There is more to selecting cross country classic skis than the price.
By Martin Wiesiolek, PSIA-RM Education Staff
There is a good chance that the classic skis in your local used gear shop will not set you up for success in learning skiing or honing your skills unless they are matched to your weight, height, typical snow conditions where you will be skiing, and your ability level.
Classic Ski Flex, Camber and Structure
For classic cross country skis to perform well, you should consider three factors:
- Ski flex design: needs to be correct for the skier’s weight and ability level
- Ski camber: needs to be appropriate for the snow conditions
- Ski base structure: should be specific to the typical snow temperature and moisture content of where you ski
For beginner to intermediate classic skiers, skier weight and ability level are the most important factors. For intermediate to advanced skiers, typical snow conditions at their favorite ski areas or races play an important role too.
Ski Flex
The ski flex (tension) must be appropriate for the skier’s weight and skill level.
The ski flex is three things:
1. Cross country skis have a certain amount of pre-tensioning that creates the camber (bowing up) in the ski. The pre-tension is the distance between the ski and the snow surface, without any pressure (weight) on the ski.
2. When the skier steps on the skis and stands evenly on both feet they create a residual tension, which is the small gap left between the ski and the snow surface underneath the binding area.
3. The camber pressure is the force necessary to press the ski down on a flat surface until only a 0.2 mm gap in the binding area between the skis and the surface can be measured. In other words, the camber pressure is the force necessary to connect the ski with the snow. The 0.2 mm gap left underneath the binding area is needed for the layers of grip wax or the skin.
Ski manufacturers provide ski flex reference tables for selecting ski length for the skier’s weight, which ensures that the ski will flex just right:
- For propulsion: a skier would apply force to one ski to cause the ski’s grip zone to come in solid contact with the snow allowing the skier to push off from that ski;
- For gliding on both skis: a skier stands on both feet with their weight evenly distributed over both skis allowing the ski’s grip zone to hover just a bit above the snow preventing the ski from gripping;
- For gliding on one ski: a skier glides with their weight on one ski and while the ski’s grip zone is in the snow because the ski is flattened, the ski glides with minimum resistance because the ski isn’t over flexed (bowing down).
If you are a beginner skier and your weight falls near the low number of the recommended weight range for the ski, select the ski length for the next lower weight range (a shorter ski).
To see the classic skis flex: https://youtu.be/NUNkdz61GkY
Consequences of a ski being too stiff and not flexing enough for a skier: Pushing off becomes almost impossible as the skier cannot compress the ski enough for the grip zone to make contact with the snow. Additionally, the skier glides on tips and tails of the ski resulting in poor ski control and in extra resistance through friction in cold/soft snow conditions.
Consequences of the ski being too soft and flexing too much for the skier: The ski is slow due to the skier compressing the ski through the camber, thus resulting in permanent contact of the grip zone with the snow.
If buying used skis, you should have a professional shop help you determine if the skis are properly flexed for your weight. If you have no access to a competent shop, you can do a paper test. Here’s how to do a paper test:
With your weight equally distributed between both feet, stand on the skis with toes at the binding clip or the spot just behind where the ski balances. A friend should be able to slide a paper back and forth beneath the skis under your feet. When you shift all of your weight to one side or the other, the weighted ski should compress enough to trap the paper against the floor.
You can also search the internet to find the flex chart for the year, make, and model of your skis.
Ski Camber (Cold, Universal, Plus)
Snow conditions should dictate the camber of the ski.
Cross country skis bow upward in the middle, and that upward bowing of the ski is called the ski camber. A low camber ski has long contact zones with the snow while a high camber ski has short contact zones.
Ski Camber for Cold and Dry Snow
For a ski to be as fast as it can be and easy to maneuver in cold, dry snow, it should generally have a low camber and a relatively long area of contact in the tip and tail sections. The same amount of friction spread over a larger area (longer contact zones) translates into less friction per square inch and therefore faster glide in dry snow.
Ski Camber for Wet and/or Dirty Snow
In snow that has a high moisture content (“wet snow”) you would want a high camber ski that will fight suction and thus remain fast and maneuverable. A low camber and more contact between the ski base and the snow would result in more suction and more opportunity to pick up dirt creating more friction and more suction. The suction effect would lead to the ski braking. Also, dirty snow, even when cold and dry, creates the same braking effect as the wet snow. A low camber ski would result in more opportunities for dirt to attach itself to the ski base, making the ski slow due to the added friction. The dirt in the ski’s base structure affects the hydrophobic nature of the base material. In short, high-camber skis should be used in wet snow to fight the suction effect and/or to reduce friction in dirty snow.
Ski Base Structure
Snow conditions should dictate the structure of the ski’s base.
When your ski glides on snow it melts a microscopically thin film of water, which is what the ski glides on. If there is too much water your ski will develop suction with the snow’s surface, slowing the ski down. If there is too little water forming between your skis and the snow, this friction will also slow down your skis.
Ski Structure for Cold and Dry Snow
For a ski to be as fast as it can be and easy to maneuver in cold, dry snow, its base finish should have a structure designed for dry, abrasive snow crystals. Fine and smooth ski base finish will prevent the sharp-edged snow crystals from snagging the base and thus slowing creating a braking effect on the ski. Such structure is achieved with stone grinding, which is a process where a ski grinding/tuning machine uses special rotating stones to remove sealed base material, flatten the base, and apply final structure to the ski base surface. For skis that will be used in cold, dry snow, the final structure will be very smooth.
Ski Structure for Wet and/or Dirty Snow
For a ski to perform well in wet snow conditions it needs to have a high camber with a wet/warm structure on the base to minimize the contact with the wet snow.
Basic rules of cross country ski camber and structure:
- Long contact zones and fine structure reduce friction of cold and dry snow
- Short contact zones and deep structure reduce suction of warmer and wet snow
Hopefully this information helps you select a perfect pair of classic skis. Don’t hesitate to contact me with any comments or questions: martin@nordicskipro.com or through https://nordicskipro.com