide. Depth and shape of structure is minimal for this kind of snow. Too much structure would contribute to crystal penetration creating drag.
With a clean flat freshly exposed and smooth base, structure – the 3rd and final step - is etched onto the bottom of your skis the same way the previous 2 steps were accomplished. The diamond bit table in our CNC stonegrinder is programmed to etch the requested structure and the ski is run over the stone one last time. Most of these skate or classic structures are compound structures – structure layered over another structure. This layering is done when dressing the stone. Compound structures provide more nuanced detail when building those structures.
*An Investigation of Ski/Snow Friction by Rasmus Nes Tjorstad 2018
*Gliding for Gold. Physics of Winter Sports by Mark Denny 2011
*Snow Temperature Behind Sliding Skis as an Indicator for Frictional Meltwater by Hasler Jud Nachbauer 2021
*The Mechanics of Snow Friction as Revealed by Micro-Scale Interface Observations by Lever Taylor Song Courville Lieblappen Weale 2017