Cross-Country Skiing Explained (Part 2): Geometry of Classic Cross-Country Skis

Copyright © 2017 Jared Manninen

From an outward appearance, classic cross-country skis look simple in their design. They are long, skinny, lightweight, and feature a graceful arc that (mostly) spans the length of the ski. The reality is, however, that the shape of classic cross-country skis, as well as the techniques used to run them, are deceptively complex. Understanding the basic geometry of classic cross-country skis will not only aid you in your selection of an appropriate sized ski, but also in your goal of becoming a better cross-country skier.

Like so many examples of extraordinary design, classic cross-country skis render the complex nature of their task into the simplest possible form. Probably the only superfluous element to find on a classic ski is the graphics of the top sheet. Other than that, every geometrical aspect of a ski serves to aid the skier in overcoming the forces of friction and gravity as they attempt to propel forward. The three geometrical characteristics of classic cross-country skis are that they’re relatively long, narrow, and lightweight. Another distinct physical characteristic of classic cross-country skis (related to their weight and shape) is that they lack metal edges.

Length

Traditionally, cross-country skis have been excessively long. Now they’re just generally long, at least in comparison to most alpine skis. The old standard by which you would measure the length of your skis was to reach into the air with one arm (while standing), then measure the height (length) to your wrist. This is no longer the case. Thanks to advances in design of the internal structure of classic cross-country skis, a shorter ski can bear weight more effectively. So, now the primary determining factor for the length of your skis is your body weight (plus whatever clothing and gear you’ll be wearing and carrying).

Looking grumpy, probably because I’m wearing too many layers. Salomon Equipe classic skis featuring “skins.” They’re around 192-197cms, which is the longest ski I can run at my weight (150lbs) and skill level. Photo © 2016 Pixie Baber.

I can’t emphasize enough how important it is to be honest about your weight in this context. If you rent or purchase skis that are designed to bear the weight of a lighter person you’ll end up dragging the grip zone and creating excess friction that will make even skiing downhill a drag (pun intended). On the other hand, if you run cross-country skis that are designed for a person more heavy than you, you’ll never be able to fully compress the ski which means that you’ll never be able to get any grip (which means you won’t be able to move forward!).

This question of appropriate length for classic cross-country skis was one of the most often asked questions I received while working at a cross-country ski center at Lake Tahoe. The answer is really simple, but the path to reach the answer is a bit more involved. If you just want the number (length in centimeters, which is how skis are measured) and don’t care about how you determine that number, just look up the ski manufacturer’s recommended length (for your body weight) for the specific model of ski for which you are researching. Using the ski manufacturer’s recommendation is the easiest method to determine the correct length for your classic cross-country skis. This is the option I recommend unless you are a more advanced skier and/or have a cross-country ski shop nearby with knowledgeable staff. Using the manufacturer’s recommendation is also the only option you have if you’re forced to buy skis online. The trick is actually finding online those recommendation charts. For some reason, manufacturers don’t actually post them on their websites. So, you’ll have to use one provided by a cross-country ski shop online. Even if all you want is the number, I do encourage you to keep reading about how that length is determined because it will give you a better understanding of how technique applies to the ski.

Now, If you have access to a gear shop that sells cross-country skis (and the employees know what they’re doing), I would recommend determining the correct length of your skis by using the “paper test.” There are two parts to this test and both require you and another person. You also need a standard sheet of paper and the actual skis you intend to buy. And please, don’t go to a shop, have them help you perform this test, and then go home to buy those skis online. First of all, that’s messed up. Secondly, there’s no guarantee that those skis (even if they’re the identical length) are going to have the same flex as the skis you tested in the store. But mostly just don’t do it because of the first reason. Small gear shops need all the business they can get, and should be supported when they provide quality customer service.

The first part of the paper test simulates the glide phase when double poling and your ability to glide straight downhill. For this part, place both skis on a solid flat surface that (hopefully) won’t scratch the base of the skis. Stand on both skis exactly where you would stand if you were wearing ski boots and clicked into the bindings. With your weight evenly distributed over both skis, have the other person run the paper under the grip zone (area where you would find the scale pattern on “waxless” skis). The person should be able to freely slide the paper back and forth from the front to the back of the grip zone. If the person cannot do this, then you are probably too heavy for those skis. Test the next length of skis. If the person can slide the paper well ahead or behind the grip zone, then you are probably too light for those skis. Test a shorter length of skis.

The second part of the paper test simulates the push-off phase of diagonal striding. For this part, leave the paper on the floor under one ski. Then transfer all of your weight to that ski (i.e. stand on just the one ski) and have the person try to pull out the paper from beneath the ski. If the person cannot remove the paper without ripping it, then those skis are probably fine so long as you’ve also passed the first test and determined that you’re not dragging the grip zone when standing on both skis. If the person can pull the paper out from underneath the ski, you are probably too light for the skis. Keep in mind that this is not 100% accurate because during the push-off phase you would be dynamically compressing the ski rather than simply standing on it like dead weight. If the other person can, but only slightly, pull out the paper while you’re standing on the one ski, it may be because your weight is shifting around while trying to retain your balance. Have the person try to pull out the paper only when you’ve “landed” on that one ski. If this works, you may be ok, but know that this may also indicate that you need to have exceptional technique in order to use these skis effectively. Test the next smallest size just in case.

The practice meadow at Tahoe Donner Cross Country ski area on the morning of January 6, 2017. © Jared Manninen

Some other notes to consider regarding the length of classic cross-country skis …

Lower end/lower valued skis tend to accommodate a wide range of weight. They are generally more forgiving in this respect. For example, a middle of the road recreational ski might be recommended for a person weighing between 140-175lbs. In contrast, a racing caliber ski will be more specific (around 10lbs. of allowance) in regard to the weight it will bear effectively.

For people who are short and heavy or tall people who are very light, you may want to deviate from using only your body weight as a guide in choosing the right length of ski. Basically, you don’t want to be the shortest person on the longest ski or the tallest person on the shortest ski. Without going into detail, just know that in both cases your technique will be affected negatively due to the extreme differences in length between you and your skis. In either of these cases, consider using a ski that’s a little closer to that length of your reach (like the old school method), but still in the ballpark for your weight.

Width

Copyright © 2017 Jared Manninen

Skis wider than 60mm don’t fit into classic tracks well, particularly in firm snow conditions. Click to read.

Probably the most obvious physical characteristic of classic cross-country skis is their width. They are quite skinny and, in fact, are often referred to as “skinny” skis in the Nordic world. This narrow width creates a small surface area, thereby reducing friction. In general, a typical classic ski will be between 40-50mm wide and fit easily into the standard tracks (approximately 60-70mm wide) laid by cross-country ski grooming machines. Beginner-oriented skis of the classic variety will be wider and closer to the 50mm mark, sometimes beyond. The wider ski, as you might imagine, will provide a little more stability for an inexperienced skier.

Another aspect of width that contributes to the efficiency of a classic cross-country ski is the lack of a pronounced sidecut. The term sidecut refers to the variation in width along the length of the ski. The three places that are measured to determine this value are the tip, waist, and tail of the ski. Often the measurement will be printed in the graphic on the top sheet of the ski and read tip-waist-tail (i.e. 41-44-44 or 48-44-46). All of the numbers are represented in millimeters which, if you recall, is different than the measurement used for the length of the ski (centimeters). Something to be aware of. In the first example (41-44-44), the tip is actually narrower than the rest of the ski and is referred to as having a “javelin” shape, while the other example (48-44-46) features a more traditional sidecut ratio where the waist is narrower than the tip or tail of the ski. That said, four millimeters of difference between the waist and tip, and two millimeters between the waist and tail and tip and tail is really not much of a factor when considering the overall length of the ski. In my case, that “curve” spans 192cms. For all intents and purposes, the ski is straight.

Classic cross-country skis don’t have sidecut to promote economy of movement when traveling in a straight line. A straight ski will track straight. Cross-country skis that feature a prominent sidecut (such as backcountry cross-country skis), on the other hand, will behave in a “squirrely” manner when trying to ski in a straight line. The ski is inclined to travel in accordance with its shape, so if it’s straight, it will go straight. If it’s curved, then it will follow that path.

The obvious drawback to the lack of sidecut is that if you want to turn, you will have to oppose the ski’s inherent nature of traveling in a straight line. This is why many alpine skiers who try out cross-country skiing for the first time freak out when going downhill. They quickly figure out that 1) the skis don’t have metal edges (described below), 2) there’s no sidecut with which to initiate a clean parallel turn, and 3) the ski is equipped with a double camber which is in sharp contrast to an alpine ski that has no camber or even a negative camber in some cases (camber is a future discussion). So yes, turning while skiing downhill in cross-country skis can be problematic, but just because the skis don’t have sidecut doesn’t mean you can’t turn. There are a number of different types of turns you can use, in fact, but they all require practice.

Weight

Except for some backcountry varieties, cross-country skis weigh next to nothing. Racing skis are well under two pounds. Even the low end models of recreation skis weigh less than four pounds. Steel-toed boots weigh more than that, but you don’t get to glide in those things! Alpine skiers are always amazed at how light cross-country skis are. I just chuckle and tell them, “Yeah, it’s so you can go uphill that much faster!”

The bottom line is that lighter skis reduce the amount of fatigue you’ll experience because you won’t be spending so much effort fighting gravity when skiing on the flats and uphill. Imagine the different experience you’d have running a 10k in those steel-toed boots rather than a pair of dedicated running shoes.

There is one thing to consider, however, when it comes to the weight of your skis. A lighter ski isn’t necessarily going to be a better ski for you. Some people actually prefer to feel some weight attached to their feet when cross-country skiing. The reason for this is because a slightly heavier ski will absorb some of your inefficiencies of technique. Visualize attaching a five-foot long stick made of balsa to one of your feet and to your other foot one made of pine. Every little twitch you make with your foot will transfer to that piece of balsa wood, whereas the pine stick (although still relatively light) will not be nearly as sensitive to your movement. So, if you want to run ultra-light (and ultra-expensive) skis, your technique better be impeccable.

Lastly, classic cross-country skis don’t have metal edges unless they are classified as some type of off-track touring or backcountry cross-country ski. The reason regular classic cross-country skis don’t have metal edges is because the metal increases the ski’s weight, changes its flex characteristics (generally makes the ski stiffer), and increases the amount of friction the ski will experience in snow. As nice as it is to have those metal edges when skiing in the backcountry and having to deal with wind-scoured, sunbaked, and crusty snow, the additional control (i.e. friction) that comes from those metal edges only conspires to slow you down on groomed trails. Unfortunately, no one has yet developed a limited-slip differential option for cross-country skis using those metal edges!

In conclusion, there are a million aspects and details about the geometry of classic cross-country skis intentionally omitted from this article. My goal was to provide an overview of the topic (without bogging you down in excessive detail as us Nordic nerds are want to do), so you would have a good starting point from which to conduct further research. Just remember that:

  • The appropriate length of classic cross-country skis is determined by their ability to bear weight without dragging the grip zone when gliding, but still allowing full compression of the grip zone (camber) during push-off.
  • The length of a person’s classic skis are related to their weight, but if the person is extra heavy or light in relation to their height, consider using a ski that’s closer to your reach (old method).
  • To determine the correct length of your skis refer to the manufacturer’s weight/length chart and, if possible, use the paper test.
  • A straight classic ski will track straight, while one with more pronounced sidecut will be easier to turn going downhill but be “squirrely” while trying to maintain a straight line when diagonal striding.
  • Classic skis are lightweight allowing you to travel further with less effort, but too light of a ski can be too sensitive and result in responding (moving or twitching) to your every slightest movement.

Please note that I write Cross-Country Skiing Explained with the beginner or intermediate cross-country skier in mind. This is the demographic for whom I most often served while working in the outdoor recreation industry at Lake Tahoe. I treat these articles as extensions of the conversations I’ve had with those customers. That said, expert skiers could take away something of value from this article. Just know that I don’t intend to specifically address racing-oriented philosophy, technique, or gear selection. There’s far more qualified people than me out there to do that!

More articles in the Cross-Country Skiing Explained Series:

For information about buying cross-country ski gear (for beginners), please read the following articles: