Lance’s Lab: Rocker Revelations

After watching their parents float through fluff and rise above backcountry crud for four seasons, my daughters finally drew the line. “We NEED fat, rockered skis!” one said. When asked why, they responded, simply, “Because they FLOAT.”

That got me thinking about how skis “float.” Imagine a rectangular, tip-less ski on a level snow surface. Gravity forces the skier/ski downward and the snow pushes back until the two forces reach equilibrium. Ski surface area keeps the skier from sinking deeper into the snow.

Now, tilt the snow surface. Split the gravity force into two vectors—one of which pulls the ski perpendicularly into the snow, the other pulls the ski parallel to the surface. The ski won’t sink as far as it did on the level surface because the perpendicular force is less. The ski will accelerate until snow friction and air drag on the skier balance the parallel force of gravity.

So, how does snow provide float? Thinking deeper: skis don’t “float” by fluid displacement like boats do in water. Wu et al.1, 2, 3 modeled both wind-packed snow and fresh powder as compressible porous media and backed it up with experiments. Interestingly, they found, as skis rapidly compress powder, trapped air provides significant float and rebound. “Dramatically different dynamic behaviour is observed for two different snow types,” they wrote, “one (wind-packed) giving a steady continuous relaxation of the excess pore pressure and the other (fresh powder) leading to a piston rebound with negative pore pressure.” Ah-ha—scientific evidence that it’s more fun skiing powder than crud.

What about tip design and planing, as opposed to float? Let’s add a tip to our imaginary ski. A straight tip at 90 degrees to the ski wouldn’t plane, but merely plow through the snow. The plowing force would oppose the parallel component of gravity and slow the ski—easy to imagine. If we adjust the tip to 135 degrees to the ski, the force on the tip would be evenly split—half plowing force, pushing the ski backward and half planing force, pushing the tip upward—pretty much like an old-school short tip. By making the tip longer, with a larger angle to the ski, say 160 degrees, most of the force of the snow translates to planing and very little to plowing—the magic of early rise.

Ski width, length, shape, tip height, tip length and tip shape all impact plane and float. Independent of the ski, various factors determine float and plane including skier weight, snow density, speed, slope angle and turn radius. How will all this evolve? My guess: designs that enhance the surfy, floaty feel in the fluff while maintaining competence on the firm will lead the way.

Ski Design: Where Rocker and Camber Meet

Rockered Tip and Tail, Camber Underfoot

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This approach, the most popular design currently, performs well over a range of conditions. Early rise tip lengths range from 300-500mm, and they plane easily in softer and cruddy conditions, while camber underfoot improves edge pressure on firm snow. The raised tail balances the tip for a smooth, fluid feel in powder. The downside: in steeper, firmer conditions, the tail can feel unsupportive. Ski It: Voilé V8, Rossignol Soul 7

Rockered Tip, Flat Tail, Camber Underfoot

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Some prefer a stiff, solid tail to finish turns in steep, icy couloirs or to pop into the next turn when carving firm. This design features the same early rise tip and camber underfoot as the first category, but finishes it with a straight tail. Jamming the tail into the snow makes for a quick anchor when ski mountaineering. Ski It: K2 Annex 98 and Annex 108, Dynastar Cham series

Slight Full Rocker

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Modern full rocker falls about halfway between a traditional cambered ski and full reverse camber, seeking to maintain the benefits of both while minimizing their drawbacks. Slight full rocker allows a ski to plane and initiate turns easily in soft snow. Having less rocker than a banana ski keeps it carving on firm snow. On some designs, a blunt tip rise can fight with crusts and crud more than a gradual early rise. Ski It: Völkl One, Two and Nunataq, G3 Empire and Empress

3D Rocker

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Rockered tips smooth the contact between tip and snow by lessening the angle, but in our thought experiment on tip design and planing, we only considered figure 11s. Throw in some turns, and things get much more complicated. When a ski is angulated in a powder turn, the edge doesn’t just carve like on hardpack, it slides down the fall line, too. Making the ski base convex, like the underside of a spoon, reduces that contact angle and extends the surfy, fluid feeling of rocker into tighter turns. Ski It: DPS’s Spoon, Lotus 138 and Lotus 120

This story first appeared in the February/March 2014 issue of Backcountry Magazine.

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