The Technical Reason Why Ski Trails Are Corrugated.

Freshly groomed XC trails last longer. Credit: North Shore Nordic Association

[Editor Note: This article was written in collaboration with Auguste Lockwood, Yellowstone Track Systems.]

Interest in winter recreation promises to grow phenomenally in winter 2020-’21, despite—or because of —COVID-19, and a lot of us are looking forward to more friends and families on more and better groomed trails.

One of the subtle, unsung, but vital elements of high-quality (read “fun”) cross-country ski, snowshoe, fatbike, and snowmobiling grooming is corrugation. You’re probably familiar with metal or plastic corrugated panels that increase strength and rigidity for roofs and siding. Those same qualities apply to trails, where consistent, durable, beautiful surfaces are essential for skiers, snowshoers, and riders as well as groomers. 

If you’re interested in either producing or using high-quality trails, first thing to know about corrugation is something about the physics of snow. Corduroy —that’s the grooming version of corrugation —has peaks and troughs that create minute pressure differentials. These differences promote the movement of free moisture up through the snowpack to the tips of the peaks, where evaporation is accelerated by increased exposure to cold air.

The phenomenon of “hot moves to cold” describes the movement of moisture from a warm material to a cold one, like condensation on a cold drink on a hot day. Corduroy increases the surface area of the trail, exposing more snow to the air where moisture transfer can happen.

Corduroy creates a strong, stable surface if it’s in place long enough before being used. In addition to faster set-up time (“mechanical age hardening”) and increased snow metamorphism, the triangular shape of the individual ridges adds strength and stiffness. Ideally, corduroy peaks form a thin glaze that extends about 1/3rd of the way down the trough, in effect building a little ice cap. With a well-distributed load, these caps are the first defense against trail flattening, friction, and melt.

En masse, these caps are extremely strong, easily holding the weight of a skier, fatbiker, etc., leaving virtually no damage or even marks. Once the caps or tips fail, the upper half of the ridge is fairly soft, but the layer just above the trail base (about 1/4 inch high) remains extremely strong.

This slightly ridged trail can last for days or weeks under the right conditions. A ridged trail sets up faster, bonds harder, and lasts longer than a flat trail.

Fat Biking On Corduroy

Fat biking promises to become a major winter sport that complements cross-country skiing and snowshoeing, and it’s new enough that grooming for it is a little mysterious. Groomers’ knowledge about any of these sports can help create a premium product for the others.

With the widely-spaced lugs of a fat bike tire, there are two types of interaction with the trail: the lugs, and the smooth rubber of the tire. A corduroy surface makes for a better riding experience by providing consistent traction. With corduroy, tire lugs fall between the peaks of the ridges, allowing the smooth rubber to evenly distribute pressure between the tips of three to five ridges at a time.

Lugs that contact the ridges will crush the snow, digging in and providing lateral traction, while the lugs between ridges grab the edge of a ridge and provide lateral traction, without damaging the trail. This creates a superior surface, as an increased number of ridges per foot of the grooming comb has several advantages.

One advantage is increased surface area, decreasing setup (consolidation) time. Another is more surface contact with a tire, since with widely spaced ridges, the tire will only be contacting a couple of ridge tips, increasing the pressure on the tips and breaking them. In contrast, on a smooth trail, tire lugs either don’t penetrate enough if the trail is too firm, thus reducing traction; or they’ll penetrate the top surface of the trail, until the weight is transferred to the smooth rubber of the tire.

The ideal density can be hard to achieve—too hard and there’s not much traction, too soft and the weight of the rider will crush the top layer of trail, creating a weak spot.

Finally, from an aesthetic point of view for fat biking, really good corduroy leaves a small line of snow on the tire. So while riding, there are three or four (depending on tire width) thin white lines that circle the tire. This produces a mesmerizing pattern on the wheels to watch on your front tire or on the back tire of someone you’re following. The lines run longitudinally around the tire if you’re going straight, but you can zigzag and make cool patterns as they interact with the regularly spaced lugs while turning.


One Comment

  1. Albert Pierce says:

    Great article. Thanks for the clear explanation of what happens to the snow and how the corduroy shape can prolong snow stability and ultimately a ski season, is very interesting.

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