Ski Life: Ask the Professor, By the Numbers

Fall Line
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Fall Line

Dear Prof,
Sometimes I hear shaped-ski sidecuts described in terms of "depth" and other times according to "radius." What's the difference?

Ehran Altiner

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"Depth" and "radius" are simply two different ways of describing sidecut. Turn a ski upside-down, look down its length, and notice how the sidewall curves inward. That curve is its sidecut. Sidecut depth is simply the depth of that curve, measured in millimeters. Radius is derived mathematically, by theoretically extending the sidecut curve into a full circle, then citing that circle's radius in meters. The greater the depth, or shorter the radius, the tighter the turn a ski can make.

Most companies talk about radius, though a few¿notably K2¿talk about depth. One key difference is that radius is length-dependent while depth is not. So you can say that all K2 Merlin IVs have a 14 mm sidecut depth. But while an Atomic Beta Race 10.26 has a 26-meter radius at 193 cm (the length in which it was designed), its radius grows shorter or longer according to its length.

In addition to all this, remember that ski shapes are universally summed up using three numbers: tip-waist-tail, expressed in millimeters. For example, the Rossignol Salto: 102-65-93. Armed with these numbers, you can easily calculate the sidecut depth of any ski (see diagram) in order to compare models.

What shape is right for you? Consider these factors: Sidecut depth/radius: If you love carving tight, round arcs, go with a tight radius (18 meters or less) or deep sidecut depth (15 mm or more). If you prefer medium arcs and higher speeds, choose a less radical sidecut (over 18 meters, under 15 mm). Waist width: Narrow waists (under 65 mm) are more precise, better on hard snow. Wider waists (68 and up) are more forgiving, better on soft snow. Taper angle (tail width in relation to tip): A ski with a wide tail (ie., Atomic Beta Ride 9.22, 108-72-100) will stay locked in a carve longer than a ski with a narrow tail (Dynastar 4X4 Powertrac, 107-70-92), which will "release" sooner, and skid more readily. The former is more precise, the latter more forgiving. Have a question for The Professor? Write Stu Campbell at .