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Corrugation for Groomed Trails

By Auguste Lockwood, for Yellowstone Track Systems


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After the extraordinary snows of 2018-’19 in the U.S. and Canada, interest in winter recreation promises to grow phenomenally. For many of us, that means 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 corrugated panels (with parallel ridges or grooves) that increase strength and rigidity for roofs and siding. Those same qualities apply to trails, where consistent, durable, long-lived, and visually beautiful surfaces (yep, aesthetics matters in winter recreation!) 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, or the dew point. Corduroy increases the surface area of the trail, exposing more snow to the air where moisture transfer can happen. Corduroy also helps the packer pan track straight for “classic” cross-country ski trails.


Corduroy creates a very strong and stable surface if it’s in place long enough before being skied, snowmobiled, etc. In addition to the faster set-up time (sometimes called “mechanical age hardening”) and increased snow metamorphism, the triangular shape of the individual ridges adds strength and stiffness. In my experience, under ideal conditions, the 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 little caps are the first defense against trail degradation (flattening, friction, melt…).


En masse, these caps are extremely strong, easily holding the weight of a skier, fatbiker, etc., with virtually no damage or even marks to the trail. Older snowmobiles also do no damage to the caps, though modern sleds with tall sharp paddles and speed-happy drivers can degrade or even destroy any surface.


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. On a snowmobile trail, the tips quickly break down, providing lubrication for the slide rails, leaving just the bases of the corduroy ridges.


This slightly ridged trail can last for days or weeks, under the right conditions. I have often seen that a ridged trail sets up faster, bonds harder, and lasts longer than a flat trail. During my experimenting with a fatbike drag, a ridged trail would be walkable hours before a flat trail, under identical temperature, snow, and wind conditions. Sometimes a single-pass corduroy trail would be walkable, and a flat trail would never set up enough to walk on.


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


Here are some fatbiking insights that suggest both benefits and complexities of any type of grooming:


- With the widely spaced lugs of a fatbike 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 do happen to 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 makes corduroy a superior surface for fatbiking, as an increased number of ridges per foot of the YTS 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.


The comb also tracks well on a snowmobile trail, since the tire tends to follow the ridges and weird lean steering characteristics are greatly reduced. This should be mentioned to anyone who has concerns about their experience with wider-spaced corduroy. 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.

With a narrower trail, this weak spot is concentrated in the center of the trail. The most difficult part to groom, leading to the dreaded trough, I predict will soon be the scourge of narrow-gauge groomers worldwide. My grooming implement design concentrates heavily on the ability to maintain a 10 inch deep trough in firm snow.


Finally, from an aesthetic point of view for fatbiking, 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.


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